EMBRYOPSIDA Pirani & Prado

Gametophyte dominant, independent, multicellular, not motile, initially ±globular; showing gravitropism; acquisition of phenylalanine lysase [PAL], microbial terpene synthase-like genes +, triterpenoids produced by CYP716 enzymes, phenylpropanoid metabolism [lignans +, flavonoids + (absorbtion of UV radiation)], xyloglucans in primary cell wall, side chains charged; plant poikilohydrous [protoplasm dessication tolerant], ectohydrous [free water outside plant physiologically important]; thalloid, leafy, with single-celled apical meristem, tissues little differentiated, rhizoids +, unicellular; chloroplasts several per cell, pyrenoids 0; glycolate metabolism in leaf peroxisomes [glyoxysomes]; centrioles/centrosomes in vegetative cells 0, microtubules with γ-tubulin along their lengths [?here], interphase microtubules form hoop-like system; metaphase spindle anastral, predictive preprophase band + [with microtubules and F-actin; where new cell wall will form], phragmoplast + [cell wall deposition centrifugal, from around the anaphase spindle], plasmodesmata +; antheridia and archegonia jacketed, surficial; blepharoplast +, centrioles develop de novo, bicentriole pair coaxial, separate at midpoint, centrioles rotate, associated with basal bodies of cilia, multilayered structure + [4 layers: L1, L4, tubules; L2, L3, short vertical lamellae] (0), spline + [tubules from L1 encircling spermatid], basal body 200-250 nm long, associated with amorphous electron-dense material, microtubules in basal end lacking symmetry, stellate array of filaments in transition zone extended, axonemal cap 0 [microtubules disorganized at apex of cilium]; male gametes [spermatozoids] with a left-handed coil, cilia 2, lateral; oogamy; sporophyte multicellular, cuticle +, plane of first cell division transverse [with respect to long axis of archegonium/embryo sac], sporangium and upper part of seta developing from epibasal cell [towards the archegonial neck, exoscopic], with at least transient apical cell [?level], initially surrounded by and dependent on gametophyte, placental transfer cells +, in both sporophyte and gametophyte, wall ingrowths develop early; suspensor/foot +, cells at foot tip somewhat haustorial; sporangium +, single, terminal, dehiscence longitudinal; meiosis sporic, monoplastidic, MTOC [MTOC = microtubule organizing centre] associated with plastid, sporocytes 4-lobed, cytokinesis simultaneous, preceding nuclear division, quadripolar microtubule system +; wall development both centripetal and centrifugal, 1000 spores/sporangium, sporopollenin in the spore wall laid down in association with trilamellar layers [white-line centred lamellae; tripartite lamellae]; nuclear genome size [1C] <1.4 pg, main telomere sequence motif TTTAGGG, LEAFY and KNOX1 and KNOX2 genes present, ethylene involved in elongation; chloroplast genome with introns (not: Mesostigma), close association between trnLUAA and trnFGAA genes [precursors for starch synthesis], tufA gene moved to nucleus; mitochondrial trnS(gcu) and trnN(guu) genes +.

Many of the bolded characters in the characterization above are apomorphies of subsets of streptophytes along the lineage leading to the embryophytes, not apomorphies of crown-group embryophytes per se.

All groups below are crown groups, nearly all are extant. Characters mentioned are those of the immediate common ancestor of the group, [] contains explanatory material, () features common in clade, exact status unclear.


Abscisic acid, L- and D-methionine distinguished metabolically; pro- and metaphase spindles acentric; sporophyte with polar transport of auxins, class 1 KNOX genes expressed in sporangium alone; sporangium wall 4≤ cells across [≡ eusporangium], tapetum +, secreting sporopollenin, which obscures outer white-line centred lamellae, columella +, developing from endothecial cells; stomata +, on sporangium, anomocytic, cell lineage that produces them with symmetric divisions [perigenous]; underlying similarities in the development of conducting tissue and of rhizoids/root hairs; spores trilete; shoot meristem patterning gene families expressed; MIKC, MI*K*C* genes, post-transcriptional editing of chloroplast genes; gain of three group II mitochondrial introns, mitochondrial trnS(gcu) and trnN(guu) genes 0.

[Anthocerophyta + Polysporangiophyta]: gametophyte leafless; archegonia embedded/sunken [only neck protruding]; sporophyte long-lived, chlorophyllous; cell walls with xylans.


Sporophyte well developed, branched, branching apical, dichotomous, potentially indeterminate; hydroids +; stomata on stem; sporangia several, terminal; spore walls not multilamellate [?here].


Vascular tissue + [tracheids, walls with bars of secondary thickening].


Sporophyte with photosynthetic red light response, stomata open in response to blue light; plant homoiohydrous [water content of protoplasm relatively stable]; control of leaf hydration passive; plant endohydrous [physiologically important free water inside plant]; (condensed or nonhydrolyzable tannins/proanthocyanidins +); xyloglucans with side chains uncharged [?level], in secondary walls of vascular and mechanical tissue; lignins +; stem apex multicellular, with cytohistochemical zonation, plasmodesmata formation based on cell lineage; tracheids +, in both protoxylem and metaxylem, G- and S-types; sieve cells + [nucleus degenerating]; endodermis +; leaves/sporophylls spirally arranged, blades with mean venation density ca 1.8 mm/mm2 [to 5 mm/mm2]; sporangia adaxial, columella 0; tapetum glandular; ?position of transfer cells; MTOCs not associated with plastids, basal body 350-550 nm long, stellate array in transition region initially joining microtubule triplets; suspensor +, shoot apex developing away from micropyle/archegonial neck [from hypobasal cell, endoscopic], root lateral with respect to the longitudinal axis of the embryo [plant homorhizic].


Sporophyte endomycorrhizal [with Glomeromycota]; growth ± monopodial, branching spiral; roots +, endogenous, positively geotropic, root hairs and root cap +, protoxylem exarch, lateral roots +, endogenous; G-type tracheids +, with scalariform-bordered pits; leaves with apical/marginal growth, venation development basipetal, growth determinate; sporangium dehiscence by a single longitudinal slit; cells polyplastidic, MTOCs diffuse, perinuclear, migratory; blepharoplasts +, paired, with electron-dense material, centrioles on periphery, male gametes multiciliate; chloroplast long single copy ca 30kb inversion [from psbM to ycf2]; mitochondrion with loss of 4 genes, absence of numerous group II introns; LITTLE ZIPPER proteins.


Sporophyte woody; stem branching lateral, meristems axillary; lateral root origin from the pericycle; cork cambium + [producing cork abaxially], vascular cambium bifacial [producing phloem abaxially and xylem adaxially].


Plants heterosporous; megasporangium surrounded by cupule [i.e. = unitegmic ovule, cupule = integument]; pollen lands on ovule; megaspore germination endosporic [female gametophyte initially retained on the plant].


Plant evergreen; nicotinic acid metabolised to trigonelline, (cyanogenesis via tyrosine pathway); microbial terpene synthase-like genes 0; primary cell walls rich in xyloglucans and/or glucomannans, 25-30% pectin [Type I walls]; lignin chains started by monolignol dimerization [resinols common], particularly with guaiacyl and p-hydroxyphenyl [G + H] units [sinapyl units uncommon, no Maüle reaction]; root stele diarch to pentarch, xylem and phloem originating on alternating radii, cork cambium deep seated; stem apical meristem complex [with quiescent centre, etc.], plasmodesma density in SAM 1.6-6.2[mean]/μm2 [interface-specific plasmodesmatal network]; eustele +, protoxylem endarch, endodermis 0; wood homoxylous, tracheids and rays alone, tracheid/tracheid pits circular, bordered; mature sieve tube/cell lacking functioning nucleus, sieve tube plastids with starch grains; phloem fibres +; cork cambium superficial; leaf nodes 1:1, a single trace leaving the vascular sympodium; leaf vascular bundles amphicribral; guard cells the only epidermal cells with chloroplasts, stomatal pore with active opening in response to leaf hydration, control by abscisic acid, metabolic regulation of water use efficiency, etc.; axillary buds +, exogenous; prophylls two, lateral; leaves with petiole and lamina, development basipetal, lamina simple; sporangia borne on sporophylls; spores not dormant; microsporophylls aggregated in indeterminate cones/strobili; grains monosulcate, aperture in ana- position [distal], primexine + [involved in exine pattern formation with deposition of sporopollenin from tapetum there], exine and intine homogeneous, exine alveolar/honeycomb; ovules with parietal tissue [= crassinucellate], megaspore tetrad linear, functional megaspore single, chalazal, sporopollenin 0; gametophyte ± wholly dependent on sporophyte, development initially endosporic [apical cell 0, rhizoids 0, etc.]; male gametophyte with tube developing from distal end of grain, male gametes two, developing after pollination, with cell walls; female gametophyte initially syncytial, walls then surrounding individual nuclei; embryo cellular ab initio, suspensor short-minute, embryonic axis straight [shoot and root at opposite ends; plant allorhizic], cotyledons 2; embryo ± dormant; chloroplast ycf2 gene in inverted repeat, trans splicing of five mitochondrial group II introns, rpl6 gene absent; whole nuclear genome duplication [ζ - zeta - duplication], two copies of LEAFY gene, PHY gene duplications [three - [BP [A/N + C/O]] - copies], 5.8S and 5S rDNA in separate clusters.

VII. ANGIOSPERMAE Lindley / MAGNOLIIDAE Takhtajan  Back to Main Tree

Lignans, O-methyl flavonols, dihydroflavonols, triterpenoid oleanane, apigenin and/or luteolin scattered, [cyanogenesis in ANA grade?], lignin also with syringyl units common [G + S lignin, positive Maüle reaction - syringyl:guaiacyl ratio more than 2-2.5:1], hemicelluloses as xyloglucans; root cap meristem closed (open); pith relatively inconspicuous, lateral roots initiated immediately to the side of [when diarch] or opposite xylem poles; origin of epidermis with no clear pattern [probably from inner layer of root cap], trichoblasts [differentiated root hair-forming cells] 0, hypodermis suberised and with Casparian strip [= exodermis]; shoot apex with tunica-corpus construction, tunica 2-layered; starch grains simple; primary cell wall mostly with pectic polysaccharides, poor in mannans; tracheid:tracheid [end wall] plates with scalariform pitting, wood parenchyma +; sieve tubes enucleate, sieve plate with pores (0.1-)0.5-10< µm across, cytoplasm with P-proteins, not occluding pores of plate, companion cell and sieve tube from same mother cell; ?phloem loading/sugar transport; nodes 1:?; dark reversal Pfr → Pr; protoplasm dessication tolerant [plant poikilohydric]; stomata brachyparacytic [ends of subsidiary cells level with ends of pore], outer stomatal ledges producing vestibule, reduction in s tomatal conductance with increasing CO2 concentration; lamina formed from the primordial leaf apex, margins toothed, development of venation acropetal, overall growth ± diffuse, secondary veins pinnate, fine venation hierarchical-reticulate, (1.7-)4.1(-5.7) mm/mm2, vein endings free; flowers perfect, pedicellate, ± haplomorphic, protogynous; parts free, numbers variable, development centripetal; P +, ?insertion, members each with a single trace, outer members not sharply differentiated from the others, not enclosing the floral bud; A many, filament not sharply distinguished from anther, stout, broad, with a single trace, anther introrse, tetrasporangiate, sporangia in two groups of two [dithecal], each theca dehiscing longitudinally by a common slit, ± embedded in the filament, walls with at least outer secondary parietal cells dividing, endothecium +, cells elongated at right angles to long axis of anther; tapetal cells binucleate; microspore mother cells in a block, microsporogenesis successive, walls developing by centripetal furrowing; pollen subspherical, tectum continuous or microperforate, ektexine columellate, endexine lamellate only in the apertural regions, thin, compact, intine in apertural areas thick, pollenkitt +; nectary 0; carpels present, superior, free, several, ascidiate [postgenital occlusion by secretion], stylulus at most short [shorter than ovary], hollow, cavity not lined by distinct epidermal layer, stigma ± decurrent, carinal, dry; suprastylar extragynoecial compitum +; ovules few [?1]/carpel, marginal, anatropous, bitegmic, micropyle endostomal, outer integument 2-3 cells across, often largely subdermal in origin, inner integument 2-3 cells across, often dermal in origin, parietal tissue 1-3 cells across, nucellar cap?; megasporocyte single, hypodermal, functional megaspore lacking cuticle; female gametophyte lacking chlorophyll, not photosynthesising, four-celled [one module, nucleus of egg cell sister to one of the polar nuclei]; ovule not increasing in size between pollination and fertilization; pollen grains land on stigma, bicellular at dispersal, mature male gametophyte tricellular, germinating in less than 3 hours, pollen tube elongated, unbranched, growing towards the ovule, between cells, growth rate (20-)80-20,000 µm/hour, apex of pectins, wall with callose, lumen with callose plugs, penetration of ovules via micropyle [porogamous], whole process takes ca 18 hours, distance to first ovule 1.1-2.1 mm; male gametes lacking cell walls, ciliae 0, siphonogamy; double fertilization +, ovules aborting unless fertilized; P deciduous in fruit; mature seed much larger than fertilized ovule, small [], dry [no sarcotesta], exotestal; endosperm +, cellular, development heteropolar [first division oblique, micropylar end initially with a single large cell, divisions uniseriate, chalazal cell smaller, divisions in several planes], copious, oily and/or proteinaceous, embryo short [<¼ length of seed]; plastid and mitochondrial transmission maternal; Arabidopsis-type telomeres [(TTTAGGG)n]; nuclear genome size [1C] <1.4 pg [mean 1C = 18.1 pg, 1 pg = 109 base pairs], whole nuclear genome duplication [ε/epsilon event]; ndhB gene 21 codons enlarged at the 5' end, single copy of LEAFY and RPB2 gene, knox genes extensively duplicated [A1-A4], AP1/FUL gene, palaeo AP3 and PI genes [paralogous B-class genes] +, with "DEAER" motif, SEP3/LOFSEP and three copies of the PHY gene, [PHYB [PHYA + PHYC]]; chloroplast chlB, -L, -N, trnP-GGG genes 0.

Age. Age estimates of crown-group angiosperms vary considerably, although many are in the range (210-)150-140(-130) m.y. (e.g. J. A. Doyle 2001; Sanderson & Doyle 2001; Wikström et al. 2001; Aoki et al. 2004; Davis et al. 2004a; Sanderson et al. 2004; Bell et al. 2005; Leebens-Mack et al. 2005; Moore et al. 2007; Soltis et al. 2008: a variety of estimates; Moore et al. 2010). Bell et al. (2010: note topology) suggest ages of (199-)183(-167) and (154-)147(-141) m.y. (see also Magallón 2009) and Iles et al. (2014) ages of (167.7-)158.7(-151) m.y. ago. Some recent estimates based on molecular data tend to be substantially older than others, Magallón (2008 and references) and Magallón & Castillo (2009) noting ages of 182-158 m.y. and 130 or 242 m.y. respectively, i.e. mostly Lower to Middle Jurassic or older, indeed, S. A. Smith et al. (2010: see esp. Table S3) suggested that crown-group angiosperms were (270-)228, 217(-182) m.y.o., and there are ages of 275-216 m.y. in Magallón (2010), (280-)246, 209(-186) m.y. in Zeng et al. (2014; see also Rothfels et al. 2015b) and ca 279 m.y.a. in Z. Wu et al. (2014). Yet again, other ages are somewhat younger, e.g. (240-)205(-175) m.y. in Clarke et al. (2011: other dates), (256-)198(-163) m.y. in N. Zhang et al. (2012, similar in Xue et al. 2012), (257.9-)208.7-193.7(-157.7) m.y. in Magallón et al. (2013) for this clade and around 195.4-185.3 m.y. in Naumann et al. (2013); see also Schneider et al. (2004). (152-)144(-133) m.y.a. is the estimate in Silvestro et al. (2015), ca 244.7 m.y. in Tank et al. (2015), and (253-)221, 206(-176) m.y., or as much as ca 242 or as little as 161-154 m.y. in Foster et al. (2016a, q.v. for caveats). Magallon et al. (2015) estimate the age of crown-group angiosperms to be around (141-)139.4(-136) m.y.a. by taking the fossil record as an indicator of their age; they argue that since quite a number of fossils are known soon after this date, their absence before is perhaps real; dates of clades within angiosperms from this paper should be interpreted accordingly (see also Sanderson 2015). Beaulieu et al. (2015) suggest that some older ages for crown-group angiosperms should be treated with caution, finding ages of around 140-130 m.y. where (270-)228-217(-182) m.y. was the previous estimate (S. A. Smith et al. 2010), although Beaulieu et al. themselves were pretty agnostic about what any "real" age might be. Ca 130 m.y. is the estimate in Laenen et al. (2014). It is amusing to see a graph of suggested angiosperm ages against the publication dates of these ages...

Indirect estimates are also interesting. Thus angiosperms are optimized as the ancestral host plants for sawfly larvae - and sawflies are estimated to be Triassic in age (Schneider 2016: see caveats there).

Note: Boldface denotes possible apomorphies, (....) denotes a feature common in the clade, exact status uncertain, [....] includes explanatory material. Note that the particular node to which many characters, particularly the more cryptic ones, should be assigned is unclear. This is partly because homoplasy is very common, in addition, basic information for all too many characters is very incomplete, frequently coming from taxa well embedded in the clade of interest and so making the position of any putative apomorphy uncertain. Then there are the not-so-trivial issues of how character states are delimited and ancestral states are reconstructed (see above).

Evolution: Divergence & Distribution. For dates of clades throughout angiosperms, see e.g. Hedges and Kumar (2009: comprehensive summary of early dates), Foster et al (2016a) and the hundreds of dates in Harris and Davies (2016: Table S4); I have not tried to incorporate all these below.

A net diversification increase possibly associated with the ε/epsilon nuclear genome duplication of angiosperms (see this node) is placed at the Mesangiosperm node (Tank et al. 2015). P. Soltis and Soltis (2016) briefly discuss the evolution of the flower in the context of this duplication. In general, the clade size imbalance at the base of the angiosperm tree should give us pause for thought when thinking about genome duplications and their relationship to diversification (see also Lunau 2004). See also Sánchez-Reyes et al. (2017) for angiosperm diversification and age/richness correlations.

Different topologies of ANA-grade angiosperms (see below) may have little effect when thinking about the evolution of characters like the evolution of habit/habitat (c.f. Barkman 2000b; Drew et al. 2014), but the patterns of variation of some of these characters, problems with the delimitation of character states and their optimisation (see above), and the highly derived nature of Hydatellaceae in particular (see also Endress & Doyle 2015) should cause some concern. The morphology of those gymnosperms - currently largely unknown - that are on the angiosperm stem clade will also affect the level at which some of the "angiosperm" apomorphies above are to be pegged. "Key evolutionary innovations" are placed on a tree by Z.-H. Wang et al. (2017) in odd places, c.f. also the tree topology, also odd.

When thinking of the further evolution of angiosperm morphology, the uncertainty in relationships above the ANA grade - see the near-basal pentatomy in the main tree here - makes life very difficult, to say the least. Note that one topology used by Endress and Doyle (2015) on which to optimize characters is [ANA-grade angiosperms [[Chloranthales + Ceratophyllales] [[magnoliids + monocots] eudicots]], in another the relationships are [ANA-grade angiosperms [[Chloranthales + magnoliids] [monocots [Ceratophyllales + eudicots]]] (see the chloroplast gene analyses of Jansen et al. 2007; Moore et al. 2007). Zeng et al. (2014) optimised the distribution of a large number of characters on a tree with a topology [monocots [magnoliids [[Ceratophyllum + Chloranthaceae] [eudicots]]] (see also below).

For the floral features of the ancestral angiosperm, see e.g. Doyle and Endress (2000), Endress (2001a), Endress and Doyle (2015) and Gottsberger (2016). Primary polyandry in which stamens develop centripetally from separate perimordia, is common in the ANA grade, Magnoliids, etc.; for further discussion of polyandry, see Pentapetalae and eusterids. Protogyny is very common in "basal" angiosperms (Routley et al. 2004: Amborellaceae are ± dioecious), while protandry is common in eudicots, and above Alismatales in monocots (see also Bertin & Newman 1993; Endress 1994b). For connections between protogyny and protandry and self compatability and self incompatability, see Routley et al. (2004 and references). Armbruster et al. (2002), Hristova et al. (2005) and Sage et al. (2009) suggest that the basic condition of the angiosperm stigma may have been dry, with the pollen tubes growing intracellularly, rather than wet, with the pollen tubes growing in the stigmatic secretion (see also Thien et al. 2009). For differences in pollen tube growth in Pinaceae and angiosperms see Rounds and Benzanilla (2013)

The "standard view" is that the ancestor of flowering plants had monosulcate pollen (e.g. Walker & Walker 1984; Doyle 2013), but pollen of Amborella is anaulcerate. However, if reticulate-perforate pollen is optimized to the second node on the tree (see Friis et al. 2009 for a discussion), it makes the pollen morphology of the common ancestor of all angiosperms ambiguous, and the exine of Amborella has been variously interpreted (J. A. Doyle 2009). For exine development and sporopollenin production, see e.g. Heslop-Harrison (e.g. 1968; papers in 1971), Ariizumi and Toriyama (2011), Chebli et al. (2012) and Quilichini et al. (2015), and for tapetum type and orbicule production in particular, see Verstraete et al. (2014 and references). Sporophytic tapetal cells play an important role in pollen wall development (Heslop-Harrison 1968; see also Blackmore et al. 2009 for a good introduction). For pollen characters of angiosperms of the ANA grade and the magnoliids, see also under the respective orders, Sampson (2000), Doyle (2005: c.f. topologies of trees used, 2008a, 2008b). The pollen morphology of Amborellaceae is not well understood and there is much variation in microsporogenesis and pollen morphology in Nymphaeales, Amborellaceae, etc. (e.g. Furness et al. 2002; Taylor et al. 2015). Wortley et al. (2015), Luo et al. (2015) and Lu et al. (2015) - note character state delimitation, sampling, treatment of variation, and optimization - place pollen variation on the tree, and their work should be consulted by those interested. Pollenkitt of many angiosperms includes hydroxycinnamic acid (HCAA) and flavonol α-1,2-linked diglycosides (3-O-sophorosides) of tapetal origin that may be antioxidants protecting the pollen against UV radiation (Fellenberg & Vogt 2015: "tryphine"; see also below). Pacini and Hesse (2005) also discuss possible functions of pollenkitt.

Armbruster et al. (2002) and X.-F. Wang et al. (2011) discuss the evolution of syncarpy and the complex patterns of gains and losses of various kinds of compita (see also Endress 2001a; Endress & Igersheim 2000; etc.). Staedler et al. (2009; see also Wang et al. 2011) thought that the presence of an extragynoecial compitum could be an apomorphy of angiosperms, and it would then have to be lost at least twice. See also Endress and Igersheim (1997 and references), Armbruster et al. (2002), and Endress (2015) for where to place compitum characters on the tree, but this will depend on the optimization procedure used and of course resolution of relationships along the backbone of the tree. For gynoecial morphology and carpel closure, see e.g. Endress (2015), also Sokoloff et al. (2013).

In Amborellaceae and some other ANA-grade angiosperms, including Hydatellaceae, the stigma has multicellular papillae. Endress (2001a) notes i.a. that the carpels (?of the ancestral angiosperm) may have uniseriate hairs, and in Trimeniaceae and Nymphaeales the apical cell of these hairs is elongated and tanniniferous. For the evolution of placentation "types", see Ickert-Bond et al. (2014c). Features such as parietal tissue (= nucellus) two to three cell layers across above the embryo sac and a stylar canal lacking an epidermal layer are plesiomorphous for ANA angiosperms (Williams 2009), but where higher up on the tree they might change is unclear.

Friedman (2006; c.f. Tobe et al. 2000) described the very distinctive embryo sac of Amborella: A third synergid cell arises from a cell division that also produces the female gamete/egg, and the result is a 9-nucleate embryo sac. In nearly all other angiosperms the polar nuclei are sister to the egg nucleus (at one end) and the central chalazal nucleus (at the other), and the egg is produced by a nuclear division. Friedman and Ryerson (2009; see also Xi et al. 2014) discuss the evolution of the angiosperm embryo sac in detail, and that of Amborella may be derived, but it will depend on how characters are optimized. Porsch (1907) early took the view that the micropylar and chalazal ends of the eight-nucleate embryo sac were identical, so the eight- (or nine-)nucleate embryo sac would then result from the duplication of a 4-nucleate unit such as has been found in many members of the ANA grade. Indeed, Tobe (2016) has described embryogenesis and fertilization in the odd genus Cardiopteris (Aquifoliales-Cardiopteridaceae) where it seems that cells that would be antipodal cells in the normal Polygonum embryo sac form the egg apparatus. Indeed, the development of the embryo sac is a complicated operation, Tekleyohans et al. (2016) bemoaning tha fact that "the remarkable ability of all FG [female gametophyte] cells, to be basically able to adopt any other FG cell identity, has complicated the interpretation of mutant phenotypes.".

For details of the fertilization process - the two male gametes are identical, if not in Plumbago zeylanica, perhaps a special case - see Kawashima and Berger (2011). Whether or not a triploid endosperm is a synapomorphy for all angiosperms or only for those angiosperms above the ANA grade is similarly unclear (Friedman 2001a, b, 2006; Baroux et al. 2002), and Friedman et al. (2003a, esp. b) and Friedman and Williams (2003, 2004) incline towards the latter hypothesis. If diploid endosperm is the ancestral condition, triploid endosperm will have evolved in parallel in Amborella and again in the mesangiosperm clade (see also Williams & Friedman 2004 and references; Xi et al. 2014). The diploid endosperm in Nymphaeales is slight and it probably functions as transfer tissue betweem embryo and perisperm (Friedman et al. 2012). For more on endosperm evolution, see above.

The triterpenoid oleanane is widely distributed in angiosperms, in the long-extinct seed plant groups giganopterids and Bennettitales, and also in some ferns (Taylor et al. 2006; Feild & Arens 2007 and references), and the triterpenoid isoarborinol is synthesized by a marine bacterium (Banta et al. 2017). For woodiness and wood anatomy, see S. Kim et al. (2004a) and Herendeen et al. (1999: a useful table), for changes in phyllotaxy, Ronse De Craene et al. (2003); chloranthoid leaf teeth - three veins converging at or below the apical gland of the tooth - may be a feature of all angiosperms (J. A. Doyle 2007; Doyle & Upchurch 2014). For a survey and evaluation of floral morphology and systematics, see Matthews and Endress (2012) and for floral evolution in ANA angiosperms in particular, see Endress and Doyle (2015). For further discussion on evolution and diversification, see e.g. Hasebe (1999), D. Soltis et al. (2005a, b), and Laenen et al. (2014: ages and diversification rates for a number of clades), see also Hickey and Taylor (1995), Rudall (2013) and Claßen-Bockhoff (2016b), all evolution of the flower, Bachelier and Friedman (2011: female gametophyte competition within a single ovule and angiosperm evolution), Taylor and Kirchner (1995: carpel evolution), Wing and Boucher (1998: ecology), Donoghue (2004: general), Whitney (2009: stronger selection for divergent flower than fruit morphology). For the evolution of microsporogenesis, see especially Furness et al. (2002b) and Taylor and Osborn (2006). What goes on in the male gametophyte in the period from pollination to fertilization is of much current interest (Williams & Mazer 2016 for literature). Williams (2008, 2009, 2012a, esp. 2012b; Williams et al. 2016) look at pollen tube/male gametophyte development, nucellus, etc., and Harder et al. (2016) at different patterns of pollen tube growth in the style, basically, the population ecology of male gametophytes. For variation in embryo size, see Verdú (2006), and for a possible chromosome base number for angiosperms, x = 7, see Oginuma et al. (2000). For various other aspects of early angiosperm evolution, see below.

Ecology & Physiology. See Early Cretaceous Evolution below for the ecology of ANA-grade angiosperms and of that of early angiosperms in general.

The mycorrhizal condition of members of the ANA grade is largely unknown, as is that of Canellales, Piperales, and most Laurales. Mycorrhizae are absent in Nymphaeales and Ceratophyllales, as might be expected for aquatic groups (Landis et al. 2002; B. Wang & Qiu 2006). However, the ancestor of Magnoliophyta may have had vesicular-arbuscular mycorrhizae, and this condition may have been a feature of the common ancestor of all seed plants, if not still deeper in the tree (see Maherali et al. 2016).

The presence of vessels is placed at the [Austrobaileyales + the rest] node here. The distinction between vessels and tracheids can be hard to make (e.g. Carlquist 2012a, c), Amborella may have vessels made up of two elements (Feild et al. 2000b), vessels in Nymphaeaceae and Cabombaceae are quite distinctive (Carlquist & Schneider 2009; Schneider & Carlquist 2009), and vessels are absent in Hydatellaceae. Overall, vessels of one sort or another may have arisen some three times in the basal part of the angiosperm tree, and vessels in taxa of the basal anagiosperm branches may be less efficent in water transport than tracheids in Pinales, for example (e.g. Sperry 2003; Pitterman et al. 2005, 2010; Hacke et al. 2005, 2015).

Sack et al. (2012) note that large simple leaves with high venation density are an angiosperm innovation, the high venation density allowing the leaf to function even when irradiance was high because the evaporating water cools the leaf.

For details of sugar transport in the phloem, which have both ecological and phylogenetic correlations, see below).

An arsenite transporter controlled by the single-copy ACR3 gene and promoting arsenic tolerance is found throughout land plants other than flowering plants (Indriolo et al. 2010), so its loss is another apomorphy.... Boyce at al. (2004) noted that the lignification of the primary cell wall was heavy in ANA-grade angiosperms (and in Drimys, a magnoliid) and gymnosperms, less in eudicots, and this had implications for ion-mediated xylem flow. However, the sampling is very preliminary. Compared with conifer lignin, primarily made up of guaiacyl units, the S-rich lignins of angiosperms is less dense and less highly condensed and the polymer units are smaller and have more β-ether interunit linkages. Wood fibres are rich in S units, while in vessel elements G lignin predominates (Wagner et al. 2015); any implications this might have for fungus-mediated breakdown of lignin is unclear. p-hydroxybenzaldehyde, a component of many lignins, is apparently absent from broad-leaved angiosperms - at least from magnoliids and most eudicots (Towers & Gibbs 1953), but it is present in monocots and in some living gymnosperms (and also some Myrtaceae, etc.).

Genes & Genomes. As will become quickly obvious, there has been extensive genome duplication, subsequent reduction in size, and rearrangement in angiosperms (Murat et al. 2017 and referemces). The ancestral angiosperm chromosome number may be 5–7 (Stebbins 1971; Raven 1975), but there have since been numerous genome duplication/polyploidy events (Wendel 2015; see also below). For 1C genome values, see Plant DNA C-values Database, consulted vi.2013 and subsequently (also Bennett & Leitch 2005, 2010). Genome size in many angiosperms is small, 1-1.4 picograms being the estimated ancestral nuclear C value (Masterson 1994; Leitch & Bennett 2005; Leitch et al. 2005). However some Proteaceae, Liliales and Asparagales in particular have very large genomes. Extant gymnosperms have quite large genomes, while the sizes of those of Ephedra in particular are very variable, and there genome size and ploidy level are quite closely connected (Leitch et al. 2005; Nystedt et al. 2013; Ickert-Bond et al. 2015a; see also below).

Gossmann et al. (2016) compared genes expressed in the gametophyte and those expressed in the sporophyte and found that there was a higher proportion of rapidly evolving and young genes in the former, perhaps connected with factors like the low tissue complexity of the gametophyte (genes are developmentally less linked) and haploidy, and Cui et al. (2015) suggested that new sporophytic genes for plant defence and the like might result from the competition between male gametophytes. There may be a similar pattern in the moss Physcomitrella (O'Donoghue et al. 2013)...

For B-function genes, etc., see S. Kim et al. (2004b) - synonymy: AP3 and PI with DEF and GLO respectively. For genome duplication in stem-group angiosperms, see e.g. Karlgren et al. (2011). The numbers of LATERAL ORGANS BOUNDARIES DOMAIN genes in Amborellales are around double that in extant gymnosperms, while elsewhere in the angiosperms numbers may be (much) higher again (Chanderbali et al. 2015).

For the evolution of the IR/LSC junction in the chloroplast genome, see R.-J. Wang et al. (2008), for chloroplast genes whose losses are synapomorphies for angiosperms, see Jansen et al. (2007), and for general chloroplast genome evolution, see Kua et al. (2012).

W. Guo et al. (2016a) summarize information on angiosperm mitochondrial genomes, and suggest that ancestrally the genome had 41 protein genes, 25 introns, was slowly evolving, and so on.

Chemistry, Morphology, etc. Some taxa in the ANA grade have been surprisingly little studied.

Xylans are more common than glucomannans in the cell wall, as in Gnetales. Magnoliales, at least, have glucoronosyl units every (6) 8 (10. 12) xylosyl residues, and they are acetylated, but there are no α-arabinosyl units (Busse-Wicher et al. 2016). For mannans, etc., see Popper and Fry (2004); Austrobaileya has mannans, although two other members of the order sampled lack them, Nymphaea also has mannans, but other Nymphaeales and Amborella were not sampled. Pribat et al. (2010) discuss the distribution of a folate-dependent phenyanaline hydrolase, which may be correlated with major plant groups when sampling is improved. Triterpenoids are produced by CYP716 enzymes, see Miettinen et al. (2017).

For root hairs see L. Huang et al. (2017), Hwang et al. (2017), Salazar-Henao et al. (2016) and references, also the discussion above. Leaf traces make connections only with xylem produced during the first year (Tomlinson et al. 2006); c.f. Pinales. In plants that have rhizomes the hypodermis is more or less suberized and has a Casparian strip (e.g. Perumalla et al. 1990b). Stomatal morphology and development in many members of the ANA grade is notably variable (e.g. Upchurch 1984; Rudall & Knowles 2013). For the plate meristem and how it relates to venation density, see Sack et al. (2012); is the plate meristem an apomorphy of angiosperms? The filiform apparatus is developed at the micropylar ends of the synergid cells (Zini et al. 2016).

Phylogeny. Evidence currently favours the hypothesis that angiosperms are sister to a clade that includes all extant gymnosperms. For a general discussion about the relationships of the major extant seed plant clades, see seed plant phylogeny.

Within early-diverging angiosperms, Donoghue and Mathews (1998) listed 16 different hypotheses of relationships that involved the first three nodes. However, Amborellaceae alone have often been found to be sister to other angiosperms (not an hypothesis that Donoghue and Mathews included), Nymphaeales are sister to the rest, and then Austrobaileyales, the three making up the ANA grade (this used to be called the ANITA grade) (e.g. Mathews & Donoghue 1999, 2000; Qiu et al. 1999, 2001 [checked for long-branch attraction - none], 2006b [some analyses], 2007; P. Soltis et al. 1999, 2000; Parkinson et al. 1999; Zanis et al. 2002; Magallón & Sanderson 2002; Kim et al. 2003; Borsch et al. 2003, 2005; Hilu et al. 2003; Nickerson & Drouin 2004; Aoki et al. 2004; P. Soltis & D. Soltis 2004; Müller et al. 2006a; Hansen et al. 2007; Duarte et al. 2008; McCoy et al. 2008; E. K. Lee et al. 2011; Zeng et al. 2014; Drew et al. 2014; Wickett et al. 2014: transcriptome analyses; Sun et al. 2014: chloroplast and nuclear data; etc.). For other studies, see Graham et al. (2000), Cai et al. (2006), Bausher et al. (2006), Chang et al. (2006), Jansen et al. (2006b, 2007), Ruhlman et al. (2007), Moore et al. (2007, 2011: position not as stable as one might like), Wu et al. (2007), Huang et al. (2010: c.f. rooting), and Iles et al. (2014).

On the other hand, Goremykin et al. (2003a), using complete chloroplast sequences, but for only 10 angiosperms, suggested the relationships [[Amborellaceae + Calycanthaceae] [eudicots + monocots]], but poor taxonomic sampling (monocots were represented by Poaceae alone) with resultant long-branch attraction may be responsible for these results (D. Soltis & P. Soltis 2004; Jansen et al. 2004; Stefanovic et al. 2004; Degtjareva et al. 2004b; D. Soltis et al. 2004). Goremykin et al. (2004) found the same general result when adding Nymphaea; it linked with Amborella - which was still not sister to all other angiosperms. Grasses themselves are highly derived monocots (see Kuhl et al. 2004 for the very distinctive genome of Poaceae). Indeed, even when looking at complete chloroplast sequences of just a few flowering plants, the inclusion of Acorus, breaking up the long branch leading to Poaceae, had a major effect (Stefanovic et al. 2004), although questions remained about the models used in the analyses (Lockhart & Penny 2005; Goremykin et al. 2005). For further discussion on the position of Ceratophyllum and Chloranthaceae, see elsewhere.

Sampling and analytical strategies are critical, the latter particularly in cases like this when there can only be relatively few taxa, but each can have very large amounts of data (e.g. Jarvis et al. 2014). In some cases large amounts of data may indeed be the solution, in others, perhaps quite surprisingly little data per taxon but improved sampling will do the trick (e.g. Rokas et al. 2005; Hedtke et al. 2006). The discovery that Hydatellaceae are sister to other Nymphaeales (Saarela et al. 2006) unexpectly did allow sampling in this area of the tree to be improved, but without affecting its topology. Fiz-Palacios et al. (2011) suggested a number (25+) of "non-conventional" relationships in their study on land plant diversification, but this, too may be a taxon sampling issue; I rarely mention these here. More data are not always an unmixed blessing, thus Barrett et al. (2012), using whole chloroplast genomes of monocots, found that adding data did not result in the gradual stabilization of support values, rather, these continued to fluctuate even when there was quite a lot of data and relatively small further amounts of data were added. Finally, some kinds of DNA data may be positively misleading when it comes to understanding relationships (Duvall & Ervin 2004; Qiu et al. 2005; Duvall et al. 2006, 2008b; G. Petersen et al. 2006b). Thus mitochondrial data tend to give odd topologies, and horizontal transfer is notably common in mitochondrial genomes (Sanchez-Puerta et al. 2008, 2011; Hao et al. 2010; W. Wang et al. 2012; c.f. Cusimano et al. 2008; Rice et al. 2013). With genome and transcriptome data now being accumulated for considerable numbers of plants, issues surrounding how best to analyse massive amounts of data become central.

An [Amborellales + Nymphaeales] clade is quite often recovered. This topology was perhaps particularly prominent in some analyses of mitochondrial genes, as in Qiu et al. (2006b), although there were several unexpected if poorly supported relationships elsewhere in their preferred tree (Qiu et al. 2010; see also Qiu et al. 2000, 2005, 2006a). Sun et al. (2014) also found this topology in most of their mitochondrial analyses, as did Barkman et al. (2000: genes from all three compartments). Soltis et al. (2007; data from D. Soltis et al. 2000) found that the relationships obtained depended on the method of analysis; Bayesian analysis favoured [Amborellaceae + Nymphaeaceae], while parsimony yielded [Amborellaceae + The Rest] (see also Mardanov et al. 2008: support weak). Goremykin et al. (2012) suggested that a wrongly specified substitution model would produce a topology [Amborellaceae + The Rest]. What kinds of characters are analysed may be important. Goremykin et al. (2009b) found an [Amborella + Nymphaea] clade after removing a relatively few (500) highly variable positions from the analysis (see also Goremykin et al. 2015), and although criticized by Drew et al. (2014), other studies hint more or less strongly that rate heterogeneity may be an issue (Barkman et al. 2000a; Stefanovic et al. 2004; Leebens-Mack et al. 2005; Finet et al. 2010; Goremykin et al. 2013). In a comprehensive series of analyses, Xi et al. (2014) thought that the problem had to do with the rate of gene evolution, not amount of data. Concatenation results tended to give a clade [Amborellaceae + The Rest], but if fast-evolving sites were removed an [Amborella + Nymphaea] clade was found, as in all their coalescent analyses. In a series of analyses of fairly well sampled transcriptome data, Wickett et al. (2014) found very little support for the [Amborella + Nymphaea] clade whatever the analytical method used. Simmons and Gatesy (2015) returned to the analysis of Xi et al. (2014) and suggested that one of the reasons that their preferred topology was found was that Selaginella, with a very long branch, was used for rooting and some of the coalescent methods they used were susceptible to misrooting caused by its inclusion, and also that the slowly evolving genes in some of their analyses had very little phylogenetic signal. There might be somewhat more support for an [Amborella + Nymphaea] clade in the plastid data, but that suggestion, too, was questioned (Simmons & Gatesy 2015 and references). Edwards et al. (2015) also tended to recover an [Amborellaceae + Nymphaeales] clade, again using sites that were parsimony-informative but evolving only slowly (see alo Xi et al. 2014; Goremykin et al. 2015), but their results were questioned by Simmons (2016); Shen et al. (2017: remove one gene!) evaluate the support for the [Amborellales + Nymphaeales] clade. The [Amborellaceae [Nymphaeales [Austrobaileyales...]]] topology is maintained here for the time being...

There are convenient summaries of the copious literature on relationships between the major angiosperm clades in e.g. P. Soltis & D. Soltis (2004), D. Soltis et al. (2005b, 2016) and Qiu et al. (2005). For information on broader patterns of relationships, see especially the notes at the mesangiosperm node; for relationships higher up in the tree, see especially monocots, eudicots, Pentapetalae/core eudicots, asterids and euasterids. Discussions focus on the topologies of the trees in this site, and in quite a few places there is still substantial uncertainty about relationships. The main tree here is more conservative than that in A.P.G. IV (2016), and in places this latter tree is more conservative than A.P.G. III (2009).

Classification. The classificatory framework, i.e. family and above, follows that of the Angiopserm Phylogeny Group (A.P.G. 1998, II 2003, III 2009, IV 2016); some infra-familial classifications are included, and these are slowly increasing in number... For full bibliographic information on all names above genus in angiosperms, see Reveal and Chase (2011), Reveal (2012) and especially Reveal's Index nominum supragenericorum plantarum vascularium (2000 onwards0; the latter is an invaluable resouce and I hope it is kept up-to-date).

Synonymy: Alismatidae Takhtajan, Arecidae Takhtajan, Aridae Takhtajan, Asteridae Takhtajan, Bromeliidae C. Y. Wu et al., Burmaniidae Heintze, Caycanthidae C. Y. Wu et al., Caryophyllidae Takhtajan, Ceratophyllidae Doweld, Chloranthidae C. Y. Wu et al., Commelinidae Takhtajan, Cornidae Reveal, Dillenidae Reveal & Takhtajan, Ericidae C. Y. Wu et al., Hamamelididae Takhtajan, Iliciidae C. Y. Wu et al., Juncidae Doweld, Lamiidae Reveal, Lauridae C. Y. Wu et al., Liliidae J. H. Schaffner, Loranthidae Tieghem, Malvidae C. Y. Wu et al., Myrtidae J. H. Schaffner, Nelumbonidae Takhtajan, Nymphaeidae Takhtajan, Orchididae Heintze, Piperidae Reveal, Plumbaginidae C. Y. Wu et al., Polygonidae C. Y. Wu et al., Ranunculidae Takhtajan, Rosidae Takhtajan, Rutidae Doweld, Theidae Doweld, Triuridae Doweld, Winteridae Doweld, Zingiberidae Cronquist - Magnoliophytina Reveal - Magnoliophyta Reveal

EVOLUTION AND DIVERSIFICATION OF THE ANGIOSPERMS (This section remains under construction, and probaly always will - new things always coming in.)

The first three sections below are more introductory, while the others attempt to summarise particular aspects of angiosperm evolution and diversification. Sections 2 and 3 include aspects of the evolution of insects and fungi respectively; their associations with angiosperms have been of great importance for both parties. Some of the issues raised here are taken up later and more from the point of view of the plant. Our knowledge of the evolution of stem-group angiosperms remains rudimentary (section 4). The evolution of flowers and fruits figure prominently in narratives of angiosperm evolution and success (see sections 4, 5A-C, 6A, 7), and they are indeed important, although not in any simple sense. In sections 5D and 5E in particular I discuss some physiological-ecological dimensions to angiosperm evolution, emphasizing aspects that seem to have had a major hand in shaping the global environment over the last 100 m.y. or more, and in sections 6B-D I look at other aspects of angiosperm evolution, in particular at the evolution of lowland tropical rainforest and the myriad linkages of plants and animals that today characterize it. In section 8 I turn to asymmetries in evolution, emphasizing relatively small groups of both plants and animals that seem to have had a disproportional (in terms of their species numbers) effect at scales from the global environment to the local community. Here, when thinking of plants in a more ecological context, measures like primary productivity, biomass accumulation, and the like can be used as indicators of importance - and species-rich clades in the euasterids then barely have a walk-on part. In section 9 I attempt a summary. Needless to say, many very important topics are ignored.

1. Important Caveats.

2. Angiosperms and Insects.
2A. Insects, Plants and Herbivory.
2B. Diversification of Phytophagous Insects.
2C. Diversification of Pollinating Insects.

3. Angiosperms and Fungi.
      Early Plant-Fungal Relationships.
3A. Mycorrhizae.
      Ecto- and Ericoid mycorrhizae.
3B. Endophytic Fungi.
3C. Mycorrhizae and Endophytes in general.
3D. Further Complexities.

4. Angiosperm History I: Evolution in Stem Group Angiosperms.
4A. Relationships.
4B. Pollination and Seed Dispersal.

5. Angiosperm History II: Cretaceous Origins.
5A. Introduction.
5B. Early Cretaceous Evolution - to the end of the Albian, ca 113 m.y. ago.
5C. Later Cretaceous Evolution - Albian to Maastrichtian.
5D. Venation Density, Stomatal Size, and Vascular Evolution.
5E. Wood and Litter Decay.

6. Angiosperm History III: Caenozoic Diversification.
6A. Flowering Plants.
6B. Latitudinal Gradients of Diversity.
6C. Gene and Genome Duplication and Genome Size.
6D. Diversification of other Plant and Animal groups associated with Flowering Plants.
6G. Discussion.

7. Flowers and Pollination.
7A. Flowers, Pollination and Fertilization.
7B. Major Clades With Monosymmetric Flowers.
7C. Major Clades With Wind-Pollinated Flowers.
7D. A Cautionary Note.

8. Asymmetries in Evolution.
8A. Plant-Animal interactions.
      8A1. Pollination.
      8A2. Seed Dispersal (much more to do here).
8B. Carbon Sequestration.
      Major Players.
      C4 photosynthesis, and Grasses and Grasslands.
      Ectomycorrhizal Plants.
      Seagrasses, Mangroves, and Tidal Saltmarshes.

9. In Conclusion.

1. Important Caveats.

When thinking about evolution in general, a well-supported phylogeny is of course essential, yet there are several parts of the Main Tree as well as of many ordinal trees where we know less than we would like. Indeed, the huge data sets being developed are likely to yield some unexpected topologies, especially as nuclear data become more widely used. But beyond this, there are critical issues of dating, working out diversification rates, optimising characters on trees, etc., that need to be understood. I discuss some of these issues briefly below, but please consult the primary literature for details.

1. The relationships of angiosperms to other seed plants continue to remain unclear (see below), and thus so do the whens, whys and hows of their initial diversification (see Davies et al. 2004b; Friis et al. 2005, 2011; Frohlich & Chase 2007; Pennisi 2009; Lee et al. 2011; Herendeen et al. 2017). Here we need to distinguish between the origin of the clade of which angiosperms are the only extant representative, i.e. stem angiosperms ("origin 1"), the origin of plants with carpels, tepals, and a heterosporangiate strobilus, i.e. the evolution of plants with flowers ("origin 2"), and finally, the origin of crown-group angiosperms, i.e. extant flowering plants and their immediate common ancestor ("origin 3"). Stem angiosperms presumably are early Carboniferous in age or even older, 350±35-305-275±35 m.y. old, if the angiosperm clade is sister to the clade including all living gymnosperms (e.g. Savard et al. 1994; Crane et al. 1995; Crane 1999; Magallón & Castillo 2009; Clarke et al. 2011), perhaps to a younger bound of Permian in age (J. A. Doyle 1998a). Even if crown-group angiosperms are as much as 270 to 182 m.y.o. (S. A. Smith et al. 2010, but c.f. Beaulieu et al. 2015), they will still have a substantial stem history. For the bulk of this some 100 m.y.+ time plants along the angiosperm stem probably had naked seeds, lacked flowers, etc..

Current evidence suggests that extant gymnosperms are monophyletic, but when including fossil taxa gymnosperms are paraphyletic with respect to angiosperms; angiosperms are derived from a gymnospermous ancestor. When integrating extant taxa and fossils in attempts to understand the relationships of angiosperms, for example, morphological analyses may yield rather less information than might be supposed (Puttick et al. 2017; see also O'Reilly et al. 2016). Overall, little progress has been made over the last fifty years or more in identifiying plants that can be placed somewhere between angiosperm origins 1 and 3 (Taylor & Taylor 2009; J. A. Doyle 2012).

2. Dating is critical, but how best to do this remains a subject of intense discussion (e.g. Magallón and Sanderson 2001; Graur & Martin 2003; Pirie et al. 2005; Renner 2005b; Bell & Donoghue 2005; Magallón & Sanderson 2005; Rutschmann et al. 2007; Sanderson et al. 2004; H. Wang et al. 2009; S. A. Smith et al. 2010; Magallón 2009; Milne 2009: sampling; Burleigh 2012; Sauquet et al. 2012; Magallón et al. 2013; Magallón 2014; Sytsma et al. 2014; Warnock et al. 2014; Clarke & Boyd 2014; Bell 2015; Schenk 2016: secondary calibrations; Foster et al. 2016a: quite a spread for different methods, increased/more representative sampling needed, also methodological changes, new information from fossils, i.e., just about everything; Matschiner et al. 2016: given ages found, cichlids get around by l.d.d., not continental drift; Saladin et al. 2017: value of fossils). Dates based on molecular, tectonic, and paleontological data are often in conflict, and the first two often give substantially older ages than the last (c.f. some Nymphaeales; Wilf & Escapa 2014 and subsequent correspondence). In all too many cases there are wildly different estimates for the same event. For instance, compare Wikström et al. (2001, 2004), Clarke et al. (2011) and Z. Wu et al. (2014) for angiosperm ages, Wikström et al. suggest a crown-group age in the Cretaceous, Clarke et al. an age in the Jurassic or earlier, and Wu et al. an age in the mid-Permian. In a study of different ways of calibrating, and two different ways of dating, Sauquet et al. (2011) found that the ranges of the means alone for each node varied by up to a factor of 10 (see also Parham et al. 2011). Relaxed ages are of course often substantially older than constrained ages - for example, the relaxed crown-group age for angiosperms is about 242 m.y., and the constrained age about 130 m.y. (Magallón & Castillo 2009). See also Sauquet et al. (2012) for Nothofagus, Crisp and Cook (2011), Martínez et al. (2012) and Condamine et al. (2015) for cycads, Barreda et al. (2010b, 2012a) and Heads (2012) for Asteraceae (and Waters et al. 2013 for a critique of the latter), Crisp et al. (2014) for Asphodelaceae-Xanthorrhoeoideae, and Franzke et al. (2016) for Brassicaceae. Beaulieu et al. (2015) suggested caution for some claims of angiosperm ages while J. W. Brown and Smith (2017) outlined a set of different problems such that, between these two articles, it becomes difficult to believe much about dating at all - and that affects many other aspects of evolutionary biology.

Fossil evidence is central to dating. However, fossils are usually more or less incomplete, and their identity, especially that of older fossils, needs to be confirmed. Recent developments in leaf identifications using a sparse code learning approch may completely overhaul this aspect of the business, although the study in question used cleared leaves, which fossil plants are remiss in not providing (Wilf et al. 2016a). Indeed, some studies have questioned what had previously seemed to be quite well established fossil identifications (e.g. Cook & Crisp 2005; Nothofagus; Biffin et al. 2010b: Araucariaceae). Fossils cannot be expected to be simply "ancestral", rather, they may lack the apomorphies of the crown-group and/or they may have evolved distinctive features of their own, and these may suggest links with unrelated groups in morphological analyses, or the fossils may be assignable to more than one node. Fossils, when treated with care, can help in the calibration of molecular trees (e.g. Gandolfo et al. 2004; Graham 2010; Clarke et al. 2011; Parham et al. 2011; Warnock et al. 2014; etc.), and their role here can be quite sobering (Wilf & Escapa 2014; c.f. Q. Wang & Mao 2015, but see Wilf & Escapa 2015); good, well-identified fossils of crown groups give minimum ages (Donoghue & Benton 2007), but if referred to a stem group their significance is more ambiguous since stem groups can persist after the evolution of the relevant crown group. A database for reliably-dated fossils has recently been developed (Ksepa et al. 2015, see also Xing et al. 2016 for other fossil databases). The fossil record is sure to have surprises (indeed - see Barreda et al. 2015; Wilf et al. 2017), although reports of Jurassic flowers (e.g. Z.-J. Liu et al. 2015; Han et al. 2016 and references) definitely need confirmation, indeed, Herendeen et al. (2017) critically review them, and none passes the muster.

Although a clade restricted to a volcanic island might necessarily seem to be younger than that island, several examples suggesting the contrary are to be found in these pages (see also Heads 2011; c.f. Franzke et al. 2016). Island ages cannot be used as a maximum age constraint for a clade without there being other evidence.

Dating Poaceae, q.v. for more details, presents particular problems. Amber fossils of Poaceae-Poöideae (Poinar 2004, 2011) from the Cretaceous of Myanmar/Burma are dated to ca 98.8 m.y.a. (Shi et al. 2012). Such dates would imply that many other dates for flowering plant clades are too young and cause a general rethinking of angiosperm evolution. But yet more: “With the discovery of a Claviceps-like fossil infecting a floret of an Early–mid Cretaceous Asian grass, we propose that the progenitor of Claviceps evolved in Asia among early grasses sometime in the mid- to Late Jurassic.” (Poinar et al. 2015: p. 17), crown-group angiosperms would probably be Triassic. Fossils identified as core eudicots have also been found in this amber (e.g. Poinar 2011, Poinar et al. 2007, 2008). Even the ca 66 m.y. age of crown Oryzoideae phytoliths from India (e.g. Prasad et al. 2011, accepted by Iles et al. 2015) will have a substantial knock-on effect on other age estimates, and hence on diversification rates (Christin et al. 2014).

Many ages for clades are given on these pages, and dates for older literature, not all mentioned here, are conveniently assembled in Hedges and Kumar (2009). However, all dates should be treated with extreme caution, since a very large number of dates in the literature, even recently-published dates, must be more or less seriously wrong - or, if with a large standard error (for example) of little real use. The original papers should be consulted for details of methods used, the actual node to which the date refers (I have tried to be accurate), the range of dates, and the topology of the tree being dated.

3. Distributions are not easy to interpret. There is abundant evidence that the present and past distributions of many plant and animal groups are very different. Thus early in the Caenozoic the distributions of a number of tropical taxa like Nypa and Cyclanthaceae that are today rather restricted were much wider (e.g. Plaziat et al 2001; S. Y. Smith et al. 2008), while in the Oligocene hoatzins, hummingbirds and parrots were flying around in Europe (Mayr 2002, 2004, 2009). Genera and families continue to be added to the list of groups in which past and present distributions are very different (e.g. Friis et al. 2011: numerous examples; Stull et al. 2012; Manchester et al. 2012; Grímsson et al. 2013; Hofmann et al. 2015; Sadowski et al. 2015). Many taxa now restricted to Southeast Asia grew in Europe and North America in the Caenozoic (e.g. Ferguson et al. 1997; Manchester et al. 2009) and many currently Australasian taxa are found in Eocene forests of Patagonia (Merkhofer et al. 2015), but such changes in longitudinal distributions are easier to understand.

Dates are of course essential when interpreting distributional patterns. Patterns that that seemed to reflect vicariance caused by plate tectonic events may be better explained by much more recent dispersal/migration events (e.g. Renner 2005b; de Queiroz 2005, esp. 2014; Wen & Ickert-Bond 2009: summaries, also Higgins et al. 2003; Nathan 2006; Yoder & Nowak 2006; Carpenter et al. 2010; Gillespie et al. 2012a; Baker & Couvreur 2012a, b; Christenhusz & Chase 2012). Even Lars Brundin's hitherto iconic chironimid midge drift-determined distributions may need reinterpretation from this point of view (Krosch et al. 2011). However, if the ages for some crucial fossils are upheld, vicariance may have to be revisited (see in part Ladiges & Cantrill 2007; Heads 2008, etc.). Thus Wilf and Escapa (2014) questioned the ages of a number of Patagonian fossils and so the dispersal-type explanations based on these ages (see also Wilf et al. 2013; Barrera et al. 2015). Smedmark et al. (2014) emphasized how difficult it was to reconstruct biogeographical events if more than one topology was allowed in places where support was weak, so using a single tree for such reconstructions was inadvisable (see also character optimization). For more on how organisms achieve the ranges that they have, about which we know little, see e.g. Gehrke and Linder (2009) and Schurr et al. (2009) and also the individual family accounts.

4. The apparently simple issue of species numbers is in fact not that straightforward even when discussing the size of extant clades. There are two aspects to this - what is really the clade of interest?, and, how many species does it contain?

A. I take it as axiomatic that comparisons between taxa simply because they have the same hierarchical rank are ill-advised, putting it mildly. Simplistic "major clade"-type comparisons are of little value (e.g. S. A. Smith et al. 2011; Igea et al. 2015 - but see Ricotta et al. 2012 and Laenen et al. 2014: Fig. 1), and that all Poaceae, for example, are assigned to tribes (see below) does not make comparisons between these tribes of any biological value. There are many examples of extreme clade size imbalance throughout the tree in which categorical ranks are less than informative about evolution and diversification. An emphasis on genera, genus size, etc. (e.g. Frodin 2004) is misplaced; the proper units of comparison of a genus are its sister taxa or other clades of the same age, not other genera (c.f. Givnish 2016: Brocchinia to be compared with rest of Bromeliaceae), but adjustments may have to be made if one is thinking about characters and diversification (Käfer & Mousset 2014: see below).

Orchidaceae, often considered to be highly diverse in terms of numbers of species when compared with other families, are a good example of the problems we face. Since they are sister to the rest of the Asparagales, the disparity in species number, although considerable, is only four-fold (ca 27,800 vs. 7,100), furthermore, Asparagales as a whole, with ca 34,900 species, are sister to commelinids, with some 24,500 species. Within Orchidaceae the highly speciose and mostly epiphytic-CAM crown Epidendroideae include around 19,560 species (figures from Pridgeon et al. 2005, 2009, 2014), i.e., about two thirds of the species in the whole family (e.g. Gravendeel et al. 2004). So answers to the questions, "Are orchids particularly diverse, and if so, why?", are not straightforward. Perhaps Epidendroideae, or a clade within them, are the hyperdiverse group (see below for another example).

B. Estimates of the number of extant species of flowering plants vary by a factor of about two - 422,127 (Govaerts 2001) to 223,300 (Scotland & Wortley 2003) - and perhaps add 20% (Joppa et al. 2010); when necessary, I use an intermediate figure of 352,000 species of flowering plants (see Paton et al. 2008); c.f. Nic Lughaddha et al. (2016) for comments on this and other estimates. However, new estimates come out every other year or so, e.g. Christenhusz and Byng (2016), where, at 295,383 species, the number is lower, but using their predictions of the rate of description of new species the two will be about the same around about 2045 AD... Interestingly, links to the numbers in Christenhusz and Byng (2016) were immediately posted on Wikipedia, however, a still more recent estimate is, at 369,434 species, very different (Nic Lughadha et al. 2016). In some groups uncertainty over species numbers is particularly great, thus estimates of species numbers of Ophrys range from 16 to 252 (e.g. Bateman et al. 2011a; Vereecken et al. 2011), and such numbers increasingly depend on the kinds of data collected and how they are analysed and evaluated, and this is a very active area of research. Furthermore, as Mallet (2013: p. 690) noted, "species counts over large areas of space and time [i.e., those in these pages] represent only a sketchy measure of biodiversity, a measure that owes more to taxonomic and metaphysical fashion than to science".

5. Many characters seem to come and go on the tree almost willy-nilly, so making character optimisation a distinctly hazardous undertaking. Thus using either parsimony or maximum likelihood, making apparently reasonable assumptions about the relative weighting of gains versus losses, or just using the simple models of evolution explicit in ACCTRAN or DELTRAN may affect the position of synapomorphies on trees, and hence our ideas of evolution (e.g. Donoghue & Ackerley 1996; Cunningham et al. 1998; Omland 1997, 1999; Ree & Donoghue 1999; Polly 2001; Webster & Purvis 2001; Ronquist 2004; Crisp & Cook 2005; Sannier et al. 2007, 2011; Remizowa et al. 2010b; Cohen 2012; O'Meara 2012; Sokoloff et al. 2013d; Sundue & Rothfels 2013; Gascuel & Steel 2014; Kriebel et al. 2014; Wright & Hillis 2014; Wortley et al. 2015). Syme and Oakley (2012) suggest that tree-based and node-based methods give very different results when it comes to allowing reversals. And of course the very definitions and circumscriptions of the features of interest are all too often problematical (see the Introduction).

6. Understanding the palaeoecological context of the evolution of angiosperms is a challenge. Ecological contexts change over time, and that of the early Caenozoic diversification of angiosperms is likely to be quite different from those of the origins of stem- and crown-group angiosperms; the past and the present are not immediately comparable. The Caenozoic context is initially connected with the bolide impact at the K/P boundary and the eruptions that produced the massive Deccan Traps, although much of angiosperm diversity - and that of the animals associated with it - as we now appreciate it seems to be a phenomenon of the later Caenozoic, and again the ecological context has changed; Meseguer et al. (2014b) attempt to deal with the problem. As we go back in the past, the proportion of no-analogue communities, i.e. those that we cannot understand by extrapolating from the present to the past, increases.

Palaeontologists face this problem on a daily basis. Thus Rothwell et al. (2000) reconstructed the palaeoecology of the small, probably short-lived conifer Aethophyllum using a combination of evidence from the fossil, the palaeoenvironment, etc. (also e.g. Strömberg 2006). Although it is tempting to read the ecology of early angiosperms from that of extant taxa of the ANA grade, this is hazardous (e.g. Wheeler & Baas 1991, 1993; Uhl 2006; Philippe et al. 2008). Little et al. (2010) challenge the reliability of aspects of leaf morphology, especially the presence of teeth, as palaeoclimatic indicators (for which, see e.g. Wolfe 1978). However, since the immediate relatives of angiosperms are unclear, working out how the ancestral crown angiosperm functioned will for now have to be a top-down process (Feild & Arens 2005, 2007), even if the present is an imperfect key to the past.

7. With time, the tree, distributions, apomorphies, and numbers of species, we can begin to think about "diversification". Although mentioned frequently below, diversification and the related "adaptive radiation", "key innovation" and "success" are both imprecise and difficult to estimate and to interpret (e.g. Heard & Hauser 1995; Bengtsson 1998; Hunter 1998; Sanderson 1998; Davies et al. 2004; Ricklefs 2007; Olson & Arroyo-Santos 2009; Ackerly 2009; Wertheim & Sanderson 2010; Yoder et al. 2010b; Stadler 2011a, b; Drummond et al. 2012; Morlon 2014; Givnish 2015b, etc.). There are at least two major issues here - first, estimating species numbers (see also above) and how they change over time, and second, thinking about diversification, whether as a species number issue, or changes in ecological roles/moving into a new adaptive zone, or changes in such features as dominance, biomass production and/or net primary productivity of clades, although these by no means exhaust the applications of the term key innovation, for example.

A. In curves showing diversity in clades over time, what can seem like an abrupt radiation, with rapid diversification after a period when there was little apparent diversity - the "broom and handle" and "stemmy" patterns evident in many clades - may be the result of extinction, diversification after the extinction event resuming at a rate similar to that before the event (e.g. Crisp & Cook 2009). Extensive sampling (>80%) is needed if accurate estimates of slowdowns in diversification are to be made (Cusimano & Renner 2010), extinction is hard to estimate (Rabosky 2010c), and diversification rates will automatically tend to increase towards the present. Simple experiments estimating future extinctions showed that these might affect estimates of clade size imbalance at nodes of up to ca 50 m.y.o. (Clarke et al. 2011). In general, estimating clade size imbalance is a remarkably tricky operation, especially in the near absence of fossils, the usual situation (Tarver & Donoghue 2011; see also e.g. Rabowsky 2010a, b). Even when there is an excellent fossil record, strategies like removing recently-radiating clades may be needed if one is to detect diversity loss in other clades (Morlon et al. 2011: whales, etc.; see also Stadler 2011b).

B. Having worked out changes in clade sizes over time, associating environmental/extrinsic and organismal/intrinsic features with these changes is not easy, although Bouchenak-Khelladi et al. (2015; see also Beaulieu & Donoghue 2013; Donoghue & Sanderson 2015; Givnish 2015b, and others) make the attempt. How are "key innovations" to be linked to particular nodes? And when they are, is the diversity at those nodes to be labelled as adaptive radiation, with members of a clade "doing" different things, more an ecological concept, or diversification, more a species number issue? Other measures such as morphological disparity (Minelli 2016; Oyston et al. 2016), dominance, biomass production and net primary productivity can all be evaluated at a variety of phylogenetic and ecological scales - see also below. The use of sister taxa when estimating the effect of putatively important features on diversification, an apparently straightforward operation (e.g. Sargent 2004), in fact needs some care, since if the feature of interest is apomorphic then ceteris paribus the clade with the feature is likely to be smaller than its sister clade - less time for diversification (Käfer & Mousset 2014)!

8. Finally, angiosperms and the organisms with which they associate form complex symbiotic systems at a variety of levels. Angiosperm physiology is mediated by fungi and bacteria both in the soil and in the plant, and this has shaped and continues to shape the environment at all scales. Features ascribed to plants may be the result of interactions between plants, fungi, and/or bacteria (Friesen et al. 2011), and this goes far beyond the ancient endosymbiotic events that resulted in chloroplasts and mitochondria. Most plants are mycorrhizal, and they also have a variety of often poorly understood fungal and bacterial endophytes. These may affect the growth of the plant, whether by fixing nitrogen, detering pathogens, improving growth rate, or the like (Kembel et al. 2014; Weiþ et al. 2016 for references). An individual plant is a microcosm or some kind of complex chimaera, as Herre et al. (2005) noted, referring to tropical plants and their endophytic fungi in particular (see below), and this idea is elaborated by, for instance, Bordenstein and Theis (2015).

2. Angiosperms and Insects.

Associations between plants and insects, whether as parasite, herbivore, detritivore, gall-former, seed-disperser or pollinator, are ubiquitous. The diversification of angiosperms is broadly contemporaneous with the massive diversification of many insect groups that are now more or less dependent on them, or the two are co-dependent, although there is some argument as to just how close the linkages are. Ehrlich and Raven (1964) provide an early statement of the idea of co-evolution that centred on the relationship between angiosperms and the insects that eat them, while Marquis et al. (2016) attempt to link herbivory to changes in secondary metabolites to the actual process of speciation; see also e.g. Brues 1924; Janzen 1980; Schemske 1983; Brouat et al. 2001; Futuyma & Agrawal 2009; Kato et al. 2010; Fordyce 2010; Janz 2011; de Vienne 2013). However, co-evolution has come to include anything from situations in which changes in the two members of the association are not directly linked, to those in which changes in one member of the association is linked to changes in the other (cospeciation need not be involved), to cospeciation (which may not involve mutual evolutionary change). However, reciprocal evolutionary change and diversification of co-evolving plant and insect or other animal groups is at best uncommon (Suchan & Alvarez 2015), and usually involves vertical inheritance of parasites/endophytes (de Vienne et al. 2013), although there is quite close co-evolution in some examples of herbivory and parasitism (Winkler & Mitter 2008; Althoff et al. 2012, but c.f. some bruchids, etc.). Host switching is often associated with radiation of a herbivore on a new host, the "escape and radiate" hypothesis (e.g. Ehrlich & Raven 1964; Fordyce 2010), and is quite common (de Vienne et al. 2013). We have to ask, to what extent is/was the evolution of the plant and insect connected?, and dating the diversification of the two partners is critical here - for radiation of insects after plant radiations, see Stirman et al. (2010), Leppänen et al. (2012) (but see above). Complicating our understanding of the interactions of insects and plants are the symbiotic bacteria and other organisms associated with both partners (Frago et al. 2012; Zhu et al. 2014).

2A. Insects, Plants and Herbivory. Details of plant-insect relationships are discussed after individual orders and families. What attracts an egg-depositing insect to one plant and prevents it laying eggs on another is often some aspect of plant chemistry (see Bernays & Chapman 1995 and Fernandez & Hilker 2007 [Chrysomelidae] for host plant selection), and plants have also evolved a variety of mechanical defences against herbivory. In general, more related plants have more similar animals eating them (Weiblen et al. 2006; Futuyma & Agrawal 2009 for literature), simply because they will tend to taste similar, having similar secondary metabolites. However, related plants may well show greater than expected diversity of traits involved in defence as they try to escape the herbivores eating their congeners growing in the same area (e.g. Becerra 2007; Becerra et al. 2009; Kursar et al. 2009); for the coexistence of members of plant species groups, herbivory, and interspecific diversity in secondary metabolites, see Sedio et al. (2017).

Plant tissues are for the most part rather nutrient-poor, and plant cell walls are made up of the rather indigestible cellulose and still more indigestible lignin. Lignin and cellulose digestion in termites occurs via their association with protozoa or the fungi they cultivate (Ni & Tokuda 2013), while some other insects and other arthropods are able to break down cellulose walls independent of any mutualistic association with micro-organisms, and this has implications for the evolution of land plants and their associated insects (Calderón-Cortés et al. 2012 for literature).

Protective metabolites may be found in latex, or they may be translocated via the vascular tissue, or there may be other specialised tissues involved. Herbivorous insects that eat plants with such defences may show distinctive vein-cutting behaviours which stop the supply of any protectants to the plant tissue and enable the insect to eat it (see e.g. Dussourd & Eisner 1987; McCloud et al. 1995; Becerra et al. 2001; Dussourd 2009, 2016).

Some insects eat only plants with particular defences that they then coopt for their own defence (e.g. Termonia et al. 2001 for chrysomelid leaf beetles). Herbivorous insects may sequester secondary metabolites from the plant in the larva and/or adult stages, ensuring some measure of protection by so doing; they often have warning colouration, i.e., they are aposematic. They may also use plant metabolites for pheromones to attract mates, or these metabolites may simply act as oviposition cues (Brower & Brower 1964 on butterflies; Nishida 2002 for a review). Secondary metabolites quite often benefit herbivores, especially generalists but also specialists (Smilanich et al. 2016), although some may protect against some herbivores, others attract/are necessary for other herbivores (e.g., glucosinolates in Brassicales). Some herbivorous insects effectively track plant secondary metabolites and are found on whatever plant has a particular metabolite, independent of the phylogeny of the plant groups concerned (e.g. Winkler et al. 2009); glucosinolates and some alkaloids are examples. Glucosinolates are found in both Putranjivaceae (Malpighiales) and Brassicales, as are the pierid butterflies that are attracted to glucosinolates, while swallowtail butterflies are found on Rutaceae and Lauraceae, the two having similar alkaloids, and on Rutaceae and Apiaceae, which both have furanocoumarins (Berenbaum 2001).

Within herbivores, there is a general decrease in host specificity both in temperate and tropical regions that follows the sequence: granivores > leaf miners > fructivores > leaf chewers = sap suckers > wood eaters > root feeders (Novotny & Basset 2005). Specialization in weevil-plant associations is similar: fruit and seed > wood > root and stem eaters (McKenna et al. 2009), while Novotny et al. (2010: around 24 guilds listed) found specialisation greatest in four guilds, leaf suckers, larval leaf chewers, leaf miners, and fruit chewers, guilds like root and phloem chewers showing less specificty. How insect larvae feed, i.e., whether they are internal feeders like stem borers and whether they can tolerate raphides, or latex, etc., may be more conserved than associations between larvae and particular groups of plants or other types of feeding behaviours (e.g. Powell 1980; Peigler 1986; Powell et al. 1999: associations with latex-containing plants; Konno 2011: chemistry; Farrell & Sequiera 2001; Lopez-Vaamonde et al. 2003, 2006). Phylogenetic conservatism may be greater in groups in which the adults tend to remain close to plants in/on which they grew up, as with beetles, compared to the situation where the adult may fly away, as in many lepidoptera (Berenbaum & Passoa 1999).

Extant angiosperms show a correlation between woodiness and tannin frequency and a negative correlation between tannins (generalized defence) and alkaloids and other secondary metabolites (specific defence). Plants that were obvious or apparent to herbivores were longer lived and had large amounts of generalized defences, while less apparent plants were more short-lived and had lower amounts of more specific defences (e.g. Feeney 1976; Silvertown & Dodd 1996; see also Levin 1976; Mole 1993; Endara & Coley 2010). Apparent plants were late successional, non-apparent plants early successional, although herbs in general came to be thought of a non-apparent (see Smilanich et al. 2016). (Insects that were specialized on their hosts ate the first group, other insects the second, and also the first - Endara & Coley 2010.) "Quantitative" defences like polyphenolics are more generalised, and butterflies such as Lycaenidae are the herbivores, while "qualitative" defences are highly toxic and butterflies like Nymphalidae are specialized herbivores (Fiedler 1996). Consistent with the apparency theory, the level of herbivory in woody plants is higher than that in herbivorous plants (Turcotte et al. 2014: see caveats about analyses).

The nature and amount of the defensive compounds produced can also be explained, and perhaps more satisfactorily, by the resource availability hypothesis, in which herbivore defence is thought of from a cost:benefit point of view (Endara & Coley 2010 for a summary). In particular, the growth rate of the plant affects the nature and amount of defences. Fast-growing plants need less in the way of defence since their leaves are short-lived and would soon be replaced even if they had not been eaten (Endara & Coley 2010). Deciduous plants in general, with their rather thin leaves, will tend to be eaten by insects more than plants with long-lived xeromorphic leaves (e.g. Coley & Barone 1996; Arnold et al. 2001; Wilf et al. 2001; Lewinsohn et al. 2005). When there are low concentrations of available nutrients, growth is slow, the leaves are long lived, and defences are laid down (Coley et al. 1985; Endara & Coley 2010). Along the same line, herbaceous light-demanding taxa are more often attacked by biotrophic fungi, fungi needing living plant tissue (therefore probably not loaded with toxic substances) to prosper, whereas woody taxa, and especially those growing in shaded conditions, were more likely to be infected by necrotrophic fungi, fungi which first kill plant tissue before digesting it (García-Guzmán & Heil 2013). Ali and Agrawal (2013) summarized the curent state of knowledge about the generalist/specialist paradigm, noting how little was really known about details of herbivore:plant interactions. Indeed, condensed tannins can be found in massive amounts in leaves and are sometimes thought to be an antiherbivore device - except that it has been suggested that they may not affect the activities of herbivores, rather, they increase the tolerance of plants to herbivory, facilitating plant recovery after herbivore damage by increasing nitrogen uptake from litter, frass, etc. (Madritch & Lindroth 2015; see Barbehenn & Constabel 2011 for a review of a difficult literature).

There is much discussion about possible correlations between geography, species richness, herbivory and defensive metabolites, and some geographical dimensions of these correlations are discussed later. Menken et al. (2009) suggest a correlation between species numbers, host specificity and feeding habits in lepidoptera in particular. Internal feeders tend to be small, have higher host specificty, and show relatively little diversification, while external feeders tended to be larger, show less host specificity, and formed more speciose clades. Nylin et al. (2014) thought that transient polyphagy in nymphalid butterflies facilitated diversification and subsequent host-plant shift (see also Ehrlich & Raven 1964), however, recent work suggests that in some butterflies, at least, there is no simple connection between host-plant shift and diversification (Hamm & Fordyce 2015). The plesiomorphic condition in lepidoptera is small size and internal feeding such as burrowing, features of the basal clades, which have jaws, and the basal Glossata, with probosces, such as Eriocraniidae (Menken et al. 2009; Imada et al. 2011), and overall the change in host plant preferences has been from specialist to generalist (Menken et al. 2009).

To summarize: The impact of insect/plant associations on plant diversification is still poorly understood (Futuyma and Agrawal 2009: also other papers in Proc. National Acad. Sci. U.S.A. 106(43)). In addition, symbionts, particularly bacteria, of the insect may affect its interaction with the plant (Frago et al. 2012). In some cases, diversification of plants can be linked to the development of particular defences, but this does not happen in any simple fashion; the mechanism by which insect diversification increases when feeding on angiosperms is also unclear (Janz 2011). However, a recent idea is to apply ideas from island biogeography to the problem. With cecidomyiid gallers, at least, if host species are closer, insect diversity may increase because it is easier to switch hosts, and if ranges of insects are large and the plants are structurally complex, diversity also increases; clade age has little to do with it (Joy & Crespi 2012; for age, c.f. Brändle & Brandl 2002 [in part]; Farrell & Mitter 1994). We need to know more about both the timing of diversification and patterns of phylogenetic relationships in both groups, and evidence for the former in particular is often lacking (de Vienne et al. 2013), worse, there can be uncertainty in both areas; this is discussed further below.

2B. Diversification of Phytophagous Insects. There is still considerable uncertainty about overall arthropod numbers, although likely to be substantially below the 20,000,000 or so once suggested; current estimates are between 1.6 and 7.4 million (e.g. Hamilton et al. 2013 and references). Phytophagous insects make up about one quarter of all described species, and over half the beetles (Janz et al. 2006: over half; Farrell 1998: ca one third of beetles; Hunt et al. 2007; Wiens et al. 2015), although there are also very species-rich beetle clades that are neither herbivores nor decomposers (e.g. Barraclough et al. 1998). There may be around (0.9-)1.5(-2.1) million beetles (Stork et al. 2015); for a comprehensive phylogeny, see Hunt et al. (2007). Clades of phytophagous insects may be more speciose that their non-phytophagous sister groups, ectophagous clades more diverse than their endophagous sister taxa, and clades that eat angiosperms more speciose compared to those that eat other plants (Mitter et al. 1988; Winkler & Mitter 2008; Wiens et al. 2015), although in the first case, at least, the asymmetry may not be significant in beetles (Hunt et al. 2007; Wiens et al. 2015) - indeed, Rainford and Mayhew (2015) find no evidence of diet switches, specifically, of switches to herbivory being connected with clade richness. Species estimates for various insect groups are given below.

Weevils include some 62,000 described species and perhaps 220,000+ species altogether. McKenna et al. (2009) suggest that crown-group diversification of major angiosperm-associated weevil clades was underway by the Aptian 125-112 m.y.a., and there was a "massive diversification" as angiosperms became floristically common. Basal Curculionidae show strong associations with monocots, but there is little evidence that early monocots were either particularly abundant or ecologically successful (Crane et al. 1995; Friis et al. 2004; J. A. Doyle et al. 2008; c.f. McKenna et al. 2009). Scolytinae, Cossoninae, and Platypodinae are the three major clades of wood-boring (endophagous) weevils, a habit that originated independently in the three (Haran et al. 2013); for Scolytinae, see under Pinaceae.

Chrysomelidae or leaf beetles, including the bruchids or seed beetles, are another very speciose herbivorous clade. Their origin has been dated to (86-)79-73(-63) m.y.a., well after the origin of the angiosperms (e.g. Gómez-Zurita et al. 2007).

There are at least 150,000 species of butterflies and moths (Lepidoptera) (Roe et al. 2009 for a summary), and it is estimated that larvae of about two thirds of these are herbivores, most being mono- or oligophagous (Bernays & Chapman 1994). A clade that include many macrolepidoptera, butterflies and some groups of moths, are well embedded in the paraphyletic microlepidoptera. Caterpillars of many of the more basal microlepidopteran clades, i.e. excluding most Apoditrysia (see below), are leaf miners, stem borers, etc, i.e., they are internal feeders and do not feed exposed on the plant. Adults of the very basal clades of Lepidoptera have jaws, and they include Micropterigidae (their larvae often eat hepatics), Agathiphagidae (they eat Agathis), and Heterobathmiidae (Nothofagus), and they may have diverged as early as the end-Triassic; Agathiphagidae may be sister either to Micropterigidae or to Heterobathmiidae (Wahlberg et al. 2013; see esp. Regier et al. 2015; Kristiansen et al. 2015; Heikkilä et al. 2015). [Heterobathmiidae + other leps] make up Angiospermivora (Regier et al. 2015), while Glossata, which make up the remaining leps, have probosces. Relationships among major groups of Ditrysia, Glossata in which the females have separate openings for mating and laying eggs and which include the majority of macro- and microlepidoptera, have been unclear (Mutanen et al. 2010; Heikkilä et al. 2015; see also more basal clades). However, it seems likely that Tineoidea (Tinaeidae) with ca 3,000 saprotrophic species (Sohn et al. 2015), para/polyphyletic at the base of Ditrysia, are fungus and detritus feeders (Regier et al. 2013, esp. 2014), which in turn suggests that the internal feeding habits of other ditrysian microlepidoptera are derived, not plesiomorphous. These include a clade made up of Yponomeutoidea, in which there have been several shifts to external feeding (Sohn et al. 2013) and the around 2,000 species of Gracillariidae, largely leaf miners, although including Epicephala, a large genus that is a pollinator/seed predator on Phyllanthus s.l. in Malpighiales-Phyllanthaceae (Kawahara et al. 2016). This clade is sister to Apoditrysia, within which Kawahara and Breinholt (2014) link large moths including the megadiverse Noctuoidea s.l. with much smaller Pyraloidea, etc., the combined clade being sister to a clade that includes the monophyletic butterflies, Papilionoidea (see also Regier et al. 2013). Cho et al. (2011 and literature; see also Mutanen et al. 2010; Wahlberg et al. 2013; Regier et al. 2013) also suggested that butterflies and large moths were are not sister groups. Papilionoidea are part of a clade that also includes Thyrididae, the picture-winged leaf moths, and Copromorphidae, tropical fruitworm moths (Regier et al. 2013). Papilionoidea (Wahlberg et al. 2013) include a small clade made up of the mostly night-flying Hedylidae, the American moth-butterflies, and relationships may be [Papilionidae [[Hedylidae + Hesperidiidae] [Nymphalidae [Pieridae + Lycaenidae]]]], although these relationships are not set in stone (Heikkilä et al. 2011; Regier et al. 2013; Kawahara & Breinholt 2014: pierids not included). Immediately downstream from this whole [butterfly + some moths] clade are groups like Gelechioidea, with over 5,000 saprophagous species (Sohn et al. 2015), and Zygaenoidea (Regier et al. 2013).

However, clade ages in Lepidoptera are quite uncertain. Thus crown-group Lepidoptera are ca 215 m.y.o. (Wahlberg et al. 2013) or ca 140 m.y.o. (Misof et al. 2014), while Grimaldi (1999) and Grimaldi and Engel (2005) thought that diversification of Glossata began in the mid- to upper Jurassic, Labandeira et al. (1997) and Wahlberg et al. (2013) suggesting somewhat older dates. Similarly, Ditrysia originated ca 160 m.y.a. (Wahlberg et al. 2013) or ca 100 m.y.a. (Misof et al. 2014). Within Ditrysia, the crown-group age of the butterfly clade is perhaps 104 m.y.o. (Wahlberg et al. 2013). Add to this the uncertainty in divergence times within Angiosperms, and it becomes difficult to say much about possible linkages between the diversification of lepidoptera and that of flowering plants (see also Regier et al. 2015 and below).

There are over 4,000(-8,000?) species of aphids (Hemiptera-Aphididae) feeding on plant sap, and they tend to be monophagous. Their diversity is greatest in temperate areas, although mymecophilous species are commonest in the tropics (Bristow 1991: Stadler & Dixon 2005). Diversification may be Late Cretaceous/early Caenozoic (von Dohlen & Moran 2000) or somewhat earlier (R. Chen et al. 2016). Hemiptera-Sternorrhyncha, with around 3,000 or more species (Burns & Watson 2013), and Psyllidae (jumping plant lice) also tend to be very host-specific.

Crown-group diversification of major angiosperm-associated weevil clades may have been underway by the Aptian 125-112 m.y.a., with a "massive diversification" of Curculionidae - now ca 90% of all weevils - 112-93.5 m.y.a. during the Cretaceous Terrestrial Revolution (KTR: McKenna et al. 2009). Plant-sawfly associations may have become reorganised around the KTR, interestingly, basal clades of sawflies are associated with angiosperms, while within monilophytes there is no association between the phylogenetic position of the plant with that of the sawfly larvae that eat it (Schneider 2016). Initial divergence within butterflies s.l. (Papilionoidea) may also have been around 110-95 m.y.a. (Heikkilä et al. 2011).

Galls, often with very distinctive morphologies, result from close associations between plant and insect (see Shorthouse & Rohfritsch 1992 and Redfern 2011 for good introductions). There are anywhere from (21,000-)132,930(-211,000) species of of galling insects (e.g. Espiritó-Santo & Fernandes 2007), the estimates are based on extrapolation and so partly depend on the numbers of flowering plants because of the specificity of many galler/plant associations. The dipteran gall midges (Cecidomyiidae-Cecidomyiinae, see Roksam 2005 for a phylogeny) are the largest group of galling insects and make up perhaps 25% of all galling insects in North America (Abrahamson & Weiss 1997). They are worldwide in distribution, but they show no particular patterns of host associations (Yukawa & Rohfritsch 2005: but see below for geography). The hymenopteran gall wasps (Cynipidae) may comprise as many as 50% of gallers locally and are north temperate; they are almost entirely restricted to eudicots (Ronquist & Liljeblad 2001; Csóka et al. 2005; Ács et al. 2009 for species limits). Smaller groups of gallers include psyllids (jumping plant lice, hemipterans) which are particularly common in Australia, and also on Fabaceae, at least in the Neotropics (Fernandes & Price 1991; Crespi et al. 2004; Espiritó-Santo & Fernandes 2007; Raman et al. 2005) and a couple of hundred species of aphids, also hemipterans (Wool 2004; J. Chen et al. 2013 for the phylogeny of hormaraphidine gallers). Some other insects, including a few Lepidoptera, are also gallers.

In general, gall-inducing insects are commonest on sclerophyllous plants growing on poor soils in warm climates between 25 and 45o N and S, or perhaps more generally in species-rich communities, whether dry or wet, but not necessarily in tropical climates (Price et al. 1998; Yukawa & Rohfritsch 2005; see Price et al. 1987 for galling in an adaptive context). Other organisms may be directly involved in the establishment of functioning galls, such as fungi in cecidomyid ambrosia galls (Rohritsch 2009 and references). Here the fungus gets its nutrients from the plant and is eaten by the midge larvae, indeed, cecidomyids may originally have been fungivorous (Roksam 2005). A complex network of parasites, hyperparasites and predators is all more or less dependent on gall larvae (Redfern 2011; see also figs and fig wasps).

2C. Pollinating Insects.

The evolution of bees is of particular importance, given the close involvement of many of them with angiosperm pollination (for bees and pollen, see Westerkamp 1996; for an account of all bee groups, see Michener 2007). Apoidea includes the spheciform wasps, since bees evolved from within the wasps, a group that feeds their larvae with insects (e.g. B. R. Johnson et al. 2013). The basic phylogenetic structure within Anthophila/Apiformes, the clade that includes all bees, is [Dasypodainae [[Meganomiinae + Melittinae] [[Andrenidae [Halictidae [Stenotritidae + Colletidae]]] [Apidae + Megachilidae]]]] (Cardinal & Danforth 2013), thus the old mellitids, here represented by Dasypodainae and [Meganomiinae + Melittinae], are paraphyletic, although they are monophyletic in Hedtke et al. (2014). Recent suggestions are that the age of stem-group bees is some (182-)149(-119) m.y., in line with some estimates of ages for the origin of angiosperms, with crown-group Megachilidae, a major clade including the leaf-cutting bees, dated at (154-)126(-100) m.y.a. (Litman et al. 2011). Crown-group Apiformes are some (132-)123(-113) m.y.o. (Cardinal & Danforth 2013), all families having diverged by the K/P boundary. Stem Apidae are some 135-120 m.y.a. (Grimaldi & Engel 2005), with their initial diversification apparanetly occurring in the early- to mid-Cretaceous 112-100 m.y.a. in association with the evolution of angiosperms (Grimaldi 1999, see also Engel 2000; Michez et al. 2009, 2012: discussion of fossils purporting to be bees; Grimaldi & Engel 2005; Almeida & Danforth 2009; c.f. Renner & Schaefer 2010).

Crown-group Apidae are dated to (95-)87(-78) m.y. (Cardinal & Danforth 2011). Within Apidae, the somewhat over 1,000 species of primitively eusocial corbiculate bees have the relationships [[Euglossini + Apini] [Meliponini + Bombini]] (e.g. Cardinal et al. 2010; Cardinal & Danforth 2011; Danforth et al. 2013; Martins et al. 2014a), although morphology-based trees suggest relationships [Euglossini [Bombini [Apini + Meliponini]]] (Canevazzi & Noll 2015 and references). Corbiculate bees are estimated to be (89-)77(-66) m.y.o. (Martins et al. 2014a, q.v. for other estimates). Within the corbiculate clade, the crown group of the stingless, rather speciose and highly eusocial meliponines is dated to (61-)58(-56) m.y. and (56-)51(-48) m.y., that of the euglossine orchid bees to (35-)28(-21) m.y. and (38-)26(-17) m.y., of bumble bees (Bombini) to (31-)21(-12) m.y. and (48-)26(-14) m.y., and of honey bees (Apini) to (30-)22(-16) m.y. and (29-)22(-17) m.y. (estimates from Cardinal & Danforth 2011 and Martins et al. 2014a respectively). Another estimate of the age of crown-group euglossines is 42-27 m.y. (Ramírez et al. 2010) and for crown-group Bombini is (49-)44.6, 27.9(-25.4) m.y. (Hines 2008: Table 1, highlighted areas). For relationships within Bombini, see S. Cameron et al. (2007; also Hines 2008), and for their classification, see Williams et al. (2008).

These general relationships are consistent with the appearance of bees in the fossil record. Both Apidae and Megachilidae, derived long-tongued bees, are known from Baltic amber of Eocene age (Danforth et al. 2006 and references). An early fossil, perhaps sister to other Apoidea, was found in amber of Upper Albian (probably Early Cenomanian, (99.4-)98.8(-98.2) m.y. - Shi et al. 2012) from Burma (Poinar & Danforth 2006), it may rather be a predatory wasp (Ohl & Engel 2007). A fossil from Late Cretaceous (96-74 m.y.a.) New Jersey amber was assigned to the extant genus Trigona, a highly derived eusocial stingless bee (Apidae-Meliponini: Michener & Grimaldi 1988); both its age (now estimated at 70-65 m.y.) and its relationships (it is placed in Cretotrigona, a stem meliponine) have since been re-evaluated (Engel 2000).

3. Angiosperms and Fungi.
Early Plant-Fungal Relationships.
3A. Mycorrhizae.
      Ecto- and Ericoid mycorrhizae.
3B. Endophytes
3C. Mycorrhizae and Endophytes in General.
3D. Further Complexities.

Early Plant-Fungal Relationships. Embryophytes and fungi established associations very early in the Silurian/Devonian (Selosse & Tacon 1998; Nebel et al. 2004). In some "bryophyte" clades mucoromycetes are associated with the gametophytes, Endogone-like fungi (Mucoromycotina) forming mycorrhizae with liverworts like Treubia and Haplomitrium (Field et al. 2012, 2014, 2015d) and with hornworts (e.g. Bidartondo et al. 2011; Desirò et al. 2013). Fungi in these liverworts are found in the thallus, and the relationship between plant and fungus seems to be one of mutualism (Field et al. 2014). Mosses - but not Takakia - usually lack mycorrhizal associations (Read et al. 2000; Kottke & Nebel 2005; Duckett et al. 2006b; Ligrone et al. 2007; Wickett & Goffinet 2008; Stenroos et al. 2010; Pressel et al. 2008, 2010; Rimington et al. 2014). Interestingly, the genome of the glomeromycote Rhizophagus irregularis has a number of similarities with those of Mucoromycotina (Tisserant et al. 2013), and both groups have Mollicutes-related endobacteria (Desirò et al. 2014); relationships between the two groups need clarifying (Field et al. 2015d). The story is getting more complex. Fine endophytes, arbusculur mycorrhizal fungi with hyphae ca 1.5 µm across that produce fan-like arbuscules and small vesicles, quite common in vascular plants, had been included in Glomus, but their SSU 18S ribosomal RNA gene groups with that of mucoromycotes, not with Glomus and relatives (Orchard et al. 2016). Members of a clade in Sebacinales-Serendipitaceae are associated with liverworts (Weiß et al. 2016), but this is likely to be a fairly recent connection.

Early vascular plants of the 407 m.y.o. Rhynie Chert formed associations with both Mucoromycotina and Glomeromycota (Remy et al. 1994; Strullu-Derrien et al. 2014). Since the nature of the plant-fungus association in many non-seeding plants can be rather different from the classic ectomycorrhizal or vesicular arbuscular mycorrhizal associations, these early plant-fungus relationships are perhaps best called paramycorrhizal associations (Kenrick & Strullu-Derrien 2014).

3A. Mycorrhizae. The evolution and ecological significance of mycorrhizae have been widely discussed (see Malloch et al. 1980; papers in Allen 1992; Read et al. 2000; Landis et al. 2002; Egger & Hibbett 2004; L. L. Taylor et al. 2009; Field et al. 2014, etc.), as has the morphology of the plant/fungus interface (e.g. Peterson & Massicotte 2004; Peterson 2013 and references) and how the fungus uses the 10% or more of photosynthesate that it gets from the plant (Leake et al. 2004). For the general economics of the exchange between the two partners - mostly sugars from the plant, but a greater variety of metabolites from the fungus, see Wyatt et al. (2014). For organic nutrient uptake by mycorrhizae and the amount of carbon in the soil, see Orwin et al. (2011: kind of mycorrhizae not specified). For comprehensive surveys of mycorrhizal associations, see Brundrett (2008: updated online resource, 2009) and Akhmetzhanova et al. (2012) in particular; Garbaye (2013) is another useful introduction.

This is a complex story. Being mycorrhizal is not a simple either/or matter, furthermore, one species of plant can have different kinds of fungal associations (see also below). Complicating any simple story about the evolution of mycorrhizal relationships is the fact that fungi are associated with the gametophytic generation in liverworts, hornworts and even monilophytes, but with the sporophytic generation in seed plants (Desiró et al. 2013), although in some lycophytes and monilophytes the same fungus is found in both generations (Winther & Friedman 2008; Field et al. 2015). Finally, the basal Agaricomycete order Sebacinales are involved in important interactions with land plants from liverworts to orchids and oaks, both chlorophyllous and echlorophyllous, and in associations that range from endophytic to ectomycorrhizal. Most of the over 1,000 species in the order are undescribed, and only recently has it been divided into two families. One is the largely ectomycorrhizal Sebacinaceae (= Group A Sebacinales of earlier studies), also found on basal Ericaceae, and it includes some orchid mycoheterotrophs and even a few endophytes (Selosse et al. 2009; Weiß et al. 2016). The other is the ecologically very diverse Serendipitaceae (= Group B Sebacinales), of which only four species have been described and of which "fruiting bodies"/basidiomes have never been seen (Weiß et al. 2016). Mention of Sebacinales elsewhere in the site reflects these recent taxonomic changes (Weiß et al. 2016 for a summary).

Its is thought that epiphytic taxa are not often mycorrhizal (Janos 1993; see other papers in Mycorrhiza 4(1). 1994; Desirò et al. 2013; Kato & Tsutsumi 2013), but c.f. Ericaceae and Orchidaceae, two of the major epiphytic angiosperm groups (e.g. Kottke et al. 2008; Martos et al. 2012). Ericoid mycorrhizae are also part of the story, but orchid mycorrhizae (q.v.) - basidiomycetes, including Sebacinales-Serendipitaceae are commonly involved (Weiß et al. 2016) - will not be discussed further here, although in patterns of fungal associations they are similar both to ecto- and ericoid mycorrhizae (Toju et al. 2016; c.f. in part Imhoff 2009). Aquatic flowering plants, hardly surprisingly, often lack mycorrhizae (see Safir 1987 and Radhika & Rodrigues 2007 and references for records; de Marins et al. 2009), and mycorrhizae are also often absent in many Caryophyllales, Proteales, Brassicales, and the like. Overall some 18% of flowering plants may lack mycorrhizae, and a further 12% are only facultatively mycorrhizal (Molina et al. 1992) - the figure for non-mycorrhizal angiosperms is only 6% in Brundrett (2009). Non-mycorrhizal species may on occasion be associated with AM fungi, vesicles sometimes being produced but arbuscules only rarely, and there may even be some movement of carbon from plant to fungus (Lekberg et al. 2015), however, there are several reports of the growth of non-mycorrhizal plants being reduced when growing with AM plants (see Veiga et al. 2013: experiments on Arabidopsis thaliana).

Endomycorrhizae or arbuscular mycorrhizae (AM) (no distinction between AM and vesicular-arbuscular mycorrhizae - VAM - is made here) are very widespread, being found in about 70% of seed plants, 74% of flowering plants, 80% of all land plants, and 92% of plant families (Blackwell 2011; Brundrett 2009), perhaps 200,000 or so species of plants being involved (Rinaldi et al. 2008). AM associations are of long standing, and if they were not to be found in the common ancestor of all land plants (see above, also Parniske 2008), they characterize at least a major subset of vascular plants. Mycorrhizae are not often found in fossil gymnosperms, although AM have been described in Upper Permian Glossopteris, the Triassic voltzialean Notophytum, etc. (Harper et al. 2013, 2015 and references). Just how many times AM associations evolved in land plants is unclear; in part, it depends in part on the very definition of mycorrhizae and on the relationships of "bryophytes".

Glomeromycota are usually the fungi involved in AM associations (Schüßler et al. 2001), but see above. Their hyphae are aseptate and intracellular, often forming vesicles and/or branching structures (the arbuscules) within the cells. Sexual reproduction in Glomeromycota seems to be very uncommon (Rosendahl 2008). The spores are multinucleate, the nuclei in any one spore not having an immediate common ancestor, so the unit of selection may be the individual nucleus (Jany & Pawlowska 2010). Hyphae from different mycelia can fuse, making the nuclear mix yet more complex (Giovannetti et al. 2004). However, Rhizophagus irregularis, which has been studied in some detail, has a number of genes that elsewhere would be involved in sexual reproduction, and how glomeromycetes reproduce is an open question (Tisserant et al. 2013 and references).

Morphological details of the fungus-plant association vary, Paris-type associations lacking arbuscules, the commoner Arum-type having both coils and arbuscules (e.g. F. A. Smith & Smith 1997; Torti et al. 1997; Peterson & Massicotte 2004). The proportion of fungal biomass inside and outside the plant depends on the group of Glomeromycota involved (Maherali & Klironomos 2007). Although details of the establishment of AM interactions are still poorly known, some of the genes involved are the same as those involved in establishment of nodulation in the nitrogen-fixing clade (e.g. Maillet et al. 2010 and references; see also Fabales). Furthermore, many important genes involved in the AM association have been detected in streptophyte clades immediately basal to the embryophytes (Delaux et al. 2012, 2015). Initial attraction of the fungus to the plant, and also hyphal branching, is mediated by strigolactones secreted by the root (Akiyama 2010 and references). These strigolactones may initially have been involved in rhizoid elongation in the gametophyte (Delaux et al. 2012). Lipochito-oligosaccharides are also involved in the signaling between plant and fungus (Maillet et al. 2011), and a karrikin receptor complex plays a role in the initial interaction between plant and fungus (karrikin and strigolactone both have a butenolide element), and this complex is found widely in embryophytes (Gutjahr et al. 2015). The invasion of plant tissue by the fungus involves appressorium formation in reponse to the production of cutin monomers, and is similar to the establishment of parasitism (Bonfante & Genre 2010), especially by oomycetes (e.g. E. Wang et al. 2012, see above). Overall, the effect of AM fungi on gene expression of the host is much less than that of endophytic or parasitic fungi (Dupont et al. 2015).

Some 290 species of Glomeromycota have been described (Öpik et al. 2010; Merckx et al. 2012), and all form AM associations; overall, individual species are widely distributed and show low host specificty (references in Schappe et al. 2017). Recent surveys suggest that the dispersal of the fungi may be limited, partly because their spores are relatively large and not dispersed by wind (Tedersoo et al. 2014b), so Glomeromycota diversity may have been considerably underestimated. Thus in a recent survey in a AM forest in New Zealand, Martínez-García et al. (2014) could associate only 8 out of the 113 glomeromycote OTUs found there with names, and Gorzelak et al. (2017) found a number of unidentified AM fungi in their Thuja plicata-dominated forests. But even if there are 1,000 or even many more species of glomeromycotes (Rosendahl 2008; N. C. Johnson 2009; Kivlin et al. 2011; see also Pickles & Pither 2013 for cautionary comments), that is still far fewer than the probably 200,000 species or so of AM plants (Rinaldi et al. 2008). A recent analysis suggests that glomeromycotes show very little local endemism, 93% of the taxa (virtual taxa) being known from more than one continent and one third found on six (Davison et al. 2015: Antarctica not included; Öpik et al. 2016; Pärtel et al. 2016; but see comments by Bruns et al. 2016) - not necessarily at odds with the findings of Martínez-García et al. (2014). How easily the fungi disperse needs re-examination.

There is some evidence for fungal host specificity or at least host preferences (e.g. Gosling et al. 2013; Martínez-García et al. 2014), and Wenner et al. (2014) found that fungi in related species (= same genus, subfamily) of Asteraceae tended to be most similar (here Glomeromycota were in a minority). Overall, the diversity of AM fungi was not correlated with that of its hosts (Tedersoo et al. 2014b), and in Panamanian forests composition of AM fungi was affected more by soil properties than tree identity, that of other fungi (no ECM fungi) was affected by both (Schappe et al. 2017). Many plants form associations with several species of fungus, and ecological specialists and generalists may form associations with different fungi (Öpik et al. 2009, 2010). A single plant can form different associations sequentially (van der Heijden et al. 2006) or unrelated species of plants may be colonized by the one fungus (Kottke et al. 2008; Walder et al. 2012). Overall, fungal associations in AM networks are nested, that is, specialist fungi, forming AM associations on only a few species of plants, are frequently found on generalist AM plants (Toju et al. 2016 and references).

In AM associations, nutrient uptake by the plant - especially of phosphorus, and recent work adds nitrogen - is increased, and water uptake is improved (Read 1991; Allen 1992; Govindarajulu et al. 2005; Leigh et al. 2009 and references; Tian et al. 2010; Bonfante & Genre 2010; S. E. Smith et al. 2011, 2015); root hairs or an AM association may be alternative ways for a plant to obtain phosphorus when it is in short supply (Schweiger et al. 1995 and references). Overall, up to 90% of the plant's phosphorus may come from AM fungi, while up to 20% of the carbon in its photosynthate moves to the fungus (Sheldrake et al. 2017). Most soil phosphorus is to be found in soil microbes, and AM efficiently scavenges soluble phosphate, but this becomes progressively more difficult to access as soils age (Turner et al. 2012). Gosling et al. (2013) found that the diversity, but often not colonization rate, of AM fungi was affected by the concentration of soil phosporus, while Sharda and Koide (2010) found that high P levels were associated with lower levels of AM associations (see also N. C. Johnson 2009). Glomeromycetes are unable to break down and utilize complex biopolymers (Tisserant et al. 2013), and they obtain at least carbohydrates from the plant (Helber et al. 2011; see also Walder et al. 2012; Kaiser et al. 2014), and Tisserant et al. (2013) noted that the colonization of plants by AM might result in a 20% net increase in photosynthesis (Kaschuk et al. 2009: crop Fabaceae). AM fungi effectively scavenge nutrients that become available as leaves, etc., decompose, but they cannot access nutrients in the complex humic substances in the soil organic matter that is left behind, thus organic phosphorus in temperate deciduous forests is in a less available pool than the phosphorus where AM plants predominate (Rosling et al. 2015; see below). Because hyphae of AM fungi from different plants may fuse, a potentially quite large number of plants from the same or different species may be put in indirect contact with each other (Giovannetti et al. 2004). As a result, the estimation of the costs and benefits accruing to the parties involved becomes more complicated (Walder et al. 2012).

AM also have other beneficial effects by improving soil structure - they produce large amounts of glycoproteins (L. L. Taylor et al. 2009; Garbaye 2013) - and drainage and hence affect weathering. Maherali & Klironomos (2007) found that such ecosystem functioning was improved if all three major types of Glomeromycota were in the one community. How changing carbon dioxide concentrations in the atmosphere might affect the soil carbon storage activities of AM is unclear (Verbruggen et al. 2012 and references).

Loss of the ability to form AM associations has been studied in detail by Delaux et al. (2014; see also Maherali et al. 2016). Delaux et al. (2014) distinguish between a set of genes involved in general symbiosis establishment, the common symbiosis signalling pathway (CSSP), but that also have other functions, and endomycorrhizal-specific genes, and the latter have been lost in parallel in different angiosperm groups. The RAM2 - Required for Arbuscular Mycorrhization - locus is lost; RAM2 is involved in the production of cutin monomers recognized by both glomeromycotes during the establishment of AM associations and oomycetes during the initiation of parasitism (E. Wang et al. 2012; Geurts & Vleeshouwers 2012). Hence loss of the ability to form AM associations can be linked to the development of resistance to oomycete infestations (see elsewhere). The CSSP is involved in the establishment of both rhizobial and actinorhizal as well as AM associations (Barker et al. 2017 and references).

For some other articles on endomycorrhizae, see Botany 94(6). 2016.

Ectomycorrhizae (ECM)/plant relations are surveyed by Itoo and Reshi (2013); ericoid mycorrhizae (ERM) are also included in this section (see below). ECM fungi are associated with relatively few seed plants, although estimates vary: 2,500-3,000 species (S. E. Smith & Read 2008), 5,600 species of angiosperms + 285 species of gymnosperms (Brundrett 2009), or ca 8,000 species (Rinaldi et al. 2008: inc. gymnosperms), other figures, apparently rather different but which take into account species not studied and so perhaps more accurate, are around 29,300 (Maherali et al. 2016: inc. gymnosperms; angiosperm numbers from Paton et al. 2008) or 16,100 (starting off with figures from Brundrett 2002). Many species of fungi are involved in ECM associations, including basidiomycetes like Boletales, Pezizales and other ascomycetes like the widespread Geococcum, and perhaps also Zygomycota. Conservative estimates are 7,750 species of fungi (including Ericaceae [see below] but excluding Orchidaceae), although the figure may be as high as 20,000-25,000 (Rinaldi et al. 2008; Tedersoo et al. 2012: Tedersoo et al. 2014b and Pickles & Pither 2013 for the care needed when estimating the diversity of ECM fungi); the fungi growing in tropical white sand vegetation have recently been tabulated (Roy et al. 2016). Of course, until the advent of molecular methods, identification of the fungi depended on their being cultivable and undergoing sexual reproduction, and although Sebacinales, for instance, can now be identified using molecular data, some remain unculturable and most are undescribed (Tedersoo et al. 2010b; Weiß et al. 2016).

Species of ECM fungi are often quite widespread, an example being the ascomycete Cenococcum geophilum, common and occurring in various successional stages of temperate forests (Meyer 1964; Visser 1995). Overall local fungal diversity is often high and depends on the phylogenetic diversity of the host plants (Nguyen et al. 2016), even fungi in some tropical lowland ECM associations being as diverse as those in more temperate climates (Henkel et al. 2012: Dicymbe-Fabaceae; Brearley 2012: Dipterocarpaceae). However, African and Malagasy ECM woodlands showed relatively less diversity, the same ECM fungi being found on unrelated species and at different successional stages (Tedersoo et al. 2011). There is in general little host specificity (but see Bruns et al. 2002), although there can be a distinct fungal succession as in regrowth of jack pine forests after burning (Visser 1995), and specificity was reported to be highest in the early stages of other successional situations when fertility was lowest (Reverchon et al. 2012: also saprophytic fungi; Hayward et al. 2015). However, in some tropical communities where ECM plants are uncommon, the specificity of the ECM fungus for the host may be quite high (Timling & Taylor 2012; M. E. Smith et al. 2013 and references), while Nguyen et al. (2016) found specificity became apparent only when comparing fungi associated with different orders or higher groups of seed plants. Tedersoo et al. (2013: Salicaceae, also sometimes VAM) found that the phylogenetic relationships of the host had a strong effect on both ECM richness and community composition (see also Pölme et al. 2013).

There is a strong phylogenetic signal in the plants that form ECM associations (e.g. Alexander & Lee 2005; L. L. Taylor et al. 2009, 2011), and this is discussed below. Garcia et al. (2015) emphasized that the variation in how ECM associations were established and maintained reflected their highly polyphyletic origin in seed plants, and there is extensive polyphyly - ca 80 origins - on the fungal side, too (e.g. Martin et al. 2010; Tedersoo & Smith 2013). At the same time, there can be extensive movements between unrelated plant hosts within small clades of fungi - thus the Strobilomyces/Afroboletus clade (Boletaceae), with perhaps 50 species, is found on five major unrelated ECM clades of seed plants from pines to peas and eucalypts (Sato et al. 2016).

Ericaceae often form mycorrhizal associations (ericoid mycorrhizae, ERM) with the basidiomycete Sebacinales-Serendipitaceae (Selosse et al. 2007; Imhof 2009 for a summary; Weiß et al. 2016); many other ERM fungi are ascomycetes (Read 1996; Garbaye 2013). Vrålstad (2004; see also Villareal et al. 2004; Brundrett 2004; Imhoff 2009; Tedersoo et al. 2010b) suggested that ECM, ERM, and associates form a single ecological guild, one of whose characteristics is the uptake of organic nitrogen by the plant (c.f. in part Lindahl et al. 2002: opposition between decomposer and mycorrhizal fungi; Clemmensen et al. 2014: ECM and ERM of different importance in forests of different ages; Talbot et al. 2008: VAM; Inselbacher et al. 2012). Interestingly, neither ECM nor ERM fungal networks are nested, that is, specialist fungi, forming associations with only a few species of plants, are often not also found on generalist plants growing in the same area - c.f. AM plants (Toju et al. 2016 and references). Serendipitaceae also form associations with "autotrophic" Orchidaceae, etc. (Setaro et al. 2012; Yagame et al. 2016), while Ericaceae-Pyroloideae, -Arbutoideae and -Monotropoideae are associated with Sebacinaceae, most species of which are ECM (see also Selosse et al. 2007). Tuberculate ECM, clusters of roots surrounded by hyphae, are another ECM variant (Paul et al. 2007; M. E. Smith & Pfister 2009).

ECM associations with Pinaceae, perhaps 200 m.y.o. or more, are probably the oldest; crown-group Fagales, probably ancestrally an ECM clade, have been dated to 120-62 m.y.a., which is not very helpful. ERM associations in Ericaceae have been dated to ca 90 m.y.o. (van der Heijen et al. 2015 and references), but the fossil (see Paleoenkianthus), on which this age is based, may not even be stem Ericaceae, and in any event Enkianthus itself does not have ERM; the origin of ERM here can perhaps be dated to around 77-65 m.y. (Wagstaff et al. 2010; Z.-Y. Liu et al. (2014). A substantial clade in Malvales, [[Pakaraimaea + Cistaceae] [Sarcolaenaceae + Dipterocarpaceae]], is also commonly ECM, and can be dated to as little as (25-)23(-21) m.y.a. (Wikström et al. 2001) or over 88 m.y.a. (split of Dipterocarpaceae and Sarcolaenaceae, Ducousso et al. 2004).

ECM/ERM seed plants are generally woody, although there are some herbaceous ECM taxa, particularly in Arctic-Alpine environments (e.g. Newsham et al. 2009). ECM help the plant acquire nitrogen and phosporus from organic material as diverse as pollen, dead nematodes (by breaking down chitin) and organic material in general, soil phosphatases being high, and also from the weathering of rocks (e.g. L. L. Taylor et al. 2009, 2011; Averill et al. 2014; Rosling et al. 2016). ECM fungi can produce extracellular enzymes that break down organic nitrogen (Perez-Moreno & Read 2000), and nitrogen may also be transferred to the plant in an organic form, e.g. as glutamine (Alexander 1989a; Newbery et al. 1997; Michelsen et al. 1998; Read & Perez-Moreno 2003; Cairney & Meharg 2003; Lindahl & Taylor 2004; Martin & Nehls 2010; Bonfante & Genre 2010; but c.f. Persson & Näsholm 2001: common in plants in vitro). The fungi may retain nitrogen in their mycelium, perhaps especially when the supply of photosynthesate from the plant is high, and the result is soil with very low N concentrations that is unfavourable for the growth of non-ECM/ERM plants (Näsholm et al. 2013), although details of the N balance between fungus and host are complex (Garcia et al. 2015; Terrer et al. 2016). Interestingly, although litter from AM plants may decompose faster than that of ECM plants, the mineral-associated soil organic matter produced is stable and the organic nitrogen is inaccessible to the AM fungi, more so than the nitrogen in the more slowly-decomposing litter of ECM plants to ECM fungi (Terrer et al. 2016 and references). It is often noted that ECM can break down soil organic matter and extract nutrients such as phosphorus and nitrogen from it (Koele et al. 2012; Shah et al. 2016). Interestingly, ECM can help mitigate the negative effects of dark septate hyphal endophytes (see below) on the plant (Reininger & Sieber 2012). For other aspects of the ecological interactions of ectomycorrhizae, see below).

There is probably an evolutionary sequence, white rot fungi → brown rot fungi → ECM fungi (e.g. Kohler et al. 2015), although too much should not be made of these decomposition types; Hibbett et al. (2000) also suggested that white-rot fungi might also be derived from within ECM clades. White rot and brown rot fungi are both wood decomposers that utilize cellulose, but only the former, widely scattered through Agaricomycetes, can mineralize the lignin so removing it from the ecosystem (Eastwood et al. 2011; Floudas et al. 2012; Kohler et al. 2015). Development of ECM associations in Boletales, at least, may be favoured by low nitrogen and high organic matter (= mostly lignin), ecological conditions that result from the activities of brown rot and to a certain extent from AM fungi. The soil organic matter accessed by ECM fungi is made up of small fragments of lignin, polysaccharides, peptides, etc (Schmidt et al. 2011; Shah et al. 2016).

ECM associations have formed perhaps 78-82 times in fungi, especially in ascomycetes and basiodiomycetes, but also in Zygomycota, and more origins are likely to be discovered especially in tropical and south temperate areas (Tedersoo & Smith 2013; see Wurzburger et al. 2016 for clades). Clade size varies greattly. Thus the ascomycete Cenococcum geophilum, the commonest ECM fungus, is the only member of the 19,000+ species of the Dothidiomycetes with this life style (Peter et al. 2016). Given the extreme polyphyly of the ECM habit, different clades may vary in details of the nature of their associations with plants, but in at least some cases physiological similarities between different clades of ECM fungi are quite extensive (Peter et al. 2016).

The fungal hyphae form a complete sheath investing the fine roots, the Hartig net, and hyphae penetrate between the exodermal/cortical cells; this sheath, not the root/root hair system, forms the interface between the plant and the soil (e.g. Clowes 1951; L. L. Taylor et al. 2009). They also form rhizomorphs of various morphologies that extend sometimes many metres from the plant, a distance that depends on the particular association (Agerer 2001; Hobbie & Agerer 2010; Águeda et al. 2014 and references) and which are involved in foraging for nutrients (see below). Usually the Hartig net involves only the outermost layer of cells of the roots, but sometimes, as in Pinaceae, the hyphae penetrate more deeply (Brundrett 2004). The fungal hyphae are septate and are not intracellular except in ERM, which usually lack a Hartig net (Ericaceae have quite a variety of associations with fungi). ECM fungi are not saprotrophic, i.e. they have few enzymes involved in cellulose and lignin breakdown and do not obtain their metabolic carbon from dead organic matter, rather, this comes from the plant, although ERM fungi may retain some saprotrophic genes from their ancestors (e.g. Michelsen et al. 1996; Jonasson & Michelsen 1996; Hashimoto et al. 2012; Lindahl & Tunlid 2014; Kohler et al. 2015). Nevertheless, the plant-fungus association hinges on the ability of the fungus to utilise the humic substances that make up soil organic matter (e.g. Schmidt et al. 2011; Rosling et al. 2015; Shah et al. 2016). Although ECM fungi have only a limited ability to break down lignocelluloses, some have coopted oxidizing enzymes that, with sugar from the plant, do break down lignocellulosic material, liberating N and P which is taken up by the plant, or they use peroxidases for this purpose (Shah et al. 2016). These enzymes are normally characteristic of brown and white rot fungi respectively, and have evolved functions that differ from those of the same enzyme in fungi in non-ECM associations, for example, rather than obtaining carbon they scavenge nutrients (Doré et al. 2015; Shah et al. 2016).

3B. Endophytic Fungi. Aside from mycorrhizal associations, other non-parasitic (at least initially) fungi growing inside plants, endophytes, have been placed in four groups. Class one endophytes are ascomycete clavicipitaceous fungi and occur in grasses with which they form very close associations (e.g. Schardl 2010; Card et al. 2014). Transmission is via the seed (vertical transmission). In other endophytes, transmission is usually via fungal spores (horizontal transmission) (Arnold 2008). Class two endophytes pervade all the tissues of the plant; the fungi involved are not particularly speciose. Class three endophytes are restricted to shoots and are very diverse. Class four endophytes, dark septate endophytes, dark because of fungal melanin (see Fernandez et al. 2013), are restricted to roots (Rodriguez et al. 2009); some of these also form ECM and/or ERM associations (Lukesová et al. 2015). (The "fine endophytes" described as growing in the roots of some Arctic plants [Newsham et al. 2009] have been considered to be VAM. However, molecular analyses suggest that these fungi, which produce fan-like arbuscules and small vesicles, are mucoromycotes, not Glomus or relatives [Orchard et al. 2016].)

All seed plants, but perhaps particularly Poaceae and Ericaceae (e.g. Petrini 1986, other papers in Fokkema & van den Heuvel 1986; Saikkonen et al. 2004), harbour endophytic fungi (Rodriguez et al. 2009; Hoffman & Arnold 2010; Friesen et al. 2011), and the number of species of fungi involved is probably very large indeed. Arnold et al. (2001) found 418 morphospecies of class three endophytes in only 83 leaves of two species of tropical trees, Ouratea (Ochnaceae) and Heisteria (Erythropalaceae) while Sato et al. (2015) found 1,245 ascomycetes, probably endophytes, in 442 root samples taken from dipterocarp forests in Sarawak (see also Bills & Polishook 1994; Frohlich & Hyde 1999; Arnold & Lutzoni 2007 and other articles in Ecology 88[3]. 2007; Ferreira et al. 2017). However, Vincent et al. (2015) found rather low beta diversity of the endophytic community in New Guinea rain forest trees - relatively few generalist species, many rare species. Most endophytes are Ascomycota, Xylariaceae and Clavicipitaceae bring notably common (Rogers 2000). A recently-described ascomycete order, Phaemoniellales, was initially based on endophytes growing in the sapwood of Peruvian Hevea (K.-H. Chen et al. 2015; see also Gazis et al. 2012). Martin et al. (2015) found 118 basidiomycete taxa (12% of all isolates) in leaves and sapwood from 192 collections of two species of Hevea; many appear to be wood decaying white rot fungi (brown rot fungi are at most uncommon as endophytes), and some have very wide distributions. Endophytes are partcularly common in Sebacinales-Serendipitaceae (Weiß et al. 2016 and references).

In other than class one endophytes, details of what the fungi are doing, let alone of any advantages accruing to any of the parties involved, are still poorly understood (e.g. see Jumpponen 2001: dark septate endophytes; S. E. Smith & Read 2008: summary; Jumpponen & Jones 2009: phyllosphere; Rodriguez et al. 2009: summary; Chaverri & Samuels 2013: Trichoderma; May 2016). The basidiomycete Piriformospora [= Serendipita] indica (Sebacinales-Serendipitaceae) can help the plant tolerate root hebivory (Cosme et al. 2016; see also Oberwinkler et al. 2013; Franken 2012), it alleviates salt stress and increases resistance to root and leaf diseases in barley (Baltruschat et al. 2008), and has other effects on the very wide variety of plants it colonizes, and typically it increases vegetative growth by up to ca 50% (see Franken 2012; Ray & Craven 2016; Weiß et al. 2016 for summaries and literature). Fungi may affect seed germination and survival (U'Ren et al. 2009), and endophytic associations are probably at least intermittently mutualistic (Carroll 1988, 1995); they may facilitate stress tolerance in the host (Rodriguez & Redman 2008). The effects can be complex. Thus dark septate endophytes may decrease the growth of conifers with which they are associated, but the effect depends on host, temperature, and association of the plant with ECM, just to begin with (Reininger & Sieber 2012). Metarhizium robertsii (Clavicipitaceae: see also below) can help its host plant alleviate salt-induced oxidative stress (Behie & Bidochka 2014). Van Bael et al. (2009) found that leaf-cutting ants seemed to dislike plants with numerous endophytes (nitrogen-fixing bacteria, esp. Klebsiella, are also an integral part of this system - Pinto-Tómas et al. 2009). Beetles fed high-endophyte Merremia umbellata (Convolvulaceae) leaves seemed to do fine, but their chances of getting captured by predatory ants increased nine times (Hammer & Van Bael 2015). Endophytic fungi have also been implicated in the protection of the host plant against fungal pathogens (Martin et al. 2012: in vitro; Raghavendra & Newcombe 2013). Lignases have been detected in a number of ascomycetes endophytic in Uruguayan Myrtaceae (Bettucci & Tiscornia 2013), while metabolites synthesized by endophytic fungi in Carapa guianensis may be antibacterial, antiviral, or trypanocidal (Ferreira et al. 2015). Interestingly, saliva from herbivores may disrupt the endophyte-plant relationship, for instance, by reducing the amount of toxic alkaloid produced by the endophyte (Tanentzap et al. 2014).

Dark septate endophytes may facilitate the uptake of nitrogen (Newsham et al. 2009; Newsham 2011), while nitrogen has been found to move from caterpillars parasitized by the ubiquitous insect pathogen Metarhizium (Clavicipitaceae, related to grass class 1 endophytes) to grasses and Fabaceae-Faboideae, plants in which the fungus is also an endophyte (Behie et al. 2012). Mutualisms can get complex here: Metarhizium can be free-living, endophytic in a wide variety of plants, some species preferring trees or herbaceous plants, at least locally, and/or a parasite of insects, and nitrogen may move to the plant from the parasitized insect and photosynthate from the plant to the endophyte (Wyrebek et al. 2011; Behie et al. 2012, 2017). Indeed, Metarhizium and fungi like the related Beauveria are widely distributed, parasitize numerous species of insects, are similarly promiscuous when it comes to being endophytes, and have been shown to benefit the plants with which they are associated; they may be important elements of the global nitrogen cycle (Behie & Bidochka 2014).

The life styles of the immediate fungal relatives of endophytes are various. Endolichenic fungi, fungi living inside lichens (but not the lichenising mycobionts whose association with algae constitutes the lichen thallus), and endophytic fungi may be phylogenetically close, while lichenising and endophyte clades tend to be exclusive (Arnold et al. 2009b; U'Ren et al. 2012; K.-H. Chen et al. 2015). Sebacinales (basal basidiomycetes) are notably diverse ecologically (Selosse et al. 2009; Weiß et al. 2012, esp. 2016), and the immediate relatives of the endophytes there may be saprotropic taxa, although these are little known. Grass endophytes (related to ergot, also ascomycetes) are perhaps derived from insect pathogens, and some species of fungi are both pathogen and endophyte (Spatafora et al. 2007; Sasan & Bidochka 2012;). As mentioned above, Metarhizium and relatives (ascomycetes, Clavicipitaceae) can be both endophytes and insect parasites (Behie & Bidochka 2014). In general, endophytic fungi are often more related to pathogens and other endophytes than to saprotrophic fungi (U'Ren et al. 2009), and they are sometimes derived from necrotrophic fungi, parasitic fungi that first kill the host cell before digesting it (Delaye et al. 2013; García-Guzmán & Heil 2013; see also Chen et al. 2015). Members of the Phialocephala/Acephala species complex may be dark septate endophytes or ecto- or ericoid mycorrhizae (Lukesová et al. 2015). Trichoderma (Hypocreales, an ascomycete) has various associations with plants and other fungi, and endophytes may become saprotrophic or mycoparasitic on the death of the host (Chaverri & Samuels 2013; see also Oberwinkler et al. 2013).

Setting up and maintaining the complex interactions betweeen endophytic fungus and host has been studied in some detail in Serendipita indica. Within minutes of the initial contact of fungal chlamydospores with the plant there are changes in the host's gene expression, changes that go on for weeks as the fungus establishes itself in the plant (Weiß et al. 2016 and references). Like the ECM association, the biotrophy involved in being an endophyte may be achieved by modifications in saproptrophic ancestors (Weiß et al. 2016). However, the extensive reprogramming of the Poa host genome by Epichloë endophytes is more like what happens when pathogenic fungi attack the plant, whereas mycorrhizal fungi have a much smaller effect (Dupont et al. 2015).

3C. Mycorrhizae and Endophytes in General. The distinction between different mycorrhizal or endophyte "types" can be less than clear, as is the very distinction between having or lacking mycorrhizae (Lekberg et al. 2005; Maherali et al. 2016). Furthermore, a single species of plant can form more than one kind of mycorrhiza, and individual mycorrhizal fungi may form simultaneous associations with more than one plant, and quite commonly with more than one species of plant, or these associations may be sequential, but occuring in the one locality; the result is the formation of very complex common mycorrhizal networks (e.g. Bruns et al. 2002; Villareal-Ruiz et al. 2004; Simard & Durall 2004; Selosse et al. 2007; McGuire 2007a; van der Heijden & Horton 2009; Kjøller et al. 2010; Kennedy et al. 2012; Hynson et al. 2013; van der Heijden et al. 2015a; Michaëlla Ebenye et al. 2017). Thus complex ECM networks may link the oldest to the youngest individuals in Pseudotuga menziesii forests (Belier et al. 2010), although this may not mean that there is nutrient excahnge between them (Michaëlla Ebenye et al. 2017). Salix may harbour ECM and/or AM fungi, not to mention dark septate endophytes (see below: e.g. Van der Heijden 2000; Becerra et al. 2009; see also Poole & Sylvia 1990; Molina et al. 1992; Bennett et al. 2017: Supplement for AM/ECM associations). Liverwort gametophytes can form associations with fungi that are also ECM on the flowering plant on which the liverwort is epiphytic. Examples include Marchantia/mycorrhizal fungus/Podocarpus and the mycoheterotrophic chlorophyll-less liverwort Cryptothallus/the basidiomycete Tulasnella/Pinus-Betula (Read et al. 2000; Bidartondo et al. 2003; Kottke & Nebel 2005 and references). Serendipitaceae from orchids formed ericoid mycorrhizae on Calluna vulgaris in vitro (Weiß et al. 2016). Genets of the suilloid Rhizopogon formed a complex web of associations with Douglas fir (Pseudotsuga menziesii, Pinaceae), the youngest and oldest plants being linked (Beiler et al. 2010; for ECM legumes, see Fabaceae) and similar networks were found in Thuga plicata and its AM fungi (Gorzelak et al. 2017). In forests at the Arctic treeline, ECM fungi from shrubs move on to seedlings germinating after fires (Hewitt et al. 2017). Villareal-Ruiz et al. (2004) persuaded a single fungal mycelium to form an ECM with Pinus sylvestris and an ERM with Vaccinium myrtillus at the same time, and the ascomycete Acephala, a dark septate hyphal species, showed similar behaviour, although not simultaneously (Lukesová et al. 2015). The fungal associates of Arbutus menziesii (Ericaceae-Arbutoideae), from Pacific North America and with arbutoid mycorrhizae, are diverse, and are found on other angiosperms growing in the same area, and in particular as ECM on Pseudotsuga (Kennedy et al. 2012) - however, temperate ECM associations usually develop from fungal propagules in the soils (Hewitt et al. 2017). There is similar lability in the ascomycete Rhizoscyphus ericae, often growing as an ERM on the hair roots of north temperate Ericaceae and as an ECM on Pinus in the same community (Grelet et al. 2010; see also Villarreal-Ruiz et al. 2004); the fungus also forms mycorrhizal associations with Jungermanniales-Schistochilaceae, leafy liverworts (Pressel et al. 2008). ECM fungi in woodlands and savannas of Africa and Madagascar are relatively uniform, and one species of fungus can form associations with plants in different families and at different successional stages, and this can facilitate ecological succession and perhaps also community shifts (Tedersoo et al. 2008, 2011). Interestingly, ascomycete fungal isolates from ECM and ERM plants, and also from those with dark septate endophytes, sometimes more or less interdigitate on phylogenetic trees and show little phylogenetic divergence (e.g. Vrålstad et al. 2002; Walker et al. 2011; Perotto et al. 2012).

Common mycorrhizal networks are not some kind of egalitarian system, rather, they may enable asymmetrical redistribution of nutrients within the system. Thus mycorrhizae of large, well-lit AM plants in such networks aquire nutrients from the soil around neighbouring plants, which did not thrive as well (Weremijewicz et al. 2016: experimental set-up).

AM presence and various aspects of root architecture such as root thicknesss, root branching, and the development of root hairs are linked (Baylis 1975; Schweiger et al. 1995; B. Liu et al. 2015), although connections between root attributes and mycorrhizal status/plant response to mycorrhizal establishment can be difficult to make (Maherali 2014). Species with thin roots may tend to forage for nutrients directly via their roots while species with thicker roots forage more indirectly via mycorrhizae, and members of the first group regenerated roots after pruning more than did the latter, although mycorrhizal colonization in the latter was higher (B. Liu et al. 2015). Thick-rooted species may be particularly common at least in the magnoliids and the [Taxaceae + Cupressaceae] clade (e.g. Kong et al. 2015; B. Liu et al. 2015). AM root architecture - complex/branched (finer) versus simple/little branched (thicker) linked with identity of the Glomus AM partner affects infection by the pathogen Fusarium oxysporium (Sikes et al. 2009). Notably fine roots are associated with ERM (modified ECM), while a few measurements of presumably ECM Fagaceae also suggested roots on the thinner side (Liu et al. 2015). In a small sample of temperate trees growing in an environment with patchy distribution of nutrients, AM trees with fine roots exploited these patches by producing more roots, while in ECM trees exploitation was via mycelial growth; the three members of Pinaceae (ECM) and two magnoliids (VAM) included had the thickest rootlets and none exploited the nutrient patches with any precision (W. Chen et al. 2016, 2017). However, a general survey of root thickness, root order, and possible links with root longevity and rate of decay, mycorrhizal type, infection by pathogens, different ways of acquiring nutrients, etc., is in order (Clowes 1951; Silva & Miya 2001; McCormack et al. 2015; Laliberté et al. 2015; Laliberté 2017 for some connections and the need to standardize measurements) and Freschet et al. (2017) provide a preliminary analysis (see also W. Chen et al. 2017: root density, thickness, and mycorrhizal type). Variation in families like Euphorbiaceae is extreme (Kong et al. 2014) and first-order "adventitious" roots in epiphytic Orchidaceae and some woody monocots like Pandanaceae are notably thick (see also Pagès 2016 for descriptions of branching patterns, root thickness, etc.), as thick or thicker than the much higher order "thick" roots (>5 mm diameter) in studies like those of Silva and Miya (2001). Some distinctive root morphologies like bottlebrush-like cluster roots in Proteaceae and dauciform roots (these have exceptionally long root hairs) are responses of the plant to nutrient-poor conditions (e.g. Schweiger et al. 1995; Shane et al. 2004b, 2006; Playsted et al. 2006; Gao & Yang 2010).

Extant forests made up of AM trees are often more diverse than forests dominated by ECM trees (Malloch et al. 1980; McGuire 2007a; Peay 2016). There are rather few species of angiosperms in ECM-dominated communities in temperate and in particular boreal areas (see below). Laliberté et al. (2013) discuss the diversity of vegetation associated with ECM in terms of the youth of the soils; tropical soils are older, more strongly weathered, and the vegetation more diverse. However, tropical ECM dipterocarp-dominated forests are as diverse as tropical forests anywhere (Read 1991, 1996; Page et al. 2012). For a comparison of carbon cycling in ECM and AM plants, with a focus on subarctic alpine Sweden, see Soudzilovskaia et al. (2015).

Overall, temperate and boreal areas have the highest ECM fungal diversity, and ECM-associated plants often dominate there (Wardle & Lindahl 2014; m 2014b). Thus Leotiomycetes, ascomycetes that are commonly ERM, become more diverse towards the poles and their ericaceous associates are also common there (Wardle & Lindahl 2014; Tedersoo et al. 2014b). ECM fungi are found in white sand vegetation in tropicl South America (Roy et al. 2016), although their diversity there is unclear. AM are found to 82o N while ECM and ERM are found to only 79o N, being limited by the ranges of their mostly woody hosts (Newsham et al. 2009). However, AM fungal species were not found beyond 74o N in the Canadian Arctic (Olsson et al. 2004) and were very uncommon just below the Arctic Circle in Alaska, and this contrasted strongly with the diversity of other ecological groupings of fungi, although the disparity in terms of numbers of clones was somewhat less (D. L. Taylor et al. 2013). The diversity of ECM fungi increases in mid to high northern latitudes and that of ERM fungi also increases towards the poles, in both cases consistent with the distribution of their seed plant associates (Wardle & Lindahl 2014; Tedersoo et al. 2014b), although the story in tundra habitats is not so clear (Gardes & Dahlberg 1996). Other groups of soil-dwelling organisms show a similar pattern, perhaps connected with the substantial vertical stratification of soils in ECM-dominated communities (Tedersoo et al. 2012; see also Delgado-Baquerizo et al. 2016). General latitudinal gradients in diversity are discussed later on.

Associations between plant and fungus may become very close. Thus mycoheterotrophic plants lack chlorophyll entirely and are nutritionally dependent on the activities of their associated fungi. Glomeromycotes are the associates in some mycoheterotrophic plants including the gametophytes of lycophytes and ferns, and the sporophytes of some monocots, Gentianaceae, etc. (e.g. Bidartondo et al. 2002; Merckx et al. 2012). Fungi from the Glomus A group are involved (Schußler et al. 2001; Winther & Friedman 2009). Mycoheterotrophs are in which the association is with glomeromycotes may be sensitive to the amount of phosphorus in the soil, as was found in a Panamanian study when they disappeared when soil phosphorus increased above 2 mg P kg-1 (Sheldrake et al. 2017), rather as increasing soil P is known to negatively affect the abundance of AM fungi in general (N. C. Johnson 2009). In mycoheterotrophic and mixotrophic Ericaceae (and Orchidaceae) modified ECM associations are common as well as associations with saprotrophic fungi, and these allow the plant to get carbon indirectly from other angiosperms also associated with the fungus or from decaying organic matter (e.g. Perotto et al. 2012; Oberwinkler et al. 2013; Hynson et al. 2013). A number of mycoheterotrophic plants are very rare, although since they spend most of their life underground and may be small and inconspicuous when in flower it is difficult to be sure, but sometimes their rarity can be linked to the restricted distribution of their fungal associate (Merckx et al. 2013b). See the papers in Merckx (2013a) for mycoheterotrophy in general.

Maherali et al. (2016) discuss evolutionary changes between ECM and AM associations, and also the non-mycorrhizal condition, in seed plants. Basically, there are few transitions between the three conditions, the mycorrhizal associations being rather stable (see also Kiers & van der Heijden 2006). The loss of mycorrhizae, a feature of clades that are usually herbaceous (Proteaceae are an exception), may be associated with a higher speciation rate that is commoner in such clades (Maherali et al. 2016 and references).

3D. Further Complexities. Sharp distinctions between different types of associations can be hard to draw (Gao & Yang 2010; esp. Vrålstad 2004; Perotto et al. 2012; Peterson 2012), and the line between mutualism - or at least prolonged symbiosis - and parasitism is a fine one (Rogers 2000; Eaton et al. 2010 and references; Oberwinkler et al. 2013). A single species or even plant may have a variety of associations with fungi, or one fungus can form different kinds of associations with different species of plants, as mentioned above. Thus Weiß et al. (2012) found the same nuclear LSU sequences of basidiomycetous Sebacinales in taxonomically unrelated plants growing in different areas, sometimes the association was mycorrhizal, sometimes endophytic, and they suggested that Sebacinales might play an important role in ecosystem integration. As mentioned, Sebacinales are poorly understood, but their members show various relationships with plants (Weiß et al. 2009, 2016; Oberwinkler et al. 2013; Varma et al. 2013).

Bacteria are also common endophytes. A diversity of bacteria and fungi, perhaps endophytes, have been isolated from the latex of Euphorbia spp., a habitat that one would not have thought to be notably suitable for such organisms (Gunawardana et al. 2015). As with fungi, the line between parasite and pathogen is not sharp, and bacteria may move from e.g. hemipterans associated with plants to the plants themselves (Caspi-Fluger & Zchori-Fein 2010). More obvious examples are nitrogen-fixing bacteria (see the N-fixing clade), but a great variety of other bacteria are involved as are plants other than members of the N-fixing clade (e.g. Hardoim et al. 2008; Lemaire et al. 2011b). It has even been suggested that plants may derive nitrogen, etc., from bacteria that enter the root and then are digested by the plant (Paungfoo-Lonhienne et al. 2010). In general, bacteria may affect the growth of the plant, whether by fixing nitrogen, deterring pathogens, or the like (Kembel et al. 2014 for references; Pennisi 2015).

Both bacteria and fungi are also found in the phyllosphere, the above-ground surface of the plant (Vacher et al. 2016 for a review). Thus some 7,300 bacterial OTUs were found on 57 species (ca 420 OTUs/tree) from Barro Colorado Island, Panama, and there may be some 11,615 bacterial OTUs in the phyllosphere of plants on that island alone (Kembel et al. 2014: use of current phylogenetic ideas does not affect results - S. W. Kembel pers. comm. xi.2014; Griffin & Carson 2015 for a review). Similarly, several hundred species of fungi have been found in the phyllosphere of the temperate Quercus macrocarpa (Jumpponen & Jones 2009). The combined fungal phyllosphere/endophyte community may be stratified in the tree canopy, and there may also be intraspecific geographic variation (Harrison et al. 2016), but little is known about such things - in/on Sequoia sempervirens, for example, basidiomycetes were surprisingly common (Harrison et al. 2016). Bacteria also flourish in the rhizosphere, where they can facilitate the uptake of organic nitrogen (J. F. White et al. 2015), and they have also been implicated in fixing nitrogen in tuberculate ECM on conifers (Paul et al. 2007). Indeed, bacteria can be very important for ECM (and AM) associations, whether facilitating the establishment of mycorrhizal associations (mycorrhization helpers) or being integral to its subsequent functioning (mycorrhizal helpers), being involved in nitrogen fixation, solubilization of nutrients in the soil, protection against root pathogens, and the like (Frey-Klett et al. 2007; Müller et al. 2016 and references).

Fungi associated with plants may have endosymbionts themselves (Frey-Klett et al. 2007). Mycorrhiza-plant associations often include bacteria as additional partners, whether growing on the surface of the mycelium and synthesising crucial metabolites or living within the hyphae. The bacterium Candidatus Glomeribacter gigasporarum (near Burkholderia) is found in the AM fungus Glomus (Castillo & Pawlowska 2009, 2010; Bonfante & Genre 2010). Such bacteria can affect the growth of the fungi, and they may be vertically transmitted like the fungal genome itself (Bianciotto et al. 2003; Hoffman & Arnold 2009). Numerous bacteria (mostly Proteobacteria) and a diversity of fungi have such relationships, although they sometimes seem to be rather casual (Hoffman & Arnold 2010; Oberwinkler et al. 2013). Viruses in endophytes may affect the ability of the host plant to grow in particular conditions. Thus Márquez et al. (2007) found that only when the endophytic fungus Curvularia protuberata (Pleosporaceae) was infected with a double-stranded RNA virus was Dicanthelium lanuginosum, the host of the fungus, able to grow in volcanically-heated soils in Yellowstone at ca 65o C.

The establishment of AM associations and a variety of aspects of the root nodulation process starting with root hair curling are connected in Fabaceae and can be linked to an autophagy-related kinase, precursors for all these activities being produced by autophagy (Estrada-Navarrete et al. 2016). Oomycete infection is part of this set of associations, and the ability to form AM associations and to allow oomycete infection is linked in Brassicales, resistance to infection being connected to the absence of mycorrhizal associations in most Brassicales (E. Wang et al. 2012; Geurts & Vleeshouwers 2012).

Several distinctive "plant" metabolites such as indolizidine (swainsonine) and ergoline alkaloids are synthesized by fungal or bacterial associates of the plant; they are toxic to animals and presumably protect the plant (e.g. Popay & Rowan 1994; Gunatilaka 2006; Kusari et al. 2012; Markert et al. 2008; Schardl et al. 2013: Convolvulaceae; Pryor et al. 2009: Fabaceae; Wink 2008); Celastraceae and especially Poaceae are also distinctive in this regard. Such compounds seem to be ordinary plant metabolites (Zhang et al. 2009; Friesen et al. 2011). Endophytic bacteria are involved in selenium uptake by Se-accumulating plants (Lindblom et al. 2013; Sura-de Jong et al. 2015). Of course, "true" secondary metabolites like terpenoids and quinolizidine alkaloids are produced more or less exclusively in mitochondria and/or chloroplasts, i.e. in endophytic bacteria whose associations with organisms are very ancient (Wink 2008). All this calls into question exactly what an individual "is" and what is selected for. Thus rather than thinking of individual plants and their genomes it may be better to think of plants as holobionts with hologenomes (Bordenstein & Theis 2015; Gilbert & Tauber 2016; Gundel et al. 2017).

4. Angiosperm History I - Evolution in stem group angiosperms.

4A. Relationships. The angiosperm stem group probably diverged from other seed plants by the late Palaeozoic (Moldowan et al. 1994; E. L. Taylor et al. 2006). So how might the heterosporangiate strobilus with short internodes with ovules enclosed in a carpel that is the angiosperm flower have evolved from the separate male and female strobili with ovules borne free on ovuliferous scales of most gymnosperms, living and fossil? Is the flower a monoaxial structure, as in most cycad strobili, or is it polyaxial, as in a pine cone, or are the flowers of some groups monoaxial and those of other groups polyaxial? The latter idea might suggest that that angiosperms are polyphyletic (see Friis et al. 2011: pp. 141-144 for a summary of such ideas), and X. Wang and Wang (2010) and Z.-J. Liu and Wang (2015) have indeed suggested that angiospermy may have arisen more than once.

Candidates for stem-group angiosperms include Corystospermales (Pteruchus, Ktalenia, etc.: Frohlich & Parker 2000), Bennettitales (especially common in the Jurassic), and Caytoniales (poorly known in the younger Mesozoic). In the much-discussed Caytonia the ovule is borne inside an inverted cupule, the cupule wall being equated with an outer integument - with interesting implications for pollination, although, like most other gymnosperms, the pollen grains end up at the micropyle (e.g. J. A. Doyle 2006, 2008b; Doyle & Donoghue 1986a, b, 1992; Doyle & Endress 2010; Friis et al. 2011). Other pteridosperm groups that have been linked with angiosperms include Pentoxylon and glossopterids, the latter a poorly known group in which ovules are borne on the leaf (Friis et al. 2011 for a summary), the diminutive Petriellales, Peltaspermales, and Doyleales, with compound seed cones (Taylor & Taylor 2009; Rothwell & Stockey).

Seed morphology and anatomy in particular, but also pollen morphology, suggest that Bennettitales, Erdmanithecales and Gnetales should be placed together (the BEG group), and Caytonia may also be part of this group (Friis et al. 2007, 2009a: four new genera in this complex, 2013; 2011: especially useful, see chapter 5; Mendes et al. 2010). Note that some Jurassic-Early Cretaceous fossils described as angiosperms seem to have a better home in the gnetalean area (Herendeen et al. 2017 and literature). Bennettitales were especially common in the Jurassic (although they may have persisted to the Oligocene - McLoughlin et al. 2011), Erdmanithecales persisted into the Late Cretaceous, while Gnetales are still extant. The reproductive morphologies of some of the early (Upper Triassic) Bennettitales are rather different from those of later fossils (e.g. Pott et al. 2010), and the interpretation of their complex reproductive structures is not easy (see Crane & Herendeen 2009 for careful analyses) Thus Crane and Kenrick (1997) suggest that the interseminal scales of Bennettitales are sterile [ovule + cupule] complexes. Stockey and Rothwell (2003) noted that in Williamsonia pollination appeared to be siphonogamous, there was no pollen chamber, and a nucellar plug filled the micropylar canal.

A more or less close phylogenetic association between Cycadeoids or Bennettitales, so-called "fossil beehives", and angiosperms has long been mooted (see also J. A. Doyle 2006 and Hilton & Bateman 2006 for morphological cladistic analyses and literature; Little et al. 2014). Interestingly, the triterpenoid oleanane, found pretty much throughout angiosperms, also occurs in Bennettitales and is scattered elsewhere (Moldowan et al. 1994; E. L. Taylor et al. 2006; Feild & Arens 2007; see also Banta et al. 2017), so it is not easy to interpret the significance of its presence. Furthermore, many morphological studies have associated Gnetales and angiosperms, the anthophyte hypothesis (see elsewhere), although the former may be best placed sister to or even inside Pinales (see below). Gnetales themselves are far from having a flower, but they do have strobili with both types of sporangia, even if only one type normally produces spores. However, the BEG clade seems to have little immediately to do with angiosperm origins.

The envelopment of the seed to produce a fruit-like structure is likely to have happened independently in a number of groups like Bennettitales, Doyleales, and angiosperms (Rothwell & Stockey 2010, 2016 and references; see also Friis et al. 2011; Tomlinson 2012 and angio-ovuly). Pollen tubes in Pinales/Gnetales and in angiosperms also probably evolved independently since at least some glossopterids had multiciliate male gametes (Nishida et al. 2004; c.f. Lee et al. 2011: cilia lost and regained).

There are number of reports of pre-Cretaceous angiosperm pollen. Some can be dismissed because of incorrect dating or contamination (Hochuli & Feist-Burkhardt 2013 for literature). However, there are several pollen types from the Middle Triassic (ca 243 m.y.a.) of northern Switzerland that are similar to angiosperm pollen, being monosulcate, columellate, semitectate and reticulate, but with a very thin nexine (Hochuli & Feist-Burkhardt 2013). Pollen grains of this type are found in a variety of habitats, so it is a little difficult to explain the absence of angiosperm megafossils if the pollen really was from angiospermous plants. Hochuli and Feist-Burkhardt (2013) reasonably elect to consider these grains to be pollen of relatives of stem-group angiosperms. If columellate pollen is ancestral in angiosperms, there may be connections with the Triassic reticular-columellar Crinopolles pollen type (J. A. Doyle 2001; Zavada 2007). Other grains from the Triassic and Jurassic are remarkably like the very distinctive pollen of Acanthaceae-Tricantherinae (see also Tripp & McDade 2014b). Some of these angiosperm-like pollen types are associated with macrofossils, for instance, the late Triassic Sanmiguelia, although whether any can be linked to the angiosperm line is uncertain (Friis et al. 2011: pp. 158-162; Herendeen et al. 2017; c.f. Cornet 1986).

Rudall and Bateman (2010) reasonably thought that the morphology of crown-group conifers, being highly derived, might be of little help in thinking about that of the ancestors of angiosperms. This in part turns the problem over to the interpretation of fossil remains, and here there has been little progress over the last fifty years or more. Similarities between the ovules of some Magnoliaceae and the cupules of Caytonia (e.g. Umeda et al. 1994) are probably superficial; features like the lobing of the integuments which induced this comparison seem to have little phylogenetic significance (e.g. Endress & Igersheim 2000; Endress 2005c). However, it has been suggested that the two integuments are of quite different origins and ages. The inner integument perhaps represents a modified dichotomising telomic system while the outer integument is leaf-like and derived from a cupule wall (see J. A. Doyle 2006; see also Skinner et al. 2004: expression of a YABBY gene in the o.i.); the inner integument of angiosperms develops first (e.g. Schneitz 1999). Although the ovule-bearing structures of Caytonia and Glossopteris can be linked with the ovules and carpels of extant angiosperms by invoking appropriate morphological gymnastics, it does not make for satisfactory reading.

Taking morphology and development together affects how one interprets fossils. Baum and Hileman (2006) proposed a developmental genetic model for the evolution of the angiosperm flower which may also help in the interpretation in the significance of particular fossils (see also Ruelans et al. 2017: gene duplications involved). Frohlich and Parker (2000) had suggested that the heterosporangiate strobilus had evolved in pteridosperms like Corystospermales from a male strobilus on which ectopic ovules developed - their "mostly male" theory of the origin of angiosperm flowers. LEAFY/FLORICAULA genes were likely to be associated with male reproductive structures, they suggested, and NEEDLY genes with female. However, work on the expression of LFY/FLO and NLY orthologs suggest that both genes are expressed in early-stage primordia, but the former are then expressed in ovules and microsporangia while the latter are expressed in the ovuliferous scale, aril, microsporophylls, etc. The expression of both genes in both male and female cones is not consistent with the "mostly male" theory (Vásquez-Lobo et al. 2007 and references; Moyroud et al. 2010; Tavares et al. 2010; see also Bateman et al. 2011b). Mathews and Kramer (2012; c.f. Kelley & Gassner 2009 for a more conventional approach) analyse ovule development in seed plants and floral development in angiosperms to think how these structures might have evolve, and i.a. they suggest that evolution is less the change in form of pre-existing structures than the assemblage of new developmental modules in the context of homeosis, heterotopy, and heterochrony (see also Harrison et al. 2005b; Pires & Dolan 2012; Lovisetto et al. 2012; Almeida et al. 2014).

To summarise: ideas of relationships between angiosperms and other seed plants remain in limbo, as they have for some time (Scott et al. 1960; Feild & Arens 2005, 2007; Frohlich & Chase 2007; Herendeen et al. 2017). As Feild and Arens (2007: p. 292) noted, "It is not hyperbole to claim ‘all bets are off’ on the question of angiosperm sister groups." In particular, it is unclear what gymnosperms/seed ferns are to be linked with stem-group angiosperms, regardless of whether or not extant gymnosperms are monophyletic or where Gnetales are to be placed on the tree (e.g. Rudall & Bateman 2010; see also above). In a comprehensive review on the bearing of fossil data on the origin of the flower, J. A. Doyle (2008b) concluded that our understanding of the fossil record was insufficient to help much in understanding angiosperm origins. Taylor and Taylor (2009) reached much the same conclusion, and as they noted, timing is important. For instance, glossopterids are not known after the Permian-Triassic boundary, i.e. some 100 m.y. before the earliest angiosperms - at least, by some estimates. Puttick et al. (2017) reanalysed the data matrix of Hilton and Bateman (2006) using a variety of methods (other data matrices, both real and simulated, were used), and found that Caytonia consistently came out sister to angiosperms, but broader groupings depended on the methods of analysis used (see also O'Reilley 2016), and with the Bayesian method, that was all you got. As Puttick et al. (2017: p. 6) noted of a Bayesian analysis they carried out on the Hilton and Bateman data set, "This [low resolution] suggests that the available data are insufficient to discriminate among the competing hypotheses, and this long-standing debate [about the relationships of angiosperms as evident in the fossil record] is largely an artefact of the false resolution of parsimony methods." Only morphology can allow us to infer what the immediate ancestors/relatives of crown-group angiosperms, and the ancestors/relatives of those plants, looked like, and morphology is currently not helping much.

Pollination & Seed Dispersal. Early seed plants are likely to have been wind pollinated. A number of gymnosperms, both living and extinct, have saccate pollen, and the sacci would seem to be wings that aid in the dispersal of the pollen, but they are more like water wings, and help float the pollen on to the micropyle, although they may also reduce the settling velocity of the pollen, a low settling velocity being characteristic of pollen involved in wind pollination (Schwendemann et al. 2007; Bolinder et al. 2015; c.f. Hall & Walter 2011 in part). There is a general correlation between saccate pollen, erect cones, inverted or downwards-facing ovules, and the presence of a pollination droplet - although perhaps not in Cordaitales. Saccate pollen has evolved more than once, and it has also been lost - but apparently never regained (e.g. Stützel & Röwekamp 1999b; Leslie 2008, 2010b; especially Leslie et al. 2015a). Pollination droplets may be involved in pollen capture as long ago as in the Middle Pennsyylvanian seed fern, Callospermarion pusillum (Rothwell 1977; c.f. Labandeira et al. 2007). They may also become a part of an insect pollination mechanism, as in some extant Gnetales (Rydin & Bolinder 2015; see also Frohlich 2001), and Little et al. (2014) and von Aderkas et al. (2014) discuss the composition of the nectar-like pollination droplet of gymnosperms - even in wind-pollinated taxa it is rich in sugars.

Insects first appeared in the Middle Silurian (Misof et al. 2014: molecular dates), to Late Devonian (Garrouste et al. 2012: fossils). Early insects are implicated in the pollination of some pre-Cretaceous gymnosperms, and a diversity of groups - beetles, Neuroptera, mecopterids (scorpion flies, Mecoptera, perhaps) and true flies (bee flies may be early Jurassic - see Wiegmann et al. 2011), thrips, etc. - may have been involved (Labandeira 1998, 2006, 2010; Grimaldi 1999; Labandeira et al. 2007; Ren et al. 2009; Peñalver et al. 2012). Crown-group ages of all these clades date to the Carboniferous (Misof et al. 2014), however Tong et al. (2015, see reply by Kjer et al. 2015) thought that a number of ages should be ca 100 m.y. older, crown-group lepidoptera being around 260 m.y.o., later Permian, and thus any link between angiosperm and lepidopteran diversification was unlikely. Stem-group ages in Wiens et al. (2015) cover quite a spread. Overall, ages for lepidopteran diversification are unclear (see also Regier et al. 2015). Note that the first couple of clades are jawed moths, but most lepidoptera are Glossata, with probosces.

Bennettitales flourished from the Triassic to the Cretaceous, and they had large, rather flower-like reproductive structures, in Cycadeoidaceae in particular producing both pollen and ovules, but how they were pollinated is unclear (Friis et al. 2011). However, Peñalver et al. (2015) found a fly with a long proboscis some 105 m.y.o. that had pollen probably from a Bennettalean plant on it. The type of fly was widely distributed and is known from deposits 125-100 m.y.o., while scorpion flies are also quite large pollinators with a diversity of proboscis lengths and may have fed on a variety of Mesozoic gymnosperms (Peñalver et al. 2015). Labandeira et al. (2016) suggest that kalligrammatid lacewings (Neuroptera), were likely to have pollinated bennettitalean plants. These Neuroptera show notable parallelisms with Lepidoptera, including eyespots and scales on the wings, long-probosces, etc., and flourished 165-120 m.y.a. (Labandeira et al. 2016). Thrip-, beetle-, fly- and moth-pollination are all known in extant gymnosperms (Kato & Inoue 1994; Schneider et al. 2002; Oberprieler 2004; Labandeira 2005). Moth pollination may even be the ancestral condition in Gnetales (Rydin & Bolinder 2015), and features of fossil "ephedroid" pollen frequently fit the insect pollination syndrome, i.a. the pollen tending to clump (Bolinder et al. 2015). Certainly, angiosperms have never had a monopoly on insect pollination (see also Erbar 2014).

In extant gymnosperms unfertilised ovules are sometimes about as large as seeds, since they keep on growing until the time of fertilization, which may be long after pollination. In angiosperms, however, ovules are small, the time to pollination is short, and the seeds are nearly always relatively much larger than the ovules. Angiosperm ovules can be aborted with little loss to the plant if pollination does not occur, but in gymnosperms the loss is much more substantial (Haig & Westoby 1989).

Sims (2012) suggested that during the middle Mississippian to Pennsylvanian average seed size increased to about 8 mm3, a value that held largely steady until the evolution of flowering plants, when it decreased; cycads (large) and pines (small) are the two ends of the size spectrum in extant gymnosperms. Seeds of Mesozoic seed plants are diverse morphologically (e.g. Anderson & Anderson 2004), and animal dispersal is likely to have been quite common (Friis et al. 2011). Lovisetto et al. (2011) discuss the evolution of fleshiness in disseminules of seed plants in general; similar genes are involved, even if fleshiness may develop in very different places/from very different structures.

Plant-Animal Interactions. For plant-arthropod associations, see Labandeira (2006), who noted that medullosan pteridosperms showed a particularly high level of herbivory in the Carboniferous compared to that of other plants of that time. Galls, often thought to have diversified along with early angiosperms, are known on Glossopteridales and Peltaspermales in particular from the Lower Permian (Schachat & Labandeira 2015). Lepidopteran diversification may have begun on Jurassic or even Late Triassic gymnosperms (Labandeira et al. 1997; Wahlberg et al. 2013), although this has been questioned and is perhaps unlikely (Grimaldi 1999; Grimaldi & Engel 2005; Regier et al. 2015: esp. Fig. 10), and anyhow we have seen that the age of lepidoptera is very uncertain.

5. Angiosperm History II: Cretaceous (or much earlier?) Origins.

5A. Introduction. Ages of fossil plants that have been called with greater or less degrees of confidence crown-group angiosperms vary greatly, and molecular estimates show an even wider spread; these latter are summarized above and range from 280-130 m. years. Thus reporting on possible pre-Cretaceous angiosperms from China, X. Wang (2010a) thought that carpels consisted of an axis (= placentae) subtended by bracts, which made the carpel walls (see also Guo et al. 2103; W. Liu & Ni 2013). The early to mid Jurassic Schmeissneria, previously placed in Ginkgoales, was considered to be angiospermous, having closed carpels (X. Wang et al. 2007; X. Wang 2009, 2010b; Z.-J. Liu & Wang 2015 for discussion), while another mid-Jurassic fossil, Xingxueanthus, had closed carpels as well as a style (Wang & Wang 2010); these were thought to be stem-group angiosperms, and they have angiospermous gynoecia. Euanthus panii, also from the same locality, was described as having 5 sepals and petals, probably 5 tetrasporangiate stamens, a semisuperior ovary with quite a long hairy style, and ovules, although details of ovules and anther contents in particular are hard to make out (Z.-J. Liu & Wang 2015) and the authors were not sure whether these were fossils of stem- or crown-group angiosperms; Herendeen et al. (2017) suggest that Euanthus represents part of a conifer cone, and the carpels of Solaranthus (= Euanthus) daohugouensis are resin bodies of a male cycad cone (Deng et al. 2014). Finally, Han et al. (2016) have described the very small - <4cm tall - Jurassic (>164 m.y.o.) Juraherba which looks faintly like an onion but with long-pedicellate, axillary flowers that perhaps have parietal placentation. Herendeen et al. (2017) review these fossils and the literature bearing on their identity and find that none is definitely an angiosperm and some that can be idenfied seem rather to be Gnetales or Ginkgoales.

If crown-group angiosperms are 280 to 186 m.y.o. (e.g. Zeng et al. 2014; Foster et al. 2016a), so certainly Jurassic if not mid-Permian (see above; c.f. Beaulieu et al. 2015) in origin, rather than Cretaceous in origin and only some 140-130 m.y.o., we have a series of problems. There will probably be different sets of gymnosperms/pteridosperms involved in the evolution of the angiosperm flower, and the ecological context for the evolution of angiosperms and of the insects associated with them will differ. How angiosperms persisted as a presumably not very diverse clade for 50 m.y. or much, much more is yet another question. In addition to the wide spread of ages suggested for crown-group angiosperms, there are very differing narratives for later angiosperm evolution. Some suggest that angiosperms achieved ecological dominance by the end of the Cretaceous; others suggest that tropical rainforest as we know it had barely developed then, and that Cretaceous angiosperms were ecologically and physiologically rather unlike many of their Caenozoic successors. There are similar tensions in the literature on the evolution of animals associated with plants.

Angiosperm diversification is often discussed in terms of the consequence of the evolution of flowers (and fruits). To oversimplify: The diversity of flowers and fruits represent adaptations to pollination and fruit dispersal and help ensure reproductive isolation; flowers allowed greater speciation rates (e.g. Hickey & Doyle 1977; Niklas et al. 1983). This may well be true - floral differences are often involved in species barriers - and there are other important changes in the life cycle like increased efficiency in producing seeds and shorter gametophyte phase (see below).

There are a couple of things to remember here. First, although what pollinated the seed plants immediately ancestral to angiosperms is unclear, insect pollination in extant gymnosperms (e.g. Zamiaceae, Gnetales) is quite common, and the same was probably true of their fossil relatives (see above). Insect pollination is by no means unique to angiosperms. Florivory by insects - extant angiosperms being for the most part notably palatable to insects, although there is little obvious difference in protection of flowers versus their leaves - may be a route by which angiosperm-pollinator relationships became established (Frame 2003), although florivory may well cost the plant (McCall & Fordyce 2010). Second, there have also been vegetative and associated physiological changes that have profoundly affected the ecology and evolution of angiosperms and indeed the climate of the whole earth (see also below). It is not simply species numbers and flowers and fruits that matter, but also the morphologicasl disparity in clades and what clades "do", i.e. their roles in the ecosystem and their effects on the biosphere as a whole, that help us understand angiosperm evolution (e.g. Bengtsson 1998 and in particular Augusto et al. 2014, emphasis on the ecophysiological dimensions; Rothman 2001; Minelli 2016: emphasis on morphological disparity).

Our understanding of the eco-physiological dimension of the evolution of extant angiosperms is improving fast (e.g. Feild & Arens 2007; Internat. J. Plant Sci. 173(6). 2012; Shah et al. 2016; etc.). Some of the differences between the leaves of early angiosperms and those of other vascular plants have long been evident (e.g. Hickey and Doyle 1977), and both these and other vegetative changes can be linked with changes in the rate of photosynthesis, nutrient cycling and acquisition, silicate breakdown and rock weathering, and the like (e.g. Knoll & James 1987; Volk 1989: emphasis on deciduous ecosystems; Sack & Scoffoni 2013; Augusto et al. 2014). One result may be the facilitation by angiosperms of the spread of the l.t.r.f. habitat in which so much biotic diversity is now to be found, and angiosperms may also be implicated in the long-term decline in atmospheric CO2 concentration that characterises the Caenozoic. The interactions of plants with their fungal associates, whether ecto- or endomycorrhizal or endophytic, the bacteria associated with them, and again, their effects on the weathering rocks, soil structure, carbon sequestration, and nutrient cycling are all part of the story (e.g. Rothman 2001; Herre et al 2005; Beerling 2005a; L. L. Taylor et al. 2009; Quirk et al. 2012; Bragina et al. 2014), indeed, recent work emphasizes that iindividual plants can usefully be thought of as holobiont, a group of more or less closely integrated organisms, the genomes of all these organisms making up the hologenome (Bordenstein & Theis 2015; Gilbert & Tauber 2016). Interactions of plants with their pollinators and seed dispersers have taken place in the context of this changing biosphere (see also Boyce et al. 2010; Marazzi & Sanderson 2010).

In what follows, I initially focus on angiosperm evolution, and later I attempt to integrate angiosperm diversification with that of some of their more important plant and animal associates.

5B. Early Cretaceous Evolution - to the end of the Aptian, ca 113 m.y. ago. Background. The climate in the late Upper Jurassic-early Lower Cretaceous was dry - certainly Pangea had a notably dry interior - but continents were drifting apart, and sea levels were rising. Atmospheric CO2 concentrations were about 1,400 p.p.m. around the mid Cretaceous, possibly the highest concentrations since the late Devonian ca 360 m.y.a., but they declined subsequently, if with the odd hiccup; there may have been a particularly abrupt decrease in the middle of the Cretaceous (Feild et al. 2011b; Barclay et al. 2010; McKenzie et al. 2016). Oxygen concentrations were also high (Shi & Waterhouse 2010; Beerling & Franks 2010; He et al. 2012; Franks et al. 2013).

The most comprehensive reviews of Cretaceous angiosperm history are those of Friis and collaborators, and this whole section draws heavily on Friis et al. (2011); see also Krassilov (1997), Dilcher (2010), Taylor (2010: focus on genes possibly involved), Cantrill and Poole 2012 (focus on Antarctica), and J. A. Doyle and Upchurch (2014). Doyle (2008b), Specht and Bartlett (2009), Endress (2010a), Doyle and Endress (2010), and others provide surveys of the floral morphology and biology of extant basal angiosperms. For the floral morphology of the immediate common ancestor of angiosperms, see e.g. Doyle and Endress (2000), Endress (2001a, 2010a), etc..

Here I assume an early Lower Cretaceous age for the appearance of crown-group angiosperms, and this section takes to the story to the and of the Aptian, ca 113 m.y.a., i.e. a little before the end of the Lower Cretaceous. However, as already mentioned, estimated ages for angiosperms are all over the shop. For example, Fleming and Kress (2013) suggested that crown-group Zingiberales were late Jurassic or early Cretaceous, and so some 150 m.y.o.; what vertebrates pollinated them (they think vertebrate pollination is the plesiomorphic state for the order) is unclear under this scenario, and as to what might pollinate ancestral angiosperms, which must be substantially older, would be anyone's guess.

The pollen of early angiosperms is more or less globose, monosulcate, with a continuous tectum, columellate infratectum, and thin endexine (e.g. Doyle 2005; Friis et al. 2011; see also Hughes 1994 and references). Fossil pollen from the Cretaceous Valanginian-Hauterivian 141-132 m.y.a. has been attributed to angiosperms, and their diversification was well under way by 137 m.y.a. as judged by these pollen remains, but it was 10-30 m.y. or more before crown-group diversification really got going (e.g. Feild & Arens 2005). Thus in the Barremian-Aptian ca 125 m.y.a. there are some 140-150 taxa recorded from Portugal alone (e.g. Friis et al. 1999, 2000a, 2010b). All in all, a remarkably diverse flora, even if recent work suggests a younger age for at least some of this material, perhaps Albian and ca 112 m.y.o. (Heimhofer et al. 2005, 2007).

Very few of these fossils can be assigned to extant families, but 85% of them are from magnoliid-type or somewhat monocot-like plants (e.g. Friis et al. 1997a, 1999, 2001, 2010a, 2011; Heimhofer et al. 2007; J. A. Doyle et al. 2008: early putatively monocot fossils; Doyle 2014b: identifications of Early Cretaceous fossils). Doyle (2001) noted the abundance of fossils with ascidiate carpels and exotestal seeds in these floras - and in extant members of the ANA grade and Chloranthaceae. Doyle and Endress (2010: relationships [Chloranthaceae [[magnoliids + monocots] eudicots]]) and Friis et al. (2011) should be consulted for possible phylogenetic placements of a number of mostly magnoliid and ANA-grade Cretaceous fossils. Indeed, many older plant fossils have remarkable character combinations (e.g. Feild & Arens 2005; Fries et al. 2011 and references). For example, Archaefructus, probably an aquatic herb from the Barremian-Aptian at least 124 m.y.a. (Sun et al. 2002), has been interpreted as having perfect flowers that are unlike those of any extant angiosperms - there is no perianth, the receptacle is very elongated, the stamens are paired, and the carpels are conduplicate - or these "flowers" are inflorescences, the paired stamens representing staminate flowers. Z. Zhou et al. (2003), Friis et al. (2003b, 2011), Crepet et al. (2004), Ji et al. (2004), Doyle and Endress (2007) and others offer interpretations of the flowers of this and other early fossil angiosperms. However, even if Archaefructus is sister to all extant angiosperms (Sun et al. 2001; Crepet et al. 2004), given that we don't know other plants from that part of the tree, its significance is unclear; it does not necessarily mean that angiosperms had an aquatic ancestry. Although Du et al. (2016) incline in that direction they prefer to think that Nymphaeales, Acorales and Alismatales, and Ceratophyllales are extant members of an early radiation of aquatic angiosperms that includes Archaefructus.

Recent morphological work suggests that Archaefructus could belong to Nymphaeales (Doyle & Endress 2007, 2010a; Doyle 2008b). Hyrcantha, also more or less aquatic, is from the same Barremian-Aptian deposits in China (Dilcher et al. 2007); it has leaves with sheathing stipules and partly connate carpels with apparent resin bodies at their apices. Of course, such fossils may represent quite distinct but now extinct clades (von Balthazar et al. 2008). Indeed, a variety of strange-looking putative angiosperms have been discovered in deposits from northeastern China, and although the identities of a number are disputed (Sun et al. 2006; Y. Yang & Ferguson 2015: some "angiosperm" fossils are near Ephedra), new fossil finds from this area continue to challenge our understanding of angiosperm evolution (e.g. Sun et al. 2011, but see Z. Zhou 2014: eudicots). However, fossils ascribed to Sarraceniaceae (asterids-Ericales) from deposits about the same age as those in which Archaefructus was found (Li 2005) have turned out to be galls of the conifer Liaoningocladus boii (W. Wong et al. 2015), which is something of a relief.

Moore et al. (2007) suggested some time between 148.6-135.5 m.y.a. for a rapid separation of the Chloranthaceae, monocot, magnoliid, eudicot and Ceratophyllum clades (see also Sun et al. 2011). Plants ca 130-120 m.y.a. belong to the ANA grade, Chloranthaceae, and Ceratophyllum areas, while magnoliids, Platanales, and Buxales predominate later. Fossils assignable to Chloranthaceae date from the late Barremian ca 130 m.y.a. onwards, some being very like extant Hedyosmum (e.g. Crepet & Nixon 1996; Friis et al. 2006b, 2011). Pseudoasterophyllites, vegetatively like Ceratophyllum, has been linked with Tucanopollis, an abundant palynomorph from Africa-South America over 125 m.y.o., and also with Chloranthaceae (Kvacek et al. 2012). A [Chloranthales + Ceratophyllum] clade would be of considerable interest and importance (see also Doyle et al. 2015; Gomez et al. 2015, c.f. Herendeen et al. 2017), but molecular evidence for it is as yet not compelling (see elsewhere for more discussion). Monocot pollen 120-110 m.y.o. has been identified as Araceae-Pothoideae (Friis et al. 2004; see also J. A. Doyle et al. 2008: Friis et al. 2010; c.f. Hoffmann & Zetter 2010), but overall monocot fossils are not very common, perhaps because monocots are largely herbaceous and may have fossilized less well. Pollen data suggest that monocots/magnoliids split in the early Aptian-mid Albian 125-105 m.y.a. (Heimhofer et al. 2005; Hochuli et al. 2006), while Jud and Wing (2012) thought that monocots and eudicots might have diverged 125-119 m.y.a. (see also Jud & Hickey 2013; W. Wang et al. 2014), initial angiosperm diversification having occurred within a mere 5-10 m.y. before that (see also Wang et al. 2016a, b). Magnoliids diversified somewhat later for the most part (Friis et al. 1997a, 2006b for reviews).

Tricolpate pollen, the signature of eudicots, is reported from the Late Barremian-Early Aptian some 125-120 m.y. (e.g. Magallón et al. 1999; Sanderson & Doyle 2001), although if the relationships of Leefructus from early Cretaceous deposits 125.8-122.6 m.y. old in China and assigned to stem Ranunculaceae (Sun et al. 2011: no associated pollen) is confirmed, these ages may need to to be revised (but c.f. W. Wang et al. 2014, 2016a, b). The functional advantage of tricolpate pollen may be that the grains germinate faster than monoaperturate pollen, even if they remain viable for a shorter time (e.g. Furness & Rudall 2004). In west Portugal and elsewhere tricolpates were initially in low numbers, but by the Early Albian ca 112 m.y.a. angiosperms, including tricolpates, were diversifying rapidly, although they were still not very abundant (Heimhofer et al. 2005; Friis et al. 2006b; Horikx et al. 2016). Kajanthus, from Portugese Cretaceous deposits around 113 m.y.a., may even be assignable to crown-group Lardizabalaceae (Mendes et al. 2014). For a critical re-evaluation of the North American Potomac floras, largely Aptian to Albian in age (125-100 m.y.o.), see Doyle and Upchurch (2014).

Thinking about the morphology of extant magnoliids and ANA grade angiosperms may help here. In these plants, distinguishing between perianth and prophylls and bracts can be difficult, there can be intermediates between perianth and stamens, the numbers of parts and their arrangement vary, pollen morphology and embryo sac development vary, etc. (e.g. Buzgo et al. 2004; M. L. Taylor et al. 2008, 2015; Endress 2008a; J. A. Doyle & Endress 2011; Abercrombie et al. 2011; Wortley et al. 2015; Lu et al. 2015). Taxa in which DEF-like and GLO-like proteins can form homodimers predominate in the ANA grade and magnoliids (DEF-like proteins cannot form homodimers above the latter node, and GLO-like proteins cannot form them in eudicots); heterodimers are formed throughout flowering plants, although the situation in gymosperms is less clear (Melzer et al. 2014). This ability to form both homo- and heterodimers may contribute to the diversity of floral morphologies in these basal clades (Melzer et al. 2014), and perhaps in their extinct relatives. In the fading borders/sliding boundaries model of floral evolution (see also Thiessen & Melzer 2007), genes whose expressions are quite tightly linked to particular floral whorls in eudicots show much less specificity in these more basal angiosperms (Chanderbali et al. 2009, esp. 2010 and references: Lauraceae and Nymphaeaceae emphasized, 2016), while Warner et al. (2009) noted that "petalness" and "sepalness" in flowers of taxa from Nymphaeales and Austrobaileyales was environmentally controlled, the exposed areas in buds of Nymphaea, for example, being sepal-like on the outside. Such factors may contribute to the the difficulty in assigning early angiosperm fossils to extant clades, but understanding floral development in these clades will clarify floral evolution in angiosperms as a whole (Chanderbali et al. 2016a), for instance, in the last case the development of fully whorled phyllotaxis in the perianth (i.e., not the monocot condition where the tepal whorls do not fully encicrcle the floral apex) and change in the control of "sepalnesss" would be a route to developing eudicot flowers.

The flowers of early angiosperms appear to be rather generalized and are small to very small, quite often less than 1 mm across - there are very small fossil waterlilies, very small Hedyosmum-like flowers (Chloranthaceae), etc. (see e.g. Crane et al. 1995; Doyle & Donoghue 1986a; Friis & Crepet 1987; Friis & Endress 1990; Friis et al. 2000, 2006b, 2010b, 2011; Endress 2001a; Weberling 2007; Doyle 2008b; Doyle & Endress 2000, 2010, 2011). The stamens are often wedge-shaped, with a massive apex, stout filaments and connectives, and anthers opening by laterally-hinged valves (e.g. Crepet & Nixon 1996; Endress 2008c and references; Endress 2011a) - although perhaps not in the earliest angiosperms. Styles were at most short, and dry stigmas and protogyny were probably the common conditions (e.g. Sage et al. 2009; Endress 2010a). There was often only a single ovule per carpel, perhaps the ancestral state for angiosperms (e.g. Doyle 2012), the stamens produced only a few pollen grains, and there is initially no evidence of nectaries (e.g. Crepet et al. 1991; Dilcher 2000; Friis et al. 2006b, 2011). However, quite "derived" features are early apparent. Thus Sinocarpus, from the Barremian-Aptian 139-122 m.y.a., had carpels that were apparently connate at the base (Leng & Friis 2003), and flowers with inferior ovaries were surprisingly common early on (e.g. Crane et al. 1995; Friis et al. 1999, 2011). Interestingly, Oyston et al. (2016) suggested that angiosperms a a whole have shown remarkably little variation in disparity, that is, the amount/extent of morphological variation in a sample of taxa, over time (c.f. gymnosperms), and this is true of individual angiosperm groups like palms; Lupia (1999) had noted that pollen disparity increased early in angiosperm evolution and was flat in the late Cretaceous-Palaeocene.

As to what pollinated these early angiosperms, little directly is known. Their tiny flowers were probably aggregated into inflorescences to attract pollinators (Friis et al. 2006b, 2011). Some sort of insect pollination is likely (Hu et al. 2008), although Hu et al. (2012 and references) suggest that pollination may have been by both insects and wind, ambophily (see also Friis et al. 2011). How pollinators handled these flowers is unknown, although extant members of early-branching clades of Lepidoptera (monotrysian "microlepidoptera"), at least, are usually very small (Regier et al. 2015 for a phylogeny). B. Wang et al. (2013) reviewed the possible role of beetles in the pollination of early angiosperms, noting that potential pollinators such as Scarabaeoidea and Chrysomeloidea had evolved by or at the early Cretaceous. Labandeira (2010 and references) suggested that many pollinating clades of Hymenoptera, Diptera, and Lepidoptera originated around the late Barremian/end Albian some 125-100 m.y.a., and although Misof et al. (2014: p. 767) note, "we dated the spectacular diversifications within Hymenoptera, Diptera and Lepidoptera to the early Cretaceous, contemporary with the radiation of flowering plants" (see also Grimaldi 1999; Lunau 2004), any connection between insect and plant diversification then is unclear, and anyhow dates in Tong et al. (2015; c.f. Kjer et al. 2015) are sometimes substantially earlier (see also Regier et al. 2015 for caveats over dating).

Pollen is a likely reward for early pollinators (Erbar 2014). Pollen was initially produced in rather low quantities, but it is found in insect coprolites and may have been eaten by the pollinator (Friis et al. 1999). The paucity of dispersed pollen morphs and the diversity of pollen morphs associated with plant remains also suggest some kind of insect pollination (Friis et al. 1999). However, there is no signal from fossil pollen that unambiguously suggests particular pollinators, and although pollen size and style length are correlated in a general way, there are no plots of pollen size over time (Roulston et al. 2000). Bees are unlikely to have pollinated early flowers, and certainly, their diversity then was low (e.g. Grimaldi & Engel 2005). Pollenkitt, produced by tapetal degeneration and rich in plastid-derived lipids that are used by bees, helps pollen grains stick together and to the pollinator, although the pollen of cycads, for example, may clump in the absence of pollenkit (e.g. Hall & Walter 2011; Bolinder et al. 2015); pollen of early flowers is not often clumped (J. A. Doyle et al. 1975; Hu et al. 2008).

Nectar, whether in the form of stigmatic secretions or from some other kind of nectary, is unlikely initially to have been a common reward (see Erbar 2014: Table 1 for floral nectaries of basal angiosperms; Gottsberger 2016). However, septal nectaries may be an apomorphy for monocots, even being found in some Alismatales, but probably not in Acorales; they may date to 120 m.y.a. or earlier. Many Laurales have paired glandular bodies at the bases of the stamens that may provide some reward to the pollinator, and the clade with these nectaries is dated to around 127-89 m.y.a. (see Laurales). Brachyceran diptera, which can have probosces over 5 mm long "especially suitable for visiting long tubular flowers" (Ren 1998: p. 86) for nectar, might have pollinated early angiosperms. These flies are known from the late Jurassic and early Cretaceous (Ren 1998; c.f. Z. Zhou et al. 2003 for dates). However, given both their age and their relatively large size, their role in angiosperm evolution is unclear; there is no evidence that they pollinated early angiosperms (Peñalver et al. 2015), "long tubular flowers" apparently evolving later. Indeed, Labandeira et al. (2016 and references) suggest that the mid-Cretaceous decrease in prominence of gymnosperms was associated with the demise of many of their insect associates, including putative pollinators with elongated mouthparts. It has been suggested that an apparent plateau in diversity in the mid-Cretaceous 125-100 m.y.o., the Aptian-Albian gap, in the middle of the angiosperm radiation, might be due to a lag period as insects became habituated to angiosperms (Labandeira & Sepkowski 1993; Labandeira 2014: complex analysis, families the units, no tree). Peñalver et al. (2015) wonder why, if Bennettitales were pollinated by insects (as they suggest), an apparently adaptively superior mode of polination, they should have gone extinct. Pieces of the puzzle are missing.

Labandeira (2000) included Colepotera, Diptera, Hymenoptera, Lepidoptera and Thysanoptera in his "big five" angiosperm pollinators. Looking at pollination in extant basal angiosperms, Gottsberger (1970, esp. 2016; see also Thien et al. 2000) suggested that the basic condition for angiosperms is to be protogynous and self-compatible, and with generalized (from the point of view of the plant) pollinators, mostly flies and beetles. The flowers are also quite often often unisexual. Thermogenic (beetle) pollination occurs in some extant members of many "basal" lineages, including the ANA grade and some magnoliid angiosperms, Araceae, etc., although it is unknown in Amborella, Acorus and Laurales (Thien et al. 2000; Seymour et al. 2003; Seymour 2010; Gottsberger 2016). Beetles are attracted to haplomorphic flowers lacking definite symmetry signals (Leppik 1957), but of course other factors such as scent are also involved, furthermore, some beetle-flower interactions may be quite recent, such as pollination by scarabeid beetles, a group that evolved only in the Palaeogene (Gottsberger 2016). Interestingly, pollinators have been observed visiting droplets on the stigma, particularly in Winteraceae, apparently as part of the pollination process (e.g. Lloyd & Wells 1992 and references), and these droplets are perhaps analogs of the pollination droplets of conifers (c.f. Frohlich 2001). An extragynoecial compitum might also function in the same way. Hu et al. (2012) list early records of pollen, tabulate pollen morphology and suggest possible pollinators of ANA-grade angiosperms, magnoliids, basal eudicots and monocots, and finally optimise a number of pollen and floral characters on the tree. Of course, unlike early fossils, many extant basal angiosperms have quite large flowers (Gottsberger 2016), and they are likely to be derived, large size, along with thermal regulation, carcass mimicry, etc., being adaptations to pollination by small flies and beetles (Davis et al. 2008).

Angiosperm seeds range in size from 10-7 to 104 grams, i.e. from about the size of an embryo sac to massively larger. Seeds of extant gymnosperms, at 10-3 to 103 grams, are rather larger than those of the basal clades of extant angiosperms (Haig & Westoby 1991; Moles et al. 2005a), which are also rather small compared with those of other angiosperms (Tiffney 1986a; Eriksson et al. 2000a, esp. b, also Haig & Westoby 1991). Eriksson et al. (2000b) estimated an average volume of less than 1 mm3 for seeds in the ca 124 m.y.o. Famalicão flora, while Friis et al. (2015b) emphasized the minute size of the fruits, the seeds, and in particular that of the embryos (length relative to that of the seed) in 125-110 m.y.o. floras from east North America and Portugal. Dispersal of disseminules of many early plants is likely to have been by wind (Wing & Boucher 1998). However, in a sample of some 100 taxa from the Barremian-Aptian (132-112 m.y.a.), ca 25% of the disseminules were probably fleshy and animal dispersed (Eriksson et al. 2000b; c.f. Tiffney 1984). The fruits were small and mostly single seeded, although some may have been aggregated (see Eriksson et al. 2000b; Eriksson 2008; Dilcher 2010, in part; Sussman et al. 2013). It is unclear what ate these fruits; Eriksson et al. (2000b) suggest reptiles and multituberculate mammals, and perhaps other mammals and birds. However, the dentition of multituberculates from that period does not suggest herbivory (G. Wilson et al. 2012), while many early birds are likely to have been carnivorous and/or aquatic (Jarvis et al. 2014), although gut contents with seeds have on occasion been recovered.

Vegetative changes were very important in early angiosperm evolution. Angiosperm leaves are very different from those of most other vascular plants (Hickey & Doyle 1977; Sack et al. 2012). Fossils show more regular and hierarchical venation, and teeth and compound leaves were early evident. How fast nutrient turnover in the litter of these angiosperms might be is debatable (Berendse & Scheffer 2009; Royer et al. 2010; c.f. G. Liu et al. 2014). The ecological consequences of such changes are discussed elsewhere.

Growth rates of early angiosperms may have been high and reproduction rapid compared with gymnosperms (J. A. Doyle & Hickey 1976; Bond 1989; Wing & Boucher 1998; Verdú 2002), although high growth rates in extant angiosperms are most evident in more productive habitats (Lusk et al 2003; Augusto et al. 2014). The reproductive cycle was relatively short, with a short time between pollination and fertilization (see below), seeds were small, the plants were small. However, since the embryos were so tiny, the seeds would need a period of dormany before germination, and this would increase generation time (Friis et al. 2015b). A small size is perhaps to be expected since plant and seed size are quite strongly linked (e.g. Harper et al. 1980; Eriksson et al. 2000b; Moles et al. 2005b) and these early angiosperms were not large plants. In another study, seed size was estimated to be 2-3 mm3 (but c.f. Eriksson 2016, barely 1/10th this volume ca 80 m.y.a.); seed contents were probably mostly endosperm, and again, given the small to minute size of the embryo with respect to the endosperm, a period of morphological dormancy was likely before a plantlet developed and germination could occur (Forbis et al. 2002; Linkies et al. 2010). In summary, there was a short pre-reproductive period and short overall generation time (e.g. Williams 2008, 2009; Crepet & Niklas 2009; Bond & Scott 2010; Abercrombie et al. 2011: pollen tube growth).

How woody the early angiosperms were is unclear. Seed size in the 132-112 m.y.o. Famalicão flora is similar to that modern, tree-dominated floras, but Eriksson et al. (2000b) thought the plants that produced these seeds were herbaceous or shrubby (see also Hickey & Doyle 1977). Similarly, Jud and Hickey (2013) thought that angiosperms around 130-120 m.y.a. were often herbaceous, but they would have maintained cambial activity. Philippe et al. (2008) thought that earliest angiosperms might have had cambium, but the wood lacked thick-walled fibres and in general cell walls were thin (see also Amborella). Fossil angiosperm wood is known from deposits of up to about 120 m.y.a. (Aptian), although its assignment to extant clades is not easy (Oakley et al. 2009).

There are other suggestions. Arguing by analogy with extant angiosperms, early angiosperms may have been fairly small (Friis et al. 2010b) tropical trees with decumbent lignotubers and sympodial growth that tolerated shady, humid and disturbed ("dark and disturbed") conditions, perhaps like extant members of the ANA grade and Chloranthaceae, although not the aquatic Nymphaeales (e.g. Feild et al. 2003a, b, 2004; Feild 2005; Feild & Arens 2005, 2007; Coiffard et al. 2006; Berendse & Scheffer 2009 for a summary). (Most eudicots have seedlings/young plants that are at least initially erect.) Leaves of plants growing in such conditions are likely to have had relatively low venation density (e.g. de Boer et al. 2012), and a vascular system with vessels, etc., is unlikely to have been at a premium. There are also suggestions that angiosperms initially grew in semi-arid (e.g. Stebbins 1965, 1974; Hickey & Doyle 1977) or at least seasonally arid (Bond & Scott 2010) conditions, and some early leaf floras from Portugal do have leaves that are small in size and xeromorphic in appearance (Friis et al. 2010b, see also 2011: pp. 46-47). Our knowledge of the dark reversal of P[phytochrome]fr → Pr may be a bit skimpy, but the general pattern of the ability of the plant to do this seems interesting (Kendrick & Hillman 1971).

Coiffard et al. (2012) give the impression that around 130-125 m.y.a. angiosperms were largely a clade of aquatics, noting that 5/11 genera that grew in the first stage of the rise to dominance of angiosperms "competed with charophytes" (p. 20955). They did not say that the ancestral angiosperm was aquatic, but Goremykin et al. (2012) do think that this was likely and Gomez et al. (2015, but c.f. Herendeen et al. 2017 for Montsechia) also entertained this possibility. Indeed, early herbaceous nymphaealean-type plants are likely to have grown in aquatic or marshy habitats (G. Sun et al. et al. 2008, see also Friis et al. 1999, 2011). Whether Nymphaeales are sister to Amborellaceae or not may have little effect on ancestral reconstructions of habitat preferences, but, as already mentioned, an important issue is where very old and long-aquatic clades like Ceratophyllales go on the tree, and how they relate to Chloranthales. Are they related, or are Nymphaeales and Ceratophyllum members of old clades of freshwater anguiosperms (Du et al. 2016)? Others have suggested that the angiosperm progenitor was a "diminutive, rhizomatous to scrambling herb" (Taylor & Hickey 1992: p. 137, c.f. the palaeoherb hypothesis). However, both very xeromorphic and aquatic plants are likely to be derived; in particular, loss of cambial activity in aquatics may be difficult to reverse (Groover 2005; Feild & Arens 2007). In any event, early climatic niche (habitat) evolution is likely to have been slow (S. A. Smith & Beaulieu 2009).

Whatever habitats the very first angiosperms prefered, there is a consensus that they were soon components of disturbed, well lit or open and mesic communities (e.g. J. A. Doyle & Hickey 1976; Hickey & Doyle 1977; Tiffney 1984; Eriksson et al. 2000b; Heimhofer et al. 2005; Green & Hickey 2005; Feild & Arens 2007; Bond & Scott 2010; Boyce et al. 2010), and mention of flood-plain, swamp and riparian habitats is common (see also Wing & Boucher 1998; Coiffard et al. 2008, 2012) - Aethophyllum, a small (but hardly herbaceous), fast-growing conifer (?Voltziales), had occupied similar habitats in the lower middle Triassic (Rothwell et al. 2000). In the Portugese Late Barremian-Aptian 124-112 m.y.a. climate and environment were unstable, which might favour angiosperms adapted to disturbed habitats (Heimhofer et al. 2005). It has been suggested that these ± weedy early angiosperms instituted a novel fire regime, being both productive and flammable, and this would have facilitated their spread (Bond & Scott 2010), although a small experiment suggests that "weedy" angiosperms would be unlikely to ignite when fresh, while "shrubby" angiosperms, ferns and conifers were more flammable (Belcher & Hudspith 2016) - the effect that the increased atmospheric oxygen then might have on this aspect of the burning process is unclear.

Angiosperms first appear in the pollen/spore record south of palaeolatitude 30o and by tha Aptian, around 125 m.y.a., they started to become quite abundant (>20% of the record, but see below) and more widespread, spreading both north and south (Crane & Ligard 1989). Similarly, fossil ephedroid pollen was common early, peaking between the Barremian and Santonian, i.e. 125-83 m.y.a., angiosperms and ephedroids apparently preferring similar habitats (Crane & Lidgard 1989).

5C. Later Cretaceous - Albian to Maastrichtian - Evolution. The Upper Cretaceous began ca 99.6 m.y.a. with the Cenomanian and ended ca 66 m.y. ago with the bolide impact in the Yucatan and the eruptions that produced the Deccan Traps. The period from 110-80 m.y.a. encompasses the so-called Cretaceous Terrestrial Revolution (KTR: Lloyd et al. 2008; Benton 2010; Meredith et al. 2011 - W. Wang et al. 2016a suggest that it lasted from 125-80 m.y.a.), and so the Albian period (ca 113-100 m.y.a.) is also included here. The sea was initially about one hundred and fifty meters above its present level. A long-term warming trend from the early Aptian culminated in the Cenomanian-Turonian thermal maximum ca 99 m.y.a. (Heimhofer et al. 2005: Coiffard et al. 2007, 2007). However, there is the possibility of some glaciation in the Aptian and Maastrichtian (Ladant & Donnadieu 2016; see also Bowman et al. 2014). During this period there were major changes in the terrestrial vegetation (e.g. Crepet 2008; Coiffard & Gomez 2011; Coiffard et al. 2012). Angiosperms continued to spread latitudinally from the more tropical areas they initially inhabited (Axelrod 1959; Hickey & Doyle 1977; Wing & Boucher 1998; Hemihofer et al. 2005 for further references). It is interesting that neither geography, i.e. the fact that continents were drifting around, nor humidity seems to have affected the early dispersal of angiosperms (Morley 2003).

Tricolporate pollen, common in Pentapetalae, is first known from around 107 m.y.a. in the mid Albian (Friis et al. 2011), and Crepet (1996, 2008) noted the first appearance of many floral characters of the Pentapetalae beginning in the Albian, but especially the Cenomanian/Turonian some 100.5-90 m.y. ago. However, in the early Albian in Portugal angiosperm pollen was only (2-)5.3(-7)% of the total, and that figure had increased only slightly, to (1-)8(-20)%, by the late Albian-early Cenomanian; polyporate pollen, a tropical floral element, also occurs in this mid latitude palaeoflora (Horikx et al. 2016). Mid-latitude floras in particular did change considerably, large trees first appearing in the fossil record in the Middle to Late Albian less than 110 m.y.a., and fossil woods became notably more common (Philippe et al. 2008; Wheeler & Lehman 2009). Nevertheless, in the late Albian ca 107-100 m.y.a. angiosperms are still described as being herbaceous to shrubby early successional plants of open habitats where they competed with, but did not necessarily displace, ferns (Peppe et al. 2008: Aspen Shale, Wyoming), and ephedroid pollen was also common between 30o N and S (Crane & Lidgard 1989). There was a fair diversity of pollen in Australia by the late Albian ca 102 m.y.a., and it showed similarities with the palynoflora of southern South America, suggesting that the climate had warmed and eudicots moved from South America to Australia via Antarctica (Korasidis et al. 2016). By the Albian-Turonian some 100 m.y.a. there was a significant number of eudicots in mid-latitude North America (ca 450 N), and by the end of the Cretaceous they were about 40% of the flora at 80o N and about double that number towards the equator (e.g. Crane & Lidgard 1989, 2000; Lupia et al. 1999). Platanoid fossils were found along river channels ca 110 m.y.a. in the Late Albian of northern Alaska (Spicer & Herman 2010; Pott et al. 2012 and references), and angiosperm abundance increased there, although diversity in the high Arctic was low and there was at most little endemism (Wolfe 1987; Spicer & Herman 2010).

Many fossils from the Aptian/Albian ca 112 m.y.a. still have odd assemblages of characters (see also Friis et al. 1995; Horikx et al. 2016: pollen; Friis et al. 2017: P 5, A extrose, pollen tricolpate). Antiquifloris, in amber at least 99 m.y.o. from Myanmar, has some magnoliid features like broad filaments with three traces and a prolonged connective, but just a single style, and the flower was about 6 mm long (Poinar et al. 2016a), while the enigmatic Prisca reynoldsii with its slender receptacle 5-7 cm long grew 94-92 m.y.a. in Kansas (Retallack & Dilcher 1981). At this time - ca 100 m.y.o. or slightly younger - there are fossils like Cecilanthus, form early Cenomanian Maryland with a floral formula of * P many; A many; G many, with a well-developed receptacle and probably one ovule/carpel, that while perhaps Magnolialean cannot really be assigned to any extant family (Herendeen et al. 2016).Indeed, groups like Laurales had become common by around 112 m.y.a. (J. A. Doyle & Upchurch 2014), and as late as the Cenomanian ca 96 m.y.a. many fossils are probably of plants of the ANA-magnoliid grade (e.g. Coiffard et al. 2006; Kvacek & Friis 2010; Friis & Pedersen 2011). A flower, the Rose Creek fossil from the earliest Cenomanian in Nebraska about 99 m.y.a., is the earliest known pentapetalan fossil, and it has five stamens that are somewhat unexpectedly opposite the petals, fused carpels and short styluli (Basinger & Dilcher 1984). Caliciflora mauldinensis, of about the same age and from Maryland, it also pentapetalan, but with a rather different floral morphology, not to mention its stellate hairs and valvate-recurved calyx (Friis et al. 2016). The floral formulae of the two are * K 5; C 5; A 5; N+; [G 5] and * K 5; C 5; A 8 [?10]; N0; G 3 respectively. Tropidogyne, one of the eudicots from Burmese amber and also of about the same age, has a floral formula of * K 5; C ?0; A 10; G [3]; N+ (Chambers et al. 2010; fossils to be taken into account also include Eoëpigynium burmensis - Poinar et al. 2007; Poinar 2011). None of these fossils is securely identifiable to an extant family, and although Tropidogyne may be Cunoniaceae, its age conflicts with other estimates for that clade; the Rose Creek flower might be rhamnaceous, but it is notably larger than those of extant Rhamnaceae (Jud et al. 2017 for references).

Fossils referable to extant angiosperm families begin to appear in east North America around 115-90 m.y.a., and by some 85 m.y.a. their diversity had increased considerably (Crane & Herendeen 1996; also Lidgard & Crane 1988; Friis & Crepet 1987; Friis & Endress 1990; Crepet et al. 2004, etc.). Many major euasterid, rosid and monocot clades seem to have radiated by around 90 m.y.a. at the latest (e.g. Sanderson et al. 2004; Jian et al. 2008 and references; Wang et al. 2009). The diversity of floral form in the Turonian (93.9-89.8 m.y.a.) of east North Americas is very considerable, magnoliids, rosids and asterid-Ericales all being represented (e.g. Crepet & Nixon 1996). Rosids in particular were common in the Late Cretaceous (Friis et al. 2010b). Flowers perhaps assignable to Saxifragales (ovary inferior, crowned by a nectary, styles more or less separate, i.e. they look very like the old - polyphyletic - woody Saxifragaceae) were especially common, as were ericalean flowers with a variety of morphologies (Friis et al. 2006b, 2011). Indeed, Saxifragales, although now not very speciose, may represent an ancient and rapid radiation (Fishbein et al. 2001; Fishbein & Soltis 2004; Jian et al. 2008).

Flowers from this period commonly have spreading petals and stamens with the anthers distinct from the filaments and short styluli or styles; they are still mostly quite small (Friis et al. 2011). At ca 2.5 cm across, the Rose Creek flower is relatively large compared to the flowers of many other Cretaceous angiosperms, while at about 1.5 mm the flowers of Caliciflora represent the other end of the spectrum. There are a few other fossils of quite large flowers, mostly terminal in position. However, the putative early bee Melittosphex (placed along the stem group of modern bees) collected from the same amber locality as Caliciflora is a mere 3 mm long (Danforth & Poinar 2011), and nectaries and "food sources" have been reported in these flowers from Lower Cretaceous Burmese amber (Santiago-Blay et al. 2005). Of course, nectar is a major pollinator reward in many extant angiosperms. With the diversification of Pentapetalae nectar produced by receptacular or gynoecial nectaries is likely to have become a common reward for pollinators (Friis et al. 2006b; Erbar 2014), although receptacular or ovarian nectaries are also found in Proteales (even some fossils of the now wind-pollinated Platanaceae are described as having nectaries) and Buxales. Angiosperm flowers from the Cenomanian-Turonian 110-90 m.y.a. have a variety of quite specialized zoophilous morphologies, and nectar secretion became common (Crepet 1996, 2008; Hu et al. 2008); Citerne et al. (2010) thought that 93.5-89 m.y.a. in the Turonian was a period of floral innovation and evolution of pollinators. Nectaries are conspicuous in floral diagrams drawn for Late Cretaceous flowers (Friis et al. 2011: fig. 16.6, 17.10). By the mid-Cretaceous pollen was more abundant and is quite often found in clumps, suggesting that pollenkitt had evolved and that pollinators were becoming more specialized (Hu et al. 2008; Leslie & Boyce 2012).

Rather surprisingly, ca 50% of end-Cretaceous mesofossil flowers had inferior ovaries, a higher proportion than in the present flora (Crepet & Friis 1987; Friis et al. 2011: fig. 16.8). Sympetaly (Actinocalyx bohrii had flowers ca 3 mm long with connate petals) and monosymmetry (evidence for the latter is indirect, seeds and other fossils assignable to Zingiberales: Rodríguez-de la Rosa & Cevallos-Ferriz 1994) appear in the Late Cretaceous, indeed, monosymmetric flowers are uncommon in the Cretaceous (Friis 1985; van Bergen & Collinson 1999; Friis et al. 2003a, 2011).

The macro- and mesofossil record is likely to be skewed to large, woody plants. W. Wang et al. (2016a) discuss the evolution of angiosperm-dominated herbaceous floras in more open vegetation. They suggest that in Ranunculaceae, at least, herbaceousness evolved in plants living in forests in which angiosperms were becoming dominant during the KTR before moving into more open conditions, and they date initial diversification in Ranunculaceae to around 109 m.y.a., the first branches in the family being represented by forest-dwelling herbs (Wang et al. 2016a: cf. topology and ages, crown-group Ranunculaceae (ca 119-(114.6-))108.8((-101.6(-ca 82) m.y. - see also their Fig. 2). They note that a similar scenario may explain the evolution of other herbaceous clades like the grasses and orchids, although the great expansion of open, herb-dominated vegetation exemplified by today's grasslands and savannas and the massive diversification of a largely epiphytic clade of Orchidaceae are very much later phenonomena, and it is also unclear exactly what kind of angiosperm forest these Ranunculaceae were growing in, given that Wang et al. (2016b) thought that Ranunculales represented a very early radiation of the eudicots and that all the other early groups they mentioned are either non-forest plants, magnoliids, or extinct clades of uncertain affinities - Platanaceae (again) are the only exceptions.

By the Late Cenomanian/Early Turonian ca 93.5 m.y.a. there were four main pollen provinces. 1. The tropical province was dominated by palm-type pollen, although conditions may commonly have been rather dry (Burnham & Johnson 2004). Indeed, fossils of palm stems of Cretaceous age have fibro-vascular bundles with two vessels per bundle as in extant palms that live in places with a dry period (Thomas & Boura 2015 and references). 2. Plants with distinctive pollen assignable to the Normapolles complex and comparable with that of some extant Fagales were both diverse and ecologically prominent in rocks from east North America to western Asia (Friis et al. 2006a); interestingly, fossil flowers with such pollen are often perfect, while flowers of extant Fagales with Normapolles pollen are generally imperfect. 3. Of the other pollen provinces, Aquilapollenites, a pollen type of unknown relationships although Santalales has been suggested, characterised an area that made up the rest of the more temperate northern hemisphere. 4. The southernmost province was characterized by Nothofagites pollen, probably also from plants that would be included in Fagales (e.g. Pacltová 1981 for a review; Kedves & Diniz 1983; Diniz 1988; Friis et al. 2006a, 2010b, 2011; Cantrill & Poole 2012). Extant Normapolles plants and Nothofagaceae are trees, although some are shrubs, and Fagales as a whole are largely ectomycorrhizal (ECM). Pinaceae, also ECM, were also part of the vegetational mix then, so ECM plants may have been an appreciable component of the vegetation, at least locally, and this has implications for carbon sequestration then (see also below).

Seeds/diaspores in the later Cretaceous remained small, and abiotic dispersal may have been prevalent (Tiffney 1984, 1986a, b), however, Eriksson et al. (2000a) noted that biotic dispersal was probably appreciable. For Tiffney, ants, fish, reptiles and archaic mammals were possible dispersers, but not birds, for Eriksson et al. birds and multituberculate mammals (see also Eriksson 2016) were likely candidates. Eriksson et al. (2000a) distinguished between the "disperser" and "recruitment" hypotheses for changes in fruit size; in the former, the evolution of animals that could handle large fruits drove the evolution of plants with large diaspores, while in the latter the increasing size of angiosperms was associated with larger seeds/diaspores that were initially dispersed by rather generalist dispersers, only later did more specialist frugivores evolve. Eriksson et al. (2000a, see also Moles et al. 2005a; c.f. Tiffney 1984) incline to the latter hypothesis, and from what is known about mammal and bird evolution (see below), this seems most likely. Increasing seed size became evident about 85-75 m.y.a., probably reflecting the increasing size of the plants (e.g. Eriksson et al. 2000a; Moles et al. 2005a, b; Eriksson 2016), while by 70 m.y.a. around a quarter of the disseminules in European palaeofloras were drupes (add berries in Annonaceae, Vitaceae, etc.), a proportion that held quite steady until the early Eocene, at least (Eriksson 2016). Interestingly, fruits and seeds are larger and the former are more likely to be fleshy in woody vegetation of the humid tropics today, or at least in areas that have bursts of heavy rainfall (S.-C. Chen et al. 2016 and references).

Around 108-94 m.y.a. (Late Albian), and again at the end of the Cretaceous, the venation density of angiosperms increased and became markedly greater than that of non-flowering plants and ANA-grade angiosperms (Feild et al. 2011b; c.f. Bond & Scott 2010 in part, discussed further below), and l.t.r.f. of sorts may have begun to spread ca 100 m.y.a. (Eiserhardt et al. 2017, see also below). Friis et al. (2006a; see also Heimhofer et al. 2005) note a dramatic increase of phylogenetic diversity and ecological abundance of angiosperms at this time. Towards the end of the Cretaceous there were changes in atmospheric CO2 concentration, etc., and some taxa in the Aquilapollenites flora went extinct, perhaps because of the loss of their pollinators (Spicer & Collinson 2014.

Eudicots replaced free-sporing plants (see also Fiz-Palacios et al. 2011: "continuous replacement"), but initially not conifers and ephedroids (see e.g. Crane &Ligard 1989; Wing & Boucher 1998; Lupia et al. 1999); cycads may also have declined. The decline of cycads and Bennettitales (cycadophytes, an ecological grouping) might be linked with the contemporaneous decline in herbivorous stegosaurian dinosaurs, but there is no indication of any even loose relationships between early angiosperms and dinosaurs (Barrett & Willis 2001; Butler et al. 2009 and references); insects associated with these gymnosperms, including the remarkable butterfly-like kalligrammatid lacewings, had also declined by ca 120 m.y.a. (Labandeira et al. 2016).

Areas where conifers remained common seem to have become more restricted, and ecological factors such as slow seedling growth, details of leaf construction, narrow stomatal apertures (ca 2 µm: Walker 2005), etc., may explain this (e.g. Bond 1989). The increase in venation density in angiosperms to 6 mm/mm2 and more around 100 m.y.a. coupled with high stomatal conductance, etc., enabled higher photosynthetic rates at a time when atmospheric CO2 concentration was declining and made angiosperms ecophysiologically more flexible (McElwain et al. 2015 and references). Furthermore, Sack et al. (2012) note that large simple leaves with high venation density are an angiosperm innovation, the high venation density allowing the leaf to function even when irradiance was high because the evaporating water would cool the leaf. However, conifers have a very high leaf area index, a measure of leaf area/unit ground surface area, of up to 21 (Maguire et al. 2005), and the leaves can be very long-lived, and such features in extant conifers make them formidable competitors with angiosperms in well-lit conditions on soils that are other than nutrient-rich (Brodribb et al. 2012; see also Chabot & Hicks 1982: stil useful). Conifer pollen and also spores from "bryophytes" and "pteridophytes" decreased in diversity, although gnetalean pollen was quite diverse at rather lower latitudes in the mid-Cretaceous (Crane & Lidgard 1989). After 100 m.y.a. there was little change to a moderate decrease in pteridophyte diversity, the beginnings of a decrease in gymnosperm and cycad diversity, and a dramatic increase in angiosperm diversity (Niklas et al. 1983; S. Brown et al. 2012).

Fires were relatively common throughout the Cretaceous, and they may have encouraged/been encouraged by a rather shrubby, low stature vegetation with a relatively short life cycle (Bond and Scott 2010; He et al. 2012; Bond & Midgley 2012; Hill & Jordan 2016 and Carpenter et al. 2016: both Australia; c.f. Belcher & Hudspith 2016, in part); Cretaceous mesofossils are often charcoalified (Friis et al. 2011). Berner (2003) noted that rocks rich in charcoal derived from plants are particularly prominent in the mid Cretaceous 120-90 m.y.a. to the Palaeocene, and fires may also have been encouraged by the relatively high atmospheric oxygen concentrations of 21-25% then (Brown et al. 2012). Indeed, it may have been in the mid Cretaceous that the presence of shrubby angiosperms and ferns increased the prevalence of intense (especially the burning shrubby angiosperms) and rapidly-spreading fires, and conifer forests then may have preferentially burned, being replaced by angiosperms (Belcher & Hudspith 2016). Frequent fires were likely to have been accompanied by increased runoff/erosion and loss of phosphorus to the ocean (S. Brown et al. 2013), while nitrogen is lost by volatilization (Forrestal et al. 2014 and references). Interestingly, fire-adapted Proteaceae-dominated heathlands are to be found ca 89-84 or 75-65.5 m.y.a. in Central Australia, numerous species of Proteaceae being involved, and Banksia may have evolved by then (Lamont & He 2012; Carpenter et al. 2015 respectively). Myrtaceae were not involved, but spores attributable to Sphagnum were abundant (Carpenter et al. 2015). Substantial amounts of inertinite, fossilized charcoal from fires in mire systems, are found through the Cretaceous, but less since, even quite recently, which is difficult to understand (Scott & Glasspool 2006; Bond 2015). It is thought that fires were unable to burn closed angiosperm forests when these finally developed in the Caenozoic (Bond & Midgley 2012).

In the Albian-Cenomanian of Europe ca 100 m.y.a. angiosperms were most evident in backswamp, flood plain, levee, and braided river habitats (Coiffard et al. 2006), and the deciduous habit was relatively common in these habitats, and, more generally with deciduous conifers, north of 65-70o N and through the Eocene (Wolfe 1987). Prisca was a common tree of such habitats in Kansas 94-92 m.y.a. (Retallack & Dilcher 1981). Turonian forests of ca 90 m.y.a. still grew primarily in disturbed and/or riparian-type habitats (see also Jud & Wing 2013; Spicer & Herman 2010: Late Cretaceous in northern Alaska; I. Miller 2013: 105 m.y.a. Albian); Platanaceae, found along channel margins in the Cenomanian, had spread onto flood plains in the Turonian. In Australia, angiosperm pollen had increased from a low level in the middle Albian ca 105 m.y.a. to about 35% of the total spores at the end-Cretaceous, pollen of free sporing plants dropping from 80% to 45% over the same period. However, not all fern families behaved the same, and there are differences between Australia and North America (Nagalingum et al. 2002). In parts of Campanian (83.6-72.1 m.y.a.) North America, angiosperms seem to have lived in rather species-poor and open woodlands (Lehman & Wheeler 2001; Wheeler & Lehman 2001, 2009). The trees, up to 1.3 m diameter, may have produced Normapolles pollen (Lehman & Wheeler 2001), so a diverse forest might not be expected, however, identifications suggested for these woods rarely include Fagales (Wheeler & Lehman 2009). Trees up to 2 m in diameter have been found in Late Cretaceous riparian swamps (Parrott et al. 2013). By ca 80 m.y.a. angiosperms had come to make up ca 40% of both floristic diversity and abundance even at higher latitudes (e.g. Axelrod 1959; Crane & Lidgard 1989; Lupia et al. 1999; see also Nagalingum et al. 2002), with monocots and magnoliids predominating between 50oN and 20oS. In latest Cretaceous-earliest Palaeocene on the Antarctic Peninsula podocarps, Araucaria and Nothofagus were major components of the vegetation, with open ericaceous heathlands at higher altitudes (Bowman et al. 2014).

When - and where - closed-canopy angiosperm-dominated forest first appeared is of considerable interest. Fleshy fruits reported in monocots 120 m.y.o. may reflect the closing of the canopy (see also Dunn et al. 2007), but they are not associated with any particular vegetation type. Rather, the Late Cretaceous may have been the "dawn of modern angiosperm forests" (Coiffard & Gomez 2011: p. 164; also Coiffard et al. 2012). Until the Mid or even Late Cretaceous angiosperms were mostly small herbs to small trees of the understory growing in dryish conditions (Bond & Scott 2010), perhaps rather weedy plants (Feild et al. 2011b). Analyses using variables like leaf area and vein density, plant height and seed size, suggest angiosperms were mostly not canopy trees, seed size remaining small (Jud & Wing 2013). However, some low latitude floras were dominated by angiosperms in the Cenomanian-Turonian ca 94 m.y.a. (Coiffard et al. 2012 for references). Later in the Cretaceous angiosperm diversity was quite high even close to the Arctic Circle (Hofmann et al. 2011; see Spicer & Herman 2010 for the high Arctic). Strömberg et al. (2013a) suggested that in the Late Cretaceous (ca 73 m.y.a.: Wyoming) angiosperms were not notably abundant compared with other co-occurring vascular plants, but they did vary considerably in niche optima and niche breadth. (Similarly, non-polypodiaceous ferns dominated in Late Campanian [73 m.y.a.] North American fern prairies, but Polypodiales, less dominant, were quite diverse - Wing et al. 2013.) However, angiosperms may have formed a canopy at least locally by the end-Cretaceous (e.g. Upchurch & Wolfe 1987; Crane & Lidgard 1990; Boyce et al. 2010), but how diverse that forest was is unclear. Afrocasia (Araceae) from over 72 m.y.a. is thought to have lived in terrestrial forest understory (Coiffard & Mohr (2016). There was an end-Cretaceous rise to dominance of angiosperms in Patagonia (Iglesias et al. 2011), with vegetation-induced warming at high latitudes at this time (Otto-Bliesner & Upchurch 1997). Wing and Boucher (1998: p. 379) concluded that diversification of flowering plants then was "the evolution of a highly speciose clade of weeds but not necessarily a major change in global vegetation", while Eriksson et al. (2000a) suggested that Late Cretaceous vegetation was open, rather dry (leaf size was relatively small - Upchurch & Wolf 1987), and disturbed by herbivores such as herds of large dinosaurs (for their speciation then, see Sakamoto et al. 2016).

It has often been suggested that early angiosperms and their insect pollinators diversified together, even if the orgin of angiosperms is as early as the Triassic (e.g. Zeng et al. 2014). However, Labandeira et al. (2016) suggest that the pollinators of bennettitalean-type gymnosperms were different from those of angiosperms, butterflies, for instance, diversifying ca 80-70 m.y.a., long after the demise of their earlier functional equivalents, the kalligrammatid lacewings, that pollinated gymnosperms.

5D. Venation Density, Stomatal Size, and Vascular Evolution. Atmospheric CO2 concentration declined from the late Jurassic-early Cretaceous to the later Oligocene ca 40 m.y.a., bottoming out in the Pleistocene (Rothman 2001; Shi & Waterhouse 2010; Franks et al. 2013). This provides the background for thinking about changes in CO2 uptake and water loss, both associated with increased photosynthetic efficiency. Overall, a combination of features unique to many, but not all, angiosperms - xylem dominated by vessels, and leaf blades with high venation density (Sack et al. 2012), precise and optimal positioning of the veins in the blade, and dense, small stomata - allowed productivity to increase and more carbon to be sequestered (e.g. Boyce & Zwieniecki 2012; Zwieniecki & Boyce 2014; note, however in arid habitats veins may be closer than would appear optimal - de Boer et al. 2016). This may have contributed to the declining CO2 concentrations, as may the decrease in continental arc vulcanism (McKenzie et al. 2016).

The ecological context for the evolution of venation density and vasculature can perhaps be provided by living members of the ANA grade (Feild 2005; Feild & Arens 2005), other than the aquatic Nymphaeales; J. A. Doyle and Upchurch (2014) also noted the similarity between leaves of early angiosperms and those of the ANA grade. Vessels in magnoliids and ANA-grade angiosperms, so-called "basal vessels", are rather different from those in core eudicots, and water conductivity was not high (Hacke et al. 2007; Sperry et al. 2007). Venation density of the leaves is low, and spacing of the veins is suboptimal (Zwieniecki & Boyce 2015), as in non-angiosperm vascular plants. Ancestral angiosperms are likely to have had low drought tolerance (Feild et al. 2011a, c); their leaves had large and distant stomata, often lacked any palisade mesophyll tissue, and the abaxial surface of the blade reflected light back inside (Feild & Arens 2007).

If early angiosperms were pioneer plants, they might be able to tolerate high herbivory because they had metabolically cheap, rather thin, rapidly expanding leaves with a low amount of fibre and low concentrations of secondary metabolites like terpenoids, phenols, and tannins; their high quality habitat allowed rapid growth and low defence (see e.g. Bond & Scott 2010), and their whole reproductive cycle was relatively short (e.g. Verdú 2002; Williams 2008, 2009). Extant angiosperms with the highest leaf venation densities are woody pioneers (Feild et al. 2011b). Royer et al (2010) estimated the SLA of fossil angiosperms 110-105 m.y.a. was low, implying fast nutrient turnover, and the disturbed habitats of early angiosperms are likely to have had elevated levels of nutrients (see also Berendse & Scheffer 2009). However, many extant magnoliids and ANA-grade angiosperms have a "slow" leaf spectrum with a higher SLA, etc. (G. Liu et al. 2014), and their venation density is low; there is no indication of a pioneer strategy for early angiosperms here.

Venation. During the 200+ million years prior to the diversification of flowering plants, the venation density of the leaves of vascular plants held largely constant at below ca 3 mm/mm2, and this despite considerable fluctuations in atmospheric carbon dioxide concentrations (e.g. Boyce et al. 2010; Lee & Boyce 2010; Boyce & Zwieniecki 2012; Boyce & Leslie 2012 for a summary). Venation density in non-flowering plants continued to hold steady through the Cretaceous (Feild et al. 2011b). Extant members of the ANA grade, Chloranthaceae, shade tolerant and succulent plants, etc., as well as fossils from the first ca 30 m.y. of the angiosperm record all have similar low venation densities of around 2.4 mm/mm-2 (Feild et al. 2011b: post Hauterivian), and they would also have lower CO2 exchange than most magnoliids and basal eudicots (Feild et al. 2011a).

Most extant angiosperms have distinctive hierarchical-reticulate and very dense venation. Small-diameter minor veins develop during the final expansion phase of the angiosperm leaf, the density of these veins contributing greatly to the dense venation of many angiosperm leaves. Before ca 113 m.y.a. (pre-Albian) about half the fossils still had leaf blades with a low venation density ca 3 mm/mm2 (e.g. Feild et al. 2011b; Boyce & Zwieniecki 2012). However, the venation density of many angiosperms doubled to abobe 6 mm/mm2 some 108-94 million years ago in the late Aptian-early Cenomanian, a change occurring independently in monocots, magnoliids and eudicots (Boyce et al. 2009; Feild et al. 2011b; de Boer et al. 2012), and the veins became much more ordered hierarchical-reticulate. This density increase greatly reduced the main element in the resistance to water flow through the plant by shortening its path through the mesophyll (Sack & Holbrook 2006; Sack & Scoffoni 2013). When venation density surpassed 6 mm/mm2 the path length for water transport inside the leaf (from vein to stomatal pore) equalled and then became shorter than the internal diffusion path of CO2 (from stomatal pore to chloroplast); add high venation density, high stomatal conductance, and the like and assimilation rates increase, even as atmospheric CO2 declined (McElwain et al. 2015).

The venation density of angiosperm leaves increased again around the Campanian-Maastrichtian boundary ca 70 m.y.a., and plants whose leaf blades had a venation density of ca 3 mm/mm2 were then a mere 4% of the total (Brodribb & Feild 2009; Feild et al. 2011b; Boyce & Zwieniecki 2012). Only after this second increase could forests assume a more "modern" physiology, and only then did trees have a venation density around 10 mm/mm2 or more like to that of plants in the most productive l.t.r.f. today (Brodribb & Feild 2009; Feild et al. 2011a, b). Very high venation densities, >20 mm mm-2, are found only in some rosids, e.g. eucalypts growing in arid environments (Boyce et al. 2010; de Boer et al. 2016). Furthermore, angiosperms alone have leaves in which veins are the same distance from each other as from the lower surface of the leaf, which allows optimal uniform delivery of water to the stomata (Zwieniecki & Boyce 2014; see above). This spacing is found even in shade angiosperms, perhaps because they maximize photosynthesis in sunflecks (c.f. monilophytes).

Venation density increase was accompanied by a decrease in stomatal size and an increase in stomatal density, which together increased stomatal gas exchange capacity. For any given stomatal area, smaller stomata allow more water to be lost, but, importantly, more CO2 to be taken up so counteracting falling atmospheric concentrations; pore depth is shallower in small than in large stomata (Franks & Beerling 2009a; de Boer et al. 2012: Fick's and Stefan's laws are relevant here). Overall, carbon assimilation per unit water loss increased (e.g. Franks & Beerling 2009; Haworth et al. 2011; de Boer et al. 2012; Franks et al. 2012). Water could be supplied to the leaf by the efficient vascular system even if humidity decreased, whether because of drying climates or the emergence of the tree into the canopy (e.g. de Boer et al. 2012; see also Boyce & Zwieniecki 2012). Small stomata also have a faster relative response time than large stomata as do grass stomata compared with those from other vascular plants (Franks & Farquhar 2006; Franks & Beerling 2009a), however, little is known about the stomatal size/response time connection (Raven 2014). Details of guard cell shape are also important (Hetherington & Woodward 2003), and the great range of relative stomatal pore area (pore area/guard cell + pore area) in vascular plants as a whole, but even when comparing Tradescantia with Triticum, is also pertinent (Franks & Farquhar 2006), although there seems to be little information about this. Drake et al. (2013) compared five species of Banksia and found that leaves with smaller stomata indeed had a higher rate of gas exchange, maximum operating stomatal conductance, and overall high productivity. Trees have small, dense stomata when compared with shrubs and herbs (Beaulieu et al. 2008: n = 101). Small stomata may also allow areas of the epidermis to be freed up for other functions (Franks & Beerling 2009a), although this seems somewhat notional.

Increased vein density comes at a cost of increased carbon allocation to the veins, partly offset by vein tapering (McKown et al. 2010; Beerling & Franks 2010). Dense veinlets allowed an easy flow of water into the mesophyll, their proto- and metaxylem having vessels with simple perforation plates (Feild & Brodribb 2013).

Overall, with a three-fold increase in venation density, there is a 178% increase in maximum photosynthetic CO2 uptake (e.g. Brodribb et al. 2007; Brodribb & Feild 2010; Feild et al. 2011a; Roth-Nebelsick et al. 2001: vein architecture; McKown et al. 2010: leaf hydraulics). Furthermore, the water potential of angiosperm leaves can decrease 50% before stomatal closure occurs, so maximum leaf hydraulic conductivity can persist in dry conditions, whereas in ferns, for example, closure occurs earlier (Brodribb & Holbrook 2004; see also Haworth et al. 2011, 2013 for stomatal opening in land plants). Increased transpiration resulting from increased stomatal conductance will promote evaporative leaf cooling (Hetherington & Woodward 2003; Boyce & Lee 2010), perhaps particularly important at times like the Palaeocene-Eocene Thermal Maximum (see below) when temperatures globally were very high.

Areas with ever-wet tropical humid climates seem to have been rather restricted in the Cretaceous (e.g. Boyce et al. 2010; Boyce & Lee 2010; Feild et al. 2009a; maps at the end of Willis & McElwain 2014). However, the increased transpiration from angiosperm leaves may have helped to drive the spread of l.t.r.f. with reliably high rainfall (e.g. Boyce et al. 2008, 2009, 2010), conditions of the Late Cretaceous becoming more humid (Eriksson et al. 2000a). Thus simulations in which the Amazon rain forest was replaced with non-angiosperm vegetation led to a decrease in the extent of ever-wet rainforest there by about 80% (Boyce & Lee 2010; Lee & Boyce 2010; see also Feild et al. 2011b; Boyce & Leslie 2012; Feild & Brodribb 2013). There was less change in the extent of rain forest in other parts of the tropics in such simulations, but this might have been different under conditions earlier in the Caenozoic; large areas of continental Africa had not yet become elevated, continents were in different positions, etc. (Boyce et al. 2010). Models suggest that atmospheric oxygen concentration, for which estimates vary by a factor of three, also affects estimates of precipitation for the Cretaceous-Cenomanian (Poulsen et al. 2015).

Vascular Anatomy. Vessels in ANA-grade plants are short, not very dense, with scalariform perforations, little difference between the pitting of the end and lateral walls, incomplete break-down of the pit membranes, and intertracheidal pit resistance lower than that of intervessel pits, etc. (Hacke et al. 2007; Sperry et al. 2007). The resistance to water flow of the scalariform perforation plates in such xylem is higher than had been estimated (e.g. Christman & Sperry 2010). Even if individual vessels may be more effective in transmitting water than individual tracheids, when comparing xylem cross-sectional area plants with such vessels may have a hydraulic efficiency little different from that of tracheid-bearing gymnosperms (Sperry et al. 2006; Feild & Holbrook 2001; Hudson et al. 2010).

Tracheids in Pinales in particular may be short and have end walls, yet their overall hydraulic efficiency is higher than might be expected because of the low resistance to water flow of the margo-torus pits. The central torus can block the pit, so localizing air bubbles developing in the cells during embolism, yet the fibrils in the margo are widely spaced compared with those in angiosperm pits, so allowing water to flow quite readily (Pittermann et al. 2005; Sperry et al. 2007; Hacke et al. 2007; Hudson et al. 2010); there are also lipid surfactants in the xylem that i.a. coat nanobubbles that are forming, so preventing the formation of embolisms (Schenk et al. 2017). Some scrambling or climbing seed ferns like Callisophyton, Lyginopteris and particularly Medullosa had long and wide - from 65-237 μm across (the upper part of the range is Medullosa) - tracheids, and their water conductivity was probably on a par with that of some extant angiosperms with vessels (J. P. Wilson & Knoll 2010). Ferns in general have quite wide and long tracheids that can have surprisingly high rates of water transport (Pittermann et al. 2011).

Thus the acquisition of vessels is no simple key innovation, rather, their evolution is likely to have been a rather protracted process (Feild & Arens 2007; Feild & Wilson 2012). They may have been of functional value initially because heteroxylic wood, i.e. wood with vessels and tracheids, allows the specialization of cells in the xylem for support, storage, etc., the heteroxylly [sic] hypothesis (e.g. Sperry et al. 2007; Hudson et al. 2010; J. P. Wilson & Knoll 2010). Indeed, despite lacking vessels (or almost so), as in most gymnosperms, the wood of Amborella has a small amount of parenchyma (Carlquist & Schneider 2001; Feild et al. 2000b; c.f. Carlquist 2012), and axial parenchyma is notably slight to absent in Austrobaileyales, Laurales, and Chloranthales as well (Herendeen et al. 1999). Plavcová and Jansen (2015) discuss the role of xylem parenchyma (in families other than those just mentioned) in the metabolism of non-structural carbohydrates. The avoidance of cavitation may also have driven the early evolution of vessels and of other aspects of wood anatomy (Sperry et al. 2007; Hacke et al. 2007; Philippe et al. 2008; Brodribb et al. 2012: angiosperm:gymnosperm comparisons).

Vessel conductance increases substantially when the peforation plates become simple and the vessels themselves, concatenated vessel elements, become long (see e.g. Christman & Sperry 2010; Hudson et al. 2010; Feild et al. 2011c). Wood with scalariform perforation plates was particularly common in the Cretaceous (Wheeler & Baas 1993; also Wheeler & Baas 1991), but as they noted, there is conflict between features of Cretaceous fossil woods and palaeoclimatic indicators, indicators that are based on our understanding of how the wood of extant plants functions (see also Philippe et al. 2008), so functional interpretation of these early woods is not easy. Eventually wide vessel elements with simple perforation plates, the vessels being well over 10 cm long, became an integral part of an efficient water transport system. However, vessel elements with long, oblique, many-barred scalariform perforation plates occur throughout the angiosperm phylogeny, as in the common ancestor of euasterids and in Paracryphiaceae, for example - see Lens et al. (2016) for vessel/tracheid evolution in asterids as a whole. indeed, it is not easy to understand vessel evolution. Thus both Sambucus, which has vessels and the xylem of which differs greatly in other respects from that of the vessel-less Viburum, and Viburnum itself have similar climatic niches, at least from present-day distributions; perhaps these striking differences reflect past events... (Lens et al. 2016)?

Less is known about the functional/ecological significance of variation in the phloem of angiosperms and gymnosperms, although the literature on phloem loading, the movement of sugars produced during photosynthesis into the sieve tubes/cells, is quite extensive (see also Jensen et al. 2012 and references for flow across the sieve plates of seed plants). Even if differences between sieve tubes (angiosperms) and sieve cells (gymnosperms) may be somewhat over-emphasized - the nuclei in both is non-functional, although they differ in how they become non-functional - they have different sieve plate morphologies, occlusion mechanisms, and ontogenetic/functional associations with neighbouring cells (e.g. see Behnke 1986; Evert 1990b; Schulz 1992).

Passive loading ("open" minor veins) in which sucrose predominates and with associated high sugar concentrations in leaf cells, symplastic transport, and numerous plasmodesmata between the sieve tubes and associated cells, are all correlated with the woody habit. This occurs both in Pinales (probably) and many woody angiosperms (Gamalei 1989; Rennie & Turgeon 2009; cautionary comments in Liesche et al. 2010). A variant of Gamalei's type 1 minor veins in which raffinose family oligosaccharides are commonly involved and loading is via specialized intermediary cells has been distinguished, and this is not connected with plant habit, but perhaps rather with a warmer climate (e.g. Rennie & Turgeon 2009: mechanism described; Davidson et al. 2010). Active phloem loading ("closed" minor veins), where a sieve tube element is loaded by the active transport of solute molecules across the cell membrane, seems to be most common in the predominantly herbaceous euasterids, and a variety of selective advantages for active loading can be suggested (Turgeon 2010b; Fu et al. 2011: see also Icacinales - some cautionary comments) Woody plants with active phloem loading are usually asterids (e.g. Buddleja, Catalpa, Ilex, Syringa), while herbs with passive transport are members of the predominantly woody rosids (e.g. Rosaceae, Paeonia, Lythrum). However, Saxifraga, herbaceous, and Cercis and Styrax, both woody, have active loading (Rennie & Turgeon 2009; Fu et al. 2011). For sugars, etc., other than sucrose in phloem loading, see Rennie and Turgeon (2009), for phloem loading and intermediary cells, see Davidson et al. (2011); see also Batashev et al. (2013) for minor-vein phloem anatomy and physiology.

Other variation in plumbing includes that in the water supply to the flower. The large flowers of at least some magnoliids may obtain their water through the xylem, whereas smaller flowers, as in the core eudicots, may be hydrated primarily via the phloem (Feild et al. 2009a, b, but sampling).

5E. Wood and Litter Decay. Another element of the ecological impact of angiosperms is mediated through litter and wood breakdown, as well as their loss by burning (Cornwell et al. 2009). About 30% of the organic carbon in the biosphere is currently locked up in lignin (Boerjan et al. 2003). Factors like leaf mass per area (MA, the inverse of specific leaf area [SLA], the relation of leaf area to dry mass, cm2 g-1) and primary and secondary venation type have been linked with features like the rate of photosynthesis, plant growth, litter decay, nitrogen content, and nutrient cycling. However, there is much within-community variation in such features in angiosperms and any phylogenetic signal in such correlations is not well understood (e.g. Cornwell et al. 2008; Wieder et al. 2009; Walls 2011). Moreover, it should not be forgotten that most carbon is to be found in the soil, particularly in the subsoil (Schmidt et al. 2011).

Both low MA and high amounts of nutrients in litter are quite common in angiosperms and are implicated in speedy litter breakdown. Thus angiosperm floras in the Cretaceous (Potomac, 110-105 m.y.a.: Royer et al. 2007) and Eocene (49-47 m.y.a.: Royer et al. 2010) had a low leaf MA, under 100g/m2; the three gymnosperms examined in the former flora had a mean of 291 g/m2. Even contemporary tropical non-riparian lowland rainforest may have only a moderate MA, e.g. ca 198 g/m2, as on Barro Colorado Island (Royer et al. 2010), and extant gymnosperms, like the extinct gymnosperms just mentioned, have a higher MA than do angiosperms (Berendse & Scheffer 2009 and references). However, low MA per se is unlikely to be the only factor speeding breakdown of angiosperm remains.

Decay is affected by the composition of plant parts. At around 20%, the lignin content of angiosperms is about half that of lycopsids, and the bark wood ratio has shifted from around 8-20:1 to ca 1:4 (Robinson 1990). Denser gymnosperm woods have more lignin and less nitrogen than angiosperms, and angiosperm woods, although denser than gymnosperm woods, decay faster (Robinson 1990; Weedon et al. 2009). In general, both lignin and polysaccharide content are negatively correlated with the rate of litter breakdown (Cornwell et al. 2008; Martínz et al. 2005: decomposition of lignocellulosic compounds). The syringyl-rich lignins that characterise many angiosperms are more easily decomposed by fungi than the guaiacyl-rich lignins of other seed plants (Ziegler et al. 1985). Brown rot fungi, which do not degrade lignin, are more common in conifer forests than lignin-decaying white rot fungi which are common in angiosperm-dominated forests (Boddy & Watkinson 1995), although it should be noted that the distinction between the two is somewhat artificial (Riley et al. 2014; Floudas et al. 2015). Mean annual precipitation and temperature are positively correlated with litter and wood turnover and so with the release of the nutrients they contain (Yin 1999; Weedon et al. 2009; Wieder et al. 2009). A higher rate of lignin decomposition is correlated with nitrogen and phosphorus release during the process (Wardle et al. 2002). Overall patterns of wood decomposition vary in detail, and differences in decay rates depends on the local decay organisms and tree species, and even on the age of the tree (Weedon et al. 2009). The chemistry of woods is complex (e.g. Kögel-Knaber 2002)!

To summarise: Litter from extant ferns and fern allies and bryophytes is slow to decompose compared to that of gymnosperms and especially most angiosperms (Cornwell et al. 2008; Lang et al. 2009), although monocot litter can be be very slow to decompose (Wardle et al. 2002: New Zealand, no Poaceae). The litter of deciduous angiosperm trees decomposes faster than that of evergreens, angiosperm wood faster than gymnosperm wood, and the litter of angiosperm forbs in particular decomposes faster than that of any other group of land plants (Cornwell et al. 2008; Weedon et al. 2009). However, not only are the properties of wood important when thinking about decay, but manganese ions (Mn+++) may be an important element facilitating the oxidation and breakdown of litter (Keiluweit et al. 2015).

Angiosperm leaves, litter and wood all have more nitrogen and phosphorus (on a %age basis) than do those of gymnosperms (Cornwell et al. 2008; Weedon et al. 2009). Fast decomposition of angiosperm litter, particularly associated with the deciduous habit (Knoll & James 1987), speeds up nutrient cycling and plant growth (Cornwell et al. 2008; Berendse & Scheffer 2009). The high photosynthetic rates of most angiosperms allow high growth rates and the nutrients they need are released by the fast decay of their litter; eudicot angiosperms in particular may utilize any flushes of nutrients produced by litter and wood breakdown, they scavenge nutrients effectively (Berendse & Scheffer 2009). The disturbed habitats of early angiosperms are likely to have had elevated levels of nutrients (Berendse & Scheffer 2009).

Graminoid litter, i.e. that of Poaceae and Cyperaceae, decomposes more slowly than that of forbs (Pérez-Harguindeguy et al. 2000; Cornelissen et al. 2001; Cornwell et al. 2008; Lang et al. 2011). Graminoid lignin, with its appreciable component of p-hydroxyphenyl units, is somewhat different in composition from that of other plants, and it is low in nitrogen which is removed before the leaf dies (e.g. Cornelissen et al. 2001; Wedin 1995). Tissues of C4 grasses have a particularly low N content which negatively affects their decomposability (Forrestal et al. 2014). Roots of Poaceae also decompose more slowly than those of other plants (Birouste et al. 2012: sample small, Mediterranean). There is also a negative correlation between litter longevity and silicon concentration in tissues {ref.?].

G. Liu et al. (2014) pointed out that many ANA-grade angiosperms and magnoliids have a high SLA and their leaves decompose relatively slowly, they are "slow return" leaves, compared with those of other large angiosperms (see also Cornwell et al. 2014). Piperales and Nymphaeales, with more quickly decomposing leaves, are smaller plants (Liu et al. 2014). The litter of ectomycorrhizal trees also tends to decompose more slowly than that of AM plants, although not as slowly as that of Sphagnum (Pérez-Harguindeguy et al. 2000; Cornelissen et al. 2001; Cornwell et al. 2008; Lang et al. 2011; Midgley et al. 2015). With ECM trees in particular the rate of decay of the ECM fungi, whether as separate hyphae or as rhizomorphs, also has to be taken into account, and the melanin in their their hyphae, perhaps involved in water stress tolerance (Fernandez & Koide 2013) is notably resistant to decay; some dark septate endophytes, also with melanin in their walls, can be either ECM or ERM on occasion (e.g. Butler & Day 1998; Lukesová et al. 2015). Organic matter in the subsoil below 30cm, a major component of global carbon, has very long turnover times, 1,000-10,000 years or more (Schmidt et al. 2011), although it is unclear whether or not there is a link to mycorrhizal type here.

It is not only the amount and rate of decay that matters, but its seasonality. In evergreen plants, whether angiosperm or gymnosperm, nutrient cycling is gradual, nutrients being released throughout the year and tending to be taken up by the plants again. In deciduous species, however, nutrients tend to become available in flushes, and some are lost to the ecosystem, and this will increase weathering (Knoll & James 1987). Fire also removes dead organic matter and affects the availability of plant nutrients (see elsewhere).

Limited data suggest that the diameter of first order roots has decreased in angiosperms, root length per unit biomass, SRL, increasing, so allowing for more efficient exploration of soil space to satisfy the increased water demands of the plant as carbon dioxide concentration decreased and also enabling more efficient nutrient scavenging (Comas et al. 2012; W. Chen et al. 2013: diameter decrease - and branching ratio increase - phylogenetically correlated, breakpoint ca 64 m.y.a.). Many magnoliids and the ANA grade taxa have rather thicker roots, thus magnoliids are prominent in the small group of tropical angiosperms with relatively broad first-order roots compared with the narrower roots of gymnosperms and temperate angiosperms (Chen et al. 2013), and there are links between these root attributes, mycorrhizal status and nutrient foraging status. Species with thin roots may forage for nutrients directly while those with thicker roots forage via their mycorrhizal associates (B. Liu et al. 2015; Eissenstadt et al. 2015). However, Maherali (2014) found no particular correlation between root diameter and response to colonization by AM fungi. Work by W. Chen et al. (2016) suggests the complexity of possible interactions. In a small sample of temperate trees growing in an environment with patchy distribution of nutrients, AM trees with fine roots exploited these nutrient-rich patches by producing more roots, while ECM trees expolited them via mycelial growth, interestingly, the three members of Pinaceae (ECM) and two magnoliids (VAM) in the study had the thickest rootlets and none exploited the nutrient patches with notable precision (Chen et al. 2016). In general, fungi with long-distance exploration-type rhizomorphs are commoner in places where there is low nitrogen availability (Hobbie & Agerer 2010).

At ca 40µm across the hair-like roots of Ericaceae with ericoid mycorrhizae are perhaps the narrowest fine roots, but it is unclear if there are systematic differences in fine root diameter between ERM/ECM and AM plants, either within Pentapetalae or in general (but see preliminary studies in Comas et al. 2012; Chen et al. 2013; references in Raven & Edwards 2001). Within monocots the very thick "fine" roots of many epiphytic orchids, and particularly of some Pandanaceae and palms, suggest that different relationships may hold there; roots in mycoheterotrophic plants also tend to be very thick (to 15 mm across in Voyria rosea (Gentianaceae) - and they often radiate from a point and are little branched (Imhof 2010; Imhof et al. 2013; Bolin et al. 2016 and references). See Leake and Cameron (2010) for the physiological ecology of mycoheterotrophy. This whole area would repay more detailed study.

6. Angiosperm History III: Caenozoic Diversification.

Background. For a summary of global climate during the Caenozoic, see Zachos et al. (2001). Atmospheric CO2 concentrations briefly spiked at a high of over 1,200 p.p.m. at the Palaeocene/Eocene Thermal Maximum, but they continued to fall throught the Caenozoic (Arakaki et al. 2011). During the recent glaciations CO2 concentrations dropped to 180-190 p.p.m., as low as any time during the whole period of land plant evolution (e.g. Zachos et al. 2008; Gerhart & Ward 2010; Boyce et al. 2010; McKenzie et al. 2016). Similarly, global temperatures were high at the beginning of the Caenozoic, and then declined, although with pronounced if sometimes short-term increases; it is important to note that global climate was unstable in the last ca 1 m.y. of the Cretaceous, perhaps because of the production of CO2 and other gases by the Deccan Trap eruptions (Renne et al. 2013), which began a little over 66 m.y. ago. Rock weathering increased, and this continued through the later Caenozoic (Pälike et al. 2012). Continental drift was active, India and the Deccan plate colliding with Asia, Australia approaching Southeast Asia, and the Atlantic Ocean widening (e.g. Morley 2003); interestingly, a 52-50 m.y.o. amber fauna from India showed little evidence of Indian insularity (Rust et al. 2010). Land connections around the southern end of the world were broken. Major mountain ranges such as the Andes and the Qinghai-Tibet plateau were elevated towards the middle-later part of this period. Thus the latter had reached 4,000 m altitude by 35 m.y.a. at the latest, and with its elevation came the development of monsoonal climate in Southeast Asia (Spicer et al. 2003; Favre et al. 2015).

There was an end-Cretaceous bolide impact in the northern Yucatan region of Mexico ca 65.5 m.y.a. and massive volcanic outpourings that now form the Deccan Traps in India a little before and after this time (Schoene et al. 2015; see also Keller 2014), the later stages of the eruptions perhaps even being geophysically connected with the bolide impact (Renne et al. 2015). This is no place to discuss the relative importance of the two events (but see Petersen et al. 2016: two major temperature spikes in Antarctica), although between the two very large amounts of carbon dioxide and other gases were injected into the atmosphere, contributing to the world-wide changes in the biota that were evident at the Cretaceous/Palaeogene (K/P) boundary (Cretaceous/Tertiary [K/T or C/T] boundary in older literature), and an estimated 75% of species may have become extinct globally (Vajda & Bercovici 2014). Indeed, changes in some groups, including dinosaurs, may have begun a little earlier in the Late Cretaceous (references in Schoene et al. 2015).

6A. Flowering Plants. A question is, how seriously did events at the K/P boundary affect the land flora and fauna? It is claimed that up to 80% plant species were lost in some places in North America at the K/P boundary (Upchurch & Wolf 1987), although other estimates are around 25-33% (Nichols & Johnson 2008; c.f. Salas-Leiva et al. 2013). "Sudden ecosystem collapse" occurred at least locally, even some common plants not surviving the K/P boundary (Wilf & Johnson 2004: p. 347); they estimate 30-57% extinction of the flora (data are from pollen) in southwest North Dakota (see also Vanneste et al. 2014a). The familial composition of Early Caenozoic forests in North America differs from that of their Late Cretaceous counterparts (e.g. K. R. Johnson 2002; Wilf & Johnson 2004). Insect-pollinated and/or evergreen taxa of seed plants suffered more than wind-pollinated and/or deciduous taxa (Collinson 1990; McElwain & Punyasena 2007), and there was a change in the composition of insect herbivores, localities like Mexican Hat, in Montana, with high post-bolide herbivory definitely being the exception (Wilf et al. 2006; Donovan et al. 2014). Diet-specific herbivorous insects were seriously affected (Labandeira et al. 2002a, b; Wilf 2008: surveys of leaf damage types). However, there seem not to have been widespread fires (Belcher 2010).

In New Zealand the iridium anomaly associated with the bolide impact was followed by a thin layer high in fungal remains, and changes there seem to be quite pronounced, Araucariaceae in particular declining substantially (Vajda & McLoughlin 2004; Pole 2008; Cantrill & Poole 2012), while in both hemispheres there were fern spikes (and, in the Netherlands, a bryophyte peak) after the impact/eruptions (Saito et al. 986; Vajda & McLoughlin 2007; Nichols & Johnson 2008: esp. Vajda & Bercovici 2014: Fig. 6). Evidence from Australia is unclear (e.g. Macphail et al. 1994; Hill & Brodribb 2006). In Colombia, there were changes in ecological structure but not extinctions (De la Parra et al. 2007). Although diversity of insect damage types then was higher than in North America and there was less decrease in damage types across the bounday, there were no obvious boundary-crossing leaf mine types, as in Patagonia (Donovan et al. 2015, 2016; see also Jud et al. 2017) and in North America (Wilf et al. 2006; Donovan et al. 2014).

It is unclear just how severe the effects of the bolide impact/eruptions were on the biota globally. In general, macrofossils tend to show higher extinction rates than sporomorphs, the macrofossil record being more and the microfossil record less local (e.g. Mander et al. 2010). No major plant group is known to have disappeared at the end of the Cretaceous (Nichols & Johnson 2008). By and large the main pollen genera persisted across the K/P boundary in interior North America, even if species did not (Tschudy & Tschudy 1986; Lupia 1999: no change in general morphological disparity, reduction in pollen species numbers), and there was no major change in the Wodehouseia spinata pollen assemblage in North Dakota during the K/P change (Nichols & Johnson 2008; Vajda & Bercovici 2014). Indeed, the macroflora there seems to have been exceptionally diverse in the million years preceding the K/P boundary, hence perhaps accentuating the subsequent decrease (Vajda & Bercovici 2014: Fig. 4). Green and Hickey (2005) found no significant changes were detected when various aspects of leaf architecture were plotted across the K/P boundary in North America, suggesting that any effect in ecosystem structure was minor and/or short-lived, this study not unreasonably suggesting a connnection between ecosystem properties and leaf morphology (.

As one moves away from the Yucatan the effects are less marked. Thus nothing much at all seems to have happened in the high Arctic (Spicer & Herman 2010). In Patagonia in particular and some other places in the Southern Hemisphere, changes at the K/P boundary were rather muted (Wolfe 1987; Iglesias et al 2007; Wilf et al. 2013), and in both Patagonia and Antarctica the K/P boundary is visible botanically mainly as a transient fern spike (Cantrill & Poole 2005a, b, 2014) or even less (Bowman et al. 2014). The extinct conifer group Cheirolepidaceae increased and Podocarpaceae decreased in Patagonia in the early Palaeocene, but with little extinction - as is true in New Zealand, too (Barreda et al. 2012b). It is interesting that even in the Deccan Traps in India spanning the K/P boundary there is little evidence of major changes in the local fossil record of frogs and turtles, and plant productivity, at least, also shows little change, although the pollen record seems to be poorly known (Cripps et al. 2005). Woods of genera like Eucalyptus, Ailanthus, Canarium, Aglaia, Garcinia and Sterculia are reported from intertrappean deposits, also bamboos and seeds of Artabotrys, but these last records are over 150 years old (Bande et al. 1988; Bande 1992 and references). Recent work (Wheeler et al. 2017).

Things are also confusing when looking at the records of individual taxa. Groups like Annonaceae, Arecaceae, and Araceae seem to have constant diversification rates across the K/P boundary (Couvreur et al. 2011a and references). Menispermaceae, today often lianes of the l.t.r.f., showed a burst of diversification close to the K/P boundary (W. Wang et al. 2012, c.f. dates in W. Wang et al. 2016a; Eiserhardt et al. 2017). Several clades of Cretaceous heterosporous water ferns did not survive to the Cainozoic, although others persisted to the present (Collinson et al. 2013). Angiosperm-dominated herbaceous floras, of which the largely north temperate Ranunculaceae are an example, may have diversified in the Cretaceous and were relatively unaffected by events at the K/P boundary (W. Wang et al. 2016). Seed ferns survived until well into the Palaeocene in Tasmania (McLoughlin et al. 2008), cheirolepidaceous conifers into the Palaeocene in Patagonia (Barreda et al. 2012b), and Bennettitales perhaps into the Oligocene in Australia (McLoughlin et al. 2011), the latter with a ghost lineage of ca 65 m. years. Some animal groups in the same area show similar patterns of persistence (Iglesias et al. 2007; Barreda et al. 2012b).

Some animal groups - not only herbivorous dinosaurs (e.g. Samant & Mohabey 2014; Sakamoto et al. 2016) - did suffer more or less severely. Thus there are estimates of about 60% loss of butterfly diversity at the K/P boundary, with a decrease in diversification in Nymphalidae (Wahlberg et al. 2009) but not Riodinidae (Espeland et al. 2015), and a mass extinction of birds, lizards and snakes in western North America, and perhaps elsewhere, too (Longrich et al. 2011, 2012). Ammonites became extinct and plankton with calcareous skeletons were greatly reduced in numbers (Schulte et al. 2010; Ohno et al. 2014), and the relatively few survivors dominated the seas for many thousands of years (Schueth et al. 2015). Molluscs other than ammonites were severely affected in Antarctica (Witts et al. 2016 and references; Petersen et al. 2016). Rather surprisingly, given text-book accounts, mammal diversification may have been little affected (Bininda-Emonds et al. 2007; Meredith et al. 2011; G. P. Wilson et al. 2012; c.f. O'Leary et al. 2013: dates probably underestimates, see dos Reis et al. 2014), although recent work suggests a much increased evolutionary rate for eutherian mammals just after the K/P boundary (Halliday et al. 2016; see also Wilson 2014). However, the diversity of multituberculate mammals was at a peak across the K/P boundary and seems unaffected by events then (Wilson et al. 2012), while Indian Intertrappean floras show little evidence of animal extinctions (Spicer & Collinson 2014).

Not surprisingly, extimates of the time that vegetation took to fully recover range from only a few thousand years in New Zealand (Vajda & McLoughlin 2007) to over a million years years (McElwain & Punyasena 2007) or considerably more. Overall, the Patagonian flora ca 4 m.y. after the bolide was notably diverse compared to its northern counterparts, suggesting that this whole area was something of a biodiversity refugium because of its distance from the impact, higher pre-impact diversity, and/or higher immigration (but from where?) or post-impact speciation rates (Iglesias et al. 2007; Barreda et al. 2012b), but patterns of leaf herbivory before and after the K/P boundary did differ, so questioning the idea of refugia (Donovan et al. 2016). Wappler et al. (2009) found that the diversity of herbivore damage in the rich Palaeocene flora in Menat, France, was high, again suggesting less effect of the K/P events there than in North America - this flora is dated to 61-60 m.y.o., ca 5 m.y. post-impact. Tropical rainforest was growing in Colorado ca 1.4 m.y. after the K/P boundary (K. R. Johnson & Ellis 2002), and immigration of taxa from less affected areas may have been important here (Renne et al. 2013). However, overall recovery in SW North America was slow, taking some 9 m.y. (e.g. Labandeira et al. 2002; Donovan et al. 2014). In North America initial recolonization may have been by swamp- and mire-loving plants, which apparently survived the impact better (K. R. Johnson 2002; Labandeira et al. 2002b), mire vegetation being least affected by the impact (Nichols & Johnson 2008). In North Dakota species growing along river channels were more adversely affected than those in the flood plain, and fast-growth ecological strategies were favoured: Leaves were thin and often deciduous, and there was a small increase in venation density from ca 3.5 mm/mm2 to ca 4.6 mm/mm2, i.e. ca 1.8 mm/mm2 (Blonder et al. 2014), only a small change given the average difference between nonangiosperms and eudicots, ca 8 mm/mm2, and any long-term effects of these changes are unclear (see also Green & Hickey 2005). A fast-growth strategy might allow the plants to deal better with changing light regimes (see the "impact winter": Ohno et al. 2014 and refs.) and heterogeneities in resource availability (Blonder et al. 2014). In higher-latitude North America dominantly broad-leaved evegreen mesothermal forest was replaced by comparable broad-leaved forest (Wolfe 1987).

Recovery of algal primary productivity in marine ecosystems may have taken as little as the order of century or perhaps even less (Sepúlveda et al. 2009: Denmark; Aberhan & Kiessling 2015), although other aspects of marine recovery took considerably longer (literature in Wilf & Johnson 2004), with changes in marine assemblages lasting for a few million years (Aberhan & Kiessling 2015). Even if there were quite low levels of extinction of fish, after the bolide impact ray-finned fish became far more common than sharks and their relatives, a major and permanent change in marine ecosystems (Sibert & Norris 2015). North American snakes and lizards took perhaps 10 m.y. to recover their Late Cretaceous diversity (Longrich et al. 2012).

Clearly, what went on in the southern half of North America should not be extrapolated globally (e.g. Wappler et al. 2009). Indeed, although the effects of the end-Cretaceous bolide impact/Deccan Traps eruptions on angiosperms sometimes seem quite muted, land plants and animals might almost be expected to differ in how they were affected by such catastrophes (Vajda & Bercovici 2014). Plants may have been unable to grow for a mere one or two years, even in places with much devastation (Spicer & Collinson 2014). Plant propagules in the soil are likely to have survived fairly transient (months, even a few years) atmospheric or other changes at the K/P boundary better than metazoan animals that lack resting stages in their life cycles (Cascales-Miñana & Cleal 2013); seed plants show a fair amount of ecological resilience (Nimmo et al. 2015 for resistance/resilience). As Green and Hickey (2005: pp. 998-999) observed, "If all the forests in North America were burned over in a single summer, that would clearly count as a dramatic ecological effect, but it seems intuitively likely that such an effect would have few or no effects that lasted longer than the time taken for the forests to regrow"; 'mass death' would seem a more appropriate descriptor rather than 'mass extinction'. Indeed, no major adverse effects on land plants around the K/P boundary were detected in recent analyses (Cascales-Miñana & Cleal 2013; Magallón et al. 2015: level of family origination was high then). There may have been decreases in the diversification rates of ferns and low diversification and high extinction of gymnosperms, but diversification rates at the level of genus increased and overall net diversity for vascular plants was high (Silvestro et al. 2015: mostly leaf fossils; see also Niklas et al. 1983).

Fawcett et al. (2009; see also Vanneste et al. 2014a, b) dated a series of genome duplications within angiosperms to about 70-57 m.y.a., around the time of the Deccan Traps/bolide impact, suggesting that polyploids forming then, or which had quite recently been produced, were at a selective advantage because of their hybrid vigour, having extra genes/alleles available for selection given the changing environment, and also being being more tolerant of environmental stress (see also Z. Li et al. 2016). As Lohaus and van de Peer (2016: p. 64) observed, "the correlations of polyploidization with both plant survival at the K/P boundary and plant invasiveness in general are related, as the plant survivors of the K/Pg mass extinction event turned into plant invaders and recolonizers of the post-cataclysmic, low-plant diversity evironment." Disaster averted by genome duplication?

There were changes in angiosperm ecology and diversity in the early Caenozoic. For angiosperm clades that crossed the K/P boundary, the average seed mass, initially rather low, increased markedly (e.g. Tiffney 1986b; Eriksson et al. 2000a; Collinson & van Bergen 2004; Sims 2010). This trend can be seen within Juglandaceae, which have many winged disseminules (Eriksson et al. 2000a; Friis et al. 2011), as well as in Fagaceae, which largely lack such disseminules (Tiffney 1986a). Xiang et al. (2014) thought that relatively open habitats after the K/P boundary may have favoured the diversification of fagalean clades with winged disseminules, although such habitats are unlikely to have persisted. Around the end of the Cretaceous flower size must have increased, flowers even in the later Cretaceous usually being only a few millimetres in size (Friis et al. 2011).

The increase in seed size that began at the end of the Cretaceous and reached a maximumin the early Eocene may be linked to a change in forest type, with closed, more humid forests made up of tall trees becoming more common (e.g. Eriksson et al. 2000a, b; Tiffney 2004; Mack 2000; Moles et al. 2005a, b; Eriksson 2008, 2016; Dilcher 2010). Large seeds are common in plants that at least initially grow in shaded habitats, providing reserves for the early growth of the seedling, although they may also be favoured by dry conditions, soils with low mineral nutrients, etc. (Leishman et al. 2000; Bolmgren & Eriksson 2005). Within individual forest communities there is great variation in seed size, in part connected with the successional status of the species, early successional species tending to have smaller seeds (e.g. Westoby et al. 1996; Eriksson et al. 2000b). Interestingly, the emergent trees, epiphytes and lianas of today's tropical rainforest canopy are quite commonly wind dispersed (Herrera et al. 2014b for references), and the evolution of minute dust seeds, as in epiphytic Bromeliaceae and Orchidaceae, and the development of a closed canopy has been linked (Eriksson & Kainulainen 2011).

Euasterids, many of which are herbaceous or shrubby, tend to have small seeds (Eriksson & Kainulainen 2011), and they diversified greatly in the Cainozoic. Seed volume decreases somewhat from the end-Eocene onwards (e.g. Tiffney 1984; Eriksson et al. 2000a; Friis et al. 2011). Seed mass of extant angiosperms currently drops quite abruptly (seven-fold) at the edge of the tropics (Moles et al. 2007: sampling in the tropics not very good). The reasons for this are unclear, but wind dispersal of smaller seeds in the open habitats that are more common outside the tropics may be involved (Lorts et al. 2008, but see above). The current prevalance of ectomycorrhizal (ECM) forests in regions outside the tropics and the often rather acid, humus-rich, and nutrient-poor soils that they favour may also affect seed size, although temperate mast-fruiting ECM Fagaceae have notably large, often animal-dispersed seeds while ECM Pinaceae and in particular pioneering Salix and Betulaceae-Betuloideae have small, wind-dispersed seeds.

In general angiosperm diversity in the tropics and warm temperate areas was rather low during the Palaeocene (Wilf 2008). However, a middle Palaeocene (ca 61 m.y.) flora in France was diverse and also supported a diverse assemblage of herbivores, as in a number of sites far distant from the Mexican point of impact of the bolide (Wappler 2009 and references). There are no particular changes in diversification rates at around this time (Silvestro et al 2015; Magallón et al. 2015). By around 64.5 m.y.a. the Castle Rock flora in Colorado is described as "an excellent example of early modern tropical rainforest in North America" (Burnham & Johnson 2004: p. 1607). A Late Palaeocene flora from Colombia ca 59 m.y.o. had a familial composition similar to that of current neotropical rainforest, including Arecaceae, Araceae, Fabaceae, Malvaceae, Menispermaceae, Lauraceae and Zingiberales, even if overall both plant (esp. beta diversity) and herbivore diversity were rather low (Jaramillo et al. 2006; Graham 2010: vegetational history of Latin America). Thus Palaeocene-Eocene palynomorph diversity in both Venezuela and Colombia was low (Morley 2007). This may reflect a rather belated recovery from the bolide impact and/or that the tropical rainforest ecosystem was just developing (Wing et al. 2009). Laminar venation density is very high (Wing et al. 2009; see also Burnham & Johnson 2004), and this is the first fossil evidence of functional equatorial neotropical megathermal rainforest (Feild et al. 2011b; see also Jud & Wing 2013). However, taxa with wild-dispersed fruits, common in today's canopy trees, lianes and epiphytes, are uncommon in Palaeocene Colombian floras (Herrera et al. 2014b).

During the short-lived Palaeocene-Eocene thermal maximum (PETM) of about 55 m.y.a. temperatures increased 3-8o or more - estimates vary - to mean annual temperatures (MAT) 31-34o C (e.g. Willis & MacDonald 2011). (Note that the MAT of l.t.r.f. today is ca 27.5oC, and photorespiration predominates over photosynthesis above 35o - Sun et al. 2012.) Over 2,000 gigatons of carbon were released in ca 10,000 years, the whole event lasting a mere 100,000-200,000 years (Zachos et al. 2008; McInerney & Wing 2011); it was perhaps associated with the impact of some extra-terrestrial body (Schaller et al. 2016). Humidity, precipitation and so rock weathering also increased during this period (Zachos et al. 2001, 2008).

In South America plant diversity and origination rates increased at about the time of the PETM, but there is no evidence of thermal damage to the leaves (Jaramillo et al. 2010) despite temperatures 5-70 above current values (Jaramillo & Cárdenas 2013). In North America (Wyoming area) mesophytic plants, especially conifers, were temporarily replaced by species that could tolerate both increased temperature and decreased precipitation, and plant and herbivore diversity and herbivore activity were high, perhaps correlated with the high temperature (Currano et al. 2008; Jaramillo et al. 2010). Movement of floras and replacement of Cupressaceae and Podocarpaceae occurred elsewhere, too (Wing & Currano 2013). Diversity was also very high in Late Palaeocene Gulf Coast floras, pollen diversity increasing ca 15% (Harrington & Jaramillo 2007). Although Citerne et al. (2010) suggest that this was a period of floral innovation, overall increases of diversification were not detected (Magallón et al. 2015; Silvestro et al. 2015).

The PETM may be associated with some marine extinctions, and there were distributional shifts in both terrestrial plants and animals and reworking of sediments and increased sediment flux (and more clay moving to the sea), but overall there seems to have been little terrestrial extinction (Wing et al. 2005; Willis & MacDonald 2011; McInerney & Wing 2011; Foreman et al. 2012; Wing & Currano 2013; c.f. Mander et al. 2010). However, at the end of the Palaeocene there was a pronounced (ca 20%) decrease in palynological diversity in the then paratropical Gulf Coast floras (Harrington & Jaramillo 2007), which Mander et al. (2010) consider to be a significant decline - sporomorphs are a more reliable indicator of presence than are meso- or macrofossils.

Fires decreased notably in many parts of the world from the mid Palaeocene to the Pliocene (Bond et al. 2005; Bond & Scott 2010; Belcher et al. 2010b; He et al. 2012; Bond & Midgley 2012), Australia perhaps being an exception (e.g. He et al. 2011; Crisp et al. 2011; Crisp & Cook 2013). Perhumid conditions had spread and large angiosperms dominated; litter decayed quickly and there were few shrubs to support fire (c.f. the Cretaceous, fire-susceptible Proteaceae-dominated heath evolved in Australia then, Carpenter et al. 2015). However, in parts of Europe there is evidence for episodic fires in a vegetation dominated by ferns and perhaps Fagales (Collinson et al. 2007). There was a small peak of fire activity in the Oligocene.

Although temperatures soon moderated after the PETM, they became gradually warmer again, peaking at similar high values during the Early Eocene Climatic Optimum of 52-50 m.y.a., a much warmer, wetter and more temperate period than now (e.g. Greenwood & Wing 1995; Upchurch et al. 2007; Zachos et al. 2001, 2008; Sluijs et al. 2009; Kroeger & Funnell 2011). Paratropical forest was common in the early Eocene from the eastern part of the Northern Hemisphere (Mayr 2009). Species diversity of both animals and plants was probably at a maximum in later Eocene forests, partly because of flatter global temperature gradients and an overall warmer earth (Jaramillo & Cárdenas 2013; Morley 2007; see below); in the earlier Eocene temperate and subtropical taxa grew together in western North America (Wilf 1987). Palaeocene and Eocene Patagonian vegetation was mostly more diverse than its North American counterparts, similarly, the diversity of herbivore damage in a fossil flora from the early Eocene in Argentina was appreciably greater than that in comparable North American floras (Wilf et al. 2005; Iglesias et al. 2007; Wilf 2008; Wilf et al. 2011). Early Eocene South American fossil floras were notably diverse, even at 47oS in Patagonia, and included lianes, diversity declining only at the end of the Eocene (e.g. Jaramillo et al. 2006, 2010; Herrera et al. 2011; Wilf et al. 2003, 2011). Diversity in western North America seems to have been comparable to that in these southern floras (R. Y. Smith et al. 2012).

In the Eocene, palm trees grew well inside the Arctic circle (Eldrett et al. 2009; Sluijs et al. 2009). Some angiosperms even grew at the then-north pole ca 60 m.y.a. (see e.g. Wolfe 1978; Daly et al. 2011 for how plants could grow at such high latitudes). The diversity in unique mixed deciduous broad-leaved and evergreen and deciduous conifer forests that grew north of 65-70o N had increased markedly since the Cretaceous, a number of new genera appearing, and these forests were remarkably speciose considering that it was dark for about a third of the year (Wolfe 1987; Collinson 1990; Jahren 2007; see also Spicer & Herman 2010); there seems to have been some local endemicity (Harrington et al. 2011). Rich forests ca 45 m.y.o. (Eocene-Lutetian) have been described from palaeolatitude 78.6o N in Canada (Jahren 2007). This Arctic flora has also been compared with that of the Pacific Northwest, although overall there may be more similarity with eastern Asia floras, especially in precipitation seasonality (Schubert et al. 2012). In the other hemisphere, palm trees grew well inside the Antarctic circle, too, with evidence of paratropical rainforest from off Wilkes Land, eastern Antarctica, in the early Eocene ca 51 m.y.a. (Pross et al. 2012). There was considerable diversity in Patagonia and the Antarctic Peninsula (Barreda et al. 2012b), where pollen diversity in an Early Eocene site from South Ellesmere Island (76o S) was equivalent to that of vegetation in the southeastern United States today (Harrington et al. 2011; see also Bowman et al. 2014).

Chrysomelid bruchine beetles, now restricted to palms, have been found in Eocene deposits in Washington and British Colombia in North America in rather mountainous localities and in Primorye, eastern Russia (Archibald et al. 2014). Palms, gingers, tree ferns are found in 64-53 m.y.o. deposits from west central North America, suggesting warm equable climates with little frost (modern lats 39.17-48.5oN: Wing & Greenwood 1995; see also Wolfe 1978). At this time, extratropical climates showed little seasonality, and plants which would seem to have mutually exclusive climatic preferences grew together. Harrington and Jaramillo (2007) noted the mixture of families that are now temperate or tropical in floras of the Gulf Coast in the late Palaeocene. Overall, latitudinal diversity gradients were flatter or even peaked in temperate regions (e.g. Archibald 2013), as they had since the Triassic (see below). Land bridges in the Palaeocene that would allow warmth-requiring plants to migrate around the northern hemisphere include the Beringian (North America-East Asia) and North Atlantic bridges during the Paleocene in particular (e.g. Tiffney 1985b; Brikiatis 2014); Morley (2003) discussed interplate plant movements during the Cretaceous and Palaeogene. Many tropical taxa whose ranges are restricted and/or interrupted had much wider distributions in the Eocene (e.g. Wing 1987; Archibald et al. 2010; Plaziat et al 2001: Nypa; S. Y. Smith et al. 2008: Cyclanthaceae; Herrera et al. 2011: Stephania; Collinson et al. 2012: survey of the middle Eocene Messel flora; Stull et al. 2016 and references: Icacinaceae), an some of these broader distributions persisted as late as the Miocene (e.g. Ferguson et al. 1997; Manchester et al. 2009: East Asian endemics). There have been similar range changes in the Southern Hemisphere, for example in conifers (Wilf 2012), Eucalyptus (Hermsen et al. 2012), etc.. Southern temperate forests and Mediterranean vegetation are perhaps the best modern analogues of this rather aseasonal early vegetation, and both are notably speciose (Archibald et al. 2010).

In South America the tropical flora did not shift south when temperatures were high during the Early Eocene Climatic Optimum, rather, a new kind of vegetation the ecophysiology of which is unknown developed in paratropical areas below 24oS, and the dissimilarity between tropical and extratropical floras increased (Jaramillo & Cárdenas 2013; see also Romero 1993 and references). Aspects of the subsequent evolution of the extra-tropical flora there are rather distinctive (see below).

A long-term cooling trend had begun and was accentuated at the beginning of the Oligocene (e.g. Wolfe 1978; Millar 2011; Pagani et al. 2005). Estimates are of a 30o C or more reduction in the MAT in the far north since the end of the Eocene (Jahren 2007), temperatures dropping 8.2±3.1oC in just 400,000 years at the beginning of the Oligocene some 33.5 m.y.a. in central North America, apparently with little change in precipitation (Zanazzi et al. 2007), although in parts of North America woodlands became more open and arid in the Late Eocene/Oligocene (Mayr 2009). Eriksson (2016) saw this opening woodland as favouring frugivores that could readily move between the patches of trees - birds (passerines) and bats.

The Antarctic ice sheet appeared ca 33.5 m.y.a. at the Eocene-Oligocene boundary and persisted through much of the Oligocene (Zachos et al. 2001; Coxall et al. 2005; Eldrett et al. 2009 and references), although cooling in the Wilkes Land shelf area and spread of more temperate Nothofagus fusca-type pollen had begun by the middle Eocene (Pross et al. 2102) and there may have been short-lived periods of glaciation as early as 42 m.y.a. (Tripati et al. 2005) or perhaps even earlier (Bowman et al. 2014; Ladant & Donnadieu 2016). Arctic ice started developing ca 7 m.y.a. (Zachos et al. 2001), becoming widespread only in the early Pleistocene 2.4-2.2 m.y.a. (Brigham-Grette et al. 2013; Knies et al. 2014), 10 m.y. later than the development of major ice sheets in the Southern Hemisphere (e.g. Zachos et al. 2001, 2008; Retallack 2009; Millar 2011; Crisp & Cook 2011).

Importantly, seasonality greatly increased, even if it was to decrease somewhat later in the Caenozoic (Wolfe 1978). This seasonality was evident in extratropical floras at the end of the Eocene or somewhat earlier (e.g. Wing 1987; Eldrett et al. 2009), but marked seasonality in fossil woods is a Neogene (Pliocene and since - the last ca 23 m.y.) phenomenon (Wheeler & Baas 1993); more or less ring porous woods showed a marked increase from the Palaeocene to the Eocene, and again from the Oligocene to the Miocene (Wheeler & Baas 2011).

Overall, tropical floras became less widespread after the Eocene (Morley 2007), extra-tropical floras became less diverse and less cosmopolitan (Archibald et al. 2010), and deciduous plants became more widespread. Although few Eocene leaf remains can be assigned to modern genera, by the end-Eocene this can commonly be done (e.g. Wing 1987; Dilcher 2000). Ecological conditions then may differ somewhat from those of today, but any differences are surely less than between Cretaceous and extant vegetation (Mittelbach et al. 2007). Diversification in microthermal deciduous forests was mostly within genera (Wolfe 1987).

Temperatures in the later Oligocene rebounded slightly and then oscillated through the Miocene (e.g. Wolfe 1978. An essentially modern Malesian flora developed around the Oligocene-Miocene boundary ca 23.6 m.y.a. (Morley 2007). The mid-Miocene ca 16 m.y.a. in particular was quite warm and wet, and the Atlantic and Amazonian rainforest were continuous then, although now separated by a band of drier vegetation (Morley 2000, 2007). In the mid-Pliocene some 6-3.6 m.y.a. MATs were 2-3o C warmer than they are now and there were novel vegetation assemblages and increased diversity (Willis & MacDonald 2011). However, this warmer period was followed by a further long-term temperature decline, latterly precipitous, to the Pleistocene.

Considerable increases in the frequency of fires over the last 10 m.y. are associated with the spread of grassland and savanna (Bond et al. 2010; Bond & Scott 2010; Belcher et al. 2010b). Fires cause the release of CO2 into the atmosphere, on the other hand, substances like inertinite are highly resistant to decay, so resulting in the sequestration of carbon, but inertinite is largely absent from the Caenozoic record (Bond 2015). Fires also affect the nitrogen cycle, volatilizing N (Forrestal et al. 2014 and refs.). The great ecological importance of grasses, including those that carry out C4 photosynthesis, developed only within the last (10-)5 m.y. (e.g. Edwards et al. 2010), although the origin of this trait goes back 20 m.y. or more. The widespread Cerrado vegetation of Brazil in which such grasses are also prominent developed at about the same time (Simon et al. 2009; Simon & Pennington 2012), as did many clades with succulent plants, whether terrestrial or epiphytic, and these often have CAM photosynthesis (Arakaki et al. 2011).


6B. Latitudinal Gradients of Diversity. Details of the relationships between groups diversifying in seasonal temperate regions and their tropical relatives have long been a matter of speculation (e.g. Bews 1927). Some questions are, what global patterns of biodiversity can be discerned, what causes them, and when have they been evident (e.g. Kier et al. 2005)? Knowing the number of taxa involved, the ages of the clades, and the ecological attributes that can be linked with these clades are all important, and the perspective changes when the focus broadens to incorporate past climate changes.

Current plant and animal diversity on both land and sea is broadly correlated with climate. This is strongly seasonal in both southern and in particuar northern latitudes, and diversity usually declines away from the tropics (Fischer 1960; Hillebrand 2004: comprehensive metaanalysis, very few exceptions; Francis & Currie 2003: families!, see Qian & Ricklefs 2004 for problems with distribution maps; Hawkins et al. 2011 for a reanalysis; Jablonski et al. 2013). Similar latitudinal diversity gradients have been most obvious in cooler/glacial periods in the past (Mannion et al. 2013; c.f. Benton et al. 2010), and most recently they may be a post-Eocene phenomenon (Archibald et al. 2010, 2012; Rose et al. 2011: temperature gradient similar to that of today; Mannion et al. 2012; also Boyero 2014 for literature). Indeed, the development of latitudinal diversity gradients in North American mammals have been dated to within the last 4 m.y. or so (Marcot et al. 2016). During the Palaeocene-Eocene climates were much less seasonal than now and the flora was much more homogeneous with considerable diversity even at higher latitides, although not at the very highest. Groups like palms that are now tropical were then found far both to the north and south of their current distributions (see above). Diversity distributions that are not strongly tropicocentric may be the normal condition for the planet, ice ages climates of the present being the exception rather than the rule.

A large literature focusses on establishing mechanisms that would cause/explain current global patterns of diversity (e.g. Willig et al. 2003; Mittelbach et al. 2007 and Hurlbert & Stegan 2014 for critical discussion). Differences in rates of speciation or extinction, differences in the amount of incident energy or habitable areas, longer times of climate stability and the like have all been invoked. However, Antonelli et al. (2015) found no differences between speciation and extinction rates of tropical and non-tropical angiosperms, although these rates were significantly higher in the Neotropics compared with tropical Asia and Africa, and also more species have moved from the New to the Old Worlds. Connections between diversity and environmental energy variously estimated (and this links with latitude), species richness and the rate of molecular evolution are likely to be independent (Davies et al. 2004b; Moser et al. 2005; Jaramillo et al. 2006). Allen et al. (2002) thought that productive environments could support more individuals (but c.f. Jansson & Davies 2015), therefore ceteris paribus more mutations and evolution. Evapo-transpiration, topographical diversity, and related factors may also be important (Kreft & Jetz 2007). Lamanna et al. (2014) looked at the alpha, beta and gamma components of functional trait space (specific leaf area, seed mass, plant height), noting that trait hypervolume was greater in temperate areas, although it was unclear how species filled this space. Attempts to explain diversity patterns along similar lines continue (e.g. Brown et al. 2004; Condamine et al. 2011; Gillman & Wright 2014; Brown 2014; Tomasových et al. 2016), however, more than one mechanism may be involved, and results of the study of such biodiversity patterns has been likened to reports from the blind men examining the elephant (Hurlbert & Stegan 2014).

Herbivore diversity is often thought to be greater in tropical than in temperate forests, and so defences should be disposed likewise (Adams et al. 2011 and references; see also Agrawal et al. 2012). Novotny et al. (2006) suggested that individual species of temperate and tropical plants (controlled for phylogenetic relationships) supported a similar number of insect species, but since there were many more species of plants in the tropics, there would be many more species of insects there. However, there may be other patterns of association (c.f. Novotny et al. 2007 and Dyer et al. 2007), and an analysis of the food plants of Californian butterflies showed plant and butterfly diversity to be at most weakly correlated, whether the caterpillars had broad or narrow host plant preferences (Hawkins & Porter 2002). Two comprehensive analyses suggest that both herbivory and allocation of resources to plant defences tend to be greater at higher latitudes away from the equator (Moles et al. 2011a, b), while Salazar and Marquis (2012) noted that although the diversity of herbivores on Piper increased towards the equator, the amount of herbivory did not (see also Moles 2013; Richards et al. 2010). Overall, specialized insect herbivores are more frequent in tropical regions, and this is connected with the greater lineage diversity of their hosts there; the numbers of generalist herbivores showed no latitudinal correlations (Forister et al. 2015; see above for herbivore specialization). How this might relate to amount of herbivory is unclear. Numbers of galling species, mostly cecidomyids, are greatest 28-38o N and S, especially in sclerophyllous vegetation (Price et al. 1998).

Wiens and Donoghue (2004; see also Kerkhoff et al. 2014; Qian & Ricklefs 2016) suggest that phylogenetic niche conservatism might contribute to the higher diversity in the tropics. Groups that are tropical in origin adapt with difficulty to seasonal temperate climates (and vice versa), and older plant clades tend to be more tropical/southerly in distribution, younger ones are more cold-tolerant and northern, in line with the ages of temperate and tropical climates. The tropical clades probably evolved in the Cretaceous, i.e. before the current latitudinal climatic patterns were established, the temperate clades are mostly (but not all) post-Eocene in age (Kerkhoff et al. 2014: New World; Hawkins et al. 2014 and references: conifers not included; Richardson et al. 2015). The older a clade, the more time it will have to speciate, and amount of speciation may also be linked with the area of biomes (Fine & Ree 2006), however, clade age and clade diversity seem not to be linked (Rabosky et al. 2012). Similarly, Linder (2008) linked the timing of diversification in particular areas to whether or not the local environment had been climatically and geologically stable during the Caenozoic, while Fine (2015) linked diversity to the greater extent of tropical environments and their climatic stability. In the early study of temperate-tropical family pairs by Judd et al. (1994), the temperate family, younger and tending to be herbaceous, often arose from within a tropical family, older and woody. Laliberté et al. (2013) discussed diversity in terms of the youth of the soils; tropical soils were older and more strongly weathered and supported a diverse vegetation, soils at high latitiudes were younger, less weathered, and supported less diversity. (They noted that there were different forms of nitrogen like NH3 and NO2 in the soil, but the mycorrhizal status of the vegetation also needs to be taken into account.) Jansson and Davies (2015) noted that current diversification rates were highest in the tropics, but outside l.t.r.f., and they thought phylogenetic niche conservatism might well play a role in explaining diversity in l.t.r.fs, but rapid diversification in the areas outside l.t.r.f. might be quite recent and connected with areas that showed little change despite climatic changes dependent on changes in the earth's orbit. However, separating historical and ecological signals in patterns of plant diversity is not at all straightforward (Ricklefs 2005).

B. T. Smith et al. (2012) refined the niche conservatism hypothesis and proposed that in New World vertebrates, at least, families with southern origins were more likely to show conservatism than those of northern origins. Southern families have not penetrated the highly seasonal Nearctic, not simply because there is a smaller southern temperate zone where they diversified less/were exposed to less competition, but perhaps also because that zone is more equable, so it is temperate in a way different to that of northern temperate zones with greater climatic fluctuations. Indeed, there are some differences in diversity patterns between the two hemispheres. High latitude southern floras were less absolutely diverse but older than comparable northern floras (Kerkhoff et al. 2014), however, the southern flora had an only somewhat lower phylogenetic diversity than that of the tropics, while that in northern latitudes was notably lower (Kerkhoff et al. 2014). In the equatorial Andes, as minimum temperatures decrease with increasing elevation, the ages of the clades of the woody plants there increases (Qian 2014: focus on families). To Segovia and Armesto (2015), this was because the woody Andean plants were representatives of an old, temperate, Gondwanan flora, for which there is also evidence in Patagonian fossils ca 52.2 m.y.o. (e.g. Wilf & Escapa 2014 and references; see also Cantrill & Poole 2012; Kooyman et al. 2014: phylogenetic biome - Nothofagus forests - conservatism; Segovia & Armesto 2015). Similarly, Leslie et al. (2012) found that most southern hemisphere clades of Pinales are older than northern clades, the latter having been more subject to major climate swings beginning in the Oligocene. Equable climates most similar to those of the early Caenozoic are now to be found mostly in tropical and to a certain extent south temperate areas (Janzen 1967; Platnick 1992; Chown et al. 2004; Ghalambor et al. 2006; Leslie et al. 2012). However, Qian and Ricklefs (2016) suggest that the age of clades tends to increase with altitude in tropical mountains in general, and there is niche convergence between unrelated clades (see also Culmsee & Leschner 2013).

Not all groups are increasingly diverse towards the tropics (Kindlmann et al. 2007 for review). A few angiosperm clades like Carex and some other Cyperaceae (Spalink et al. 2016), Polygonaceae, Poaceae-Poöideae, and Ranunculaceae are most diverse away from the tropics (e.g. Escudero et al. 2012b; Kostikova et al 2014b), gymnosperms are somewhat odd, Cactaceae, especially at the species level, are most diverse at ca 20o N and S (Mutke & Barthlott 2005), and the diversity of forest understory herbs also does not increase towards the tropics (Ramos & Skillman 2015). Looking at N-S curves at a global level, Africa-Europe is the odd man out, and there is no clear latitudinal trend in species richness (Mutke & Barthlott 2005). In China, mosses showed weaker latitudinal diversity gradients than liverworts (and angiosperms), perhaps because the former can handle a wider diversity of environments than the latter, a number of which are epiphyllous and need humid conditions (S.-B. Chen et al. 2015); more globally, species diversity in mosses shows non-significant increases towards the equator, unlike the increase in vascular plants (Möls et al. 2013). Interestingly, much marine diversity is bimodal, lower at the tropics (Chaudhary et al. 2016), thus Wooley et al. (2016) found that brittle stars living below 2,000 m showed diversity greatest at 30-50o from the equator.

Thinking about these latitudinal patterns in the context of the interactions between angiosperms and their fungal associates, interactions which affect plant diversity, soil fertility, carbon content, etc., may provide another way of looking at the problem (see below). The distribution of ECM-dominated communities, particularly pronounced polewards and now especially prominent in the northern hemisphere, may contribute to the current diversity gradients, and bears on the hypothesis that diversity is in some way linked with productivity (e.g. Willig et al. 2003). Contemporary ECM-dominated communities are common in more extreme and unproductive environments and they are species-poor (Gillman & Wright 2006; Cusens et al. 2012; c.f. Adler et al. 2011: focus on herbaceous communities). ECM plants are abundant in boreal forests in particular, but also in many temperate forests. Although overall diversity of soil fungi increases towards the equator (less so in Africa), the diversity of ECM (but not AM) fungi increases in mid to high northern latitudes and ERM fungi also show a diversity increase towards the poles, if not at the highest latitudes. These patterns are consistent with the distribution of their seed plant associates (Wardle & Lindahl 2014; Tedersoo et al. 2014b: see details of distributions of functional types and taxonomic groups; Davison et al. 2015; see also Pärtel et al. 2016). Thus the diversity of ECM fungi like Amanita may peak in more temperate climates (Sánchez-Ramírez et al. 2015), while in Russula diversification rates are highest in extratropical lineages/those associated with Pinaceae (Looney et al. 2015). Diversity gradients of soil bacteria are similar. Although bacterial diversity increases towards the tropics in the southern hemisphere, in the north it is largely flat across latitudes, and this may be connected with often lower soil carbon content in the southern hemisphere, many bacteria depending on the decomposition of organic matter for energy (Delgado-Baquerizo et al. 2016; see also Lauber et al. 2009) - interestingly, bacterial diversity may be flat with increasing altitude (Fierer et al. 2011: E. Peru) or even increase (J. Wang et al. 2011: Yunnan). Furthermore, different bacterial groups predominate in the two hemispheres (Delgado-Baquerizo et al. 2016). Interestingly, phylogenetically diverse Mediterranean shrub communities, which tend to be more productive and have more fertile soils, have richer bacterial communities, but only in terms of species numbers, not phylogenetic diversity (Goberna et al. 2016). Hardly suprisingly, arbuscular mycorrhizal fungi do not show reversed diversity patterns (Gorzelak et al. 2017).

One might almost expect organisms that are dependent on plants, have at least some host specificity, yet have more or less passive means of finding their host, would be more diverse if their host plant is more abundant, the "common host hypothesis" (Kindlmann et al. 2007). This idea has been invoked to explain the increased diversity of aphids, although not necessarily of myrmecophilous species, away from the tropics (Bristow 1991; Stadler & Dixon 2005). To the extent that this hypothesis is true for ECM plants and their associates, this has to be factored in to the reverse diversity clines. Along the same lines, there may be a latitudinal gradient in the specialisation of mutualistic interaction networks. Paradoxically, it might seem, specialization decreases towards the equator, and in environments with low plant diversity, pollinators (to a lesser extent) and fruit dispersers become more specialized on the plants that are there - and the mutualistic networks are less stable (Schleuning et al. 2012). In the distinctive ecosystem of Sarracenia pitchers the diversity of organisms from invertebrates to bacteria in the pitchers increases with increasing latitude, the reverse of the normal trend, but here it may be because the numbers of predatory Wyomyia mosquito larvae decrease (Buckley et al. 2003; Kindlmann et al. 2007).

6C. Gene and Genome Duplication and Genome Size. There is a fast-growing literature on gene and particularly genome duplication and its evolutionary consequences, as well as on the related issue of genome size where the focus tends to be on its physiological consequences.

For a review of polyploidy, which emphasizes how difficult it is to make generalizations about what one would have thought was a much studied event, see Soltis et al. (2016b) and other papers in American J. Bot. 103(7). 2016; also P. Soltis & Soltis 2012; M. S. Barker et al. 2012, 2016b; etc.). Perhaps 15% of angiosperm speciation events are associated with genome duplication/polyploidy (see Otto & Whitton 2000 and Meyers & Levin 2006 for general overviews). Polyploidy has occurred many times and at all levels of the tree from events that led to the formation of species like Sporobolus (Spartina) anglica, formed by allopolyploidization in the 1870s, to the common ancestor of small groups of genera deep within Poaceae and Brassicaceae, or much larger groups such as the asterids, core eudicots and all seed plants - and of course it occurs in individual cells of the plant body - endoreduplication. Overall, angiosperm genomes have been duplicated several to many, many times, so that of Brassica, for example, is estimated be multiplied 288 times, i.e., it should have 1,440-2,016 chromosomes given an ancestral number of 5-7 and no subsequent losses (Wendel 2015), yet its diploid chromosome number is only 38, its genome size is unremarkable (also e.g. Wolfe 2001 for chromosome number reduction), while the genome of Arabidopsis thaliana is one of the smallest known. Genome duplications have been reported in most land plants (see Szövényi et al. 2014; Devos et al. 2016 for mosses), but they are uncommon in conifers (Scott et al. 2016; c.f. Z. Li et al. 2015). For genome duplication and number of 35S rDNA loci - not always linked - see Hidalgo et al. (2017).

It is often suggested that gene/genome duplications may facilitate subsequent morphological evolution by allowing the subfunctionalisation and neofunctionalisation of genes (e.g. Ohno 1970; Renny-Byfield et al. 2014; Rensing 2014: review). One of the gene copies may be lost (for a nice example, see de Martino et al. 2006), perhaps particularly in important housekeeping genes, which thus revert to being single copy genes (de Smet et al. 2013; Wendel 2015 and references). In a comparative study of core genes found in all the angiosperms examined, Z. Li et al. (2016) note that multicopy genes tend to represent ancient duplications and are involved in signaling, transport, development and metabolism (but not in mosses like Physcomitrella - Devos et al. 2016 for literature), while single-copy genes are younger and are often involved in the maintenance of genome stability/integrity and organelle function. Novel regulatory pathways may evolve, and there are other changes (e.g. Veron et al. 2007: retention and evolution of MIKC-type networks after genome duplications; Guo et al. 2013; Murat et al. 2013; Conant 2014: Saccharomyces; Vanneste et al. 2015). As Jiang et al. (2013) noted, features of the gene/genome such as complexity of the gene, GC content at the third position, etc., also affect what happens after duplication. Such changes may happen faster when chromosomes or the whole genome are duplicated, but change may be slower if individual genes have duplicated and lie in tandem, perhaps because of co-regulation (Lan & Pritchard 2016: animals; see also Jiang et al. 2013). In biased fractionation, genes in one of the genomes may be preferentially retained, a process perhaps associated with allopolyploidy, while autopolyploidy goes along with the absence of genome dominance and of biased fractionation (Garsmeur et al. 2013; Wendel 2015; Steige & Slotte 2016 and references). Conant et al. (2014) summarize various models of duplicate genome evolution.

Genome duplications may reduce the probability of extinction by e.g. increasing genetic variation and environmental tolerance (Crow & Wagner 2006 and references; see also van de Peer 2009a; Franzke et al. 2011; Kagale et al. 2014; Z. Li 2016, etc.), and individual mutations are less likely to have an immediate effect. Along these lines, Fawcett et al. (2009; see also Vanneste et al. 2014a, b; Lohaus & van de Peer 2016) dated a series of genome duplications within angiosperms to about 70-57 m.y.a., around the time of the Deccan Traps/bolide impact, suggesting that these polyploids were at a selective advantage because of their hybrid vigour, also having extra genes/alleles available for selection given the changing environment, etc. (see also Visser & Molofsky 2014 for possible advantages of polyploids). For genome size (and other aspects of the genome) and plant invasiveness, see Suda et al. (2014).

Genome duplication is increasingly being invoked as an explanation of the evolution and diversification of angiosperms (e.g. Vision et al. 2000; Bowers et al. 2003; Blanc & Wolfe 2004a; Schlueter et al. 2004; Adams & Wendel 2005; Maere et al. 2005; de Bodt et al. 2005; de Martins et al. 2006; Chapman et al. 2006; Cui et al. 2006; Jaillon, Eury et al. 2007; Soltis et al. 2009; van de Peer et al. 2009b; Duarte et al. 2010; Barker et al. 2010; Jiao et al. 2011; Mühlhausen & Kollmar 2013: myosin motor proteins; Guo et al. 2013; Vanneste et al. 2014a, b; Tank et al. 2015; P. Soltis & Soltis 2016). Species-rich clades and genome duplications are specifically linked, for example, Soltis et al. (2009: p. 336) associating genome duplications with "a dramatic increase in species richness" in Poaceae, Fabaceae, Brassicaceae and Solanaceae (for Poaceae, see also Salse 2016), and P. Soltis and Soltis (2016) tentatively link the origins of a number of key innovations in angiosperms to ancient polyploidy events. However, since duplication and subsequent diversification may be separated by tens of millions of years - hence the whole genome duplication radiation lag-time model (see van de Peer et al. 2009a; Schranz et al. 2012) - establishing direct connections between the two can be difficult, putting it mildly. Estimates of timing of such events and subsequent diversification in angiosperms were summarized by Tank et al. (2015), and the lag-time ranges from 0 to almost 50 m. years. This lag period may reflect the time that it takes for the genome to become diploidized (Dodsworth et al. 2016). A decrease in genome size after polyploidy in Veronica (Plantaginaceae) was linked with increased diversification rates (Meudt et al. 2015b), again suggesting a lag between polyploidy events and subsequent diversification. However, Kellogg (2016a) is sceptical about claims of connections between polyploidy and subsequent diversification.

Wood et al. (2009) suggested that polyploidy is quite common during speciation (15% speciation events in angiosperms, 31% in ferns), but the diversification rate of polyploids is not remarkable. Indeed, polyploidy is quite commonly an evolutionary dead end, with recently-formed polyploid plants speciating less and in particular showing higher extinction rates than diploids (Mayrose et al. 2011, 2014). Scarpino et al. (2014) proposed that the prevalence of polyploidy was the result of a ratchet mechanism, being irreversible (but see below); diploids speciated more. A distinction between short- and long-term effects may help clarify the discussion between Mayrose et al. (2011, 2014: emphasis on the former) and Soltis et al. (2009, 2014: emphasis on the latter).

Isozyme duplications suggest past polyploidization in clades like Magnoliaceae, Aesculus, and Salix/Populus (Soltis & Soltis 1990). The notably small stomata of some fossils when compared with extant members of these clades also may imply subsequent polyploidization, extinct members perhaps having chromosome numbers half of any of those known in extant members (Masterson 1994: Lauraceae, Magnoliaceae [one point], Platanaceae), this thesis being largely based on the assumption that there is a correlation between stomatal size, DNA content and chromosome number. However, the examples just mentioned are from woody groups that are unlikely to have had herbaceous ancestors, so ceteris paribus polyploidy would be less likely in such groups, smaller stomata might be expected given given falling CO2 concentrations, and genome size and stomatal size are not always coupled (see below). Chen et al. (2014), commenting on the small genomes and absence of polyploidy in Fagaceae, suggested that there extensive interspecific hybridization substituted for polyploidy, the two providing similar evolutionary benefits.

Genome duplication may be linked with genome size, and there has been much recent work on genome size. Nuclear genome size varies 2000-fold within angiosperms alone (Greilhuber et al. 2006; see also Characters), although most angiosperms have rather small genomes (e.g. Soltis et al. 2003c) and genome size can be correlated positively with cell size. Thus Furness et al. (2015) found a correlation between genome size and pollen size in Liliales, while Franks et al. (2012) found a correlation between guard cell length and nuclear and genome size among north temperate herbs. Stomatal size is negatively correlated with stomatal density; trees, with rather small genomes, have the highest stomatal density (Beaulieu et al. 2008, see also Bainard et al. 2012, c.f. Rupp et al. 2010 for Polystachya [Orchidaceae]). Stomatal size, and hence genome size, may have been inversely correlated with atmospheric CO2 concentration over the last 300 m.y. or so (Franks et al. 2012: Fig. 3 - Fig. 4 has problems with ancestral state reconstructions), so both increasing and decreasing in size; Lomax et al. (2013: c.f. fig. 2A and 2B) thought that maximum genome size (derived from maximum guard cell length) may have been steadily increasing from 360 m.y.a. (the Mississippian), but a time-binned average shows a decrease over the last 250 m.y., there was no signal of increased genome size in the early Caenozoic and grasses (e.g.) have notably small stomata (Franks & Beerling 2009). Franks et al. (2012) do not suggest any mechanism that facilitated changes in genome size. Indeed, Hodgson et al. (2010) thought that changes in stomatal size could have driven changes in genome size, and they emphasize the complexity of the relationship between stomatal amd genome size. Surprisingly, of the species they examined, stomatal length was highest, at up to around 2µm, for vernal geophytes, not species in shade. However, Jordan et al. (2014) found that in Proteaceae, at least, changes in genome size drove changes in stomatal size, the two often being correlated, but some species showed substantial changes in stomatal size without change in genome size, and this was related to the ecology of the plants involved.

There is often little correlation between genome size and chromosome/genome block number and in particular ploidy level (e.g. Leitch & Bennett 2004; Weiss-Schneeweiss et al. 2005; Bennett & Leitch 2005; Lysak et al. 2007, 2009, 2016; Schnable et al. 2009; Peruzzi et al. 2009; Bliss & Suzuki 2012; Escudero et al. 2012a: Carex; Vaio et al. 2013; Gorelick et al. 2014; Jordan et al. 2014; Gunn et al. 2015; Hohmann et al. 2015; c.f. in part Jakob et al. 2005), and in Hippeastrum chromosome morphology (bimodal) is largely unaffected by substantial changes in genome size/genome copy after polyploid events (Poggio et al. 2014). There is no correlation between chromosome number and genome size in Cycadales, and there chromosome number changes are probably caused by fissions or fusions (Gorelick et al. 2014). Genome size tends to be smaller in island taxa (Hidalgo 2017 and references).

Genome downsizing after autopolyploid events was analysed by Zenil-Ferguson et al. (2016), who found interactions between monoploid number, ploidy level and genome size, many and duplicated chromosomes perhaps negatively affecting the meiotic process. Frajman et al. (2015 for references) discuss genome downsizing after polyploidization; since genome duplications have been common in seed plants and there are no comparable changes in genome size or chromosome number, these must be decoupled (e.g. Bennett & Leitch 2005; Leitch et al. 2005; Hodgson et al. 2010; Schneider et al. 2015; Lazarevic et al. 2015; Wendel 2015); for biased fractionation, see above. Systematic signal in genome size may be more apparent at lower taxonomic levels, i.e., when polyploidization has been more recent (Frajman et al. 2015 for references), and there may be a connection between genome size and chromosome number is some ferns (Vanneste et al. 2015 and references). Note that changes in the amount of repetitive DNA can have major effects on C-values (e.g. Jakob et al. 2004) independent of chromosome number. A recent study suggests that genome size per se does not drive evolution, rather, the rate of change in genome size, especially in angiosperms, is correlated with speciation rate (Puttick et al. 2015). Bennetzen and Kellogg (1997) floated the idea that increase in genome size might be irreversible, which could be true of some gymnosperms (e.g. Nystedt et al. 2013), but not angiosperms.

The rate of root meristem growth seems to be negatively correlated with genome size, and since holoparasites in particular have little need for roots (Gruner et al. 2010), they may have much enlarged genomes (e.g. Piednoël et al. 2012). For the GC content of genomes, which shows interesting correlations with genome size, karyotype morphology (esp. holocentric chromosomes) and some aspects of ecology, see Smarda et al. (2014). Genome size, but not ploidy level, increased the length of the cell cycle, particularly in perennial monocots (Francis et al. 2008). In extant plants, seed size is more or less correlated with genome size (Beaulieu et al. 2007a; Linkies et al. 2010) and more so with plant habit. Seed size and plant height are correlated, but for a possible negative correlation of genome size and plant size, see Beaulieu et al. (2007b). Recent work on Liliaceae suggests that species with large genomes have larger cells, the plants are larger and have larger flowers; Liliaceae are geophytes, and there fast growth/large genomes (= large cells) may be an advantage, although in more extreme conditions where the growth period is short genomes may be smaller and polyploidy is common (Carta & Peruzzi 2016). Obviously, extending this approach to other groups in which geophytes predominate is the next step.

Plant genomes, and perhaps particularly those of flowering plants, are very dynamic, and the importance of genome duplication is in the opportunities it provides for gene and genome, and hence plant, evolution. However, the significance of much of the variation in genome size is unclear, and strong correlations between genome size and other features remain hard to come by (Garcia et al. 2010). Moreover, different major groups of plants behave differently. Ferns, for instance, tend to have have very high chromosome numbers (n = 57 on overage), and here genome duplication may have been accompanied by gene silencing but not chromosome loss (Haufler 1987; Barker 2013 and references), indeed, in general, heterospory is accompanied by relatively low chromosome numbers (Barker 2013). Gymnosperms show relatively little variation in chromosome number and genome size (Murray 2013; Scott et al. 2016), the latter is, however, on average the largest of the main plant groups (Leitch & Leitch 2013).

6D. Diversification in other Plant and Animal Groups associated with Flowering Plants.

Other Embryophytes. What about the diversification of embryophytes other than angiosperms? There was no simple replacement of gymnosperms, other vascular plants and "bryophytes" by angiosperms during the Cretaceous-Caenozoic. Mosses and liverworts for the most part seem to have undergone bouts of rapid diversification earlier, but in both there has also been extensive Caenozoic diversification (Cooper et al. 2012; Feldberg et al. 2014; Laenen et al. 2014). Diversification in the speciose pleurocarpous mosses, about 40% of all mosses, seems to have been early-Cretaceous and rapid, with subsequent semi-stasis. Many mosses, especially members of Hypnales, are epiphytic (Shaw et al. 2003b; Newton et al. 2006, 2007; see also Kürschner & Parolly 1999), and their initial radiation is at about the same time as the early rise of the angiosperms during the KTR (Laenen et al. 2014). Porellales, largely leaf-epiphytic liverworts, diverged from the terrestrial Jungermanniales somewhere between the Late Carboniferous to Triassic, but they, too, diversified in the Cretaceous and early Caenozoic (Heinrichs et al. 2007; Feldberg et al. 2014; see also Ahonen et al. 2003; Forrest & Crandall-Stotler 2004; Cooper et al. 2012), and much divergence within liverwort families has been Caenozoic (Cooper et al. 2012). However, Lejeuneaceae initially diversified in the Cretaceous and neither here nor in Cephaloziineae were there rate changes in the Caenozoic (R. Wilson et al. 2007a, b; Feldberg et al. 2013). Laenen et al. (2014) suggest that, as in gymnosperms, there may also have been massive extinction events.

Ferns found the low-light environment created by the dominant angiosperms to their liking, as the title of one study, "Ferns diversified in the shadow of the angiosperms" (Schneider et al. 2004). About one third of all leptosporangiate ferns are epiphytic - ca 3,000 species, about 10% of all epiphytes and the great majority of non-angiosperm vascular epiphytes. Epiphytic ferns commonly grow on angiosperms and prefer humid conditions (see Watkins et al. 2007a, b; Watkins & Cardelús 2012 for their adaptations), and are major components of the epiphytic vegetation, particularly in the rainforests of the Antipodes and Oceania (Dubuisson et al. 2009). The adoption of the epiphytic habitat was perhaps facilitated by the evolution of a distinctive new photosystem that allowed them to grow in shady conditions, although that may have been acquired considerably earlier around 179 m.y.a. (Kawai et al. 2003; F.-W. Li et al. 2014). Most epiphytic ferns are Polypodiales, and their initial diversification (it happened in several clades) began in the Palaeocene, perhaps around the PETM (Schneider et al. 2004a, b; Schuettpelz 2007; Dubuisson et al. 2009; esp. Schuettpelz & Pryer 2009: Supplemental Tables 2, 3; Watkins et al. 2010), although Watkins and Cardelús (2012: p. 701) talk about a "Cretaceous Pteridophytic stampede into the canopy" and Testo and Sundue (2016) did not find particularly rapid diversification among epiphytic ferns. However, Testo and Sundue (2016) emphasized that although many fern clades are quite old - Cretaceous or even older (Testo & Sundue 2016: Fig. 2, S1, etc.) - their diversification, both on the forest floor and in trees, was a Caenozoic phenomenon. Although Trichomanes and relatives (Hymenophyllales) diversified in the early Cretaceous, but they are commonly epiphytic on tree ferns, a relatively old clade (Schuettpelz 2007; see also Schuettpelz & Pryer 2009). About half - 190/380 species - of clubmosses, Lycopodium s.l., are also epiphytic, and their diversification may have begun in the Late Cretaceous (Wikström & Kenrick 1997, 2001; Wikström 2001).

The complex of changes that occurred in angiosperm leaves did not occur in gymnosperm leaves (e.g. de Boer et al. 2012), and gymnosperms may have been at least locally disadvantaged by the temperature changes happening at around the K/C boundary (Blonder et al. 2014). Podocarps with flattened foliage units - i.e., including cladodes - are often shade tolerant and their diversification may have occurred somewhat after the venation density of angiosperm leaves increased, (94-)64(-38) versus 109-60 m.y.a. (Biffin et al. 2011a; Brodribb & Feild 2009; Biffin & Lowe 2011). Diversification within extant genera of both Cycadales and Pinales is quite recent, mid to later Caenozoic (e.g. Oberprieler 2004; Nagalingum et al. 2011; Crisp & Cook 2011; Davis & Schaefer 2011; Leslie et al. 2012; c.f. Salas-Leiva et al. 2013, in part). Extinction may have been higher in gymnosperms than in angiosperms, hence contributing to lower diversity in the former, certainly, gymnosperm clades have long stems and shallow crowns (Crisp & Cook 2011). Brodribb (2011, see also Brodribb et al. 2012) and others have emphasized that conifers, and Pinaceae in particular, an ECM clade, are very successful in high light but other than high-nutrient conditions, some can tolerate extreme cold, and a few species dominate a considerable area of the earth's surface (see below, also Augusto et al. 2014).

Animals. Here I focus on some animal groups that are common in tropical rainforests today.

Ants make up only ca 2% of known insect species, with about 13,000 species described. However, they make up one third of insect biomass, overwhelmingly dominate in samples collected when l.t.r.f. canopies are fogged (86% of arthropod biomass, to 94% of arthropod individuals), their 15% of total animal biomass is greater than that of terrestrial vertebrates, and they are the major consumers of plant resources in the canopy (e.g. Davidson et al. 2003; Rico-Gray & Oliveira 2007; Pie & Tschá 2009; Ward 2014; Barden & Grimaldi 2016). For ants and plants, see Chomicki and Renner (2017b) and papers in Proc. Royal Soc. B 284(1850). 2017.

Ants are are sister to the Apoidea, spheciform wasps + bees (e.g. B. R. Johnson et al. 2013; Ward 2014). Crown-group diversification of ants may have begun 176.4-132.6 m.y.a. (Moreau et al. 2006: depends on calibration used), 143.2-108.6 m.y.a. (Brady et al. 2006), or 158-139 m.y.a. (Moreau & Bell 2013, 2014). On the other hand, Grimaldi and Engel (2005) date stem ants to only some 120 m.y.a., the oldest fossil stem-group ants being from the Middle Albian some 105 m.y.a., and crown-group ants are estimated to be ca 95 m.y.o. by LaPolla et al. (2013). One scenario suggests that ants and plants have coexisted for at least 120 m.y. (Chomicki & Renner 2015), between 100 and 60 m.y.a. the rise of angiosperm-dominated forests being tracked by ant diversification (Moreau et al. 2006); the ants initially evolved in pretropical forests, moving in to tropical rain forest as it developed (Moreau & Bell 2013; see also E. O. Wilson & Hölldobler 2005).

Recent work is clarifying ant evolution. The extinct Sphecomyrminae, perhaps sister to all other ants, were quite diverse in Burmese amber deposits ca 99 m.y.o. (Barden & Grimaldi 2014). Eusocial ants are known from this amber, although they are all members of extinct basal clades (Barden & Grimaldi 2016). Interestingly, highly specialized staphylinid social parasites of ants and termites have also been found in the same amber (Yamamoto et al. 2016). Brownimecia, known from fossils 94-90 m.y.o. from New Jersey (Engel & Grimaldi 2005 and references), is sister to extant ants (Barden and Grimaldi 2016). Fossil evidence suggests that the ecological dominance of ants began only in the Eocene, 50-35 m.y.a., modern ants first becoming common in the Eocene fossil amber record (e.g. Grimaldi & Engel 2005; Stadler & Dixon 2005; Dunn et al. 2007; Rust et al. 2010; LaPolla et al. 2013; Ward 2014; c.f. in part Wilson & Holldöbler 2005; Moreau et al. 2006; Rico-Gray & Oliveira 2007), perhaps around the PETM ca 55.8 m.y.a. (Ward 2014). Ants make up less than 1% of the insects in Cretaceous deposits (Zherihin & Eskov 1999), 5-12% in early to mid-Eocene deposits 52-42 m.y.a., but around 20% in Miocene Dominican amber (Barden & Grimaldi (2016), and to over 40% by the end of the Eocene (Grimaldi & Agosti 2000).

The subfamilies Dolichoderinae, Formicinae, Myrmicinae and Ponerinae include ca 90% of all ants (Ward et al. 2015), and of these Formicinae and Dolichoderinae are the major groups of canopy-dwelling ants in the l.t.r.f. today, while Dolichoderinae, Formicinae and Myrmicinae are all much involved in close associations with plants, hemipterans and fungi (Chomicki & Renner 2017b). All authors suggest that extant ant subfamilies and several of the tribes are Cretaceous in age (Moreau & Bell 2013; Ward 2014; Blaimer et al. 2015; Ward et al. 2015). The crown-group ages of Formicinae and Dolichoderinae have been estimated to be late Cretaceous-early Palaeocene in age, 80-70 and 70-60 m.y.a. or (90-)80, 75(-65) and (69-)66.5, 53(-?) respectively (Ward 2014; Moreau & Bell 2013). However, Blaimer at al. (2015) date crown-group Formicinae to (137-)117.5, 104(-88) m.y.o. in the Aptian/Albian, late Lower Cretaceous, rather older than many other estimates, and although there was little divergence for the next 20 m.y., four of the major tribes had diverged by the beginning of the Palaeocene ca 66 m.y.a. (crown-group Camponotini and Melophorini are both early Eocene) and there had been a substantial amount of divergence by the beginning of the Eocene ca 56 m.y. ago. Crown-group Pseudomyrmecinae, a small clade of arboreal ants more or less closely associated with its plant hosts has been estimated to be (78.7-)71.7(-65.7) m.y.o., but the crown-group age of all Pseudomyrmecinae minus two small clades (= ca 230 species) is only ca 49 m.y. (Chomicki et al. 2015), however, the pseudomyrmecine crown-group age in Moreau and Bell (2013) is only ca 40 m. years. Ward et al. (2015) dated crown-group Myrmicinae to (109.5-)98.5(-88) m.y.o. in the early Upper Cretaceous, with the six tribes diverging in the late Cretaceous (Maastrichtian) to early Eocene 71-52.3 m.y.a., however, the first three of these to diverge have stem groups of 25-45 m.y., and no fossil myrmicines are known from the Cretaceous.

Extant members of several basal ant clades live largely underground (Rabeling et al. 2008), although details of the relationships of these ants need clarifying (Ward 2014). Many ground-dwelling ants are carnivores, some basal clades eating termites (Bourguignon et al. 2014). The tree-loving Formicinae and Dolichoderinae eat plant materials (Rico-Gray & Oliveira 2007). Ants, fungi, hemipterans and tropical rain forest plants are associated in a variety of ways (e.g. Oliver et al. 2008). For general information about ant-hemipteran-plant interactions, see Huxley and Cutler (1991) and Rico-Gray and Oliveira (2007). Rhizobiales (and other bacteria, including γ-proteobacteria) are endosymbionts of herbivorous ants, overall, bacteria play an important role in the nitrogen metabolism of the insect, for instance, glutamate is converted into essential amino acids (Davidson et al. 2003; Russell et al. 2009). Members of Chaetothyriales, the black yeasts, a poorly-known group of often melanized ascomycetes, are commonly associated with ants, both carton-builders - the fungus is integral to the construction of the carton - and domatium-formers (see below), as well as being commonly found on the cuticle of leaf-cutting attine ants (Vasse et al. 2017). Clades of chaetothyrialean fungi tend to be either carton-formers or domatium dwellers, but they show little geographical signal (Vasse et al. 2017).

Some important associations betwen ants and plants are as follows:

1. Ants and extrafloral nectaries. Ants can obtain sugar directly from extra-floral nectaries. The earliest such nectaries in the fossil record are found on the lamina of ca 49.5 m.y.o. Prunus fossils from western North America (DeVore & Pigg 2007). Extra-floral nectaries on plants today are common in some Pentapetalae, Dioscorea, etc., but not in early angiosperms, gymnosperms or magnoliids, and it is estimated that they have evolved 457 times (Weber & Keeler 2013). Clades of plants with extrafloral nectaries have a rate of diversification that is double that of their sister clades that lacked these nectaries (Marazzi & Sanderson 2010; Weber & Agrawal 2014; c.f. Käfer & Mousset 2014). About a third of woody broad-leaved angiosperms and vines in Panamanian forests have extrafloral nectaries or other ant attractants (Schupp & Feener 1991); extra-floral nectaries provide a rather generalized resource for ants, rarely being monopolized by a single species of ant (e.g. Blüthgen et al. 2004). Marazzi and Sanderson (2010) suggest a crown-group age for a clade of Senna with extrafloral nectaries of some 40.8-30.6 m.y.

2. Ants, hemipterans, and honeydew. Ca 41% of ant genera obtain sugars indirectly from plants by way of honeydew produced by their hemipteran associates that tap the host's phloem. Myrmecophilous aphids are commonest in the tropics, although they also serve as "temperate-zone extrafloral nectaries" (Bristow 1991: p. 116; see also Stadler & Dixon 2005). This very largely mutualistic association is known as trophobiosis and the aphids are the trophobionts. Honeydew is an important food/energy source for many arboreal ants, and the species of ants involved may dominate their local communities, forming huge colonies, even if the ants themselves tend to be quite small, and they may also occupy disturbed habitats (e.g. E. O. Wilson & Holldöbler 2005; Blüthgen et al. 2004; Oliver et al. 2008). Associations of plants with ants that take honeydew are more specialised than those with ants that visit extra-floral nectaries, and a single species of ant often dominates (Blüthgen et al. 2004; Rico-Gray & Oliveira 2007). Plants benefit from such associations in about 3/4 of the cases studied, and the differences in herbivore communities in plants with and without these associations can be considerable (Styrsky & Eubanks 2006). The larger ant families commonly involved in hemipteran-honeydew associations are Myrmicinae, Formicinae, and Dolichoderinae, and they are common in other subfamilies in the immediate clade formed by these three groups (Oliver et al. 2008; Styrsky & Eubanks 2006; Chomicki & Renner 2017).

3. Ants and domatia. Ants, particularly Dolichoderinae (Oliver et al. 2008), along with scale insects, etc., live together in plant domatia that have a variety of morphologies. These associations are young, dating to the Miocene and later, i.e. within the last (19-)16 m.y., and have evolved around in plants ca 160 times (Chomicki & Renner 2015) and in ants ca 40 times (Davidson & McKee 1993). Both the plant and ant clades involved tend to be quite small (Cecropia and Hydnophytum and relatives are among the larger) and the plant clades in particular are scattered phylogenetically; all told around 680(-?1140) species of plants are involved (Chomicki & Renner 2015; see also Ueda et al. 2008: Macaranga). In the New World, some species of Pseudomyrmex have close associations with plants, as in the swollen-thorn acacias, Vachellia spp.; these associations date to (15-)13(-11) m.y.a., although the crown age for Pseudomyrmex as a whole, most species of which have more generalized associations with plants, is around 36 m.y. (Chomicki et al. 2015). Ants living in domatia tend to be quite closely related to honeydew-collecting clades (Oliver et al. 2008) and to clades that have close associations with fungi (Chomicki & Renner 2017b).

4. Ant gardens. Ant gardens are known from both the America and the Australian-Southeast Asian region. They consist of a mass of plant material and soil all assembled in trees by ants, and the ants bring along seeds of myrmecophilous plants and of other plants that grow in these gardens (Davidson 1988: America; Rico-Gray & Oliveira 2007). Some of these myrmecophilous plants have domatia in which the ants live - examples include Psychotria s.l. (Rubiaceae), Dischidia (Apocynaceae) and Lecanopteris (Polypodiaceae), all Old World (Chomicki et al. 2017) while seven families of plants grew on the nests of Amazonian carton-ants (Davison 1988 and references; see also Ule 1902, 1905). Around 13 origins of domatium-bearing ant-garden plants have been documented in the Australo-Malesian region and they have arisen within about the last (9-)6 m.y., although one of the ants involved, Philidris, is estimated to have begun diversifying (20.2-)13.2(6.2) m.y.a. (Chomicki et al. 2017: c.f. humming birds). The seeds of non-domatium-bearing ant-garden plants also attract ants which carry them to the gardens, although some may lack obvious elaiosomes (Davidson 1988; Dunn et al. 2007), and the ants may later take nectar, pearl bodies, etc., from the plants (e.g. Davidson 1988).

5. Ants and seeds. Ants disperse the seeds of many angiosperms other than plants with domatia or growing in ant gardens. They are attracted by elaiosomes, quite common on small seeds or fruits (Beattie 1985; Rico-Gray & Oliveira 2007) that vary considerably in their morphological nature and chemistry, often being a source of important nutrients for the ants (e.g. Bresinsky 1963; Kubitzki et al. 2011; Turner & Frederickson 2013). Elaiosomes are eaten by ants that do not eat the seeds themselves, discarding the seeds on their rubbish piles (c.f. granivorous ants), so at the same time aiding seed dispersal and perhaps in the establishment of the seedling. The fatty acids in the elaiosomes that attract carnivorous ants may mimic those in their animal prey, the elaiosomes being "dead insect analogue[s]" (Carroll & Janzen 1973: p. 235; Hughes et al. 1994); oleic acid, a very common fatty acid in many insects, is a particularly potent attractant and may be found in ant-dispersed seeds that have no nutritional value for the ants - cheaters (Pfeiffer et al. 2010; Turner & Frederickson 2013)! Myrmecochory is particularly common in the ground flora of the east North American and European forests, and some 1,500 species in Australia, many of which are woody and grow outside the rainforest, and a number of South African species are also myrmecochorous (Sernander 1906; Berg 1975; Orians & Milewski 2007; Milewski & Bond 1982; Bond et al. 1991; Lengyel et al. 2009, and references). Diaspore dispersal over 2 m is uncommon, but distances up to 77 m have been recorded (Gómez & Espadaler 1998). (Gastropods may also be involved in the distribution of seeds with elaiosomes - Türke et al. 2011.) Overall, myrmecochory is commonest in smaller, perennial plants (Leal et al. 2015).

A very conservative estimate is that some 11,500 species of angiosperms in 77 families are myrmecochorous and the trait has evolved 100, or even 140 or more times (Lengyel et al. 2009, 2010). Thus caruncles have evolved ca 13 times in Euphorbia alone (Horn et al. 2012). Myrmecochorous clades have about twice as many species as their non-myrmecochorous sister clades (Lengyel et al. 2009), and adoption of myrmecochory in clades like Polygalaceae-Polygaleae seems to be linked to their diversification (Rico-Gray & Oliveira 2007; Forest et al. 2007b; Lengyel et al. 2009, 2010, see also Fokuhl 2008). Myrmecochory may have first evolved when ants themselves became abundant, measured as the time when they make a substantial component of the insect fauna trapped in amber, that is, in the Eocene (Dunn et al. 2007 and references); one estimated early date for the evolution of myrmecophory (in Proteaceae) is (58-)44.5(-32 m.y.a. (Dunn et al. 2007; Lamont & He 2012). Interestingly, perhaps half the species of stick insects (Phasmatodea) lay eggs that mimic the seeds of myrmecochorous plants (seed capitula mimic elaiosomes), and such eggs are first known from the Late Eocene (Hughes & Westoby 1992; Sellick 1997).

6. Leaf-cutting ants. The ecologically very important leaf-cutting attine ants (Atta, Acromyrmex: ca 40 species) are from the New World tropics. They are young, stem and crown-group ages of leaf cutter farmers being estimated at only (16-)13, 9(-7) m.y. and (14-)11, 8(-6) m.y. respectively (Schultz & Brady 2008), and they cultivate Leucocoprinus gongylophorus (Agaricaceae). The beginning of the association between agaricalean fungi and ants, with the fungus growing on detritus or arthropod frass, can be dated to ca 55 m.y.a. (Schultz et al. 2015) or ca 30 m.y.a. (Nygaard et al. 2016). Fungus-farming attines initially formed associations with coral fungi, Pterulaceae, and in particular Lepiotaceae - although L. gongylophorus is also cultivated by an isolated lineage of attine ants in a clade that otherwise forms associations with Lepiotaceae or coral fungi - but, interestingly, like them growing on arthropod frass (Schultz et al. 2015). For chaetothyrialean fungi and attine ants, relationships between which are poorly understood, see Vasse et al. (2017).

Close ant-plant relationships are largely lowland tropical phenomena, being less evident at higher altitudes and latitudes. In Central America, ants maintain their abundance until about 1,000 m. altitude, although the numbers of species peak at lower altitudes, while above 2,500 m. there were very few species of ants at all, although those species that were there might have quite large numbers of individuals (Longino et al. 2014). Thus in general extrafloral nectaries become less frequent with increasing altitude and latitude (Rico-Gray & Oliveira 2007), so extrafloral nectaries in Andean centropogonid lobelioids, found on the outside of the inferior ovary, occur mostly in species growing at lower altitudes where ants are to be found (Stein 1992). In the New World there are no associations with ants with those species of Cecropia growing above 1,500 m altitude (Gutiérez-Valencia et al. 2017). In the Malesia-W. Pacific area the association between ants and myrmecophilous species of Psychotria s.l. (= Hydnophytinae) breaks down at higher altitudes (Chomicki & Renner 2017a) while in Papua New Guinea the ant-plant associations became more generalized and interconnected, and there seemed to be less in the way of benefits for the plant (Plowman et al. 2017).

Hemipterans include Auchenorrhyncha, various kinds of leaf hoppers (spittle bugs are the larvae of some), and Sternorrhyncha, scale insects and mealy bugs (Coccoidea: >8,000 spp.), whiteflies (Aleyrodoidea: >1,500 spp.) and aphids (Aphidoidea: >4,400 spp.), all told around 14,000 species of sap-sucking insects (Iluz 2011). Hemiptera in general have associations with γ-proteobacteria, thus almost all aphids harbour Buchnera aphidicola, an obligate bacterial endosymbiont (Meseguer et al. 2017: also other associations in hemipterans). Interactions can become very complex and changes in the bacterial symbiont are sometimes quite common, however, these do not seem to affect the ecology of the aphid (e.g. Meseguer et al. 2017).

There are other important insect associations that indirectly involve aphids. Many caterpillars of lycaenids (blues, hairstreaks: Lepidoptera) are herbivorous but also produce substances prized by ants (around 75% of lycaenids are associated with ants), although about 5% eat ants, homoptera or homopteran honeydew, for example, the ca 190 species of Miletinae (see e.g. Lohman & Samarita 2009; Kaliszewska et al. 2015; Cottrell 1984: butterflies that eat ants/homoptera or their products). There are some 5,000 lycaenids, about a quarter of all butterflies, and their caterpillars, along with those of their sister-group Riodinidae, which has 1,400< species, have single-celled pore cupola organs, perhaps involved in reducing ant aggression (de Vries et al. 1986; Pierce et al. 2002). The age of the clade of lycaenids whose members are commonly associated with ants has been dated to around 71.7 m.y. (Wahlberg et al. 2013), the [Lycaenidae + Riodinidae] clade being somewhat over 95 m.y.o. (Espeland et al. 2015).

Crown-group termites are around 149 m.y.o. (Bourguignon et al. 2014: mitochondrial genomes). Termites are derived from cockroaches and are very important globally in plant decomposition. They depend on plant material for their nutrition, basal clades having protozoa in their guts that can break down lignins (Sugimoto et al. 2000; Bignell et al. 2011; Ni & Tokuda 2013 and references), while Macrotermitinae "cultivate" lignin-decomposing fungi. The speciose primitively soil-eating Termitidae (crown-group age ca 54 m.y.a.) become common about the same time as ants, common predators of termites, in the Caenozoic (Bourguignon et al. 2014). However, termites with social castes are known from Burmese amber ca 99 m.y.o. (Engel et al. 2016), and there are tiny aleocharine staphylinid beetles that are likely to have been social parasites either on these termites or on ants (Yamamoto et al. 2016).

McKenna et al. (2015) found that most beetle families started diverging in the Jurassic, and of the later-diverging groups associated with angiosperms, the Chrysomeloidea and Scarabaeoidea began diversifying in the very early Cretaceous. Polyphagous beetles (not including whirlygigs or carabids, but including 90% of all beetles) show low extinction rates, and beetles in general seem largely unaffected by the K/P boundary (D. Smith & Marcot 2015). Phytophaga, the phytophagous beetle sister taxa [weevils (Curculionoidea) + leaf beetles (Chrysomeloidea)] include well over 125,000 species, about half of all herbivorous insects. It has been suggested that they may have diversified largely in parallel with angiosperms (Farrell 1998), although initially diversifying on gymnosperms in the Jurassic (e.g. Labandeira et al. 1994; Farrell 1998; McKenna et al. 2009; c.f. Rainford & Mayhew 2015). Ages for Phytophaga are (169.5)161.5(-155.5) m.y., of Chrysomeloidea (159.5-)145(-124.5) m.y., and of Curculionoidea (160.5-)149.5(-138.5) m.y. (McKenna et al. 2015) - Hunt et al. (2007) suggested that the latter were ca 22 m.y. older. Chrysomelidae, leaf beetles s. str., may diversify (86-)79–73(-63) m.y.a. in the Late Cretaceous-Eocene, especially in the early Caenozoic (Gómez-Zurita et al. 2007; also Winkler & Mitter 2008). However, an association between phytophagy, whether of gymnosperms or angiosperms, and diversification is questionable (Hunt et al. 2007) although here, as elsewhere, dating is critical - in general the ages in Hunt et al. (2007) are (7-)22-46(-59) m.y. older than those in McKenna et al. (2015) and with no or little overlap. Thus Smith and Marcot (2015) suggest that an uptick in family numbers of polyphaga in the mid-Cretaceous, contemporaneous with angiosperm diversification, may rather be due to the first appearance of beetle-bearing amber from that period, while in a careful analysis that utilized twice the number of sister-group comparisons in Mitter et al. (1988), Rainford and Mayhew (2015) found no evidence that plant-feeding clades were more species-rich than their sister taxa with different diet preferences. There may have been a major diversification of herbivorous beetles in particular and insects in general around the PETM (Farrell 1998; Wilf & Labandeira 1999; Wilf et al. 2001; Lopez-Vaamonde et al. 2006).

Within Scarabaeoidea the Pleurosticti and Glaphyridae are the main plant-eating groups, both including clades that eat flowers/pollen in particular (Cetoniinae in the former), and Pleurosticti are dated to (142-)128-109(-96) m.y.b.p. (there is a 40-50 m.y. stem) and Glaphyridae to ca 141 or 101 m.y.b.p., with the flower-eating clades originating 79-62 m.y.a. (Ahrens et al. 2014). Under this scenario, there was diversification of the plant-eateing groups ca 100 m.y.b.p., although more in the Palaeogene; feeding on dinosaur dung by scarabs seems unlikely. However, Gunter et al. (2016) suggested that early scarabs did eat dinosaur dung, and when dinosaurs started to eat more angiosperms, this indirectly helped facilitate the switch of scarabs to eating angiosperms. Under this scenario, pleurosticts are much older - (177.5-)158-142.5(-101.5, 91) m.y.o. - and there was an increase in diversification ca 100 m.y.b.p., and perhaps some decrease (it depends on the group) across the K-P boundary (Gunter et al. 2016).

Bee diversification began (132-)123(-113) m.y.a. (Cardinal & Danforth 2013; ca 112 m.y.a.: Grimaldi 1999; ca 125 m.y.a.: Ronquist et al. 2012), with families diverging by the beginning of the Caenozoic; most diversification occurred within the last 100 m.y. (see also Engel 2000; Grimaldi & Engel 2005). The plesiomorphic condition for pollination specificity in bees seems to be oligolecty, that is, bees initially pollinated one or a few species of plants, all more or less related (Danforth et al. 2006; Sipes et al. 2006; Praz et al. 2008; Michez et al. 2008; Litman et al. 2011 and references: note early ages for bee diversification; also Sedivy et al. 2013; c.f. e.g. Moldenke 1979), and this may have facilitated early angiosperm evolution. Several species of oligolectic bees may pollinate a single plant species, and the floral morphology of the latter is likely to be rather unspecialized (see also below). As mentioned above, there is no signal of pollinator type in pollen protein content, etc. (Roulston et al. 2000). Xylocopinae entered the Caenozoic as four clades that had diverged about 20 m.y. before, but in the early Caenozoic diversification increased considerably; before the Caenozoic there is likely to have been extinction in these clades (Rehan et al. 2013). Colletidae, a group of generalist bees, showed no obvious burst of Tertiary diversification (Almeida et al. 2011).

Lepidoptera, butterflies and moths, with ca 160,000 described and perhaps 500,000 total species, are the biggest insect clade almost totally dependent on plants both as adults and as larvae (Powell et al. 1998; Mutanen et al. 2010). Adults of basal clades (non-ditrysian) are for the most part small (Hepialidae are an exception), and although members of the two basal clades, both small, have jaws, probosces are otherwise the norm (Regier et al. 2015). Caterpillars of these extant basal lepidopteran clades are plesiomorphically mostly internal feeders (e.g., leaf miners, stem borers), and apart from most members of the basal clade ([Micropterigidae + Agathiphagidae]) they eat mostly woody superrosid angiosperms - certainly magnoliids, members of the ANA grade and monocots do not figure prominently in their diet, but fungivory and detritivory is scattered (Regier et al. 2015: Fig. 10). Relationships between some of the main clades are poorly supported, perhaps reflecting very fast initial evolution (Mutanen et al. 2010). The basal Ditrysia are also microlepidoptera, and within this clade Tineoidea s.l. have recently been found to be paraphyletic; all five of the basal dityrsian clades are fungivores and detritivores, although one is also phytophagous (Regier et al. 2014). Most other Ditrysia are herbivores, clades like Gracillariidae being mostly internal feeders (Kawahara et al. 2016), perhaps a derived condition (Regier et al. 2013). Diversification of the non-ditrysian lepidoptera may have begun in the Jurassic and that of the ditrysians in the Cretaceous (Labandeira et al. 1997; Wahlberg et al. 2013), or both may be dated to the Cretaceous (Grimaldi 1999; Grimaldi & Engel 2005); stem-group ages of Lepidoptera range from under 200 to about 300 m.y.a. (Wiens et al. 2015). Wahlberg et al. (2013) estimated the start of diversification of the speciose Macroheterocera group of moths, with ca 100,000 spp., at about 90 m.y. ago. However, Regier et al (2015) thought that the very different estimates for both lepidopteran and angiosperm diversification made it difficult to think about mutual connections in their evolution.

Papilionoidea (butterflies) are well embedded in monotrysian lepidoptera. Heikkilä et al. (2011) suggested that the main clades (= families) diverged quickly in the early Cretaceous, Wahlberg et al. (2009) thought that these clades were largely of late Cretaceous origin while Wahlberg et al (2013) dated initial butterfly diversification to ca 104 m.y.a. - eudicots radiated ca 100 m.y.a. and lepidoptera in general shortly after ca 90 m.y. ago. Estimates for diversification within clades representing extant subfamilies are after (e.g. Vane-Wright 2004; Wahlberg et al. 2009; Wheat et al. 2007; Heikkilä et al. 2011), more or less at (Simonsen et al. 2011), or before (Michel et al. 2008; Pohl et al. 2009: 113-84 m.y.a., gene duplications) the K/P boundary. Overall, much diversification of of Nymphalidae-Nymphalinae and -Papilioninae seems to have occurred 65-33 m.y.a. (Wahlberg 2006; Zakharov et al. 2004). Although diversification of Pieridae may have begun in the Late Cretaceous (112-)95(-82) m.y.a. (Braby et al. 2006), much speciation was Caenozoic (Simonsen et al. 2011 gives a range of divergence times). Caterpillars of these groups tend to show rather high food-plant specificity. Some butterfly (and other herbivore) clades that survived the K/P boundary may initially have eaten several different food plants, but subsequently they diversified on a more restricted set of plants and/or they shifted their food preferences (Janz et al. 2006; Nylin & Wahlberg 2008; Fordyce 2010; Nylin et al. 2014; but c.f. in part Hamm & Fordyce 2015).

Mammals have a substantial fossil history before the Cretaceous, but their crown-group age is around 77.8-76.5 m.y.a. (e.g. dos Reis et al. 2014), with notable diversification in the early Caenozoic (Bininda-Emonds et al. 2007; see also Stadler 2011a). A radiation of multituberculate mammals, now extinct, began ca 85 m.y.a., and their adoption of a more herbivorous diet may be associated with the increasing prominence of angiosperms (G. P. Wilson et al. 2012). Primates initially may have been arboreal omnivores eating plants and their associated insects, but even in the Palaeocene ca 46% were probably frugivores, the figure rising to ca 76% in the Holocene, and the relationship between plants and primates has been described as a "facultative mutualism" (Goméz & Verdú 2012: p. ; Sussman et al. 2013). Although radiation of extant primates began in the late Cretaceous, e.g. 71-63 m.y.a. (Springer et al. 2012), (89.8-)84.8, 78.8(-64.9) m.y.a. (Fabre et al. 2008) or (98.6-)87.2(-75.9) m.y.a. (Perelman et al. 2011), nearly all diversification is Caenozoic. "Old" estimates date the split within Anthropoidea/Simiiformes to the later Eocene ca 43.4 m.y.a., crown-group ages of Old World and New World monkeys (Catarrhini and Platyrrhini) being around 31.6 and 24.8 m.y. respectively (Perelman et al. 2011). Recent discoveries in Amazonian Peru of fossil teeth ca 36 m.y.o. similar to those of African anthropoids (Bond et al. 2015) are consistent with these dates and with the movement of (ancestral) platyrrhines from the Old to the New World.

In bats, both frugivory and nectarivory have arisen in parallel, even within New World bats, and some combination of insectivory with these modes of nutrition is common (Datzmann et al. 2010; Rojas et al. 2011). Crown-group diversification of phyllostomid bats occurred between 43.1 and 33.4 m.y.a., probably in the late Eocene, with diversification of the more specialized fruit-, pollen- and nectar-eating bats dated to around 26-16 m.y.a. in the late Oligocene to mid-Miocene (Datzmann et al. 2010; Rojas et al. 2011; Jones et al. 2005; Teeling et al. 2005). However, only a few species of species of bats are involved, for instance, there are only ca 15 species of nectar-eating Old World pteropodids (Datzmann et al. 2010: see also below).

Birds. Jetz et al. (2012) and Jarvis et al. (2014: skeleton tree) provide phylogenies for all birds; crown-group Aves are estimated to be 115.9-94.8 m.y.a. (Lee et al. 2014). Radiation of important seed-dispersing birds such as Columbiformes (pigeons) began some (63.6-)54.4(-46.1) m.y.a. (95% CI) in the earlier Caenozoic (e.g. Tiffney 1986b; Pereira et al. 2007; Jarvis et al. 2014), but extant Columbidae are a mere 33.4 m.y.o. (Jetz et al. 2012). Bird pollination is also likely to be a Caenozoic phenomenon, three groups, Trochilidae (hummingbirds), Nectariniidae (sunbirds, etc.) and Meliphagidae (honey eaters) being most important (Cronk & Ojeda 2008). Hummingbirds diversified only in the Pliocene (Bleiweiss 1998a; McGuire et al. 2007, 2014), while that part of Passeriformes, the songbirds, that includes sunbirds and honey eaters did not begin to diversify until the Oligocene around 30 m.y.a. (Jarvis et al. 2014; c.f. estimate in Friis et al. 2011, up to 65 m.y.a.) or a little later (Moyle et al. 2016: Pardalotus diverged ca 21 m.y.a.). Meliphagids are basal oscines, and Selvatti et al. (2015) suggest that they diverged from the rest ca 35 m.y.a. and speciated/radiated ca 27 m.y.a. (Early and Late Oligocene respectively), but in Moyle et al. (2016) the crown-group age of the group is a mere 11 m.y. or so. Passerida initially diversified 26-20 m.y.a., Zosteropidae and [Dicaeidae + Nectarinidae] - Promerops is in this part of the tree - are all less than (31-)27.6, 27.1(-23.1) m.y.o. (Selvatti et al. 2015).

Parrots (Psittaciformes) are important in both pollination and seed dispersal, but very differing ages have been suggested for their diversification. Wright et al. (2008) offer two sets of ages based on different geological calibrations. Older crown-group ages are around 82 m.y.a., with the split of the cockatoos from the rest of the family bar the few Strigopoidea being dated to around (82.9-)74.1, 70(-63.9) m.y.a.; corresponding younger ages are around 50 m.y.a. and (51.4-)45, 42.7(-38) m.y.a. respectively (Wright et al. 2008). The younger ages are consistent with the fossil record and the age for Psittaciformes suggested by Jarvis et al. (2014); no fossil parrots from the European Palaeogene, with rather rich records, can be placed in extant crown groups (e.g. Mayr 2002, 2009). Loriinae, mostly Australasian, are nectarivorous, nectarivory having evolved at least three times, the major nectarivorous clade, ca 53 species of lories and lorikeets, evolved within (15-)13 m.y.a. (Schweizer et al. 2014, 2015).

6F. Discussion. We can now return to the issue of when l.t.r.f. as we know it appeared. The question is, to what extent can "the ecological opportunities provided by humid megathermal forests" (Feldberg et al. 2014: p. 1) that seem to have driven the diversification of epiphytic ferns, etc., in the Late Cretaceous translate to the existence of l.t.r.f., particularly a l.t.r.f. that might look something like that of today and have a broadly similar complexity of ecological relationships? What were the interactions between animals and plants in tropical forests in the Late Cretaceous and Palaeogene?; what birds, butterflies and bees were involved in pollination?; what vertebrates were involved in fruit dispersal?; how important were ants? Is there any direct connection between the diversification of animal groups that are now more or less directly dependent on plants for food, etc., and of the plants themselves? Dating plant and animal clades is critical here, so the problems with dating that have frequently been mentioned should be borne in mind.

Tropical rainforest today is characterized by having an annual rainfall of at least 1800 mm, at most three months having less than 100 mm per month. There is little variation in the temperature, the mean for the coldest month being around 18o, and there is (almost) no frost (e.g. Morley 2000; Burnham & Johnson 2004). Much woody angiosperm diversification has been in such forests, and epiphytes and lianes (but see below) are common there. Estimates of the numbers of tree species (10< cm d.b.h.) in tropical forests in general, i.e., including drier forests, are between around 40,000 and 53,000, both the Neotropics and the area from India to the Pacific having at least 19,000-25,000 species and Africa having fewer than a quarter of these figures (Slik et al. 2015); for the Amazon alone, there are an estimated 16,000 species (ter Steege et al. 2016), even if at the more local level diversities in all three areas can be similar (Ricklefs & He 2016). The most diverse families of woody plants in neotropical rainforests are Fabaceae (by far), Moraceae, Annonaceae, Euphorbiaceae, Malvaceae, Lauraceae, Sapotaceae and Myristicaceae (Burnham & Johnson 2004: from Fig. 2).

Saxifragales, although now with very few species in the tropics, may represent an ancient and rapid radiation (Fishbein et al. 2001; Fishbein & Soltis 2004; Jian et al. 2008). Molecular estimates of the diversification times of the major rosid clades are around (114-)108-91(-85) m.y.a., and of Fabidae and Malvidae very soon after, (113-)107-83(-76) m.y.a. (H. Wang et al. 2009). The origins of several clades within Malpighiales and Ericales, major components of today's lowland tropical rainforest, are to be pegged to the Mid Cretaceous or slightly later. Initial malpighialean diversification was rapid, and relationships have been hard to disentangle (Wurdack & Davis 2009; but c.f. Xi et al. 2012), many clades originated some time in the late Aptian/Albian, (119.4-)113.8(-110.7)/(105.9-)101.6(-101.1) m.y. ago (Davis et al. 2005a: high and low estimates). Palms diversified 102-98 m.y.a. (see Eiserhardt et al. 2017 - spread rather greater), 124-101 m.y. are some ages for crown-group Menispermaceae (Jacques et al. 2011; W. Wang et al. 2016b), and all this might suggest a mid-Cretaceous origin of l.t.r.f. (Eiserhardt et al. 2017 and references).

Areas of current very high diversity like western Amazonia are wet, less seasonal, relatively nutrient-rich, and have a mosaic of ecological conditions - and they are young, less than 20 m.y.o., indeed, many neotropical clades characteristic of l.t.r.f. are relatively young (Burnham & Johnson 2004; Hoorn et al. 2010).

Epiphytes. See Zotz (2016) for an overview. Around 24,750 species of flowering plants, 9% of the total, are epiphytes (Feild et al. 2009a; Boyce et al. 2009, 2010; Boyce & Lee 2010; Boyce & Leslie 2012; Zotz 2013), and they are common in l.t.r.f. (see Wagner et al. 2015 for possible host specificity). About 70% are Orchidaceae-Epidendroideae (Ramírez et al. 2007; Gustafsson et al. 2010; Conran et al. 2009), around 7% are Bromeliaceae (Givnish et al. 2008a, 2014a), and add Gesneriaceae (570-700 spp.) and Ericaceae (630 spp.), which together account for ca 5%, and the great majority of epiphytes are included. Bromeliaceae are all New World and Oligocene or even Pliocene in age, while highly speciose and commonly epiphytic-CAM crown Epidendroideae include around 19,560 species (figures from Pridgeon et al. 2005, 2009, 2014), i.e., about two thirds of the species in the whole family, and they may have diverged from each other only 37.9-30.8 m.y.a (Givnish et al. 2015: c.f. stem age of subfamily!). Epiphytic angiosperms commonly have small seeds (of the families just mentioned, Ericaceae have the largest), and in Orchidaceae, at least, association with a fungus is needed for germination (Eriksson & Kainulainen 2011). Ca 3,000 species of ferns are another major element of the epiphytic flora (Schuettpelz & Pryer 2009). Knowledge of epiphytic assemblages is rather rudimentary (Mendieta-Leiva & Zotz 2015).

CAM-type photosynthesis is particularly prevalent in clades that either grow in arid terrestrial environments or are epiphytes; succulent epiphytes are also quite often CAM-type plants. All told, some 17,000-18,000 or more species especially in Crassulaceae, Bromeliaceae, Cactaceae and Orchidaceae-Epidendroideae, all succulents of one sort or another, have CAM or its variants (Winter & Smith 1996b; Sayed 2001). The origin of CAM clades is largely contemporaneous with that of C4 clades, being Miocene and younger, indeed, there seems to have been a "global surge" of succulent CAM plant diversification within the last 10 m.y. like that of C4 grasslands (Edwards & Ogburn 2012: p. 726), and overall, diversification rates are moderate to high in CAM clades (Ferrer et al. 2014).

Succulence of some form, whether of root, stem or leaf, occurs in some 690 genera and 12,500 species (Nyffeler & Eggli 2010b; see also von Willert et al. 1990; Eggli & Nyffeler 2009). Succulents include species which either avoid drought, although they are rarely found in the driest conditions, or are salt tolerant - usually mutually exclusive strategies (Ogburn & Edwards 2010). They include many C4 chenopods, Cactaceae, Crassulaceae, and epiphytes, perhaps particularly orchids. The venation density of the leaves of succulent plants tends to be low (Sack & Scoffoni 2013).

Lianes. For general information, see Schenck (1892), Putz and Mooney (1991), Rowe et al. (2004) and Schnitzer et al. (2015) and references. Lianes may make up some 25% (10-44%) of both stem density and species richness of woody plants in tropical forests, and they are especially prominent in drier and disturbed forests (e.g. Schnitzer 2005, 2015a), althouth climbing Arecaceae, mostly Calamoideae, are a feature of wetter forests (Couvreur et al. 2015) and Menispermaceae are also quite diverse in l.t.r.f. (R. Ortiz-Gentry pers. comm.). Stem-twining vines seem to be commoner in old forests, tendril vines in younger forests (Schnitzer & Bongers 2002). Lianes have an important effect on carbon cycling (increased), storage (reduced) and sequestration (reduced) since liane stems are thinner and the vessels are closely packed, overall wood density being low, and the carbon the plant sequesters goes mainly into leaves, and these break down fast (van der Heijden et al. 2013, 2015; Durán & Sánchez-Azofeifa 2015; Isnard & Feild 2015; Ewers et al. 2015). Indeed, although the biomass of lianes is often less than 10%, liane leaves may be up to 40% of the total (Putz 1984; Isnard & Feild 2015). Lianes have a rather negative effect on l.t.r.f. diversity and regeneration, an effect exacerbated in the New World by their increasing abundance, perhaps caused by climate change (Schnitzer et al. 2005; Schnitzer 2015b), although this increasing abundance is less evident in perhumid forests (J. R. Smith et al. 2017). Indeed, they can be thought of almost as ruderals doing best in early successional vegetation with their thin, metabolically inexpensive stems and throw-away leaves (Ewers et al. 2015: lianes as structural parasites!). There may be some connection between lianes and the phylogeny of the plants on which they grow, probably mediated by such things as bark characteristics - but in communities like savannas these features have evolved in parallel (Zulqarnian et al. 2016: lianes and mature forests?). Liane stems are poorly insulated and water in the very wide and long vessels can freeze easily, with disastrous consequences for the plant - some of the reasons that liane abundance decreases rapidly north of the Tropic of Capricorn, ca 23.5o N (Schnitzer 2005: exceptions interesting).

Burnham (2009, 2015) noted that there were few lianes through most of the Mesozoic, a number in the Palaeozoic, and they reappeared in some numbers in the fossil record only with the evolution of angiosperms. Today, ca 8,700 species of scandent plants of one sort or another are recorded from the New World alone (Gentry 1991), for a total of perhaps double that number, and they are found in 162 families of seed plants (Scnitzer et al. 2015 and references). Liane woods from the Cretaceous-Palaeogene were dominated by Menispermaceae (S. Y. Smith et al. 2013a); crown-group Menispermaceae are around 109.1-106.3 m.y.o. and there was a burst of diversification close to the K/P boundary (W. Wang et al. 2012). About 400 species of Menispermaceae are vines/lianes, and they are common in l.t.r.f. today. There are around 800 species of lianes in Vitaceae-Vitoideae, the crown-group age of which is ca 91 m.y. (Smith et al. 2013; Wen et al. 2013). There are ca 400 species of Bignoniaceae, especially Bignonieae and largely New World (ca 50 m.y.o.), ca 535 species of Arecaceae, most in Calamoideae-Calaminae and largely Old World (maximum age 48-40 m.y., or early Eocene to Miocene), Sapindaceae-Sapindoideae-Paullinieae (?age), ca 360 species of Celastraceae (?Cretaceous, especially Africa: Bacon et al 2015), and perhaps 400 species of Malpighiaceae lianes (Gentry 1991), largely New World (Late Cretaceous, ca 75 m.y.a., or younger). Ther are ca 190 species of vines/lianes in the [Smilacaceae [Philesiaceae + Rhipogonaceae]] clade, and diversification in Smilax, which includes most of these plants, began at the end-Eocene ca 40 m.y.a. (Qi et al. 2012), while most of the 640 species of Arecaceae-Calamoideae are lianes, ?age. In South East Asia-Malesia around 600 species of Old World Piper are climbers/lianes (age very uncertain, see Piperaceae), there are ca 500 species of Annonaceae in the palaeotropics that are lianes (Couvreur et al. 2015), and Combretaceae and Loganiaceae are also important components of the liane community there (Addo-Fordjour & Rahmad 2015; also other papers in Parthasaranthy 2015). Although nearly all the 975 species of Cucurbitaceae are lianes or vines, many are relatively small and herbaceous and grow outside l.t.r.f..

Clades of parasitic and mycoheterotrophic angiosperms that currently live in l.t.r.f. or similar conditions are of particular interest. Stem-group Rafflesiaceae, now largely restricted to l.t.r.f., are estimated to have diverged from other Malpighiales some 95 m.y.a., divergence within the family beginning (95.9-)81.7(-69.5) m.y.a. (Bendiksby et al. 2010). Naumann et al. (2013, q.v. for discussion) estimate the stem age of Rafflesiaceae to be ca 65.3 m.y.a., around the K/C boundary, although stem-group Tetrastigma, on which Rafflesiaceae are parasitic, is estimated to be only some (68-)57, 51(-36.4) m.y.o. (P. Chen et al. 2011b; Lu et al. 2013). Fruits of crown-group Vitaceae are known from the Deccan Traps at around or a little before the K/C boundary ca 66 m.y.a. (Manchester et al. 2013). Mycoheterotrophic clades of Dioscoreales often grow in similar habitats, and some are estimated to have diverged (118-)109-79(-68) m.y.a., the mycoheterotrophic habit being established well before the beginning of the Palaeocene (Merckx et al. 2008a, 2010). The stem age of the mycoheterotrophic Gentianaceae-Voyrieae is only (65.2-)54.0, 46.8(-40.1) m.y.a. (Merckx et al. 2013). Eupolypod ferns diversified in the late Cretaceous-early Caenozoic (H.-M. Liu et al. 2014 and references).

Characteristic of today's l.t.r.f. is the diversity of various animal and fungal groups directly dependent on plants that live there, whether mycorrhizae, endophytes, herbivores, gallers, frugivores, decomposers, or pollinators. The connection between animal and plant can also be indirect: animals eat other animals. That the diversification of orb-weaving spiders, insect-eating bats (vespertilionids), stinging wasps, etc., was more or less contemporaneous with that of angiosperms is because they were eating insects, many of which were eating plants (see also Hawkins & Porter 2003; Penney 2004; J. S. Wilson et al. 2012b). Thus Pompilidae, wasps which now almost exclusively eat spiders, radiated in the Caenozoic after a Mid/Late Cretaceous origin (J. S. Wilson et al. 2012b), the speciose carnivorous Carabidae-Harpalinae (19,800 or so spp.) showing a similar pattern after originating in the Cretaceous Aptian/Albian ca 115 m.y.a. and then doing nothing for ca 32 m.y. (Ober & Heider 2010). However, the spider-eating archaeid spiders began diversifying around 200 m.y.a. (Wood et al. 2015). Diversification of parasitic and hyperparasitic wasps such as the hyperdiverse Chalcoidea (the chalcid wasps), perhaps half a million species strong and most ultimately dependent on flowering plants, is most intensive in the Caenozoic (Heraty et al. 2013). Similarly, the fleshy fruit "niche" was exploited by particular groups of flies, particularly by Drosophilinae, and the relationships between particular fruits and flies may be very close (Ashburner 1998: alcohol dehydrogenase in flies; Harry et al. 1996, 1998: fig-breeding Lissocephala). The sugar-rich fruits of angiosperms may have provided a habitat for budding yeasts such as Saccharomyces cerevisiae; they have a genome duplication ca 100 m.y.a. that is perhaps connected with their ability to exploit this habitat (Wolfe & Shields 1997; Conant & Wolfe 2007), much evolution here is Caenozoic (Guzmán et al. 2013).

In the following paragraphs emphasis is placed on direct relationships. Plant-feeding insects make up at least one quarter of all described species, and maybe 26% coleoptera (Weins et al. 2015; c.f. Janz et al. 2006; Farrell 1998; Hunt et al. 2007), although quite how many insect species, or arthropods in general, there are is unclear. Ølgaard (2000; see also Basset et al. 2012: insects; Hamilton et al. 2013; Stork et al. 2015) estimated that there were around (2.75-)4.9-6.7(-10) million species, down from the some 30,000,000 estimate based on the early tree-fogging experiments.

There are well over 100,000 species of extant phytophagous beetles in some five clades, particularly the chrysomelids and curculionids, that eat angiosperms. Initial diversification has been dated to the Jurassic, with over 100 extant beetle lineages diverging before the beginning of the Cretaceous 140 m.y.a. (Farrell 1998, but c.f. dates; see also Mayhew 2007; Hunt et al. 2007). Beetle and angiosperm evolution seem not to be tightly linked, and there is no strong association between diversification and the adoption of herbivory, or shifts from gymnosperms to angiosperms as a food source (Hunt et al. 2007; Weins et al. 2015; see also Rainford & Mayhew 2015). Diversification may have begun first on monocots and then moved on to broad-leaved angiosperms (Reid 2000). About two thirds of herbivorous beetles eat only one or a few species of angiosperms, i.e. they are are mono- or oligophagous. Herbivorous beetles and herbivory in particular and insects in general increased with the warming trend of late Palaeocene-Eocene (Farrell 1998; Wilf & Labandeira 1999; Labandeira et al. 2002b; Wilf et al. 2001; Lopez-Vaamonde et al. 2006, Wilf 2008).

There is no simple connection between the diversification of plants and the insects associated with them. Kergoat et al. (2005b) suggested that diversification of bruchids and Fabaceae may have occurred more or less together. More commonly, bouts of insect diversification have occurred (well) after the appropriate angiosperm host clades originated, particularly in herbivores (implicit in Futuyma 1983; see Funk et al. 1995; Berenbaum 2001; Percy et al. 2004; Lopez-Vaamonde et al. 2006; leaf-mining Gracillaridae; Winkler & Mitter 2008; McKenna et al. 2009; Janz 2011; Cruaud et al. 2012b; Kergoat et al. 2015). Diversification and overall diversity of phytophagous insect groups may at least initially increase after they adopt new hosts (Janz et al. 2006; c.f. Hamm & Fordyce 2015). However, close co-evolution/co-speciation seems to be the exception rather than the rule, and is most evident in shallow rather than deep clades (Berenbaum & Passoa 1999 for references; c.f. Farrell & Mitter 1998); a looser relationship may be more common (see Futuyma & Mitter 1996). Patterns of relationships between plants and herbivores/gallers may differ from those shown by plants and pollinators. The complexity of such relationships is evident in fig wasps, which are both gallers and pollinators. Crown group Ficus may be (101.9-)74.9(-60.0) m.y.o. and the age of the fig wasps, at (94.9-)75.1(-56.2) m.y., is similar (Cruaud et al. 2012b), although at most of the nodes within these groups the wasps seemed to be older than the figs. Around 30% of figs are pollinated by more than one species of wasp, and Yang et al. (2015) found that in dioecious species, where the figure may be over 40%, the wasps were sister species, and there co-speciation was possible, however, in monoecious figs around two thirds of the co-pollinators were not sister species. Plant clades with "specialized" pollinators may diversify despite there being little evolution of their major pollinators, which are themselves not notably diverse in the first place (see below).

Seed-dispersing animals and the plants they dispersed may have diversified roughly in parallel (see below: e.g. Tiffney 1984, 2004; Wing & Tiffney 1987; Collinson & Hooker 1991; Dilcher 2000; c.f. Herrera 1989; Eriksson et al. 2000a; Moles et al. 2005a). Vertebrates are important in the dispersal of the rather large propagules of rain forest trees and the pollination of the rather widely dispersed individuals that produce them (e.g. Regal 1977); mammals in particular are also herbivorous. However, the acquisition of fleshy fruits is not linked to notable increases in diversification of clades with them (Bolmgren & Eriksson 2010 and literature; c.f. Eriksson & Bremer 1991).

Returning to the issue of the evolution of angiosperm-dominated l.t.r.f.. The "museum" hypothesis is that current centers of diversity are centers of origin, they are areas in which there has been little disturbance over the last 50-100 m.y. (Stebbins 1974). Initial rapid diversification may have slowed down ("ancient cradle"), or diversification rates may have increased towards the present, the "recent cradle" theory (see Couvreur et al. 2011c for references), recent diversification patterns in palms fitting this model (see Eiserhardt et al. 2017 for a critque of such ideas).

There have of course been forests in the tropics earlier, as in Carboniferous era, but they were species poor with around 120 species in areas up to 105 km2 and they lacked epiphytes, even if lianes were quite common, and so are not comparable with modern l.t.r.f. (Cleal et al. 2012). Morley (2007) plots the extent of tropical forests through the Caenozoic. A biota from the late Early Eocene from Messel, Germany and about 48 m.y.o. is notable both for its floristic richness and the diversity of interactions betweren plants and what appear to be quite specialized herbivores (Dunne et al. 2014).

Some clades now prominent in l.t.r.f. seem to have been little affected by events at the K/P boundary. Thus the diversification rate for palms was more or less constant for some 65 m.y. after their origin ca 100 m.y.a. (Couvreur et al. 2011b, esp. c). Annonaceae (Couvreur et al. 2011a; see also Erkens et al. 2012) and Araceae (Nauheimer et al. 2012), and the liverworts Lejeunaceae (R. Wilson et al. 2007) and Cephaloziineae (Felberg et al. 2012) show similar constant rates of diversification. Many major clades in Malpighiales, conspicuous small trees of today's l.t.r.f. are Cretaceous in age (Davis et al. 2005a; see also Xi et al. 2012b). The presence of palms has been used to suggest that there were rainforests in mid-Cretaceous Laurasia ca 100 m.y.a., although this suggestion was tempered by noting that rainforest assembly had probably been a gradual process (Couvreur et al. 2011c).

Palms in particular are almost iconic plants of l.t.r.f. today, so either 1) the rainforest of 100 m.y.a. was rather different from that of today (see e.g. Feild et al. 2011b), 2) l.t.r.f. of "modern" aspect remained very restricted in extent for millions of years, 3) not all palms have the same ecological preferences, and/or 4) there are methodological problems with the analyses (see e.g. Quental & Marshall 2010). Coiffard and Gomez (2009) thought that early palms may have been swamp plants like living basal Arecales (their examples were Calamus, Nypa, and Mauritia). Most palm fossils in the New World may have been from plants growing in seasonally dry tropical forest (Burnham & Johnson 2004). Indeed, no palm fossils with a single wide metaxylem element in their stem vascular bundles are known from the Cretaceous, and there is an association between extant palms with two-vessel bundles and climates with a dry period; Arecoideae have a single vessel, and they are conspicuously l.t.r.f. plants (Thomas & Boura 2015). Couvreur et al. (2011c) did acknowledge the absence of Cretaceous fossils that might support their hypothesis, and they also noted that Coryphoideae, more frequent outside l.t.r.f., were Palaeocene in age.

Late Cretaceous megathermal forests from western Africa ca 72 m.y.a. are supposed to provide "[t]he first evidence for typical closed, multi-stratal forest synusiae" (Morley 2007: p. 6). However, little recent work has been carried out on this flora, from which relatively very large seeds/fruits/infructescences 1.2 (Annonaceae seeds)-10 cm long have been reported (Chesters 1955; Monteillet & Lappartient 1981). Large seeds imply large plants (Morley 2007), but there I have seen no estimates either of the size of the plants in these forests or of the venation densities of their leaves.

There is no evidence for l.t.r.f. in the New World Cretaceous (Burnham & Johnson 2004, but c.f. Eiserhardt et al. 2017 and above for references suggesting a mid-Cretaceous origin). There may have been l.t.r.f.-like forests in the increasingly moist climates that were developing then (see also Schönenberger 2005; Wing et al. 2012; etc.), but wind-dispered fruits, conspicuous in the canopy, etc., of current neotropical rainforests, were at best very uncommon (Herrera et al. 2014b). However, the Castle Rock flora, from the early Palaeocene in Montana and ca 64.5 m.y.o., is described as "an excellent example of early modern tropical rainforest in North America" (Burnham & Johnson 2004: p. 1607), and l.t.r.f. may have first developed around the K-Pg boundary (Eiserhardt et al. 2017 for references). A later Palaeocene flora from Colombia ca 59 m.y.o. had a familial composition similar to that of current neotropical rainforest, including Arecaceae, Araceae, Fabaceae, Malvaceae, Menispermaceae, Lauraceae and Zingiberales, even if overall both plant (especially beta diversity) and herbivore diversity were rather low (Jaramillo et al. 2006; Graham 2010: general vegetational history of Latin America). The venation density in this flora (Wing et al. 2009; see also Burnham & Johnson 2004) is very high, and the flora is the first fossil evidence of functional equatorial neotropical megathermal rainforest, i.e. l.t.r.f. (Feild et al. 2011b; see also Jud & Wing 2013; Ricklefs & Renner 2012). Dunne et al. (2014) found a "modern" trophic structure in rich forests in Europe in the early Eocene, but exactly when this first appeared and what it implies in the context of animal-plant relationships is unclear. The first fossil record of l.t.r.f. from Asia is from the early Eocene 52-50 m.y.a. of western India (Rust et al. 2010).

During the early Caenozoic, angiosperms with their often dense veinlet reticulum, high leaf specific conductivity, and small and dense stomata (Watkins et al. 2010; Feild et al. 2011b; de Boer et al. 2012; Feild & Brodribb 2013), in turn associated with high rates of photosynthesis and transpiration, may have facilitated the rise to dominance and spread of widespread closed tropical forest with reliably high rainfall (e.g. Boyce et al. 2008, 2009, 2010; Boyce & Lee 2010). Features like vessel length and perforation plate morphology (scalariform perforation plates became less common) that would enhance xylem conductivity, and wood parenchyma, but not ray morphology, changed across the K/P boundary (Wheeler & Baas 1991). Such changes may be connected with changes in venation density and water conductance needs, but their interpretation is not easy (see Wheeler & Baas 1993; Philippe et al. 2008; Lens et al. 2016: c.f. Sambucus and Adoxa). Fast decomposition of angiosperm litter, particularly associated with the deciduous habit (Knoll & James 1987), may also have speeded nutrient cycling and plant growth (Cornwell et al. 2008; Berendse & Scheffer 2009; Liu et al. 2014). Jaramillo et al. (2010 and references) note that high levels of CO2 and high levels of soil moisture improve the plant performance performance when temperatures are high, as earlier in the Caenozoic, perhaps also another element in angiosperm success.

Meredith et al. (2011: p. 523; c.f. O'Leary et al. 2013) observed that the KTR of 125-80 m.y.a., during which angiosperms increased from 0 to 80% of the vegetation in many parts of the world, was "a key event in the diversification of mammals and birds".

If "modern" or even "early modern" l.t.r.f. is dated to around the middle part or end of the Palaeocene some 60 m.y.a., one expectation might be that the distinctive fauna that currently inhabits it is of similar age. However, in the ca 58 m.y.o. Cerrejón flora, although insect damage was quite high, most was from external feeding, and specialized feeding types like leaf mines and galls were uncommon (Wing et al. 2009). As we will see, our understanding of ecological inter-relationships in early l.t.r.f. is not very good.

In extant l.t.r.f. there are numbers of closely related species (= species in the same genus!) growing together (c.f. Turner et al. 2013), although both the extent of this phenomenon and its history are problematic. Tropical forests as we think of them, the "modern archetypal tropical rain forest" of Burnham and Johnson (2004: Table 1), with multi-layered vegetation, lianes, epiphytic ferns, bromeliads (New World) and orchids, the trees in particular having large seeds and relatively large leaf blades with dense venation and entire margins, seem to be a Caenozoic (Palaeocene-Eocene) phenomenon (Upchurch & Wolf 1987; Wing 1987; Eriksson et al. 2000a; Morley 2000: general account; Pennington et al. 2006a; Schuettpelz 2006; Boyce et al. 2009: Schuettpelz & Pryer 2009; Crane & Carvell 2007: the early Caenozoic fossil record; Burnham 2009: climbers; Watkins et al. 2010).

Many angiosperm groups have diversified since the Cretaceous, including speciose clades that often include many herbaceous and/or shrubby taxa (e.g. Tiffney 1985a, b????). Magallón et al. (1999) noted that major core eudicot clades like Fabaceae (19,000+ species: e.g. Bruneau et al. 2008b; Bello et al. 2009) and (most of) Lamiales that together represent about 45% of core eudicot diversity appear only in the upper Cretaceous (Maastrichtian) and Caenozoic. Woody Fabaceae are prominent in today's tropical forests, while core Lamiales with monosymmetric flowers include many species pollinated by recently-evolved generalist pollinators like birds and corbiculate bees. Diversification of Asteraceae (23,000+ species: K.-J. Kim et al. 2005; Funk et al. 2009c for a summary) is also Caenozoic in age (but c.f. Barreda et al. 2015)

Diversity in families that one might thinks of as being characteristic of l.t.r.f. may be quite recent. Indeed, the modern Malesian flora has been dated to ca 23.6 m.y.a. (Morley 2007), while. Diversification in a largely epiphytic clade in Orchidaceae-Epidendroideae that contains about 19,560 species, about 2/3 of the whole family, can be dated to within the last 37.9-30.8 m.y. (Givnish et al. 2015). Even in old clades like Myristicaceae, crown-group diversification is also Caenozoic at some 21-15 m.y.a., and there is little molecular divergence between its extant members - all very recent indeed given possible stem ages for the family that are over 100 m.y., and this extensive stem-group history is largely unknown (Sauquet et al. 2003; J. A. Doyle et al. 2004, 2008; Scharaschkin & Doyle 2005; Richardson et al. 2004; but c.f. in part Couvreur et al. 2011a, c). Meliaceae are characteristic of extant l.t.r.f., but their ancestors may have been a deciduous tree of seasonal or montane habitats. Crown-group ages of the speciose rainforest clades in the family are a mere 23 m.y.o. (Late Oligocene/Early Miocene), although their stem-group ages are Eocene, and Keunen et al. (2015) suggested that they may have been quite species-rich in pre-Late Oligocene times, but extinction then obscured this early history. Indeed, a number of plants (and animals) have very different past and current distributions are (e.g. Nypa (Arecaceae, Roridula (Roridulaceae), Leea (Vitaceae and Phytocreneae (Icacinaceae), so the composition of past l.t.r.f. will be hard to predict.

Next few paragraphs little more than notes.... Around 53.5% of vascular plants are herbs, which would suggest that there are about 132,000 species of herbaceous flowering plants (Fitzjohn et al. 2014; Ølgaard 2000: 65,000 species). The high assimilation rates facilitated by high venation density may, in conjunction with a shortening of the life cycle, particularly the pre-fertilization gametophytic stage (e.g. Williams 2012b), etc., have enabled the evolution of annual herbs (Boyce & Leslie 2012). This life form is exceedingly uncommon in other vascular plants, fossil or extant (Boyce & Zwieniecki 2012). Maybe species of flowering plants are annuals.

Palaeocene-Eocene diversification in l.t.r.f. took place in a warmer world with flatter temperature gradients and with the continents in rather different places than they are now. Boreotropical migration and extensive long distance dispersal were important in shaping the assembly of tropical forest biomes (Thomas et al. 2015 and references; Moncrieff et al. 2016 for biomes and history). The role of India in "seeding" tropical diversity is unclear. Early Eocene dipterocarp resins have been found there, and its biota seems not to indicate particular isolation (Rust 2010). The beginning of diversification in Piper subgenus Ottonia, a small clade with species in both the Amazonian and Atlantic rainforests in South America, may be dated to some 55.3 m.y. (Molina-Henao et al. 2016), around about the PETM, although fossil records of the genus from Maastrichtian Colombia that could be associated with the Schilleria clade of Piper led to estimates of the stem age of New World Piper of (106-)93, 69(-67) m.y.a. or even more (Martínez et al. 2012, esp. 2014: note topology). Much diversification in the genus is Oligocene in age or younger, thus the Macrostachys and Radula clades, which between then include around half the 1,300 neotropical species of the genus, began diversifying around ca 45 m.y.a. (Martínez et al. 2014).

Diversity may have been high in the early Eocene l.t.r.f., although what plant-animal associations looked like then is unclear. Major diversification of birds, both pollinators (hummingbirds, various passeriform groups) and fruit-dispersers (pigeons and parrots) (e.g. et al. ; Jarvis et al. 2014; see also Wright et al. 2008 for parrots), the successfull generalist orchid and bumble bees, fruit-eating mammals, especially fruit-eating and nectarivorous bats and fruit eating primates, and arboreal and seed-eating and -dispersing ants (Ward 2014), was in or after the Oligocene ca 40 m.y. ago. Conspicuous elements of today's vegetation like grasslands and savanna are still younger, most being less than (15-)10 m.y. old.

Increased plant productivity and diversity would allow animals that ate, pollinated (e.g. birds, butterflies, bees) or dispersed (e.g. ants, mammals, birds) angiosperms to diversify (see also Boyce et al. 2010). Thus Novotny et al. (2010) noted that congeneric rather than confamilial, etc., plant species tended to share herbivore species, so as phylogenetic diversity increased, so would the number of herbivore species.

Exactly how, when and where plants and animals that now show "mutualistic" relationships evolve? Our understanding of the ecological-evolutionary connections between animals, in particular insects, and plants remains unclear (e.g. Futuyma 1983; Janz 2011); there is no simple underlying theory to explain the variety of the interactions. We know little about the details of long-term evolutionary-ecological interactions of plants and the organisms associated with them (see above; Fine et al. 2004 for habitat specialization and herbivore activity in the Amazon, also Janzen 1974a). Furthermore, the sheer complexity of the matrix of defensive compounds inside the plant make simple explanations of the evolutionary dynamics of plant-insect herbivore relationships in particular difficult. As Berenbaum and Zangerl (2008: p. 806) note, idiosyncracy is central to the nature of chemical co-evolution, a problem that can only be exacerbated by the difficulty of understanding ecological relationships over time. Yet, as Grimaldi and Engel (2005: p. 625) note, "Despite the fact that the mechanism is obscure as to how insects diversified with angiosperms, the overall patterns are extremely clear that the angiosperm radiations had a profound impact on insects, and vice versa."

Paragraph out of place. Light conditions in the closed forest habitat pose a particular challenge. The PHYA/C gene pair duplicated before the origin of crown-group angiosperms, and PHYA in particular may have been very important in angiosperm evolution. It is intimately involved in germination and in etiolation responses of the seedling, especially in shady conditions such as occur on the forest floor, for example, preventing seedling etiolation in response the the far-red light that dominates the spectrum there. Furthermore, it is involved in the germination response of hydrated seeds to very brief pulses of light (the very low fluence response - VLFR) such as sunflecks (Mathews et al. 2003). Unfortunately, how ANA grade angiosperms might respond to manipulation of their PHY genes is unknown.

7. Flowers and Pollination.

Here I discuss some aspects of angiosperm reproduction in a rather conventional way, i.e. as flowers (and fruits) being a key to understanding angiosperm diversification. I outline the general distribution of particular flower "types" with respect to phylogeny. For details about the interaction of particular groups of pollinators with the flowers they pollinate, see below.

7A. Flowers, Pollination and Fertilization. Most narratives of angiosperm evolution focus on flowers and fruits and their influence on speciation. The flower is considered a key innovation - or, better, a group of innovations - and the success of angiosperms has been attributed in considerable part to the evolution of flowers (e.g. Eriksson & Bremer 1992; Dilcher 2000). Burger (1981) saw insect pollination as a key to the diversification of angiosperms, insects being able to find isolated plants in small populations, and angiosperms were able to subdivide the environment effectively. Frame (2003) emphasized flexibility in construction of the flowers, the speed of the reproductive cycle, the closure of the carpels, and the fact that flowers are edible hence attracting potential pollinators as contributing to the sucess of angiosperms. Monosymmetry in particular has often been thought of as a key innovation (e.g. Donoghue et al. 1998; Neal et al. 1998; Endress 2001; Sargent 2004: see below). Pollination, especially by insects, but also bats and other mammals as well as birds, and seed dispersal, especially by mammals and birds, may interact; both can increase outcrossing, gene flow in general, and, given a heterogeneous environment, speciation (e.g. Kay et al. 2006; Kay & Sargent 2009).

Adoption of syncarpy was an important evolutionary event (e.g. Armbruster et al. 2002 and references; Friis et al. 2006b). It has evolved seventeen or more times, and a compitum, which allows pollen tubes from one stigma to fertilize the ovules in more than one carpel, evolved in three quarters of these cases (Armbruster et al. 2002). A style/compitum allows competition among male gametophytes (Mulcahy 1979; Endress 1980a); note that there are some cases, perhaps most notably in Loranthaceae, of competition between female gametophytes where the embryo sac becomes as tubular and invasive as a pollen tube (Bachelier & Firedman 2011 and references. Several/many pollen grains can potentially fertilize a single ovule, and since over 60% of the genes expressed in the sporophyte are also expressed in the gametophyte (Erbar 2003 and references), gametophytic competition can directly benefit the sporophyte. Syncarpy also allows "excess" pollen landing on one stigms to "switch" and pollinate ovules normally served by pollen landing on another stigma (Carr & Carr 1961; Armbruster et al. 2002 and references), incidentally reducing pollen tube competition. Interestingly, many basal angiosperms, often apocarpous, have rather few ovules per carpel, but the advantages of syncarpy are more evident if there are numerous ovules per carpel, however, sometimes several pollen grains per stigma are needed for the pollen to germinate in the first case, and the amount of pollen arriving is marginal for ensuring full seed set, a compitum of some sort might be generally useful (Armbruster et al. 2002). Indeed, many members of the ANA grade, for example, do have an extragynoecial compitum (e.g. Williams et al. 1993; Lyew et al. 2007; Williams 2009 - see also Igersheim & Endress 1997; Endress & Igersheim 2000; X.-F. Wang et al. 2011). An extragynoecial compitum allows pollen tubes to switch between carpels, as do compita developed at the tip of otherwise free styles, as in Apocynaceae-Aaclepiadoideae, some Malvaceae, etc. (Armbruster et al. 2002). For pollen tube competition, whatever the morphology of the gynoecium, see also Erbar (2003).

Closed carpels also protect the ovules and often become much elaborated as the seeds mature, so promoting dispersal (Armbruster et al. 2002 and references). The evolution of the carpel may have allowed the parental sporophyte to control both fertilization and the allocation of resources to the seed (Lord & Westoby 2012 and references).

A number of changes in the gametophytic phase of the angiosperm life cycle result in its usually being notably shorter than that of extant gymnosperms (Williams 2012b). The pollen tube growth rate in angiosperms is much higher than that of extant gymnosperms. Figures are 80-600 µm/hour in ANA-grade angiosperms, overall in angiosperms 60-20,000 µm/hour, with Fagaceae at the low end of the spectrum, versus 10-20 µm/hour in gymnosperms with Gnetum at the high end of the spectrum) (Hoekstra 1983; esp. Williams 2008, 2009; also Rudall & Bateman 2008). Fertilization occurs within about 24 hours of pollination in most angiosperms as compared to seven days or often far more in extant gymnosperms (Williams 2008). The development of callose plugs and callose in the pollen tube wall probably enables the pollen tube to grow faster, and callose tubes and fast growth rates are both apomorphies of angiosperms. Although callose synthase genes are expressed in at least some gymnosperm pollen, the pollen tube there is cellulose-based (Williams 2008; Parre & Geitmann 2005: mechanical properties of callose; Abercrombie et al. 2011). Harder et al. (2016 and references) discuss different patterns of pollen tube growth in the style, basically, the population ecology of male gametophytes. There is much recent research in this whole area (see Williams & Mazer 2016 and refences).

In gymnosperms, there is much growth of the female gametophyte after pollination but before fertilization, and although growth is less in Gnetum, even there the ovule increases appreciably in size after pollination. The ovule grows little after fertilization since reserves for the developing ovule have been sequestered in the large female gametophyte that has developed. On the other hand, there is usually little or no increase of angiosperm ovule size after pollination and before fertilization. The female gametophyte, the embryo sac, is tiny. After fertilization, however, resources are channelled to the developing embryo, a transfer mediated by the evolutionarily novel endosperm tissue; it is as if growth of the ovule had resumed (Haig & Westoby 1991; Leslie & Boyce 2012; Sakai 2013; Little et al. 2014). Since few reserves are committed to angiosperm ovules with their tiny mature female gametophytes, little is lost when unfertilized ovules abort, but the converse is true in gymnosperms, however, as partial insurance, ovule growth in gymnosperms may depend on pollination, at least (Little et al. 2014).

Endosperm, tissue involved in the nutrition of the embryo and with both maternal and paternal genomes (usually with a diploid maternal and a haploid paternal contribution) is unique to angiosperms, although little is really known about its origin (e.g. Baroux et al. 2002; Friedman & Williams 2004; Nowack et al. 2007). (Comparable tissue in gymnosperms is the massive female gametophyte.) Why there should be variation in embryo sac development in the ANA grade and sporadically elsewhere, too, that affects the balance of maternal and paternal genes in the endosperm is unclear. However, a higher ratio of paternal genes in diploid compared to triploid endosperm (1:1, vs 1:2) may lead to more "selfish" behaviour of individual endosperm tissues as they scavenge nutrients at the expense of other ovules in the carpel (e.g. Friedman et al. 2008; esp. Friedman & Ryerson 2009) - hence perhaps the rather low ovule number (per carpel) of many ANA-grade angiosperms which have this higher ratio.

Overall, angiosperms tend to become mature at a younger age than do gymnosperms (Bond 1989; Verdú 2002). Within gymnosperms, some Gnetales (Ephedra) are mature by about 7 years, however, 20-100 years are ages for most other gymnosperms (B. Wang et al. 2015). The whole angiosperm life cycle is speeded up (Stebbins 1965, 1981), the evolution of carpels faciltating maximum seed production, seed dispersal, and seedling survival (Stebbins 1981). Seedlings of angiosperms grow faster than those of gymnosperms; again, time to maturity is reduced (e.g. Bond 1989; Coiffard et al. 2006).

7B. Major Clades With Monosymmetric Flowers. Successful animal pollination entails that the pollinator follows a more or less complex and specific set of cues. Early angiosperms seem to have had what would be described as rather generalized flowers (but see below). A number of large clades (2,000+ species each) can be characterised by specialized floral features that seem likely to affect diversification/speciation. Five of these major clades, Orchidaceae, Zingiberales, core Lamiales (= [Gesneriaceae + The Rest]), Fabaceae, and Asteraceae, have a preponderance of members with monosymmetric flowers, although reversion to polysymmetric flowers has also occurred within these clades, most notably in Fabaceae-the mimosoid clade. Together with some rather smaller clades, e.g. Campanulaceae-Lobelioideae, Caprifoliaceae, some Iridaceae, they include almost one third of all angiosperms. Diversification in many clades with monosymmetric flowers seems to be greater than that in their sister taxa with polysymmetric flowers (Sargent 2004; Kay & Sargent 2009; c.f. Kay et al. 2006), perhaps because pollinator fidelity is increased. However, using current ideas of sister taxa, putting examples into a broader phylogenetic context (as in Sanderson & Wojciechowski 1996), and correcting for whit might be called "apomorphy lag", the fact that evolution of the apomorphy of interest does not necessarily happen when the clade of interest diverges from its sister tax (Käfer & Mousset 2014), such stories may well become more ambiguous.

Diversification as a possible result of the acquisition of monosymmetry can be studied at much finer evolutionary scales. For instance, Stebbins (1974) suggested that monosymmetry had evolved more than 25 times within angiosperm families, Westerkamp and Claßen-Bockhoff (2007) noted that it was found in 38 families. In fact, monosymmetry has evolved hundreds of times (see also Endress & Matthews 2006a; Endress 2008a; Jabbour et al. 2009, esp. 2014), the precise number of origins depending critically on definitions and implicit or explicit optimizations (e.g. Endress 1997a; Donoghue et al. 1998; Reeves et al. 2003; Cubas 2004; Jabbour et al. 2008); for the evolution of monosymmetry in asterids, see also Ree and Donoghue (1999) and Donoghue and Ree (2000). Reyes et al. (2015, 2016) recently found almost 200 changes in symmetry in angiosperms, of which up to perhaps 1/3 were reversals from mono- to polysymmetry. In most monocots and eudicots examined CYC or CYC-like genes are involved, and in eudicots are expressed mostly on the adaxial side of the flower although sometimes affecting the abaxial stamen pair (e.g. Song et al. 2009; Hileman & Cubas 2009; Hileman 2014b: Fig. 4, c.f. definition of monosymmetry and Fig 2a). Extreme parallelism, and even within Asteraceae different CYC2c paralogs are involved in the expression of the monosymmetric (ray flower) phenotype (Chapman et al. 2012). However, in the magnoliid Aristolochia CYC genes seem not to be so involved in the development of monosymmetry (Horn et al. 2014), while recent work suggests that CYC genes are also little involved in the development of monosymmetry in flowers of Orchidaceae (Madrigal et al. 2017).

Indeed, as more becomes known about floral morphology and development, a clear definition of monosymmetry becomes elusive. From a structural point of view, many flowers are monosymmetric at some stage of their development (Endress 2008a, also 1999, 2001b; see also Characters). Monosymmetric pentapetalous angiosperm flowers may have very different developmental patterns (Bukhari et al. 2016). Melastomataceae are particularly difficult to categorise, some species having strongly monosymmetric flowers, others polysymmetric, while yet others are intermediate, and this is why they are not included in the list below. Many highly reduced flowers are monosymmetric, not only in Poaceae (see below), but also in the speciose Piperaceae, etc.. On the other hand, many inflorescences are functionally the equivalent of polysymmetric or haplomorphic flowers, the often strongly monosymmetric peripheral flowers in Asteraceae, some Brassicaceae (e.g. Busch & Zachgo 2007), Adoxaceae and Apiaceae being the visual equivalent of petals to the pollinator. Inflorescences may also be the functional equivalent of a single monosymmetric flower, as in Euphorbia-Pedilanthus, etc., while the polysymmetric flowers of Iridaceae may be the functional equivalent of a small inflorescence with three monosymmetric flowers (e.g. Guo 2015b).

Taxa. There are five particularly large clades with monosymmetric flowers. 1. Orchidaceae (ca 27,800 species) are ground-dwelling or epiphytic and in part mycoheterotrophic herbs of small, sometimes moderate size producing as many as millions of tiny, wind-dispersed seeds per flower. The tepals are more or less free and the flowers, with their distinctive and complex morphologies centred on the gynostemium (stamens and stigma-style all congenitally fused) and labellum (median tepal of the inner whorl), are often inverted. Deceit pollination is particularly common, but pollinators also visit for fragrance and nectar rewards. Although the diversity of floral form in Orchidaceae is great, it is attained by variation on a rather limited basic theme. 2. Zingiberales (2,100 species) are quite large tropical plants, mostly herbs, and they have large flowers. Their fruits usually have only a moderate number to few seeds that are animal dispersed. Flowers in Zingiberales vary considerably in orientation, the parts that are petal-like, number of stamens, etc., and from this point of view are more variable than Orchidaceae. (Note that many taxa in Commelinales, sister to Zingiberales, also have monosymmetric flowers, and it is possible the the common ancestor of the two had such flowers.) 3. Core Lamiales (23,360 species) are often more or less herbaceous plants perhaps particularly abundant oustide the tropics, although there are many tropical members; woodiness is more common in the tropics. Individual flowers are moderate to quite large in size, each usually producing quite many and small seeds; in general seed dispersal is by wind. Although clades like Lamiaceae have only four seeds per flower, they are still quite small. Most of the monosymmetric 4. Fabaceae (19,400 species, of which 3,300 are in the polysymmetric mimosoid clade) have inverted keel flowers with more or less free petals (Stirton 1981). The plants are either trees, especially common in neotropical forests, or herbs. Dispersal is either autochorous (ballistic) or animal-mediated; there are rather few seeds per fruit. Many Fabaceae are nitrogen-fixers, and the family is noted for the diversity of secondary metabolites that it produces, sometimes in association with endophytic fungi.

Of the smaller but still quite large clades in which monosymmetry predominates, 5. Campanulaceae-Lobelioideae include some 1,200 species of laticiferous herbs or shrubs often with slit-monosymmetric flowers that have plunger-type secondary pollen presentation devices. Their dehiscent fruits have many small seeds, although some clades have fleshy fruits. 6. Caprifoliaceae comprise some 850 species; the flowers are often rather weakly monosymmetric and the often indehiscent and dry fruits have at most few seeds. 7. Some 200 species of Lecythidaceae-Lecythidoideae (Ericales) are trees that are a prominent element of neotropical forests. The polystaminate androecium alone is monosymmetric, the fruit is large, and the seeds are few and large. In 8. Iridaceae, monosymmetry of the flowers of the speciose Gladiolus is obvious. However, from the point of view of the pollinator the flowers of Iris, Moraea, etc., are also monosymmetric; a single Iris flower consists of three strongly monosymmetric meranthia or part-flowers (see also Westerkamp & Claßen-Bockhoff 2007). All told some 750+ species have monosymmetric flowers of one sort or another. The fruits have a moderate number of seeds. 9. Polygalaceae, with some 1,050 species most of which have monosymmetric flowers, are phylogenetically close to Fabaceae, although the exact relationships of the two are unclear (see Bello et al. 2009). They have keel flowers that are superficially like those of many Fabaceae, although different parts are involved (Westerkamp & Weber 1999). The fruits usually have few seeds and myrmecochory is common (Lengyel et al. 2009).

The final clade to be mentioned is 10. Asteraceae (25,100 species). These are mostly herbaceous to shrubby plants with small flowers that are aggregated into heads or capitulae; in most species at least some flowers of each capitulum are monosymmetric and secondary pollen presentation is widespread. Only a single usually quite small seed per flower is produced, and dispersal is often by wind. Each capitulum is functionally a single polysymmetric or haplomorphic flower which produces quite numerous seeds. Asteraceae are also noteworthy for the diversity of secondary metabolites they contain. Exactly where acquisition of monosymmetry is to be fixed within Asterales is unclear.

What pollinates such flowers? Different pollinator groups have different floral preferences. Adult Lepidoptera prefer to visit radiate flowers, that is, polysymmetric flowers with a definite number signal (e.g. three-merous, five-merous flowers) and that have enclosed nectar (e.g. Leppik 1957). In Apidae, honey bees (Apini) frequent polysymmetric (radiate) flowers with relatively accessible nectar (for a review of the cues used, see Horridge 2009). Halictidae-Rophitinae (sister to all other halictids) prefer to visit flowers of asterids, especially those of the lamiids, although other plants are also visited (Patiny et al. 2008). Oligolectic (specialist) bees prefer to visit shallow often polysymmetric flowers with easily accessible rewards (Wcislo & Cane 1996); Asteraceae are the group above that are most notably commonly pollinated by such bees. However, derived polylectic (generalist) euglossine and bumble bees (see below) in particular pollinate flowers with complex, monosymmetric corollas that the insect has to learn to work before visits are effective (see Westerkamp & Claßen-Bockhoff 2007 for monosymmetry and bee pollination). Thus oligolectic basal megachilids pollinate polysymmetric and sometimes rather large flowers, while the more polylectic derived megachilids commonly pollinate monosymmetric Fabaceae and Lamiaceae (Westerkamp 1997), or polysymmetric Boraginaceae (Litman et al. 2011; Sedivy et al. 2013). Bumble bees in particular appear to have an innate preference for monosymmetric flowers (Leppik 1957; Kalisz et al. 2006; Rodríguez et al. 2004). Birds also visit monosymmetric flowers, and in the gullet-type ornithophilous syndrome stamens in the mouth of the flowers often deposit pollen on the head of the pollinator.

CYC genes have independently been coopted in the development of monosymmetric flowers (Reyes et al. 2016 and references). However, in Zingiberales and Commelinales CYC-like genes are expressed abaxially in the flower, while in eudicots CYC genes are expressed adaxially (Reyes et al. 2016); this difference may be connected with the inverse orientations of the flowers in the two groups. Similarly, obliquely symmetrical flowers may appear inverted from the pollinator's perspective. In angiosperms in general with 3- or 5-merous flowers the perianth part that is differentiated is the odd member (and sometimes also adjacent members) of the inner whorl (see e.g. Reyes et al. 2016: fig. 4), Pelargonium being a notable exception. (A survey of the position of the differentiated petal in the context of floral orientation would be interesting....)

The evolution of monosymmetric flowers may be connected with the evolution of dichogamy (separation of the time of pollen dispersal and stigma receptivity), in particular, to that of protandry (Kalisz et al. 2006; for dichogamy, see e.g. Bertin & Newman 1993; Routley et al. 2004), but dichogamy is advantageous in other situations, too. Monosymmetry, self sterility, apomixis, dioecy and other features tend to be common in families with high diversification rates (Ferrer et al. 2014). Monosymmetry, presence of a corolla, and reduced number of stamens relative to perianth parts have been found to act together, clades with them having a doubled diversification rate compared with those that lack this combination perhaps because more precise interactions with pollinators improve the efficacy of pre-zygotic isolation barriers (O'Meara et al. 2016). In asterids and monocots in particular, mnosymmetry and polyandry tend to be mutually exclusive (Jabbour et al. 2008; Reyes et al. 2016, q.v. for possible molecular reasons for this), certain Neotropical Lecythidaceae-Lecythidoideae (Ericales) being spectacular exceptions.

Most of the plant clades just mentioned are more or less herbaceous, and Qian and Zhang (2014) discuss the evolution of life forms (woody vs herbaceous) in angiosperms. Among extant seed plants, only angiosperms are herbaceous, and the annual habit, known from ca spp of angiosperms, does not occur in other vascular plants. There are long-standing suggestions of a correlation between the rate of molecular evolution and plant habit: Molecular evolution is faster in herbs/annuals (e.g. M. A. Wilson et al. 1990; Bousquet et al. 1992: esp. chloroplast genes; Gaut et al. 1992: chloroplast rbcL gene, grasses evolve notably faster even than other monocot herbs, 1996, 2011: summary; Barraclough et al. 1996: rbcL gene; Andreasen & Baldwin 2001; Soria-Hernanz et al. 2008: ITS, correlation not very strong; Rydin et al. 2009b; Barker et al. 2009; Korall et al. 2010 [ferns]; Müller & Albach 2010; Yue et al. 2010; Frajman & Schönswetter 2011; Comer et al. 2015; c.f. Whittle & Johnson 2003: comparisons of branch lengths of species pairs, ?sampling). Both chloroplast and nuclear genes show an increased rate of molecular evolution (Gaut et al. 2011), but not all genes are equally affected (Yue et al. 2010). The rate of molecular evolution in woody angiosperms is faster than that of those conifers studied, although the rate of non-synonymous substitutions is lower (Buschiazzo et al. 2012). In a series of extensive analyses of both monocots and eudicots, S. A. Smith and Donoghue (2008) confirmed that there are usually substantial increases in the rate of molecular evolution in herbs as compared to trees, shrubs, or simply to plants with long life cycles. Thus within the commelinids, Arecaceae, Bromeliaceae and Rapateaceae, all three with long life cycles, have a low rate of molecular evolution, and it could be argued that none is very diverse, although this will depend on how comparisons are made/the methods that are used. Y. Yang et al. (2015: focus on Caryophyllales) also demonstrated this correlation, taking into account synonymous and non-synonymous sites in protein-coding genes, and found substitution rates in herbaceous lineages that were up to three times those in their woody relatives (see also Luo et al. 2015: rosids, also within Rosaceae).

This correlation may be connected with mutation rate or population size and thence to speciation rate (Gaut et al. 2011 and references). There is a general correlation between rates of molecular evolution (substitution rates) and the rate of speciation, more molecular evolution occurring in speciose clades (Webster et al. 2003, see also Barraclough & Savolainen 2001), although this is not always the case (Müller & Albach 2010: Veronica). Herbs also show an increased rate of climatic niche evolution (S. A. Smith & Beaulieu 2009). The evolution of herbs from trees has been linked with higher speciation rates of the former (Dodd et al. 1999), although Verdú (2002) suggested that is was not so much the herbaceous habit per se that was important, but the associated condition, length of time to maturity (see also Baker et al. 2014: Amazonian trees). However, in the fire-prone Mediterranean ecosystem neither diversification nor molecular evolution differs between seeders and resprouters, two ways in which plants survive fires there, yet seeders, having shorter generation times, should perhaps have diversified more (Verdú et al. 2007). In the related "rate of mitosis" hypothesis, mitosis rates in the apical meristem may slow when the plant is near its maximum height, hence leading to reduced plastid substitution rates in trees (Lanfear et al. 2013; Barrett et al. 2015; Bromham et al. 2015a). Overall, trees may have a distinctive evolutionary rhythm, speciating rather slowly; any one species may have quite large numbers of individuals, and although they may be rather dispersed they are long-lived, the species themselves also being rather long-lived (Petit & Hampe 2006).

To summarize: Much angiosperm diversity, but not necessarily biomass production or net primary productivity (see below), is concentrated in groups that are annuals or herbaceous or shrubby perennials and that have animal-pollinated flowers (see also Ferrer et al. 2014); disseminules are small, rarely fleshy (Eriksson & Bremer 1991, 1992), any animal dispersal often being by hooks and the like. Several of the large groups with monosymmetric flowers mentioned above (core Lamiales, Asteraceae) are largely made up of such plants. Overall diversification rates/species numbers are high in these clades, particularly in Asterales and Lamiales (Magallón & Sanderson 2001; Magallón & Castillo 2009; O'Meara et al. 2016), although these rates are best associated with particular clades within those orders (see the euasterids). Indeed, O'Meara et al. (2016) noted that species with monosymmetric flowers are perhaps less abundant than expected, in part because the apparently unitary "monosymmetric flower" had to be assembled from separate and independent characters like a monosymmetric corolla and reduced stamen number relative to perianth parts, and it took some time, over 60 m.y., for these features to come together.

7C. Major Clades With Wind-Pollinated Flowers. Monoecy and dioecy (the latter ca 16,160 species, ca 6% of angiosperms: Käfer et al. 2014), are associated with features such as woodiness, biotically-dispersed fruits, and the wind pollination syndrome, and dioecy has evolved hundreds of times (Renner 2014; Käfer et al. 2017). Clades in which wind pollination predominates are usually not notably speciose, the adoption of abiotic pollination often being associated with a decrease in speciation rate (e.g. Dodd et al. 1999). A clear exception is 1. Poaceae, 10,050 or so species of frequently monoecious largely herbaceous wind-pollinated plants with single-seeded fruits (in most species the flowers can be categorized as being reduced-monosymmetric). 2. Cyperaceae-Juncaceae, also with more or less reduced flowers and often with single-seeded fruits, contain about 4,800 species. 3. Fagales include about 1055 species, nearly all monoecious trees with much reduced flowers, the staminate flowers being borne in catkins and the pistillate flowers usually having an inferior ovary; here the the single-seeded fruits are often quite large.

In wind-pollinated angiosperms there has been selection for small flowers, monoecy is common, and the pollen is smallish, smooth and often some kind of porate. Most wind-pollinated taxa have few to a single ovule per flower, and this is coupled with a high pollen:ovule ratio, pollen loads on a single female flower being quite high; the fruits very often have a single seed (e.g. Linder 1998; Culley et al. 2002; esp. Friedman & Barrett 2008, 2009 [useful table], 2011). In Fagales and Leucadendron (Proteaceae) monospermous fruits seem to have evolved before wind pollination (D. W. Taylor et al. 2012; Welsford et al. 2016). Fagales are sister to Cucurbitales, many of which are also monoecious, and 1-seeded fruits may be a synapomorphy for the combined clade. Interestingly, many fossil Normapolles plants (Fagales) have perfect flowers and may even have had nectaries (Friis et al. 2011; see also fossil Platanaceae).

Kay and Sargent (2009) noted that Poaceae and Cyperaceae/Juncaceae were exceptions to the rule that it is animal pollination that leads to an increase in speciation rate, the two clades being about seven times more diverse than their animal-pollinated sister clades. Clades immediately below Poaceae are small, and their flowers are small, but are probably pollinated by insects; the relationships of the [Thurniaceae [Cyperaceae + Juncaceae]] clade are unclear. The mostly animal-pollinated Cucurbitales, probably sister to the overwhelmingly wind-pollinated Fagales, have ca 2,300 species, i.e. about twice as many species as in Fagales.

For the evolution of dioecy, see Renner and Ricklefs (1995), Vamosi et al. (2003), Dufay et al. (2014), Renner (2014), and Käfer et al. (2014, 2017) and references. Correlates of dioecy are tropical or island distributions, woody habit, abiotic pollination, inconspicuous flowers and inflorescences, and fleshy fruits, and these may also be correlated among themselves, e.g. tropical, woody, fleshy fruits, although dioecy seems not to evolve notably frequently within clades with these features (Vamosi et al. 2003). Clades in which dioecy predominates are not notably speciose, and it has been suggested that dioecious clades diversify less (Heilbuth 2000; Vamosi & Otto 2002; Kay et al. 2006, etc.; c.f. in part Leslie et al. 2013). However, subsequent analyses suggest that the evolution of dioecy seems to have little effect on diversification (Sabath et al. 2015), or, correcting for the lag in evolution of diocy in dioecious clades, an increase, dioecy being ssociated with families with moderate to high diversification rates, but reversals to monoecy are also common (Ferrer et al. 2014; Käfer & Mousset 2014; Käfer et al. 2014). Dioecy tends to be associated with pollination by wind (Sabath et al. 2016) and if insect pollinated, the floral displays tend to be dimorphic, those of the staminate plants being showier and so more visited, and it has been suggested that for this and other reasons extinction is thus perhaps quite likely (Käfer et al. 2017 for literature). Dioecy may be relatively uncommon because reversions back to monoecy may be quite frequent. After all, in dioceous taxa the ability to express genes allowing the development of both carpels and stamens must persist in both staminate and carpellate individuals, so Dollo's Law is not really applicable (see Käfer et al. 2017; Goldberg et al. 2017).

7D. A Cautionary Note. But what is really known about the relationships of such features just mentioned to the diversification of the clades that have them? Using Poaceae as an example, we can see how complex and difficult a question like "Are Poaceae diverse, and why?" can be. A series of points:

1. The first three clades of Poaceae that are successively sister to the remainder contain some 26 species out of the 11,000+, and these three "basal" clades are forest plants (e.g. Givnish et al. 2010b).

2. Ca 1,300 species of bamboos are woody and have a distinctive, synchronized monocarpic flowering habit; they are arguably ecologically distinct from the rest of the family.

3. Poaceae-Poöideae (ca 3,850 spp) are noted for their association with fungal endophytes, an association that could be ca 40 m.y. old (Schardl et al. 2004). The presence of these endophytes affects the palatability of foliage to herbivorous mammals and of seeds to granivorous birds, and animals eating the infected material may not thrive. The level of aphid infestation and that of their parasites and parasitoids, and even the pattern and rate of decomposition of dead grass, are also affected (e.g. Madej & Clay 1991 - birds; Omacini et al. 2001 - aphids; Lemmons et al. 2005 - decomposition). A variety of alkaloids, including loliine (pyrrolizidine) and ergot alkaloids, are produced by the fungi; the distinctive loliine alkaloid is primarily active against insects (Schardl et al. 2007). Various aspects of root growth may also be affected (Sasan & Bidochka 2012). Poöideae are largely a cold-tolerant group. Poöideae are also noteworthy in that they are by far the largest temperate clade in a largely tropical family.

4. Grasses are well known for the diversity of silica bodies in their leaves, and these play a role in protection against herbivory (but probably not against that by mammalian grazers), while silicon concentration itself is correlated with the rate of breakdown of plant tissues, and so with nutrient cycling (see silica).

5. About 75% of the PACMAD clade, some 4,500 species, have C4 photosynthesis (Sage et al. 1999; Grass Phylogeny Working Group II 2011, and are ecologically very distinctive (see below). C4 plants tend to be less attractive to herbivorous animals because of their lower nitrogen concentration and greater amount of fibrous tissue (Caswell et al. 1973).

8. Clade Asymmetries.

When thinking of overall patterns of seed plant diversity and evolution, there are a number of striking examples of what may be called clade asymmetries involving quite small clades of animals and plants. There are two rather different kinds of these asymmetries. One includes small (in terms of species numbers) clades of animals involved in the pollination and seed dispersal of relatively very large numbers of plants. As we will see, these are quite well known, especially on a fairly local scale, but here the scale is global. The other kind of asymmetry is that in more physiological-ecological relationships, where relatively small clades of plants have major effects on biome functioning globally, particularly through their effects on carbon cycling. Both kinds of asymmetries have major implications for species persistence, the ecological structuring of communities and ecosystems, and the way one thinks about diversity and evolution in general.

8A. Plant-Animal interactions.
8B. Carbon Sequestration.


As Ollerton et al. (2011: p. 321) noted, "if a policy-maker or conservation planner were to ask an ecologist the straightforward question, 'How many species of flowering plants are pollinated by animals?' the answer would be: 'We do not know'.". They did provide an estimate - around 308,000. However, for questions like "How many species of plants are dispersed by [such and such a group of] bats?", and "How many species are pollinated by [such and such a group of] of bees?", it is much more difficult to obtain reliable estimates.

Estimates of the numbers of species of particular groups of bees, birds or bats may be fairly accurate, but the same certainly cannot be said of the numbers of species of plants that they service. Many observations in the literature do not allow one to distinguish between a visit of an animal to a plant that is casual or one that results in pollination, and assigning a pollinator to a plant is not simple. Thus of the seven birds visiting the flowers of Butea monosperma, only one was an effective pollinator - and so was a squirrel (Tandon et al. 2003), and similar examples are common. Furthermore, studies of such plant-animal relationships still often focus on only one of the partners.

Pollinating bees, for instance, were initially categorized as such based largely on observations on plants visited for pollen, but pollination also occurs when bees are nectaring; add variation in time - from season to season, within a season, as well as time of day that the stigma is receptive - and space, and characterizing pollination relationships becomes difficult (e.g. Fishbein & Venable 1996; Waser et al. 1996; Kandori 2002; Ollerton et al. 2003, 2007; Thompson 2009; Crone 2013). Relationships between plant and pollinator are by no means constant (e.g. Aizen et al. 2012; Natalis & Wesselingh 2013), and even if they are constant locally, this may not be true across the range of the species (Newman et al. 2014; van der Niet et al. 2014 and references: pollinator ecotypes). Rosas-Guerrero et al. (2014) suggested that secondary pollinators might often be ancestral pollinators. Some insects visiting flowers may not be effective pollinators at all, but scavenge pollen remaining after pollination had actually occurred, or they are otherwise irrelevant to the pollination process, the plant and insect having some kind of commensal relationship (Linsley 1958; Linsley et al. 1973; Michener 1979; Roubik 1989; Roulston et al. 2000: p. 618).

The estimates below are based on very scanty observations sometimes extrapolated to flowers in the same immediate clade with a similar floral morphology; floral syndromes have been used (see Rosas-Guerrero et al. 2014 for justification), despite the reservations mentioned elsewhere. Interestingly, Grant (1966) noted that flowers with ornithophilous syndromes were particularly prominent in North America where the hummingbirds that pollinated them were migratory, while in tropical America such syndromes were less evident. Perhaps birds in the tropics would have a better chance to learn the local flora while for migrating birds consistent signals in different places would be desirable. Nectarivorous birds may visit a variety of flowers usually pollinated by other animals (e.g. Muruyama et al. 2013), while other animals may on occasion pollinate flowers visited by hummingbirds (e.g. Snow & Snow 1980; Brown & Hopkins 1995; Fleming et al. 2005). Thus only 2-3% of the species in Neotropical cerrado vegetation have ornithophilous flowers, but hummingbirds take nectar from a similar number of species that do not have ornithophilous flowers, and some of these are pollinated (Muruyama et al. 2013); birds may visit flowers with "inappropriate" morphologies if preferred flowers are not present or there is competition for those that are there (Temeles et al. 2002, 2009). The pollination behaviour of birds on islands may differ from that on the mainland, the birds being more promiscuous in the plants they visit, a phenomenon called interaction release (Traveset et al. 2015) - and plants may also become more promiscuous in their pollinators (Serrano-Serrano et al. 2017, see also below). Of course, many birds also eat insects they find on flowers, although this may be irrelevant when thinking about pollination relationships.

All this emphasizes the amount of salt needed when reading the discussion below: Estimates of numbers of pollinators rely in part on floral syndromes, anecdotal evidence, and other suspect data.


Plant groups with species pollinated by euglossines include Araceae, e.g. the very speciose Anthurium and Spathiphyllum, visited for fragrances (e.g. Hentrich et al. 2010b and references), Bignoniaceae, Gesneriaceae-Gesnerioideae (ca 300 spp., both male and female bees involved - Wiehler 1978), Lecythidaceae-Lecythidoideae, 900 to perhaps 2,000 species of Orchidaceae-Epidendroideae (for the latter figure, most Stanhopeinae, Zygopetalinae and Cataseteinae may be visited by male bees for fragrances: Williams 1982; numbers from Pridgeon et al. 2009), and Zingiberales, especially Costaceae (ca 19 species of Costus - Salzman et al. 2015) and Marantaceae, also Apocynaceae, Convolvulaceae, Euphorbiaceae (perhaps 70 species in Dalechampia alone - Armbruster 1993; Armbruster et al. 2009b), Fabaceae (including the nectarless Swartzia), Solanaceae, and Rubiaceae (see Dressler 1968; Cameron 2004; Roubik & Hanson 2004; Ramírez et al. 2011; Schiestl 2012, and references).

In any one community there may be up to 50 species of bees, and bee populations are often notably stable (Roubik 1989; Roubik & Hanson 2004; Zimmermann et al. 2009). Hentrich recorded ca 23 species of bees visiting ca 48 species of plants in Nouragues, French Guiana, while Ramírez (2009) noted that one species of Euglossa might visit 74 species of plants from 41 families - and that at a single locality.

Overall, these 200 or so species of euglossine bees are likely to be the major pollinators of well over 4,000 species of Neotropical plants (Wiehler 1976; Williams 1982; Ramírez et al. 2002 for a summary of the literature; Ramírez pers. comm.).

Age: Crown group euglossine diversification probably began only 42-27 m.y.a., montane clades diverging only in the last 8-4 m.y. (Ramírez et al. 2010); another estimate of crown-group age is slightly younger, some (35-)28, 26(-17) m.y.a. (Cardinal & Danforth 2011; Martins et al. 2014).

Flowers with a diversity of morphologies in temperate and Arctic-Alpine floras in particular are pollinated by bumble bees. Ericaceae, including Rhododendron and Vaccinium, are common in Arctic communities, but practically all conspicuous flowers in these habitats are visited by the bees (e.g. Heinrich 1979; Ranta & Lunberg 1981; Tomono & Sota 1997; Kudo et al. 2011 and references). I have not found estimates for the number of species pollinated there, but it is likely to be appreciable. Bumble bees are also prominent in alpine environments, where hundreds of species in large genera like Gentiana, Rhododendron (Corlett 2004 and references) and Pedicularis (perhaps 600 species here alone - Macior 1994; Williams et al. 2009; Eaton et al. 2012) largely depend on them for pollination. Lamiaceae and many other Lamiales, Fabaceae, particularly Faboideae, and Impatiens (Balsaminaceae) are also often visited (Williams et al. 2009). Most of the 600-700 species of Ranunculaceae-Delphinieae are bumble bee-pollinated (Jabbour & Renner 2012b), this having long been recognised in Aconitum in particular (see map in Kronfeld 1890). In South America bumble bees pollinate genera like Rubus, Scutellaria, Lathyrus and Lupinus, all of which have diversified substantially there (Asmussen & Liston 1998; Hines 2008).

Bumble bees in Europe and elsewhere visit monosymmetric flowers (Raine & Chittka 2007a, b; Goulson & Darvill 2004), and they can also handle polysymmetric flowers quite easily (Laverty 1994; Sedivy et al. 2013). Many bees are more specialized in pollen collection than in nectar foraging (for nectaring, see Raine et al. 2006); learning to handle pollen flowers is quite difficult even for bumble bees (Strickler 1979; Goulson & Darvill 2004; Benton 2006; Raine & Chittka 2007b; Ghoulson 2010). Bumble bees are effective buzz pollinators (Goulson 2010). Some 4,000 species of angiosperms, mostly core eudicots, are buzz pollinated, and the distinctive floral syndrome has evolved many times (Buchmann & Hurley 1978; Buchmann 1983). ?Other bees doing this.

Local diversity of bumble bees can be quite high, with 4-12 species occuring in one community (Hines 2008 and references), and one fifth or more of the world's bumble bee species (40<) are found in the Sichuan-Chongqing region of China alone (Williams et al. 2009). In the Front Range of the Colorado Rockies ca 18 species of bumble bees pollinate ca 43 species of plants (Macior 1974).

Estimates of the numbers of species of plants pollinated by bumble bees are hard to make, but upwards of 3,000 is a plausible number.

Age: Bumble bees diversified over a similar time frame as did the euglossine bees, i.e. about 47-25 m.y.a., although their stem group (they split from meliponines) age may be considerably more, 100-80 m.y.a. (Hines 2008; see also Martins et al. 2014); Cardinal and Danforth (2011) suggest a somewhat more recent age for crown-group bumble bees of (31-)21(-12) m.y. ago. The Eocene-Oligocene boundary of ca 34 m.y.a. was a time of sharp cooling and increase of seasonality, and bumble bees flourish in cooler climates, being facultatively endothermic (Hines 2008 and references). The bees moved into South America about 8-6 m.y.a., perhaps along with the plant genera of northern origins that they now pollinate (Asmussen & Liston 1998; Hines 2008).

Corbiculate bees are estimated to be (89-)77(-66) m.y.o. (Cardinal & Danforth 2011), the crown-group stingless meliponines are (61-)58, 51(-48) m.y.(Martins et al. 2014).

Age: Stem-group Apis is late Eocene/earliest Oligocene, with the diversification of extant species beginning a mere ca 13.5 m.y.a.; it is unclear if diversification began in Europe (fossils) or Asia (Kotthoff et al. 2013: they favour the first hypothesis). Other estimates for the age of the group are (30-)22(-16) m.y.o. (Cardinal & Danforth 2011; Martins et al. 2014)

The evolution of flowers which have oils as their primary reward may have begun in the Eocene (Renner & Schaefer 2010). However, Cardinal and Danforth (2013) estimated that Centradini and Tetrapedia, which take oil from Malpighiaceae in particular, evolved in the Late Cretaceous (87-52 and 92-66 m.y.a. respectively); see also Neff and Simpson (1981) for the bees. Martins et al. (2014a) found Centradini to be paraphyletic, Epicharis diverging (102-)91(-79) m.y.a. and Centris (95-)84(-72) m.y.a., about contemporaneous with the stem-group age of Malpighiaceae, estimated at (100-)86(-73) m.y.a. (Xi et al. 2012b: other estimates are 75-60(-32) m.y.a. (Wikström et al. 2001; Davis & Anderson 2010; Renner & Schaefer 2010). This is broadly consistent with some kind of co-evolutionary story, Martins et al. (2014) suggesting that the evolution of oil collection in the bee clade was in the common ancestor of Epicharis and other bees (corbiculate bees later lost the ability to collect oils). However, crown ages of Epicharis are (39-)28(-18) m.y.a. and of Centris (58-)44(-36) m.y.a., all (much) younger. The malpig Eoglandulosa warmanensis from the Eocene Claiborne Formation in Tennessee, U.S.A. in deposits of ca 34 m.y.o. has distinctive paired calyx glands and may have been pollinated by such bees (Taylor & Crepet 1987; Friis et al. 2011). However, details of the evolution of the association between the bees and malpigs are not clear.

Some 1600-2,000 species of oil flowers in 11 families are pollinated by females of 365-447 species of bees (Alves-dos Santos et al. 2007; Martins et al. 2013, 2014 for general statistics).

There are four groups of these flies. The Moegistorhynchus longirostris group includes two species of flies that pollinate 21 species of plants in two families and are suspected of pollinating 7 more species (and one more family). In the Moegistorhynchus-Philoliche group 6 species of flies visit 36 species of plants in 3 families, pollinating perhaps another 45 species (and two more families), while in the Prosoeca ganglbaueri group 3 species of flies pollinate 39 species of plants in 7 families, with 39 more species (three more families) as possible nectar sources. The fourth group, which includes just Stenobasipteron wiedemanni, pollinates 19 species in six families; in perhaps 9 species the relationship is obligate, and the fly may pollinate another 12 species (Manning & Goldblatt 1995; Goldblatt & Manning 2000, 2006; Potgieter & Edwards 2005; Johnson 2010; Newman et al. 2014).

All told, some 12 species of flies pollinate around 108 species of plants, although the latter figure is likely to be a considerable underestimate. Iridaceae figure particularly prominently, with about 34 species being pollinated by Nemestrinidae - but 100 more are thought to be pollinated by the flies (Manning & Goldblatt 1996, 1997; Goldblatt & Manning 2000)!

Age: Nemestrinids are an old group known from as far back as the Jurassic, but


In the estimates below of numbers of plants visited I have focussed on larger clades of plants and on literature covering flower visitation in particular regions. As a result taxa like Brachychiton are probably included (in the general figures for bird visitation in Australia), as the 40 or more Old World bird-pollinated species of Erythrina (Bruneau et al. 1997), while species like Holmskiodia sanguinea are probably not (see also Porsch 1936 and references for early literature). I also focus on the major groups of birds that pollinate plants. Not mentioned, for example, is the role played by New World orioles (Icterus), tanagers, and other more or less casual flower visitors (e.g. Stiles 1981) in pollination, although they are responsible for some records of "bird-pollinated flowers"; orioles are particularly important pollinators in drier forests (Stiles 1985). For a recent summary of many aspects of bird pollination, see Fleming and Kress (2013).

Hummingbirds may be trap-liners (especially hermits), generalists, or territorial, while "parasitic" hummingbirds visit flowers normally visited by other species of hummingbirds (e.g. Feinsinger & Colwell 1978). There is considerable diversity in morphology and behaviour of hummingbirds, especially in the most species-rich assemblages (Abrahamczyk & Kessler 2014), and male and female birds can have bills of different lengths, and/or differ in aggressiveness, etc., and so may pollinate quite different plants (e.g. Bleiweiss 1999; Temeles et al. 2009, 2013). Depending on the birds and plants in the community, different foraging types of hummingbirds will visit different species of plants (Feinsinger & Colwell 1978); trap-lining birds tend to visit fewer species of plants than do territorial birds (Snow & Snow 1980). As is well known, hummingbirds often visit herbaceous plants, in part because they do not need perches when feeding (e.g. Stiles 1981; Kress & Beach 1994; Fleming & Muchhala 2008). Self incompatability was commonest is woody plants pollinated by other than trapliners and also less marked in herbaceous taxa generally (Wolowski et al. 2013: Itatiaia).

Trap-lining hermits, which one would think tended to disperse pollen over longer distances (e.g. Wolowski et al. 2013) tend to be commoner at lower altitudes in the Andes, and several clades have independently moved into high-altitude habitats (Bleiwiss 1998b; McGuire et al. 2007 and references). Andean hummingbirds have notably small mean ranges when compared with other birds (McGuire et al. 2014), but there are still usually several species of hummingbirds in any one place, up to 25-30 being recorded from a single local assemblage (Graham et al. 2009, 2012), and the common species at least visit several species of plants. Figures in Fleming et al. (2005) are 3-28 hummingbird species per site, pollinating 14-51 species of flowers, mostly herbs, whether epiphytic or not, to trees. In an Andean rainforest at around 2000 m, 79 species of flowering plants (in twelve families and 29 genera) were visited by 26 species of hummingbirds, of which the eight commonest visited 74% of the plants (Dziedzioch et al. 2003), while in the Monteverde forest, Costa Rica, 23 species of hummingbirds (excluding uncommon and rare species) visited 181 species of plants in 60 genera and 28 families, 8.8% of the total flora (Murray et al. 2000; see also Maglianesi et al. 2014a). Even in southeastern Brazil, home to relatively few hummingbirds, four species of birds visited 23 species of plant belonging to 21 genera and 14 families, individual species of birds visiting between three and eighteen species of plants (Sazima et al. 1996: species visited only once excluded), while Buzato et al. (2000) recorded 86 species of plants (23 families, 44 genera) being visited by 15 species of hummingbirds, although at any one site there were 28-33 species of plants visited by four species of birds. For hummingbird pollination in the Brazilian Cerrado, see Ferreira et al. (2016). Specialization in plant-hummingbird networks is most evident in places where quaternary climate change, as measured by metres moved/year to stay in same climate, is low and species richness high (Dalsgaard et al. 2011), hence it would be expected to be less in areas in North America with few (and migratory) species of hummingbirds.

Some 1,000 or more species of Ericaceae-Vaccinioideae-Vaccinieae and Gesneriaceae-Gesnerioideae (around 600 species of the latter alone - Wiehler 1978; see also Perret et al. 2007; Rodriguez et al. 2010; Clark et al. 2015; Serrano-Serrano et al. 2017), also with their centres of diversity in the Colombian-Ecuadorean region, may be pollinated by hummingbirds (Luteyn 2002; Weber 2010). To these plants can be aded some 250 species of Salvia (Wester & Claßen-Bockhoff 2011), 225 species of Heliconia (Pedersen & Kress 1999), 500-600 species of Acanthaceae (E. A. Tripp & L. McDade, pers. comm.; Tripp & Manos 2006; Tripp & Tsai 2017), about 365 species of the Centropogon alliance of the Campanulaceae-Lobelioideae (L. Lagomarsino, pers. comm. 9.iii.2017; see Stein 1992), around 505 species in 39 largely unrelated genera of Rubiaceae, about 12% of the neotropical species (C. M. Taylor, pers. comm. 22.viii.2015), some 55 species of Erythrina (Bruneau 1997), over 40 species of Penstemon (P. Wilson et al. 2006, 2007; Wessinger et al. 2016), hundreds (perhaps 1,060-1,300) of species of Bromeliaceae, mostly at higher altitudes in the Andes, not in drier habitats or in terrestrial lowland forest habitats (e.g. Benzing et al. 2000a; Kessler & Krömer 2000; Givnish et al. 2014), about 125 species of Loranthaceae, perhaps 100 species of Fuchsia, 138/555 species of Passiflora (K. Porter-Utley & J. MacDougal, pers. comm.), and so on. Just about all sizeable sympetalous families in the hummingbirds' ranges are visited, even including ca 45 species of Asteraceae (Vogel 2016), as are polypetalous Rosaceae, Melastomataceae and Symplocaceae, ca 27 species of Costus (Costaceae: Salzman et al. 2015), etc. (see e.g. Snow & Snow 1980; Sazima et al. 1996 for other examples). Some 390 species of Cactaceae have reddish flowers, and these flowers are notably often tubular (ca 20%), and all told there are perhaps 120 species with flowers that might be considered bird-pollinated (Gorostiague & Ortega-Baes 2015; 187 species is the estimate in Mutke et al. 2015), although pollinator specificity does not seem to be high and I have not included cacti in the total.

The overall imbalance of species numbers of hummingbirds/plants pollinated is probably similar to the euglossine bees just mentioned, with some 338 species of birds pollinating over 5,000 species of plants (over 4,500 species above, but this is a considerable underestimate); Abrahamczyk and Kessler (2014) thought that the number was ca 7,000 species, but they, too, thought that this was an underestimate). In any event, the ratio is almost 12:1 to over 20:1. At regional scales, in North America north of 24oN, where the hummingbirds are migratory for the most part, and in South America south of 27oS about eighteen and six species of hummingbirds visit some 184 and 56 species of plants respectively (Abrahamczyk & Renner 2015; see also Cantrill & Poole 2012 and references for hummingbird pollination in southern Valdivian forests).

There are other, more general, estimates. Thus Muruyama et al. (2013) estimated that perhaps 20% of the species in Amazonian rainforest were pollinated by hummingbirds, compared to 2-3% in Cerrado vegetation; if there are around 60,000 species of flowering plants in Amazonia, that would suggest 12,000 species were pollinated by the birds there alone. Kress and Beach (1994) estimated that hummingbirds pollinated 14.9% of the 276 species of plants they examined at La Selva, Costa Rica, and when extrapolated to Amazonia this would yield a broadly comparable number of 8,940 species. However, these 274 species are not a random sample of the flora; Stiles (1985) estimated a total of 56 bird-pollinated species at La Selva (Kress & Beach 1994 recorded 41 species), only 3.8% of a total flora of 1650 species (Hartshorn & Hammel 1994 estimate 1,280 species of flowering plants). The numbers offered by Stiles (1985) extrapolated to the whole of Amazonia give a figure of 2,625 hummingbird-pollinated species there.

Focussing on the numbers of clades of hummingbird-pollinated plants, we find that at the northern and southern ends of the overall range of hummingbirds - i.e. southern South Ameria, much of North America - the clades are small (Abrahamczyk & Renner 2015), as are hummingbird pollinated clades of Rubiaceae (C. M. Taylor pers. comm. 22.viii.2015), Acanthaceae-Ruellia (Tripp & Tsai 2017) and Gesneriaceae (Perret et al. 2003, 2007; Roalson et al. 2007; Martín Rodriguez et al. 2009; Clark et al. 2015) throughout their ranges (but c.f. Serrano-Serrano et al. 2017). There are numerous origins of bird-pollination in Ruellia (Acanthaceae), and speciation in such clades is increased, but reversals to other pollination modes are also frequent (Tripp & Tsai 2017). However, the long-billed hummingbird Ensifera ensifera visits some 37 species of Andean Passiflora supersection Tacsonia alone as well as a few other species (Abrahamczyk et al. 2014; Abrahamczyk et al. 2017: 62-64 spp ), while some 50 species of a clade in Centropogon are pollinated by the sickle-bill hummingbirds Eutoxeres condaminii (Abrahamczyk et al. 2017). But such stories get more complex. Although there is a large clade of Andean centropogonid lobelioids that is likely to be plesiomorphically pollinated by straight-billed hummingbirds, there may have been (8-)14(-23) origins of the straight-billed pollination syndrome when reversals from the sickle-billed and bat-pollinated floral syndromes are taken into account (Lagomarsino et al. 2017).

There are two, perhaps three, cases where hummingbird-plant interactions may be quite old and where clades of hummingbird-pollinated plants may be quite large. The first case is Gesneriaceae-Gesnerioideae. There Serrano-Serrano et al. (2017) noted that diversification increased (25.5-)18.5(-5) m.y.a. and that hummingbirds may have arrived in South America 25-20.3 m.y.a. at about this time, the birds spurring this diversification. They estimated that there were (41.5-)31.5(-21.5) shifts to hummingbird from insect (bee) pollination, the latter probably being the original condition for Gesnerioideae, and these shifts were often near the base of large clades that subsequently had very high subsequent diversification rates. Overall ca 60% of Gesnerioideae they studied (351/590) were bird-pollinated (Serrano-Serrano et al. 2017; see also Roalson & Roberts 2016), although they discuss additional factors, including the adoption of the epiphytic habit, that may have increased speciation. The second case is the Heliconia-hummingbird relationship. The predominant pollinators of New World Heliconia are hummingbirds and most Heliconia are from the New World; Heliconiaceae make up one of the single most important hummingbird-pollinated clades. Most estimates of the age of crown-group Heliconiaceae are 40-30 m.y.a. - 32-28 m.y. (Kress & Specht 2006), ca 32 m.y. (McKenna & Farrell 2006), or (47-)39(-32) m.y. (Iles et al. 2016). Diversification in New World Heliconia is most evident from a little over 30 m.y.a. onwards (Iles et al. 2016). The third possible case are the bird-pollinated Ericaceae-Vaccinieae where some clades may also be quite large, although little is known about the age or phylogeny of this group. Perhaps hummingbirds that early evolved in association with these groups of plants became the templates, as it were, for a variety of younger plant clades as they adopted bird pollination - the subsequent evolution of hummingbird-pollinated plants was "facilitated by this pre-existing relationship" (Iles et al. 2016: p. [12]).

Age: Hummingbirds and swifts are sister clades and probably diverged by the Eocene 47.4-36.9 m.y. (McGuire et al. 2014; Jarvis et al. 2014; Prum 2015; Mayr 2016). Rather surprisingly, Eurotrochilus, assignable to stem-group hummingbirds, although quite similar to Trochilinae and apparently a nectar-eater, is known from Oligocene Europe in deposits ca 34.3 m.y.o. (Mayr 2004, 2009, 2014; Louchart et al. 2008), there are also fossils from the Late Eocene of the Caucasus (Louchart et al. 2008), and there are stem-group hummingbirds that appear not to have been nectar feeders from deposits ca 48 m.y. old. Crown-group hummingbirds are restricted to the New World. The trap-lining hermits, Phaethornithinae (Bleiwiss 1998a) - topazes may be in the same clade (McGuire et al. 2014) - are sister to other hummingbirds. Hummingbird diversification may have begun in South or Central America, perhaps in lowland South America, as late as the early Miocene (24.7-)22.4(-20.3) m.y., and much speciation occurred about 13-12 m.y.a. along with the uplift of the Andes (Bleiweiss 1998a; McGuire et al. 2007, 2014; Abrahamczyk & Renner 2015; Prum 2015: Archilochus + The Rest). Tripp and McDade (2014a) estimated crown-group diversification to have begun (29.9-)28.8(-28.4) m.y.a. while ca 29 m.y.a. is an estimate from Claramunt and Cracraft (2015: Phae. inc.), although around 67 m.y. is suggested by Fleming and Kress (2013).

Plants visited by Old World nectarivorous birds are mostly woody and the birds usually perch on the twigs when feeding (Stiles 1981; Fleming & Muchhala 2008). Compared with hummingbirds, the relationships between bird and flower sometimes seem rather indiscriminate (see also Stiles 1981). From Africa to Australia, the birds often fly in mixed flocks (c.f. in part Brown & Hopkins 1995), with up to nine species of lories alone occuring together in New Guinea (Schweizer et al. 2015). Any one species of bird may visit many species of plants, and any one species of plant may be visited by many species of birds, although bird-plant relationships are not totally promiscuous (Gill & Wolf 1975; Ford et al. 1979; Rebelo 1987; Brown & Hopkins 1995; Franklin & Noske 2000). Here I discuss bird pollination in Australia, South East Asia-Malesia, New Zealand, New Caledonia, Africa and Hawai'i separately.

In Australia there are about 75 species of honeyeaters (E. T. Miller et al. 2016), a number of lorikeets, and a few species of other bird groups, for a total of about 111 species that pollinate flowers; Stiles (1981) estimated that there were ca 310 nectar feeding birds in the whole Australasian region (this includes New Guinea), a figure that he thought was very definitely an upper estimate. Estimates in Fleming & Muchhala (2008) are 56 genera and 242 species, of which over 60% are honeyeaters, but perhaps only about half the honey eaters, ca 80 species, are nectarivorous, individual species varying considerably in their diet (see also Recher 1981).

Keighery (1980, 1982) recorded about 21 species of birds visiting about 750 species of flowers in Western Australia alone (estimates lower in Brown et al. 1997, but pollinators listed); Western Australia may be the epicentre of bird pollination in that region. In monsoonal northwestern Australia, some 24 species of birds visited 116 species of plants in twenty eight families (Franklin & Noske 2000). Ford et al. (1979) thought that in Australia some 300 species of both Proteaceae and Myrtaceae were pollinated by about 100 species of birds, the brush flowers of Myrtaceae in particular being pollinated by lorikeets, and these may eat pollen (Stiles 1981 and references); all told, Ford et al. (1979) estimated that around there were around 800 species pollinated by birds in Australia. There are about 1,100 Australian species of Proteaceae, and many species, including members of the large genera Grevillea and Banksia, are likely to be pollinated by birds (Maynard 1995), and a moderately conservative estimate is ca 350 species are pollinated by birds (E. T. Miller pers. comm. 12.ii.2017). In addition, there are about 70 species of bird-pollinated Loranthaceae in Australia (Barlow 1984), while Toon et al. (2014) discuss the evolution of bird pollination of ca 37 species of Fabaceae-Mirbelieae/Bossiaeeae some time between over 25 m.y.a. (stem age) to ca 17 m.y.a. (crown age).

A (gu)estimate is that around 120 species of birds pollinate 1,000< species of plants in Australia. (Cheke and Mann [2008] had suggested that honey eaters alone visited about 450 species of flowering plants from 100 families.) The proportion of ornithophilous flowers may be relatively high on that continent since the copious nectar produced incurs little cost to the plant, an advantage given the nutrient-poor soils so common there; there is less ornithophily in the more nutrient-rich east coast forests (Orians & Milewski 2007).

It is more difficult to estimate numbers of bird-pollinated plants outside Australia, although in the general area China and India to New Zealand the same groups of birds are involved. Here I do little more than list groups of plants where bird pollination may be expected to preponderate.

In the area from South East Asia to Malesia, there are a few groups of plants that are likely to be visited by birds. These include Ericaceae like Rhododendron, where 80 species from the island of Papua may be bird-pollinated (Stevens 1976; see also Corlett 2004), Paphia and Dimorphanthera, also largely Papuan and with around 75 species with red, tubular flowers, and the old genus Agapetes, with about 95 species in its centre of diversity in southwest China and adjacent Myanmar and India that also often have red, tubular flowers (see also below). In addition, species of the widespread Aeschynanthus (Gesneriaceae) typically have red, gullet-type flowers; there are about 185 species in the genus. There may be some 36 species of bird-pollinated Loranthaceae in China (Qiu & Gilbert 2003) and 165 in Malesia (Barlow 1998). The ca 14 species of the largely New Guinean Tapeinochilus (Costaceae), are likely to be pollinated by sunbirds (O. Gideon, in Salzman et al. 2015). However, Brown and Hopkins (1995) note the apparently unspecialised morphologies of many of the flowers visited by birds in a site they studied in southeastern Papua New Guinea. They describe "knob" flowers from Schefflera and "fluffy cups" from Syzygium, etc.; apart from one species of Loranthaceae, none of the 17 species of flowers they list as being pollinated by birds are in groups mentioned above, although they do include both Myrtaceae and Proteaceae, important nectar sources in Australia. The 17 species of plants pollinated by three nectarinids in a locality in Sarawak included Malvaceae-Bombacoideae, Musaceae, Sapotaceae, Myrsinaceae and Zingiberaceae (Momose et al. 1998); two spiderhunters pollinated 8 species of Zingiberaceae in four genera (Sakai et al. 1999b) as well as three species of Loranthaceae (Yumoto et al. 1997) at localities in Borneo.

New Zealand has a mere 12 species of pollinating birds, of which only three - two meliphagids and the white eye - were recorded as making almost 90% of the visits to flowers (Kelly et al. 2010). They are probably major visitors to 29 species of plants, and all told 85 species, or perhaps double that number, may be visited by the birds (Kelly et al. 2010; Lee et al. 2013). However, understanding plant/flower interactions in the islands is particularly difficult. One of the common plant visitors, the silver- or white-eye Zosterops lateralis, arrived from Australia in 1832 and 1856 (it is quite often a nectar robber - Anderson et al. 2011), three other species have suffered serious recent declines, and the stitchbird, Notiomystis cincta, is in a monotypic family, Notiomystidae, unrelated to any of the other passeriforms so far mentioned (Kelly et al. 2010). Furthermore, on some smaller islands in particular flowers with morphologies very unlike those of bird pollinated plants on the main islands are visited and apparently effectively pollinated by the two meliphagids and the stitchbird (Castro & Robertson 1997: Kapit Island, pollination release?), although this has also been noticed elsewhere in New Zealand (Kelly et al. 2010). Finally, understanding plant-pollinator relationships is difficult because of the extensive recent human-caused changes to the New Zealand avifauna, and seed set in a number of plants appears to be pollinator limited (Kelly et al. 2010 and references).

In New Caledonia some 17 species of Cunoniaceae, mostly Geissois, have red, brush-type inflorescences that are thought to be bird-pollinated (Hopkins et al. 2014), and there are ca 20 species of Metrosideros, also with brush-type inflorescences, on the island (Wright et al. 2000a; Pillon et al. 2015).

Africa, especially The Cape. There are about 200 species of probably bird-pollinated Loranthaceae in all of Africa (Polhill & Wiens 1998), while there are 64 bird-pollinated species of Iridaceae (Goldblatt & Manning 2006) and 13 species of Cyrtanthus (Amaryllidaceae) in southern Africa alone (Snijman & Meerow 2010). There are probably over 400 species of Aloe, nearly all in Africa, and many in southern Africa are bird pollinated (Rebelo 1997) although bees also pollinate a number (Symes et al. 2009; Hargreaves et al. 2012). Perhaps another 287 species of bird-pollinated plants can be added from southern Africa (Rebelo 1987: 424 species, from which Proteaceae, etc., have been removed). At a more local scale, Rebelo (1987) estimated that perhaps 318 species of plants were pollinated by six species of birds (including the sugarbird, Promerops caffra) in the Cape region alone of South Africa, while Rebelo et al. (1984) suggested that ca 86 species of Proteaceae in the South African Fynbos were pollinated by Promerops caffra, although nectariniid sunbirds also visited them (Rebelo 1987). The figure in Johnson (2010) are somewhat different, although high, ca 20 species (unspecified) of angiosperms are visited by Promerops but ca 44 species are visited by Nectarinia famosa and 66 species by Anthobaphes violaceae, the orange-breasted sunbird, both nectariniids.

Perhaps 950 African species are pollinated by birds, which are mostly Nectariniidae and a few Zosteropidae; the sugar bird is placed in Promeropidae. The relatively high frequency of ornithophily in the Cape flora may be connected with the prevalence of nutrient-poor soils, as in Australia (Rebelo 1987; Orians & Milewski 2007).

Age: Recent phylogenies are clarifying avian evolution. Old World pollinators belong to two major clades. 1. Psittaciformes include parrots and cockatoos, and Loriinae, mostly Australasian, are nectarivorous parrots, nectarivory having evolved at least three times (Schweizer et al. 2014). Their age has been estimated as the early Eocene ca 59 m.y.a. (Fleming & Kress 2013; see also Fleming & Muchhala 2008), and late Palaeocene may indeed be the stem age of the clade (Prum et al. 2015). However, the crown-group age of parrots is late Oligocene, and the some 53 species of lories and lorikeets, by far the biggest clade of nectarivorous parrots, are a mere (14.8-)ca 10(-4.8) m.y.o. (crown-group age) or (15-)13 m.y.o. (stem group) (Schweizer et al. 2014, 2015).

2. The other clade is Passeriformes, a large clade including over half the species of extant birds which began diversifying towards the middle of the Eocene around 50 m.y.a. (Prum et al. 2015: Acanthisitta, the rifleman, diverging from the remainder; ca 4 m.y. older, Claramunt & Cracraft 2015) or, a rather different estimate, around 82 m.y.a. (Barker et al. 2004). However, Jarvis et al. (2014) estimated that the part of Passeriformes that includes all the Passeri or oscines below may not have begun diversifying until the Oligocene ca 30 m.y.a., with the nectarivorous taxa being embedded in clades that are Miocene in age (Prum et al. 2015); general estimates in Moyle et al. (2016) are in the same ball-park. Within the oscine Meliphagida, which started off in Australia, Moyle et al. (2016) estimate the time of divergence of Acanthorhynchus from other meliphagids at less than 12 m.y.a. (see also Driskell and Christidis 2004; c.f. Marki et al. 2016). However, Marki et al. (2016) found that Myza, from Sulawesi, was sister to other meliphagids, diverging ca 24 m.y.a., and that the rest of the group, including Acanthorhynchus, did not radiate until ca 20 m.y. ago. Within the Passerida, Yuhina, Timaliidae, is paraphyletic to white-eyes s. str., and the whole clade (= Zosteropidae/Zosteropinae) began diversifying 8.1-6.3 m.y.a., Zosteropidae s. str. diversified 5.6-4.5 m.y.a., and the speciose Zosterops itself a mere ca 1.8 m.y.a. (Moyle et al. 2009). Within the finches, sunbirds and spider hunters are both members of Nectariniidae-Nectariniini while flower peckers are Nectariniidae-Dicaeini, and they began diversifying in the middle Eocene around 45 m.y.a.; the stem group is perhaps Palaeocene in origin (Barker et al. 2004). On the other hand, comparable estimates in Fleming and Muchhala (2008) are 30 and 35 m.y. respectively (see also Friis et al. 2011). Promerops has been mentioned on occasion; its stem age has been dated to ca 39 m.y. (Fleming & Kress 2013) or (39.5-)33.4(-28.3) m.y. (Beresford et al. 2005: note calibration).

Hawaii is noted for the remarkable radiation of Drepanididae/Drepanidinae, the Hawaaian honeyeaters, which were/are (almost 2/3 the species have become extinct since human arrival on the islands ca A.D. 1250 - Wilmshurst et al. 2010) both nectarivorous and insectivorous. All told, about 7 species of Drepanidinae seem to be nectarivorous, to which can be added the 5 extinct species of the unrelated Mohoidae (look-alikes of Meliphagidae) that were endemic to the islands. These few species of birds are/were probably pollinators of some 178 species of plants, of which around 125 species alone are Campanulaceae-Lobelioideae (Brighamia and Cyanea), although the main nectar source is probably Metrosideros, especially the protean M. polymorpha (Carlquist 1970; Lammers & Freeman 1986; Givnish et al. 1995; T. J. Givnish pers. comm. x.2013).

Age: Hawaiian Drepanidinae are oscine cardueline finches, their ancestor being something like a rose finch. It is thought that much diversification of clades representing extant drepanid species took place 5.8-2.4 m.y.a., especially after the formation of Oahu ca 4 m.y.a. (Lerner et al. 2011). However, Givnish et al. (1995) estimated that the age of the common ancestor of Cyanea (Campanulaceae), one of the main nectar sources for the birds, was of the order of 17.4-8.7 m.y.a., about three times estimates of the age of diversification of Drepanidinae, their pollinators. Pender et al. (2013) date the diversification of both birds and plants to within the last 17 m.y., while the arrival of Metrosideros, another important nectar source, on the islands has been dated to (6.3-)3.9 m.y.a., probably initially on Kaua'i (Percy et al. 2008).

To summarize some differences between Old and New World nectarivorous birds and the plants that they pollinate: Hummingbirds usually hover and tend to pollinate herbs and epiphytic plants, while the Old World nectar-feeding birds perch when feeding and more frequently pollinate larger trees (e.g. Stiles 1981; Kress & Beach 1994; Fleming & Muchhala 2008). Hummingbirds, all New World, are thought to be more specialized than both other New World flower pollinators like orioles and also their Old World ecological counterparts, and in the Old World, although African sunbirds show at least a moderate degree of specialization, honeyeaters and lories are rather generalized (Schweizer et al. 2014). Similarly, bird-pollinated flowers, at least, in the New World seem to be more specialized (Stiles 1981; Fleming & Muchhala 2008) than their Old World counterparts, and Papuan plant-pollinator relationships may be less specialised that those in Australia (Brown & Hopkins 1995). Johnson and Nicholson (2008; see also ) compared hummingbirds with other New World passerine pollinators and sunbirds with other African pollinators - thus neither Meliphagidae, honeyeaters, nor lories and lorikeets were involved in the comparison. Flowers visited by hummingbirds and by sunbirds tended to produce medium amounts of nectar with high concentrations of sugars, of which sucrose was a major component (ca 1/2), while flowers visited by other birds in both regions tended to produce larger amounts of more dilute nectar, with sucrose being at most 0.5% (Johnson & Nicholson 2008). Assemblages of hummingbirds in any one site in the New World are more diverse than pollinator assemblages in the Old World (Fleming & Muchhala 2008), although lories can be quite diverse locally in New Guinea (Schweizer et al. 2015). Overall, the diversity of plants pollinated by birds is lower in the Oriental region than in the New World or the Australo-Papuan region (e.g. Corlett 2004).


For good introductions to bat pollination, see Dobat and Peikert-Holle (1985) and Fleming and Kress (2013).

Phyllostomid bats are relatively small and often hover when they feed, and the plants they pollinate are trees as well as vines and epiphytes; they are active in the l.t.r.f. as well as in deserts (Fleming et al. 2005, 2009). In any one site 1-6 species of bats may pollinate 4-19 species of flowering plants (Fleming et al. 2005). In the Monteverde forest, Costa Rica, 7 species of bats visited 33 species of plants, 1.6% of the total flora (Murray et al. 2000). Phyllostomines are important pollinators of columnar cacti, Agave, and Malvaceae-Bombacoideae, with 16 species recorded as pollinating 90 species of plants including some of these cacti (Arizmendi et al. 2002). Clairmont et al. (2014) found pollen of 11-14 species of plants (2.4-3.9/night) in the excreta of three phyllostomines in Cuba, although the figure was 17-21 species (3.5-5.3 species/night) in two species of fruit-eating bats; the study was carried out in the wet season.

Estimates are that in the New World at least 500 species of plants in 27 families (Vogel 1969) or 590 species of plants in 43 families (Dobat & Peikert-Holle 1985) are pollinated by bats. Although other estimates are substantially lower, e.g. 364 species in 44 families (Fleming et al. 2009), such figures need to be revised substantially upwards. Thus Fleming et al. (2009) list 20 bat-pollinated species in the Centropogon alliance (Campanulaceae-Lobelioideae), although L. Lagomarsino (pers. comm. 9.iii.2017) estimates that about 167 species of that group may be pollinated by bats and 110 species is the estimate in Dobat and Peikert-Holle (1985). Similarly, 5 bat-pollinated species of Passiflora are listed by Fleming et al. (2009), while Jørgensen et al. (2012) estimate the number to be 17, 8 known to be bat-pollinated, 9 likely to be so.

Age. The bats have diversified within the last 30-12 m.y. (Datzmann et al. 2010), crown-group ages of the two major clades involved, Glossophaginae and Lonchophyllidinae, being around (20.8-)17.5(-14.2) and (11.4-)11.1(-10.9) m.y., with stem group ages of ca 21.6 and 20.6 m.y.a. respectively (Baker et al. 2012). Comparable ages in Rojas et al. (2011) are 20.1-12.9 and 23.5-22 m.y. respectively. Similarly, Fleming et al. (2009) date the evolution of bat-pollinated flowers to the Miocene ca 20 m.y.a (see also Fleming & Kress 2013).

IIIB. Pteropodidae. Pteropodid bats, about 173 species, are predominantly frugivores or nectarivores. Many pteropodids visit flowers and eat fruit, and on Samoa 2 species of bats visited 78 species of plants in 37 families, many of them for both flowers and fruits, and some also for leaves (Banack 1988). In the Old World there are only 5-6 genera and 12-15 species of nectar-feeding bats (again, other species visit flowers on a more opportunistic basis) in the macroglossine Pteropodidae (e.g. Marshall 1983; Fleming et al. 2009). The bats are found from Africa to Australasia and the Pacific (Fleming & Muchhala 2008). They are on average larger than phyllostomid bats, they tend to hold on to the plant when feeding, and the flowers they pollinate are robust and are usually borne on trees (Fleming et al. 2009).

Bats pollinate fewer than 200 species of flowers in the Old World (Dobat & Peikert-Holle 1985); 57 genera in 22 families is the estimate in Marshall (1983: records from only 19/44 genera of bats), 168 species in 41 families is the figure in Fleming et al. (2009).

Age: The crown-group pteropodid clade may be around 56 m.y.o. (Fleming & Kress 2013), older than the phyllostomids, 38-24 m.y.o. (Almeida et al. 2009) or 28-18 m.y.o. (Teeling et al. 2005).

The New World phyllostomid bats are more efficient pollinators than hummingbirds, transporting larger amounts of pollen, and bat-pollinated flowers evolved from bird-pollinated ancestors (Muchhala & Thomson 2010 and references) - note, however, that pollen transport in trap-lining hummingbirds may be remarkably effective with multiple paternities in the plants they visit because the birds do not clean themselves very much and are aggressive (Krauss et al. 2017). The bats, all New World, are thought to be more specialized than their Old World ecological counterparts. Assemblages of phyllostomid bats in any one site in the New World are more diverse than in the Old World (Fleming & Muchhala 2008). Overall, the general diversity of plants pollinated by bats is lower in the Oriental region than in the New World or the Australo-Papuan region (e.g. Corlett 2004).

DISCUSSION. [This is still choppy...]

Introduction. Ollerton et al. (2011) estimated that the number of animal-pollinated plants as 308,006, 87.5% of some 352,000 species of flowering plants (the latter number from Paton et al. 2008). Van der Niet and Johnson (2012) estimated that one quarter of plant speciation events might involve pollinator differences. However, details of con- and heterospecific pollen movement and related topics like the establishment and efficacy of barriers to crossing between species are not the main issue here (see e.g. Armbruster 2014 for a recent review). Rather, the focus is on numbers of species of plants and animals involved in particular sets of pollination relationships, since this is one way of allowing us to think about ideas of co-evolution (the term will be defined precisely when used), mutualism, and the like.

Indeed, floral variation and plant-pollinator interactions have long been of central interest to biologists. Darwin (1876: p. 371) observed that the "beaks of humming-birds are specially adapted to the various kinds of flowers they visit", although one might now add the width of the tube as another important variable (Temeles 1996; Temeles et al. 2002, 2009). Stebbins (1970: p. 308) noted that in animals major groups tended to be rather invariant, differing in characters related to survival, and in plants reproductive features showed comparable invariance: The "flower must become a highly integrated stucture, with all of its parts precisely adjusted to each other" for cross pollination by animals with specialised habits to be successful. Fifty years or so ago plant-pollinator relationships were often thought of in terms of what might be thought of as mutual co-evolution, with almost lock and key-type relations between particular flowers and their pollinators implying 1:1 relationships between the two (e.g. Grant & Grant 1965), as the often-published pictures of birds with long, curved bills next to flowers with a long, curved corolla tube suggest. Stebbins (1970), although cautious, conveyed the same general idea, as when he noted that euglossine bees obtained fragrances from orchids, extensive speciation in both being the result, and also when he discussed intermediates between different pollination syndromes.

Unfortunately, the terminology here is complex. Ollerton et al. (2007) distinguished between phenotypic generalization and specialization, thinking about the plant, and ecological and functional specialization, thinking about the pollinator, ecological generalization emphasizing the numbers of pollinators, and functional generalization their (difficult to quantify) diversity. Thus a species pollinated by 100 species of flies might be ecologically generalized yet functionally specialized. As Ollerton et al. (2007: p. 725) noted, functionally and phenotypically specialised South American hummingbird flowers might be visited by more than one species of bird (also functionally and phenotypically specialized), and so were "to some degree ecological generalists", the birds likewise being ecological generalists. Similarly, Rebelo (1987) suggested that both plants and pollinating birds in the South African fynbos were generalists, while Cameron (2004) thought that most species of orchids attracted many species of euglossine bees, and most species of bees were attracted to several different species of plants. Pauw and Stanway (2014) found that pollinators in some South African communities were often specialists, 154/217 species visiting only one species of plant, but the plants were generalists, 38/62 species being visited by more than two species of pollinator (the former figures might change if other communities had been studied). And in Ollerton's earlier work on South African stapeliads, he noted that specialised flowers were specialized on common, ubiquitous insects, and flowers pollinated by the most specialized insects had easily accessible nectar that attracted other visitors, too (Ollerton et al. 2003).

As one tries to get one's brain around the terms, one also has to remember that there is much debate over such things as (1) the nature and extent of flower-pollinator co-evolution, (2) the existence of wide-ranging pollination syndromes versus sometimes quite local guilds facilitating the success of both plant and pollinator (S. D. Johnson 2010; see also F. Zhang et al. 2012), and (3) what exactly pollinators might see and respond to (Waser et al. 1996; Chittka et al. 1999; Fenster et al. 2004; Waser & Ollerton 2006; Raguso 2008; Ollerton et al. 2009a); see also the papers in Ann. Bot. 113(2). 2014. Rodríguez et al. (2004) and Horridge (2009) discuss the bee's point of view, the former, thinking about monosymmetry in particular, the latter more generally. Function(al) groups/types or guilds are groups of animals with similar mouthparts, for example, so generating similar selection pressures on the plants they pollinate (Armbruster 2014 for literature). Note, however, that a group of meliphagids with rather similar morphologies living in arid areas of Australia occupy ecological pollination space as fully as mesic meliphagids with different morphologies (E. T. Miller et al. 2016).

Oligolecty vs Polylecty, Generalization vs Specialization. Thinking about pollinator:plant relationships leads to a series of apparent paradoxes (see also Johnson & Steiner 2000). Snow and Snow (1980) noted that although a particular species of plant might have but a single hummingbird pollinator, i.e., the plant was a specialist, the same bird, specialized though it might be, might pollinate several species of plants, i.e. it was a generalist. Similarly, specialized fruit-eating phyllostomid bats are generalists when looking at bat:plant networks, a single species of bats may eat fruits of several species of plants (Mello et al. 2011a). Thus the flowers visited by polylectic pollinators like hummingbirds and bumble bees are specialized, often being monosymmetric, with concealed nectar, and so on.

At the same time the morphology and behaviour of these polylectic pollinators themselves is by no means "primitive" or generalized. For example, the southwest African nemestrinid fly Moegistorhynchus longirostris has a tongue 40-90 mm long (S. D. Johnson 2010), polylectic bees such as Apidae have large brains and a well developed sensory system and individuals learn how to pollinate diverse flower morphologies ranging from simple to complex, while bumble bees in particular can thermoregulate (e.g. Heinrich 1976; Laverty 1994; Raine & Chittka 2007a, b) and are overall pretty intelligent (Loukola et al. 2017), and so on. The plesiomorphic condition for pollination specificity in bees is likely to be oligolecty (Danforth et al. 2006; Sipes et al. 2006; Michez et al. 2008); polylectic behaviour in bees is often derived (e.g. Müller 1996; Michener 2007; Praz et al. 2008; Sedivy et al. 2008; Litman et al. 2011; Danforth et al. 2013; c.f. Moldenke 1979b). Floral specialization has increased over evolutionary time, and unspecialised flowers, probably pollinated by several species of oligolectic bees or other pollinators, precede specialised flowers, probably pollinated by one or a few species of polylectic pollinators. Monosymmetric flowers in which precise interactions between plant and pollinator are needed for effective pollination are largely a Caenozoic phenomenon, and many of them are likely to have polylectic pollinators. Of course, there are relatively young, very speciose clades of plants that are commonly pollinated by oligolectic bees, Asteraceae being a prime example (q.v. for details).

Oligolectic bees visit relatively few species of flowers, and/or a group of rather closely related plants, i.e. within a single family, or a tribe within that family, so they are specialized from that point of view (oligolecty often refers to pollen-collecting behaviour, not nectaring, where bees tend to be more catholic - Waser et al. 1996). The bees may have few obvious morphological adaptations for pollination (Michez et al. 2012), and the flowers they visit are often what would be described as unspecialized, apparently being accessible to a wide variety of pollinators, with radial symmetry, poorly concealed nectar, etc., although oligolectic bees may also pollinate monosymmetric flowers (e.g. Bawa 1990; Sedivy et al. 2008). The bees, which are often solitary, are in general most diverse in arid and often extratropical regions, they tend to be short-lived, and the flowering times of the plants they pollinate are also often rather short (Linsley 1958; Michener 1979). Thus in arid and semi-arid areas like deserts, the Great Basin, and parts of California and Chile there are many species of oligolectic/specialist bees (e.g. Waser et al. 1996) that at least sometimes compete for the same resource (see in part Moldenke 1976, 1979a, b; Petanidou & Ellis 1996; Lindberg & Olesen 2001; Stang et al. 2007). For example, the creosote bush, Larrea tridentata (Zygophyllaceae), common in the American southwest and with an open and "unspecialized" floral morphology, is the focus of visits by 22 species of oligolectic bees and regularly visited by another 22 species of polylectic bees, not to mention other more transient visitors (Hurd & Linsley 1975). Similarly, 12 species of oligolectic Andrena bees are major visitors to Camissonia campestris (Onagraceae: Linsley et al. 1973: see also Cruden 1971: Nemophila; Ehrenfeld 1979: Euphorbia; Waser et al. 1996: Ranunculaceae). So rather paradoxically, individual specialist pollinator species may be less effective in and necessary for pollination than are generalists (e.g. Ehrenfeld 1979; Olesen 1997 and references); that a pollinator specializes says nothing about its success in pollinating.

In many situations one can think of both plant and pollinator as being specialists - and at any particular time and place, this may well be true. Certainly, from the point of view of the plant promiscuity of its pollinator may be more apparent than real. For example, even if a single polylectic (generalist) bee species, or colony, or even an individual bee, may visit many species of plants, on any one trip a particular bee may be much more selective (e.g. Heinrich 1976; Chittka & Thomson 1997; Heard 1999; Hagbery & Nieh 2012 for general pollen/nectar constancy), so being functionally mono- or oligolectic (specialist). Similarly, pollinators like hummingbirds may be widely distributed and pollinate many unrelated species of plants, but at any one place the plant-pollinator ratio is rather lower than the figures from, say, the whole of Costa Rica, might suggest (e.g. Rebelo 1987); situations where male and female birds have bills of different lengths will also lower the ratio. Interestingly, hummingbirds with smaller ranges show greater community-level, i.e. local, specialisation on flowers with different morphologies (Sonne et al. 2016). Specialization may also increase as resources decrease (Tinoco et al. 2016) and in species-rich hummingbird communities there are higher levels of specialization and modularity, as in communities where there is competition between closely-related hummingbid species (Martín González et al. 2015). (Note that on islands hummingbirds tend more to be generalists (Traveset et al. 2015), and the modularity of hummingbird communities is less (Martín González et al. 2015), and the plants are pollinated by a greater variety of pollinators (e.g. Serrano-Serrano et al. 2017)). In southern Africa Goldblatt and Manning (2006; see also Johnson 2010) estimated that there were often about six species pollinated by a particular long-tongued fly at any one locality - quite a number, but still substantially fewer than the 20-30 species that typically depended on that pollinator - and the pollen from different plants might be deposited on different places on the pollinator. Similarly, a single species of orchid bee may visit several species of orchid locally, but the pollinaria may be deposited on different places on the bee (Williams 1982).

Plant-Pollinator Interactions and Species Number Imbalances. There are marked asymmetries in the numbers of players in the plant-pollinator relationships discussed here (e.g. Bronstein 1994: question 2; S. D. Johnson & Steiner 2000; Johnson 2010: Table 1; Mello et al. 2012), and plant-pollinator ratios of 10:1 are at the low end of the spectrum. Wiens et al. (1983) expected to find similar asymmetrical relationships in plants with wind or water pollination, also in those pollinated by social bees, passerine birds, most flies, and perhaps beetles; they themselves looked at pollination of South African Protea species by small mammals.

These asymmetries are evident at all scales. Sekercioglu (2006) suggested that some 600 species of birds visited plants for nectar, 350 more being casual visitors, and that they visited 500 (3.7%) of a total of 13,500 plant genera (his estimate). By extrapolation, and using the same figure of 352,000 species of flowering plants and assuming that there is no variation in pollen syndrome within a genus(!), some 13,000 species may be bird-pollinated. Overall, one species of bird would pollinate around 21 species of plants. In the relatively well-studied Costa Rican flora, Stiles (1981) estimated that 55 species of hummingbirds pollinated mostly or exclusively around 300 species of plants. Bees are quite a diverse group, with some 17,500 species (Michener 2007), but it is particular groups of bees, not notably speciose, that play a disproportionately important role in current plant:bee interactions (see above; c.f. in part Cappellari et al. 2013: Fig 2B, C). In New Caledonia 43 species of native bees are the major pollinators of a flora of over 3,050 species of flowering plants, a plant:bee ratio of 71:1, and although this must be a considerable overestimate, it bears clarifying, while in New Zealand the ratio was estimated to be 57:1 (Donovan et al. 2013: but recent disruptions of plant-pollinator interactions caused by humans).

Smaller-scale examples show similar imbalances. Long-tongued dipteran Nemestrinidae in southern Africa have plant-pollinator ratios anywhere from 6:1 (highly conservative) to much in excess of 30:1; thus Prosoeca ganglbaueri alone pollinates 20 or more species (van der Niet & Johnson 2012). Also in southern Africa, the satyrid mountain pride butterfly Meneris/Aeropetes tulbaghia pollinates 19 species of plants belonging to 8 genera and 4 families (Johnson & Bond 1994). In the Cape region of South Africa, Rebelo (1997) estimated that perhaps 318 species of plants were pollinated by five species of sunbird and the Cape sugarbird, Promerops caffra, although he thought that the 15:1 plant:bird ratio was particularly high; figures for a diversity of pollinators given by Johnson (2010) are 420:24. Thus Promerops is particularly attracted to 80 or more species of Cape Proteaceae (Rebelo 1997; Johnson 2010: 20 spp.), while perhaps 37 or 42 species (Geerts & Pauw 2009 and Johnson 2010 respectively), mostly other than Proteaceae, depend for their pollination on the malachite sunbird, Nectarinia famosa. The convolvulus hawk moth (25 spp.), tabanid flies (3 species pollinating 48 species of plants) (Johnson 2010), spider hunting wasps, Hemipepsis (Johnson 2010; Shuttleworth & Johnson 2012) which pollinate especially Orchidaceae, Apocynaceae:Asclepiadoideae and Asparagaceae:Scilloideae, and so on. In Western Australia, the New Holland honeyeater, Phylidonyris novaehollandiae, visits flowers of 142 species of plants in 32 genera and ten families (Brown et al. 1997). The long-billed hummingbird Ensifera ensifera pollinates 37 species of the Andean Passiflora supersection Tacsonia as well as other plants (Abrahamczyk et al. 2014), so there is a bit of phylogenetic clumping of plants there. Interestingly, many of these examples are from the southern hemisphere (Johnson & Steiner 2000), and Johnson (2010) suggested that in Apocynaceae and Orchidaceae pollinator specificity was greater in southern Africa than in the northern hemisphere.

Species of the polylectic Apidae visit over twice as many families of flowering plants as do species of Halictidae and over five times those of Colletidae (Waser et al. 1996), and in the Mediterranean, Bombus and Apis were the most generalized visitors, visiting the most plant species (Olesen et al. 2007b). In the humid tropics polylectic bees, which often live longer, are proportionally more common than oligolectic bees (Michener 1979). Although in Venezuelan forests, at least, fruit set in specialized (monosymmetric, gullet types) flowers may be less than that in generalized flowers (Ramírez 2003), this cannot be linked to the particular pollinators involved.

In such asymmetric relationships the idea of pollinators and frugivores being keystone species or keystone clades that show phylogenetic niche conservatism (e.g. Fleming et al. 2005; S. D. Johnson 2010: see below) readily spring to mind. Many species of plants have specialized flowers and many species of bees, for example, are oligolectic, visiting a relatively few species of plants. Pollinator-plant relationships are not nested in any simple fashion: Specialist flowers interact more with polylectic pollinators, while generalist flowers often interact both with oligolectic and polylectic pollinators (e.g. James et al. 2012; c.f. Bascompte et al. 2003; Pawar 2014). For a polylectic pollinator that serves as a hub (when diagramming out plant-pollinator relationships) the effect of the extinction of a single species of plant may be slight, but the extinction of the pollinator may affect some of the plants it pollinates more seriously. For oligolectic pollinators, the relationship will tend to be the reverse; plant specialists or species with low numbers of interactions are more likely to go extinct than species with more diverse sets of interactions (Aizen et al. 2012; James et al 2012). The consequences of such changes will depend on the overall patterns of modularity and connectedness of the plant-animal relationships, connectance being notably higher in polylectic bumblebees or hummingbirds, less in agaonid wasps and oligolectic bees - and in the examples given by Jordano (1987) also euglossine bees (see also Waser et al. 1996; Lindberg & Olesen 2001; Rezende et al. 2007; Stang et al. 2007; Olesen at al. 2007a; Vamosi & Vamosi 2012; also Vásquez & Simberloff 2002: disturbance and pollination; Colles et al. 2009; Armbruster 2012). Thus the removal of a single species of bumble bee from a subalpine community in the Rockies perturbed general pollination relationships, even though there were other polylectic pollinators there (Brosi & Briggs 2013).

The high degree of asymmetry in pollinator/plant interactions is compatible with recent work demonstrating the apparent inevitability of the development of such asymmetries. In an experimental study, communities in which generalist species, more highly nested, developed as the population of the whole system increased, even as the whole became less resilient to perturbations, unstructured systems being more resilient (Fontaine 2013; esp. Suweis et al. 2013), although the number of species in simulations was held fixed in the latter study. [elaborate or delete: Rezende et al. (2007) suggested that extinctions might have some phylogenetic signal, but this is certainly not always true (Ramírez et al. 2011).

Co-evolution? Plants involved in the pollination asymmetries discussed here represent only ca 3% (11,380) of all animal-pollinated plants (308,006: Ollerton et al. 2011 above), although this figure is likely to be a considerable underestimate. A question is, how did the morphological features in the animal partner that we now see as the features of a supergeneralist/"specialized" pollinator arise? That there are pervasive interactions between plants and their pollinators is incontestable, but how did the morphologies of the two evolve (Thompson 2009; Guimarães et al. 2011)?

We often think that relationships between animals and plant directly reflects the past, with plants and their pollinators, for example, being involved in some kind of evolutionary pas de deux that results in the apparently co-adapted morphologies of the two that we see today. Individual plant-pollinator interactions may be very precise, witness the deposition of pollinaria by the orchid Catasetum on a visiting euglossine bee, and the complex morphologies of the staminate and carpellate flowers of this orchid (for which, see e.g. Darwin 1862a). Plants and hummingbirds may appear be mutually adapted, there having been "complementary trait evolution" (Maglianesi et al. 2014a: p. 3325). But that characters are complementary in plant and pollinator does not mean that they have been equally labile evolutionarily. What are the relationships that might be considered co-evolutionary, and what groups of plants and animals show them (In the following, I do not discuss fitness trade-offs, for which, see Armbruster 2014 and references)?

1. The majority of examples given in the seminal paper by Ehrlich and Raven (1964) had to do with insects and the food plants that their larvae ate. This particular form of coevolution has been described as "escape and radiate coevolution" (Thompson 1994). A plant develops defences against an insect, reducing herbivory, but the insect then evolves counter-measures to the defences, neutralizing them, and then speciates on the plant clade - which may develop further defences, and so on. The ability to eat the plant can be thought of as a key innovation, subsequent diversification may involve conventional geographical isolation and divergence (Althoff et al. 2014). There are many cases where an insect previously unassociated with a plant becomes able to develop on it, and a good example of this kind of coevolution is pierid butterflies that have diversified on Brassicales, especially Brassicaceae.

2. In simple coevolution, that is, co-cladogenesis or parallel cladogenesis (Janzen 1980), both plant and pollinator/frugivore diversify together, there being some kind of reciprocal evolution. However, there is little evidence for such a mutual 1:1 co-evolution in plant-pollinator interactions (Jordano 1987; Waser et al. 1996; Sazima et al. 1996; Chittka et al. 1999; Fenster et al. 2004; Waser & Ollerton 2006; Raguso 2008; Placentini & Varassin 2007; Winkler & Mitter 2008; Ollerton et al. 2009a; Fleming & Kress 2012: pp. 182-188). Coevolutionary diversification is reciprocal natural selection that leads to an increased diversification of both parties, but, as Althoff et al. (2014: esp. p. 88) showed, this is very difficult to demonstrate. Plant-pollinator relationships do not seem to involve reciprocal bursts of speciation (Hembry et al. 2014).

3. Sequential evolution/radiation, plants diversifying earlier that their bird or bat pollinators, but with an overall similar pattern of cladogenesis, has also been invoked to explain both plant-pollinator relationships (Fleming & Muchhala 2008: plant family ages used) and also the relationships between mycoheterotroph and its associated fungi. Fleming and Kress (2013: esp. table 5:3) list a number of examples of pollination and frugivory where they think particular plant groups become ""colonized" by a group of animals which then radiates in parallel with the plants" (ibid., p. 183). Their examples include phyllostomid bats with Agavaceae and columnar cacti, Hawaiian Campanulaceae with honeycreepers, Asian Loranthaceae with Dicaeidae, frugivorous phyllostomid bats with Piper, Araceae, Cyclanthaceae, and Ficus, and Southeast Asian hornbills/birds of paradise with Meliaceae/Myristicaceae. (Similar delayed sequential speciation - the delay is 65-170 m.y. - has also been invoked for speciation in Thismia and its glomeromycote fungal symbiont - Merckx & Bidartondo 2008.) However, it is unclear how sequential cladogenesis might work (see Fleming & Kress 2013: Fig. 5.4b; c.f. Winkler & Mitter 2008).

4. In a more diffuse coevolution one or both partners is "represented by an array of populations that generate a selective pressure as a group" (Janzen 1980: p. 611). Complex geographical patterns of infraspecific variation in pollinators and the plants they pollinate are consistent with such an idea (e.g. Cotton 1998: hummingbirds; Steiner & Whitehead 1990, 1991: oil-collecting bees; Johnson 2010). Species of both plants and animals converge and specialize on a core set of mutualistic traits, rather than directly on each (Thompson 2005: p. 289; Fleming & Kress 2013), and there is no suggestion that a single clade of either plant or animal is involved.

5. A plant may also evolve to "fit" the morphology and/or senses of the pollinator, advergent evolution (Johnson et al. 2003; Anderson & Johnson 2009; Pauw et al. 2009; S. D. Johnson 2010; Newman et al. 2015). Many of the asymmetries discussed here can be interpreted as representing groups of plants that have independently converged on the morphologies of super-generalist pollinators; the plants may evolve, but their pollinators are less likely to (Guimarães et al. 2007, 2011, p. 881: "non co-evolutionary cascading events"; Thompson 2009). Wiens et al. (1983) included such relationships in their idea of unilateral evolution, in which plants evolve profoundly altered floral features to attract non-specialised (from one point of view, but see below) pollinators, while Stiles (1981) contrasted the behavioural flexibility of the pollinator (hummingbirds in this case) with the evolutionary flexibility of the plant. Long-tongued euglossine bees that pollinate a variety of Zingiberales, for example, have less specialized on these flowers than the plants have specialized on long-tongued pollinators (Borrell 2005; see also S. D. Johnson 2010), and relationships between orchids and their pollinators, although often quite precise, are unlikely to signify reciprocal co-evolution (e.g. Williams 1982; Ackerman 1983; T. Jermy in Szentesi 2002; Jersáková et al. 2006; Ramírez et al. 2011). In pollination niches, positive interactions occur in the plants involved as they adapt to the pollinator (Johnson 2010), and this is associated with the idea of pollination guilds, "large numbers of species (typically about 20-30) deepnding on a single pollinator species or small set of pollinators of the same functional type" (Johnson 2010: p. 504). Trait-tracking by rare or non-rewarding species in a diffuse co-evolutionary setting (Johnson 2010) would also fit here.

There may be variation in the pollinator that has a geographical component, but there is no speciation, although there may be infraspecific variation in corolla tube length in the plant when different pollinators become involved (Newman et al. 2014). Thus ages of patterns of infraspecific variation in the bee-fly Megapalpus capensis show quite extensive covariation with biome ages, but there is just the one bee-fly species, and it pollinates 12 species or so (7 separate clades - de Jager & Ellis 2016 and references; Struck 1997, see Röschenbleck et al. 2014) in various biomes.

Knowing the relative timing of plant and pollinator diversification is critical in determining the evolution and nature of plant-pollinator relationships - unfortunately, given the state of dating phylogenies. One example that may be quite old is the Heliconia-hummingbird relationship. The predominant pollinators of New World Heliconia are hummingbirds and most Heliconia are from the New World; Heliconiaceae make up one of the single most important hummingbird-pollinated clades. Most estimates of the age of crown-group Heliconiaceae are 40-30 m.y.a. - 32-28 m.y. (Kress & Specht 2006), ca 32 m.y. (McKenna & Farrell 2006), or (47-)39(-32) m.y. (Iles et al. 2016). Diversification in New World Heliconia is most evident from a little over 30 m.y.a. onwards (Iles et al. 2016). Diversification of crown-group hummingbirds has been dated to as late as the early Miocene (24.7-)22.4(-20.3) m.y.a., much speciation occurring about 13-12 m.y.a. along with the uplift of the Andes (Bleiweiss 1998a; McGuire et al. 2007, 2014; Abrahamczyk & Renner 2015; Prum 2015), however, Tripp and McDade (2014a) estimated crown-group diversification of hummingbirds to have begun (29.9-)28.8(-28.4) m.y. ago. Iles et al. (2016) suggest that there was diffuse coevolution (sensu Tripp & McDade 2014a; see also Ehrlich & Raven 1964 - ecological interactions between two parties driving adaptations in both) between hummingbirds and Heliconia, the two becoming associated early in the history of the former in the New World. Some changes in bill length and flower curvature may have involved mutual changes, as perhaps in situations like the pollination of Heliconia by Eulampis (= Anthracothorax) (e.g. Temeles & Kress 2003). However, much of the early fossil history of stem-group hummingbirds is from Oligocene Europe in deposits ca 34.3 m.y.o. (Mayr 2004, 2009, 2016; Louchart et al. 2008) and from the Late Eocene of the Caucasus (Louchart et al. 2008). Furthermore, Heliconia itself may have a very long stem history - ca 45 m.y. (Iles et al. 2016), about which precisely nothing is known. Be all this as it may, perhaps hummingbirds became templates, as it were, for a variety of younger plant clades as they adopted bird pollination - hummingbird pollination was "facilitated by this pre-existing relationship" (Iles et al. 2016: p. [12]).

Asynchrony in timing of the evolution of the two partners in relationships that may now seem to represent an obligate association for at least one of the two (e.g. Fleming 2004; Ramírez et al. 2011; Schiestl & Dötterl 2012) suggests that there was no reciprocal co-evolution. If plants diversified before their pollinators, for example, this would suggest that their current pollinators are "exploiting" a mutualism that had evolved with other pollinators, and if the pollinating animals diversified first it is the plants that are doing the exploiting. In a summary of studies examining timing of parallel insect-plant phylogenies - the insects are all herbivores of one sort or another - Winkler and Mitter (2008) found that equivalent ages of the two partners had only rarely (four studies only) been demonstrated, in seven cases the insect radiation was younger than that of the host plant, and there were no examples of the opposite relationship.

Members even of some of the classic examples of apparently strict co-evolution show disparities in diversification times, and mutual reciprocal evolution between plant and pollinator seems to be the exception rather than the rule. Figs may be a partial exception (Cruaud et al. 2012a, b and literature), although more than one species of wasp pollinates a single species of Ficus in about 30% or more cases. Indeed, although co-speciation is possible in dioecious figs, it is less likely in monoecious figs where co-pollinating species of wasp are not closely related in about two thirds of the cases (Yang et al. 2015 and references). Much of the divergence in Yucca seems to have occurred before that of its main pollinator, Tegeticula, but only a mere 6-4 m.y.a., indeed, given the vagility of the moth, it is difficult to imagine how strict co-evolution might work. Initial diversification in Yucca may have been in association with Parategeticula, a poor flier and now rather uncommon (Althoff et al. 2012). Similarly, crown-group euglossine bees can be dated to 42-27 or 38-34 m.y.a. with especially rapid diversification 20-15 m.y.a. (Ramírez et al. 2010) or (35-)28(-21) m.y.a. (Cardinal & Danforth 2011). Ages of three immediately unrelated clades of orchids that these bees pollinate suggest that they speciated up to 12 m.y. later, (31-)27-18(-14) m.y.a. (Ramírez et al. 2011). In the apparently less tight association of aroids with scarab beetles, the beetles are thought to have diversified well before the plants (Schiestl & Dötterl 2012).

In several of these quite small but very successful pollinator groups that pollinate many species of plants it can be difficult to maintain that there is much evolution of the pollinator going on (see esp. Thompson 2009; Guimarães et al. 2011; Suchan & Alvarez 2015). As Kay and Sargent (2009: p. 638) noted "a plant's reproductive success is tied to the attraction and manipulation of its floral visitors". Plants exploit pre-existing perceptual/sensory biases of the pollinator, effectively manipulating their behaviour, and both plant and pollinator are benefited (e.g. Ackerman 1983; Chittka 1996; Schaefer & Ruxton 2009, 2010; Ramírez et al. 2011; Schiestl 2010; Schiestl et al. 2010; Ramírez et al. 2011; Schiestl & Dötterl 2012; Shrestha et al. 2013; Hembry et al 2014; c.f. Strong et al. 1984), and such plant responses can happen very rapidly (Gervasi & Schiestl 2017: bumblebees and Brassica rapa). Dyer et al. (2012) found that flower reflectance curves in Australia matched those in the northern hemisphere, and had probably evolved to match the colour discrimination abilities of their hymenopteran pollinators, which did not differ between the hemispheres; the evolution of the hymenopteran visual system predates angiosperm evolution. Similarly, colour perception is not an apomorphy for bees (Chittka 1996) while the sucrose-rich nectars that characterise hummingbird-pollinated flowers are associated with the ability of hummingbirds, but not starlings, thrushes, swifts, mocking birds, etc., to digest sucrose (Baker et al. 1999) and with the very ability of the birds to taste sweetness (Baldwin et al. 2014). Sensory exploitation also occurs in pollination by deception (e.g. Schaefer & Ruxton 2009; Moré et al. 2013), although benefits there are one-sided, and includes cases where plants produce odours that attract saprophagous, necrophagous and coprophagous insects (Jürgens et al. 2013). Darwin (1876) is turned upside down, for example, one might say that the corollas of various species of plants are specially adapted to the beaks of the particular hummingbirds that visit them, indeed, the morphology of the pollinator may be more a witness to past than to present plant:animal interactions - although this simply pushes the problem back in time.

If the effects on the pollinator in such interactions were independent of some of the other interactions in which it is involved, there could be evolutionary specificity in the response of the animal (see in part Bawa 1990). However, this seems to be uncommon. In southern Africa a number of species of plants belonging to unrelated families depend on the services of a small group of nemestrinid pollinators (S. D. Johnson 2010; see also Huang & Shi 2013 and above), and Pauw et al. (2009) described rather diffuse co-evolution between individual species of these flies and and a group of species with long-tubed flowers. Interactions here take place in a complex geographical mosaic, with some of the flies having considerable infraspecific variation in proboscis length and a number of pollination ecotypes (e.g. Pauw et al. 2008; Newman et al. 2014; van der Niet et al. 2014; Anderson et al. 2014; see also Bascompte & Jordano 2007).

Suchan and Alvarez (2015 and references) suggest that any mutual coevolution that would lead to the pollinator developing particular adaptations might happen over a relatively short period, a brief co-evolutionary fling, as it were. Moreover, today's pollinators could have ecologically "taken over" older clades in which there were already established animal-plant relationships (suggested by Janzen 1980), with current plant-pollinator relationships as it were superposed on an earlier set of relationships. For example, the earliest evidence of birds visiting flowers comes from the Middle Eocene of Germany ca 47 m.y.a. where eudicot (tricolpate) pollen was found in the gut of Pumiliornis tesselatus. Pumiliornis, although of uncertain relationships, is not close to any of today's major clades that pollinate flowers, such as parrots, sunbirds and hummingbirds (Mayr & Wilde 2014). Indeed, whether any extant clade of bird-pollinated plants dates back to 47 m.y.a. is debatable, but if they do they would have had to switch their partners (Mayr & Wilde 2014). Indeed, stem-group, but quite highly derived, hummingbirds are first known from Europe in the Oligocene only some 34.3 m.y.a. (e.g. Mayr 2004; Louchard et al. 2008; Mayr & de Pietri 2014). It has been suggested that plants like Ericaceae-Vaccinioideae-Agapetes and Bignoniaceae-Tecomaria capensis have evolved in the Oligocene in the Old World and have flowers that were originally pollinated by hummingbirds, but now by sunbirds, Nectariniidae, while hummingbirds moved to the New World where they and their flowers are now found (Mayr 2005, 2009, 2016). Although bird-pollination is common in the [Styphelioideae + Vaccinioideae] clade, there is no evidence that this was the original condition for the clade, which may be only ca 37.8 m.y.o. (Wagstaff et al. 2010). Similarly, although diversification of Nectariniidae has been dated to ca 45 m.y.a. in the Eocene (Barker et al. 2004), Jarvis et al. (2014) suggest that that the part of the very speciose Passeriformes to which Nectariniidae belong may not have begun diversifying until the Oligocene ca 30 m.y.a., while estimates from Prum et al. (2015) are still younger, less than 20 m.y. ago. Bird pollination in the Australian bacon-and-egg peas, [Mirbelieae + Bossiaeeae] has evolved many times, and diversification of the meliphagid pollinators seems to be contemporaneous with that of the peas, crown-group ages for origins of bird pollination in the latter being (23.2-)16.8(-10.4) m.y.a. (cpDNA data) or (19.2-)13.4(-7.6) m.y.a. (ITS) (Toon et al. 2014), while the meliphagids are dated to 29.4-15.9 m.y.a. (Joseph et al. 2014).

There is no evidence that both hummingbirds and plants they pollinated co-evolved in Europe and then moved to the New World. Although the early Eocene avian faunas of Europe and North America are rather similar, there is no evidence of hummingbirds - either crown or stem group - in North America that early (Mayr 2009). Old - ca 30 m.y. - clades of hummingbird-pollinated New World plants would be consistent with Eocene bird pollination. Crown-group diversification in hummingbirds is estimated at (24.7-)22.4(-20.3) m.y.a., much speciation occurring about 13-12 m.y.a. along with the uplift of the Andes (Bleiweiss 1998a; McGuire et al. 2007, 2014; Abrahamczyk & Renner 2015; Prum 2015), however, other estimates of crown-group diversification are somewhat older, (29.9-)28.8(-28.4) m.y.a. (Tripp & McDade 2014a). Turning to the plants they pollinate, most estimates of the age of crown-group Heliconiaceae are (47-)39, 32(-28) m.y.a. (Kress & Specht 2006; McKenna & Farrell 2006; Iles et al. 2016) while hummingbird pollination in Gesneriaceae has been dated to (25.5-)18.5(-5) m.y.a. (Serrano-Serrano et al. 2017). Diversification in two groups of bird-pollinated New World Acanthaceae may have been more recent than that of their hummingbird pollinators, e.g. crown-group neotropical Ruellia, in which bird pollination is common, is ca 9 m.y.o. (Tripp et al. 2013c; Tripp & McDade 2014; McGuire et al. 2014; Tripp & Tsai 2017). Of the five examples studied by Abrahamczyk et al. (2017), in three (Loranthaceae-Tristerix, Ericaceae-Arbutus, Loranthaceae-Tristerix) the hummingbirds are thought to be notably older than the plants that they pollinate, in two (Grossulariaceae-Ribes, Asteraceae-Chuquiraga) the ages were about the same. A mismatch is less clear in the sword-billed hummingbird, which split from other hummingbirds ca 11.6 m.y.a. (McGuire et al. 2014), while the stem age of Passiflora section Tacsonia whose species it largely pollinates is ca 10.7 m.y. and the crown age for the bird is ca 7.1 m.y. (Abrahamczyk et al. 2014: many support values rather low). The fifteen species of hummingbirds in the West Indies pollinate some 101 species (90 endemic) of plants in associations dated 9-5 m.y.a., and bird and plant diversifications are roughly contemporaneous (Abrahamczyk et al. 2015).

Recent work on hummingbird pollination in North America north of 24oN and southern South America south of 27oS (Abrahamczyk & Renner 2015) can perhaps be understood in the same way. Seven species of migratory hummingbirds pollinate ca 130 species of plants in the western United States alone, particularly at high elevations (Grant 1994; Grant & Grant 1968; Brown & Kodric-Brown 1979). These associations are dated to 9-5 m.y.a. (bee hummingbirds ca 6.8 (stem) and 5.6 (crown) m.y.o.: Abrahamczyk & Renner in Abrahamczyk et al. 2015; Abrahamczyk & Renner 2015), and the ages of plant and associated hummingbird diversifications in both North America north of 24oN and South America south of 27oS are thought to be largely contemporaneous, although much older in S. South America (Patagona and Sephanoides both ca 15 m.y.o.) than in North America (Abrahamczyk & Renner 2015). Bird pollination in neither area led to massive diversification within a few clades of plants, but there was repeated evolution of bird pollination in unrelated clades. Individual clades of plants pollinated by hummingbirds were usually small, only 8/ca 70 hummingbird-pollinated plant clades in North America and 0/35 clades in South America including five or more species (Castilleja is an example of moderate diversification). Only 18 (8 are Bees) and six species respectively of birds are involved, yet P. Wilson et al. (2006, 2007; see also Wessinger et al. 2016) estimated up to 21 shifts from bee to bird pollination occurred in Penstemon s.l. alone, where over 40 species are pollinated by hummingbirds; Abrahamczyk and Renner (2015) estimated ten shifts, but either way the plant clades involved are small. Speciation within these little clades may primarily be geographic, despite the extensive gene flow probably mediated by the birds (Abrahamczyk & Renner 2015). One-on-one coevolution is unlikely here since no single species of birds uses just a single species of plant as a nectar source (Abrahamczyk & Renner 2015), and although such dependencies seemed to occur, albeit uncommonly, in some other hummingbird-plant interactions, this was for only part of the year (Abrahamczyk et al. 2017).

diffuse coevolution on plant traits by interaction networks of animals, animal traits change at the same time (Poelman & Kessler 2016: plant-insect interactions)

The diversity in bill length and pollinating behaviour of extant hummingbirds is unlikely to be the result of interactions between particular extant plant and bird clades. There cannot be simple one-on-one co-evolution since many more independent clades of plants than birds are involved (e.g. Abrahamczyk & Renner 2015). Indeed, it is unlikely that there is even loose coevolution - "a long shared evolutionary history" - between guilds of hummingbirds and guilds of the plants they pollinate (Cotton 1998: p. 639), even here Cotton noting that "the floral traits of many flowers have undoubtedly evolved to exploit hummingbirds as pollinators" (ibid.: p. 645; see also Borrell 2005). Abrahamczyk et al. (2017: p. 1) note "the apparently rapid 'addition' of further plant species" to the Centropogon/Eutoxeres association, described as a co-evolutionary mutualism in which the partners seem to be perfect fits for each other, and asEAbrahamczyk et al. (2017) note the bird is much older than the plant, ca 21.5 vs 3.6 m.y. old.

These are largely temperate bird-plant associations in a group that is predominantly lowland and montane tropical, and the plants in North America are serviced largely by hummingbirds that migrate over long distances, while elsewhere in the range of the family any movement of the bird is usually over shorter distances, so the representative nature of such associations may perhaps be questioned, but, given the discussion above, this seems unlikely. Thus most of the neotropical hummingbird pollinated genera in Rubiaceae are unrelated, with 39 or more cases of adoption of bird pollination, and the clades of plants involved are small (C. M. Taylor, per. comm. 2015). Although Roalson et al. (2007), Perret et al. (2003, 2007), Martín Rodriquez et al. (2009), and Clark et al. (2015) note that clades of Neotropical Gesneriaceae tend to be small, this is not always the case (see ). There may be larger clades in Heliconia (see Iles et al. 2016), Centropogon (a clade of 34 spp. studied by Abrahamczyk et al. 2017), Ruellia, the Aphelandra pulcherrima group, and other Acanthaceae (Tripp & McDade 2014a and references), montane neotropical Vaccinieae, and Erythrina that are pollinated by hummingbirds, and in Parkia, Stenocereus, Agave subgenus Agave and Burmeistera clades of plants are pollinated by bats.

It has been suggested that oligolecty may facilitate the evolution of large numbers of sympatric species of plants because each is pollinated by different species of pollinators (e.g. Linsley & MacSwain 1958), however, the behaviour of many polylectic pollinators could have the same effect, since at any one time or place they may visit only one or a few species of plants. Schemske and Bradshaw (1999) in a classic paper discussed possible links between pollinator behaviour and pollination preferences of hummingbirds and bumble bees as drivers of speciation (see also Gegear & Burns 2007, floral features considered more or less separately).

9A2. Seed Dispersal.

Bats in which fruits are a major component of the diet are phyllostomids (New World) and pteropodids (Old World), and together they account for about 350 species, about 30% of all bat species (Baker et al. 2012).

Major diversification of fruit-eating phyllostomid bats, a clade of 68 species and twenty genera (Carolliinae, Glyphonectarinae, Rhinophyllinae, Stenodermatinae), began (27-)22(-18) m.y.a. in the late Oligocene to mid-Miocene (Datzmann et al. 2010; see also Rojas et al. 2011) or (18.6-)17.0(-15.4) m.y.a. (Baker et al. 2012), the stem-group age being around 17.0 m.y.a. (Baker et al. 2012). The fruit-eating habit may have evolved three times or so. Of the more obligate frugivores Carollia has a stem-group age of about 19-17.5 m.y., while that of the [Stenoderma + Ariteus] clade, 8 species in eight genera, is ca 10.3 m.y.a., with a crown-group age of (6.2-)5.4(-4.6.) m.y.a. (Baker et al. 2012, q.v. for other estimates, all broadly similar; slightly younger in Rojas et al. 2011). They eat soft fruits of secondary species as well as harder, more fibre-rich fruits of plants of primary growth and in the canopy (Muscarelli & Fleming 2007; Rojas et al. 2011). Phyllostomids may take nectar from 500 genera in 27 families (). The bats can be very abundant, and species of New World phyllostomids are wide-ranging compared to Old World pteropodids (Muscarella & Fleming 2008).

Old World fruit-eating bats (pteropodids) tend to disperse seeds of later-successional trees in families like Sapotaceae, Meliaceae, Arecaceae and Rubiaceae (Muscarella & Fleming 2008), and these tend to be canopy trees with larger fruits (least so for Rubiaceae). Pteropodids tend to be larger (less so in New Guinea) and they more commonly fly long distances (Cristoffer & Peres 2003). They take fruits from 144 genera of plants in 55 families (Marshall 1983: records from 19/44 genera of bats). On Pacific islands where they occur they may be represented by but a one or two species that are the major dispersers of 17 species of plant or more (Samoa: Banaack 1988; Vava'u, Tonga: McConkey & Drake 2015). Crown dates for pteropodids are around (29-)24(-20) m.y. (Teeling et al. 2005) or (30.3-)25.1(-22.7) m.y. (Bininda-Emons et al. 2007).

The ecological importance of these bats in terms of the services they provide plants is considerable (Freeman 2000; Muscarella & Fleming 2008).

Figs are commonly dispersed by bats (Muscarella & Fleming 2008) and birds (Snow 1981). New and Old World bats search for food in different ways and have selected figs with different qualities; overall fig morphology is quite diverse. In the New World some 21 species of Artibeus bats (phyllostomids) are the predominant ficivores. The bats are slow feeders and spit out larger seeds, fibre, etc., but they commonly disperse the tiny fig achenes. Like other bat-dispersed taxa in the New World, including Cecropia (Urticaceae) and Trema (Cannabaceae), particularly in Mexico (Lobova et al. 2009), species of Ficus can be found in early successional communities (Muscarella & Fleming 2008), and the altitudinal ranges of the bats and figs are similar (Fleming 1986). Bat-dispersed fruits in the New World often tend to be greenish or dull-coloured and odoriferous (Compton 1996 [a whole series of papers]; Korine et al. 2000; Shanahan et al. 2001; Harrison 2005). In the Old World fruits dispersed by pteropodid fruit bats are often yellow, etc., and are sometimes quite large (Kalko et al. 1996; Harrison & Shanahan 2005; Lomáscolo et al. 2008, 2010); bat-dispersed figs in Malesia, also dull-coloured, may not smell (Hodgkinson et al. 2003).

Although it is debatable to what extent figs really are keystone species, they are eaten by a great variety of vertebrates (Shanahan et al. 2001). Kattan and Valenzuela (2013) summarize the information pro and con the importance of New World Ficus as a food resource, noting that in some places the local figs, although at times producing large amounts of food for frugivores, nevertheless did not do so throughout the year. However, "[a]ll accounts agree that figs are among the most important foods of specialized frugivores [birds] in Africa, southeast Asia, and Australia..." (Snow 1981: p. 9), but in the New World specialised fig-eating frugivores were less prominent. Leighton and Leighton (1983) suggested that the loss of figs in the Bornean forests that they studied would deplete frugivore populations in general, which would impact the dispersal of other plants. Similarly, Terborgh (1986: p. 339) observed, "[s]ubtract figs from the ecosystem and one could expect to see it collapse"; in Cocha Cashu, Peru, fig species were less than 1% of the total (12 out of ca 2,000), but they sustained nearly the entire frugivore community for three months of the year. There is no comparative study of the nutritional content of Old and New World figs (Snow 1981).

New World phyllostomid fruit bats have networks that are more highly nested and have higher connectance than fruit-bird networks, but were less robust to extinctions (see also Guimarães et al. 2011; Suweis et al. 2013 for this phenomenon); at the same time, there was low complementary specialization within a network (Mello et al. 2011a, b).

Fruit-eating birds: frugivory is common in the toucan-barbet clade, where the clade [Psilopogon + The Rest] had been dated to the late Oligocene (Prum et al. 2015).

Fruits in some families dispersed by specialist frugivores may have evolved before the particular plant:frugivore relationships that are evident today (Snow 1981). New World Piper, whose seeds are dispersed by Carollia, a phyllostomid bat, presents problems. Major diversification of fruit-eating phyllostomid bats began (27-)22(-18) m.y.a. in the late Oligocene to mid-Miocene (Datzmann et al. 2010; see also Rojas et al. 2011, perhaps 3 m.y. later), but this is (a very long time) after the beginning of diversification of New World Piper that has been variously estimated at 111-34 m.y.a. (e.g. Martínez et al. 2014), so how this apparently close relationship (Fleming 2004) developed is unclear.

8B. Carbon Sequestration.


Bengtsson (1998), Augusto et al. (2014), and others have emphasized that species numbers are only one metric of evolutionary importance or success, however, numbers are quite easy to come by and they are usually the metric of choice. Estimates of biomass, productivity, even area occupied, whether for clades or for individual species, are much harder to obtain, but are other possible metrics, and from the point of view of global ecosystem functioning are of great importance. Here the focus is on particular clades of seed plants, sometimes quite small in size, that have a disproportionately large effect on communities, ecosystems, or even the global environment by fixing and/or sequestering large amounts of carbon.

It has been noted in a rather general way that in nearly all communities there are common species; one can almost expect 25% of the species to contribute around 50% of the biomass or over 80% of the individuals, and the slope of biomass accumulation is less than that of species number accumulation (Gaston 2011; see also Gaston 2010; M. D. Smith & Knapp 2003). The monodominance discussed here is more extreme, a single species representing 60% or more of the canopy individuals and/or basal area (Connell & Lowman 1989; see also Peh et al. 2011b), although it is sometimes extended to include members of the one clade (quite restricted) that grow together. Species that are dominant only in early successional phases (Type II transitional dominance, including Shorea albida!: Connell & Lowman 1989) are usually not considered further, although Newbery et al. (2013) described the long-lived Microberlinia bisulcata (see below) in the Cameroons as a "transient dominant". Note that even if they are not strictly successional, some of the vegetation types mentioned may have been affected by human activities, as is discussed later.

Pitman et al. (2001, 2013) noted a surprisingly large number of common (>1 individual/ha) and widespread species of trees at least 10 cm across in the terra firme forests of western Amazonia. Some 83 of these 150 "oligarchic" species (from several families) occurred per hectare in Peru, and although they represented over 50% of the individuals, no species was close to being a monodominant, even by the somewhat relaxed definition sometimes adopted below. Building on this work, ter Steege et al. (2013) found that half the individuals with stems at least 10 cm across in Amazonian rainforests were accounted for by only 227 species in 41 families in 11 orders; they called these species "hyperdominants". In any one plot 32 of these species represented about 40.7% of the individuals (medians; ranges 0-78 species, 0-93% individuals: ter Steege et al. 2013). However, a few of these common species may be dominants as defined here (see Arecaceae, Fabaceae below). Fauset et al. (2015), also looking at these Amazonian forests, found that about 1% of the trees (ca 160 species) were responsible for around 50% of the above-ground carbon storage/biomass production, but again, single species or even species from the one family had relatively small effects. The maximum percentage of biomass represented by a single species was a mere 1.93% (VAM/ECM Eperua falcata: Fabaceae) and the maximum total biomass of any one family in the top 20 biomass accumulators was 4.96% (Fabaceae, 1/4 the species). Bastin et al. (2015) found that a mere 18 hyperdominant species (1194 species in total recorded) produced 50% of the above-ground biomass in Central African rainforest, but for 17 of these species that amounted to 3.6% or less biomass production - interestingly, at 20% Gilbertiodendron deevrei (Fabaceae-Dialieae) was an exception (see also below). Overall, the dominance mentioned in such literature is very different from that under discussion here (see also Arellano et al. 2016 and references).

The estimates of area, biomass, carbon accumulation, primary productivity, etc., given below should be taken with more than a grain of salt, and they can vary very widely from author to author (e.g. Brown & Lugo 1984; Botkin & Simpson 1990; Dixon et al. 1994; Chmura 2011). Different countries may have different definitions of "woodland", "grassland", "peat soils", and the like, and the distinctions between communities such as sea-grass, marine salt-marsh and mangrove are not always clear. It is also easy to forget that there are three different kinds of tons, and which is being used is not always mentioned. Ideally, biomass estimates for both above and below ground are needed, and the latter in particular should include dead plant biomass; residence times ("stored carbon") of dead biomass can be (much) longer than that of living biomass (for estimates of living woody biomass, see Galbraith et al. 2013). Frequently estimates of above-ground biomass only are given, and any estimation of the one from figures of the other should be undertaken with caution (e.g. Cairns et al. 1997; Pan et al. 2013); here the focus is mostly on below-ground carbon accumulation. Indeed, soils accumulate over three quarters of the carbon (as soil organic matter) in ecosystems, carbon above-grpound biomass being less than a quarter, and it has also been estimated that half the global carbon is in subsoil more than 30cm deep, not always sampled (Schmidt et al. 2011). Furthermore, mycorrhizal biomass, particularly that of ectomycorrhizae, may have to be incorporated into such figures, given that a significant proportion of tree productivity may be diverted to their upkeep (see below).

Major Players.

C4 photosynthesis, Grasses and Grasslands.

C4 photosynthesis, grass, and grasslands together play a major role in global ecology. Overall only somewhat over 2% of angiosperms - perhaps some 7,500 species - are C4 plants (R. Sage et al. 2012). They can be divided into three main groups: grasses, sedges, about which rather little is known ecologically, and core eudicots; in none is the origin of C4 photosynthesis monophyletic. The C4 photosynthetic syndrome has evolved 22-24 times in grasses, and 66-68 times in angiosperms as a whole (R. Sage et al. 1999, 2011, 2012; Ludwig 2011b). It is found in ca 1,500 species of Cyperaceae, 4,500 species of Poaceae, 500 species of chenopod-style Amaranthaceae as well as in other core Caryophyllales, ca 340 species of Euphorbiaceae (Euphorbia subg. Chamaesyce section Anisophyllum), and so on (Arakaki et al. 2011; R. Sage et al. 2012). Perhaps the immediate drivers of the evolution of this distinctive syndrome are the notable decreases of CO2 in the atmosphere at the beginning and again towards the end of the Oligocene along with briefly increasing temperatures, which together would lead to an increase in photorespiration, and temperature, high light levels, etc., are all important, with C4 photosynthesis favoured above ca 23oC (Still et al. 2003: fig. 1) In grasses, C4 photosynthesis is associated with arid climates, wheras in rosids it is more immediately associated with succulence (Lauterbach et al. 2016 and references). However, we still understand rather little about details of the origin and spread of C4 photosynthesis (e.g. Edwards et al. 2010; Kellogg 2013a; Cowling 2013; Heckmann et al. 2013; Williams et al. 2013; Christin & Osborne 2014; Y. Wang et al. 2014; etc.), although parallelisms at the morphological molecular levels have been noted several times (e.g. Brown et al. 2011; Külahoglu et al. 2014; Aubry et al. 2014).

The global distribution of C4 vegetation is ca 18.8 x 106 km2 occupying somewhat over 15% of the total land area (Still et al. 2003: see map). All told C4 photosynthesis accounts for about 23-28% of terrestrial gross primary productivity (35.3 Pg C yr-1, vs 114.7 Pg C yr-1), although the biomass of C4 plants is less than 5% of the global total, 18.6 vs 407.9 Pg C yr-1 (figures from Still et al. 2003: see also Lloyd & Farquhar 1994; Ehleringer et al. 1997; Gillon & Yakir 2001; Retallack 2001; R. Sage et al. 2012). Most of the difference is in the woody biomass, that of C3 plants being 352.7 PgC and that of C4 plants zero (obviously Caryophyllales not factored in); in both cases root and leaf biomass was estimated to be about equal, that of C3 plants being about twice as much as C4 plants, 36.6 vs 18.6 Pg C (Still et al. 2003). Other estimates of total biomass are similar: 15.6 vs 488.5 Pg C (Ito & Oikawa 2004).


Grasslands often have distinctive soils. Root systems in mature grasslands are dense, and the soils are up to 1 m deep with good crumb structure and much organic matter (mollisols: Retallack 2001, 2009). The total carbon sequestration in grasslands is greater than that of the forests they replaced, and in particular the proportion of the biomass sequestered in the soil increases. Grassland soils are notably moister than corresponding woodland soils because woodlands have a lower albedo, so they reflect incoming radiation less, they are warmer and so transpire more, and so their soil is drier. Somewhat paradoxically grasslands support a cooler, drier climate, yet one that allows increased weathering, which consumes carbon; erosion from grasslands leads to a loss of organic carbon in sediment that is an order of magnitude larger than the corresponding loss from forests (Retallack 2009). Evidence from palaeosols suggests that grasses replaced woodland (Retallack 2001, 2013a). Savanna trees allocate relatively more carbon to below-ground biomass than do forest trees, while in both Cerrado and African savanna there has been the recent evolution of woody plants with massive stem and root systems underground, but with little permanent above-ground biomass (Scheiter et al. 2012; Pennington & Hughes 2014; Bond 2016a). Nutrients are also rapidly mobilized and when lost in run-off they ultimately support ocean productivity (Volk 1989). Nitrogen is volatilized in fires, but C4 grasses have low nitrogen requirements.

Productivity estimates for grasslands in particular, including both C3 and C4 species - are that they currently account for 11-19% of net primary productivity on land and 10-30% of soil C storage (Hall et al. 2000); soil C storage estimates in Averill et al. (2014) are (12.3-)14.5(-17.7) kg C m-2, NPP figures being (477-)576(-675) kg C m-2 yr-1. Gibson (2009) suggests that grasslands store 650-810 GtC, ca 33% of the global total, and 55-95% of that is stored underground, the higher values being in higher-latitude (probably = Arctic) areas. More general figures for tropical savannas and grasslands together - the grasses are likely to be C4 grasses - in Carvalhais et al. (2014: Tables S1 + S2) are ca 338 Pg total C, a carbon density of around 17.7 kgC m-2, and a mean turnover time of (12.2-)16(-22.1) years; the last set of figures is only slightly higher than those for l.t.r.f.. Comparable figures for temperate grasslands and shrublands are (145-)187(-249) Pg total C, a carbon density (13-)16.7(-22.2) kgC m-2, and a mean turnover time of (32.8-)41.3(-54.6) years; the residence time for carbon is much longer here, largely because of the cooler temperatures.

Only some 600 species of grass dominate ecologically worldwide, and most of these are C4 photosynthesizers (Edwards et al. 2010). Andropogoneae, with some 1,200 species and 90 genera, are notable in responding positively to annual burning (Forrestel et al. 2014; Visser et al. 2015), and may dominate in fire-prone grasslands. In African, Australian and North American grasslands and savannas in particular members of the ASH clade (Andropogon, Schizachyrum, Hyparrhenia), with some 244 species, are prominent among the dominants (Estep et al. 2014). However, dominance relationships brtween grasslands herbs may change as climate changes (Yu et al. 2015).

The great expansion of C4 grassland is geologically very recent, occuring only within the last 10 m.y., and within the last 3-2 m.y. in particular (e.g. Strömberg & McInerney 2011; McInerney et al. 2011; R. Sage et al. 2012). Similarly, the extensive and very speciose Brazilian Cerrado savanna vegetation with flammable C4 grasses and African savannas also developed within the last (10-)5 m.y. (Simon et al. 2009; Simon & Pennington 2012; Maurin et al. 2014). Associated with the spread of grasslands is a pronounced increase in fires ca 10 m.y.a. (e.g. Bond & Midgley 2000; Keeley & Rundel 2005; Bond & Scott 2010). Overall, grasslands are a long-term carbon sink and they contribute to long-term global cooling (Volk 1989; Retallack 2001); as Retallack (2009: p. 100) noted, "grasslands did not merely adapt to climate change, but were a biological force for global change".

Chenopods. C4 eudicots are often found in some combination of arid, ephemeral, warm to cold (at least in the winter), disturbed and/or saline conditions (Ehleringer et al. 1997; Kadereit et al. 2012). In the rather cold Gobi deserts of Mongolia 15-17% of the species are C4 plants, and over 50% of these are chenopods; chenopods contribute 30-90% of the biomass there, although overall C4 plants are only 3.5% of the total Mongolian flora (Vostokova et al. 1995; Pyankov et al. 2000). Similar fast-growing C4 Chenopodioideae (and some Polygonaceae), some of which like Haloxylon aphyllum are arborescent - it can reach 10 m in height with a trunk 1 m across (Winter 1981) - also dominate the halophytic vegetation of the somewhat warmer Central Asian Turanian deserts (Winter 1981). Succulent C3 chenopods are common in the Gobi in true desert conditions, and also in moist, saline soils (Pyankov et al. 2000).

Age: C4 photosynthesis may have originated in the Oligocene ca 33 m.y.a., but C4 grasses became diverse - and made a corresponding major contribution to overall vegetation biomass - only in the late Miocene 9-8 m.y.a., the process being complete as recently as the late Pliocene 3-2 m.y.a. (e.g. Edwards et al. 2010; Strömberg & McInerney 2011; McInerney et al. 2011; Strömberg et al. 2011; Arakaki et al. 2011; R. Sage et al. 2012; Bouchenak-Khelladi et al. 2014; etc.); for further details, see Poaceae. C4 grass-rich and fire-prone savannas in Africa and the Cerrado in South America developed at about the same time (Simon et al. 2009; Simon & Pennington 2012; Maurin et al. 2014; Pennington & Hughes 2014).

Ectomycorrhizal Plants.

Introduction. The ectomycorrhizal (ECM) habit predominates in only a few clades of seed plants, but these plants occupy about half the forested area of the globe (L. L. Taylor et al. 2011). Estimates of the number of ECM species based on a comprehensive survey of mycorrhizal plants and which takes into account species not studied and so, although high, may be in the right ball park, are around 29,300, i.e. very roughly only 2% of seed plants (Brundrett 2009; Maherali et al. 2016: inc. gymnosperms, but not taxa with ericoid mycorrhizae; see also Brundrett 2009; angiosperm numbers from Paton et al. 2008) or 16,100 (starting off with figures from Brundrett et al. (2002); previous "high" estimates were ca 8,000 species (Rinaldi et al. 2008), while at the estimate of 3% of seed plants in Merckx et al. (2013c) is similar. ECM plants include some 1000 species of Fagales, Pinaceae (210 spp.), Dipterocarpaceae (680 spp.) and relatives (together 915 spp), and Fabaceae-Detarioideae (250 spp. - Brundrett 2009; 450 spp. - B. Mackinder pers. comm. viii.2012), while some Salicaceae (Salix, Populus, 485 spp.) and other plants are also ECM (see Wurzburger et al. 2016 for clades in which ECM and ERM are known). The map shows very approximately areas where Ericaceae (olive: ericoid mycorrhizae, ERM, see below), ECM Pinaceae (red), and ECM Fabaceae-Detarioidieae communities (blue) predominate (from end-papers of Specht 1979a; White 1983; White et al. 2000; Matthews et al. 2000; Andersson 2005 - see also Read 1991). The area shaded red and olive green above 50o N in the map - tundra and much of the boreal forest - is mostly in the permafrost region.

There may not be that many ECM species of seed plants, but there are 7,750 described species of ECM fungi - but probably many more, perhaps up to 25,000 (Blackwell 2011; esp. Rinaldi et al. 2008; Weiß et al. 2016; see also Pickles & Pither 2013); and there can be impressive single-site fungal diversity (Horton & Bruns 2001: examples mostly Pinaceae-dominated forests). Indeed, Sánchez-García and Matheny (2017) suggest that in Tricholomatineae, at least, the ecological success of ECM plants may have positively affected fungal diversification as much as the ECM habit itself being an (ecological) key innovation. Overall, there are about as many (or considerably more) species of ECM fungi as ECM plants, although there may be relatively fewer species of fungi in Ericaceae (see below) and Orchidaceae (Rinaldi 2008, but c.f. van der Heijden et al. 2015a). (There are relatively far fewer species of AM fungi than their hosts.) ECM associates of taxa like Alnus show considerable host specificity (Bruns et al. 2002; Walker et al. 2013; Wicaksono et al. 2017), while species in a small clade of boletes - [Strobliomyces + Afroboletus], ca 50 species - are associated with Fagales, Pinaceae, Pabaceae, Fabaceae-Detarioideae and Dipterocarpaceae (Sato et al. 2016: host shifts associated with changes in diversification rates?).

The almost 4,000 species of Ericaceae with ectendomycorrhizal ericoid mycorrhizae (ERM) are not included in these totals. Nevertheless, Vrålstad (2004), Villareal et al. (2004), Brundrett (2004), Imhoff (2009), Tedersoo et al. (2010b) and others (see also above) have suggested that ECM and ERM form a single ecological guild - and together they would be found in ca 3% of flowering plants (Brundrett 2009). One of the characteristics of the guild is that the fungi are intermediaries in the uptake of nitrogen from soil organic matter by the plant (e.g. Read 1991, 1996; Peay 2016; c.f. Persson & Näsholm 2001), although ERM fungi are more or less saprotrophic, unlike ECM fungi (Kohler et al. 2015; Peay 2016). Another similarity is the architecture of the plant-fungal networks (Toju et al. 2016 and references). ERM fungi include ascomycetes, the basidiomycete Sebacinales-Serendipitaceae, etc., but the number of species of fungi involved is unclear (Weiß et al. 2016). ERM are included in the discussion below, but orchid mycorrhizae (see Orchidaceae), also often considered to be modified ECM and with similar plant-fungal networks, are not discussed further (see also Imhof 2009). It should also be remembered that some dark septate endophytes can also form ERM and ECM associations, indeed, sometimes a single species forms both kinds of associations (Lukesová et al. 2015).

ECM/ERM plants are especially common in subarctic to (cool) temperate habitats, and in tropical montane vegetation, basically, in areas with predominantly organic-bound nutrients (Comas et al. 2012), the fungi being able to access soil organic matter (Shah et al. 2016; Peay 2016: see below). However, ECM plants are also common in lower altitude tropical West Malesian dipterocarp forests, African Miombo (ca 2.6 x 106 km2 alone - see Iimberlake et al. 2010) and Sudanian woodlands, and also considerable areas of the Guineo-Congolian coastal and Ituri rainforests (e.g. Malloch et al. 1980; White 1983; Safer 1987; Connell & Lowman 1989; Hart et al. 1989; Read 1991; Sanford & Cuevas 1996; Torti et al. 2001; Peh et al. 2011b); Sudanian and Miombo woodlands are biogeographically close (Linder et al. 2012). Australian Eucalyptus woodlands are also ectomycorrhizal. ECM plants are not often common in Amazonian forest, although they are known from acidic white sand vegetation and other less fertile soils (Roy et al. 2016; Peay 2016).

ECM/ERM plants are generally woody, although there are some herbaceous ECM taxa, perhaps particularly in Arctic-Alpine environments (e.g. Newsham et al. 2009). ECM/ERM plants tend to dominate the communities in which they grow, although these communities are often not notably diverse, in part perhaps because of the dearth of readily available nutrients (L. L. Taylor et al. 2009; Reich 2014 for literature; see below). Many ECM plants are large individuals, each representing a substantial amount of standing biomass, and, very importantly, the soil is often acid, litter commonly accumulates, and peat formation is common (e.g. Yu et al. 2010; Page et al. 2011). A very approximate estimate of the number of ecologically dominant plant ECM species is 1,000. This includes ca 160/388 species of Dipterocarpaceae, 11/13 Nothofagaceae, and 50/165 Fagaceae from Malesia alone (data from Ashton 1981; Soepadmo 1972), however, P. S. Ashton (pers. comm. vii.2012) noted that only ca 13 species of dipterocarps were major dominants. Ca 20 species of Ericaceae are widespread in boreal forest and tundra habitats, a few species of Vaccinium are very abundant locally in montane forests in Malesia, and Rhododendron similarly dominates locally in the Himalayas-Yunnan region.

Vegetation dominated by particular ECM species often also includes other ECM/ERM plants. Thus in the Mediterranean Maquis vegetation, Cistaceae (close to Dipterocarpaceae), Fagaceae and Pinaceae, all ECM plants, are all important components of different aspects of the vegetation. There are extensive oak-pine ECM forests in the eastern United States and Mexico, while in western North America the oak-pine forests include a substantial element of Arbutus menziesii (Waddell & Barrett 2005), an ericaceous tree with arbutoid mycorrhizae. Forests with ECM Fabaceae, Dipterocarpaceae and Phyllanthaceae are common in tropical Africa. Boreal forests are dominated by ECM Pinaceae with some ECM Betulaceae and Salicaceae, and the understory often includes ERM Ericaceae (Read 1993).

Evidence suggests that the establishment of conspecific seedlings in ECM forests is often less affected than might be expected given the density of the parental trees, i.e. ECM species are less subject to the Janzen-Connell effect (Terborgh 2012). Thus seedlings of ECM trees can germinate close to their parents (e.g. McGuire 2007b; Michaëlla Ebenye et al. 2017) while seedlings of AM trees do so less readily (van der Heijen & Horton 2009; Laliberté et al. 2015; Bennett et al. 2017: Fig 2a), in part at least perhaps because the ECM investing the root helps protects the roots against pathogen attack (e.g. Koele et al. 2012; Laliberté et al. 2015), and this would facilitate the development of the dominance shown by many ECM trees. Dicymbe seedlings quickly tap in to the ECM network in Dicymbe forests in Guyana (McGuire 2007a, b). Interestingly, Corrales et al. (2016) suggested that despite strong negative plant-soil feedback to seedlings, monodominant stands of Oreomunnea mexicana formed because the ectomyccorhizal fungal associated with the plant could obtain nitrogen directly from organic matter. In temperate forests the regeneration of the more abundant species (not necessarily ECM) may show weaker negative density dependence that that of the less common species (D. J. Johnson et al. 2012a, b; c.f. Dickie et al. 2012; Liu et al. 2015: Fusarium oxysporum specialist on Ormososia glaberrima), perhaps enhancing their dominance, although this is not a general latitudinal effect (Comita et al. 2014). In more species-rich areas of South American forests there are stronger negative density dependence effects, fungal pathogens and insect herbivores driving up diversity (Mangan et al. 2010: Panama; Terborgh 2012: review, esp. Peru; Johnson et al. 2012a, b: Bagchi et al. 2014; Comita et al. 2014; see also Kulmatiski et al. 2008 and Schnitzer et al. 2011: grasslands).

Many ECM plants have a distinctive ecological syndrome: They are trees, often locally dominant and casting deep shade; they are mast fruiters, i.e., they fruit more or less simultaneously yet intermittently over large areas and they have large seeds; and the soils they grow on are poor in inorganic nutrients, often with deep leaf litter and accumulating humus, yet the organic nitrogen is accessible to them (e.g. Connell & Dawson 1989; Alexander 1989b; Richards 1996; Torti et al. 2001; Newbery 2005; Peh et al. 2011b; Terrer et al. 2016: see also Koenig & Knops 2000, 2005; Norden et al. 2007; Veller et al. 2015; Pearse et al. 2016 for mast fruiting). However, although Fabaceae-Detarioideae have most of these features, not all are mast fruiters, and some other Fabaceae that also have most of them are AM plants (see also Torti et al. 1997, 2001). Similarly, Nascimento and Proctor (1997) noted that there was little difference between the soils on which the ECM Peltogyne dominated and those in adjacent more diverse forests. Furthermore, ECM/ERM plants from cooler areas like Pinaceae, Salicaceae, and some Betulaceae, and perhaps ERM Ericaceae, have a somewhat different ecological syndrome. Although they are trees or shrubs, they have much smaller seeds and do not show mast fruiting, but like other ECM plants, they can dominate the communities in which they are found, and they grow on acidic and peaty soils - and they can tolerate soils with toxic metals (e.g. Read & Perez-Moreno 2003; Nara et al. 2003; Cairney & Meharg 2003). Indeed, Koele et al. (2012) found no particular correlations between foliar traits and ECM status. (Mast fruiting has been described from a number of trees and lianes from tropical French Guiana in South America, but these are not known to be ECM plants nor to be particularly dominant in their communities - see Norden et al. 2007.)

Before going further, a digression on lignin decomposition is in order. White-rot basidiomycete fungi are the most important decomposers of lignin. The capability to degrade lignin may have evolved in the ancestor of the Agaricomycetes clade (372-)290(-222) m.y.a. around the beginning of the Permian, in particular, in the [Auriculariales + The Rest] clade which have fungal class II peroxidases and other components of the lignin decay system, and this is consistent with the fossil record, the earliest fossils of white-rot (lignin-decomposing) fungi being around 260 m.y.a. (Floudas et al. 2012: also other similar dates; Kohler et al. 2015: ca 294 m.y.o.; Nagy et al. 2015). The white-rot fungi show extensive expansion of the gene families involved in the decay process - they can break down both lignin and cellulose - and they also have many gene families of as yet unknown function (Nagy et al. 2015, 2016). Eastwood et al. (2011) suggested an age of around 219.6 m.y. for the [Russulales + Agaricales] clade, while a larger clade in which the basal pectinations are white rot fungi can be dated to 250-234 m.y.a. (Kohler et al. 2015). Dacrymycetes are sister to Agaricomycetes and are brown rot fungi that lack lignin-decomposing enzymes, in particular, the dacrymycete Calcera cornea is a brown-rot fungus with a soft-rot, not white-rot, ancestor (Kohler et al. 2015; Nagy et al. 2015, 2016). The basal agaricomycete Sebacinales and Cantharellales also have other life styles.

Peroxidases of white rot fungi play a central role in lignin degradation by mineralizing the lignin, at the same time producing CO2, etc., and white rot fungi also have several enzymes that can degrade crystalline cellulose. Brown rot fungi can access much of the crystalline cellulose in the cell wall quite readily, but they do not destroy the lignin; they may have evolved from white-rot fungi (e.g. Floudas et al. 2012; Lundell et al. 2014; Kohler et al. 2015; Nagy et al. 2016), although Dacrymcetes are brown rot fungi without any obvious white rot ancestry (Nagy et al. 2015). Indeed, the distinction between the two groups of fungi is somewhat artificial (Riley et al. 2014; Floudas et al. 2015), and they are better thought of representing two ends of a spectrum of fungi that have various combinations of lignin- and cellulose-degrading enzymes, furthermore, agaricomycetes and also ascomycetes that cannot break down lignin may have enzymes that are involved in the breakdown of cellulose (see also Soudzilovskaia et al. 2015; Nagy et al. 2015). Some ascomycetes, soft rot fungi, can also break down lignin (Shary et al. 2007 and references), and many xylariaceous fungi can break down lignin and cellulose, "the most efficent of them rival[ling] basidiomycetes in substrate degradation" (Rogers 2000: p. 1414). The ECM ascomycete Oidiodendron maius is saprotrophic, breaking down Sphagnum peat, and it has both cellulose and some lignin-decomposing enzymes (Kohler et al. 2015), while UV light also decomposes lignin (Austin & Ballaré 2010), although I do not know how globally important these processes are/were ecologically.

ECM fungi are not saprotrophic, i.e. they have few enzymes involved in cellulose and lignin breakdown and do not obtain their metabolic carbon from dead organic matter, rather, this comes from the plant, although ERM fungi may retain some saprotrophic genes from their ancestors and break down lignin (e.g. Michelsen et al. 1996; Jonasson & Michelsen 1996; Hashimoto et al. 2012; Vohník et al. 2012; Lindahl & Tunlid 2014; Kohler et al. 2015). Given the extreme polyphyly of the ECM habit, different clades may vary in details of the nature of their associations with plants, but in at least some cases physiological similarities between different clades of ECM fungi are quite extensive (Peter et al. 2016). The hyphae of ECM fungi form a complete sheath investing the fine roots, the Hartig net, and hyphae penetrate between the exodermal/cortical cells; this sheath, not the root/root hair system, forms the interface between the plant and the soil (e.g. L. L. Taylor et al. 2009), and they also form rhizomorphs of various morphologies that extend sometimes many metres from the plant, the nature of the rhizomorph depending on the particular association which is in turn linked to overall nitrogen availability (Agerer 2001; Hobbie & Agerer 2010; Águeda et al. 2014 and references) and which do a lot of the foraging for nutrients (see below). Usually the Hartig net involves only the outermost layer of root cells, but sometimes, as in Pinaceae, the hyphae penetrate more deeply (Brundrett 2004). The fungal hyphae are septate and are not intracellular except in ERM, which usually lack a Hartig net (Ericaceae have quite a variety of associations with fungi: Weiß et al. 2016 and references). The ecological effect of these plant-fungus associations hinges on the ability of the fungus to utilise the humic substances that make up soil organic matter (e.g. Schmidt et al. 2011; Rosling et al. 2015; Shah et al. 2016). Although ECM fungi have only a limited ability to break down lignocelluloses, some have coopted oxidizing enzymes that, with sugar from the plant, do break down lignocellulosic material, liberating N and P which is taken up by the plant, or they use peroxidases for this purpose (Shah et al. 2016). These enzymes are normally characteristic of brown and white rot fungi respectively, and have evolved functions that differ from those of the same enzyme in non-ECM associations, e.g. rather than obtaining carbon they scavenge nutrients (Doré et al. 2015; Shah et al. 2016).

ECM plants differ from AM plants in their nitrogen metabolism, the former obtaining N from the persistent litter while the latter have less litter and soil with mineral C produced by the activities of soil organisms and inorganic nitrogen, the source of N for the plant (e.g. Read 1991; Philips et al. 2013; Bardgett et al. 2014; Lin et al. 2016). Even when ECM and AM forests have similar litter inputs, there is more litter accumulation in the former (Lin et al. 2016: largely temperate plantation comparisons; see also Baskaran et al. 2016: modelling). Trees from higher-latitude habitats in particular often have both long-lived leaves and fine roots, the latter being well-defended (as by ECM: Laliberté et al. 2015 and references). Furthermore, ECM taxa like oaks and pines are efficient at removing N and P from the leaves before they die, and the result is nutrient-poor humus, unsuitable for AM plants which are often faster-growing and need nitrogen (Phillips et al. 2013). ECM and ERM plants can utilize N as complex organic compounds (see above), and this is affected by pH and litter polyphenols, which also vary in tandem at the infraspecific level (Northup et al. 1995). Indeed, Averill et al. (2014; also Soudzilovskaia et al. 2015: top/organic soil layer) noted that there was about 70% more carbon per unit nitrogen in the soils of ecosystems dominated by ECM plants; the nitrogen remained accessible to the fungi, but not to their microbial competitors. However, Averill et al. (2014) thought that litter with a higher C:N ratio would lead to less overall C storage in the soil because microbes would respire relatively more C to obtain N. Low rates of litter decay (often accompanied by high MA - leaf mass per area - values) are features of both gymnosperms and angiosperms that are able to grow in stressful, nutrient-poor environments (Berendse & Scheffer 2009).

The litter of some ECM trees tends to decompose more slowly than that of other vascular plants, whether deciduous or evergreen (Pérez-Harguindeguy et al. 2000; Cornelissen et al. 2001; Alexander & Lee 2005; Cornwell et al. 2008; Lang et al. 2011; Phillips et al. 2103) and conifer litter decomposes more slowly than that of angiosperms (e.g. Wardle et al. 2008; Cornwell et al. 2008b; Weedon et al. 2009); litter accumulation is also notable in tropical ECM communities (Torti et al. 2001). However, the generality of correlations between ECM plants and their litter composition is perhaps questionable (esp. Koele et al. 2012; see also Liu et al. 2014: Fagaceae; Corrales et al. 2016: Oreomunnea). Litter from ERM plants may be the most recalcitrant (Read 1991; Cornelissen et al. 2001; Alexander & Lee 2005). Interestingly, although litter from AM plants decomposes faster than that of ECM plants, the mineral-associated soil organic matter produced is stable and the organic nitrogen is inaccessible to the fungi (Terrer et al. 2016 and references).

In Rhododendron, at least, nitrogen in stable protein-tannin complexes formed by the plant are more easily accessed by its own ERM associates than by ECM or AM roots (Wurzburger & Hendrick 2009). Nitrogen mobilization is also affected by the polyphenols in the litter (e.g. Northup et al. 1995). Enzymes, etc., produced by the ERM plant/fungus association contribute to the formation of acidic mor humus that ERM plants like and AM plants do not (e.g. Read 1991). Brown rot fungi are associated with conifer forests in particular, but they are unable to destroy lignin, which accumulates in large amounts and binds nitrogen and cations (Eastwood 2011 and references). ECM forests may have few bacteria and litter-decomposing fungi, and, as noted below, ECM fungi themselves also affect litter decomposition (Ekblad et al. 2013). Although Näsholm et al. (2013) interpret ECM fungal activities in conifer forests somewhat differently, with nitrogen in some circumstances being retained in fungal mycelium, the consequences are similar; non-ECM plants will be at a disadvantage in the nitrogen-poor conditions that result. Similarly, Newbery et al. (1997) found that in some forests on poor soil in Cameroon the phosphorus in soil and litter was preferentially accessed by the dominants, ECM Dialeae (Fabaceae), again, ECM plants can exploit the rather extreme soil conditions they themselves make. At the same time, it should be noted that some ECM communities grow in quite nitrogen-rich habitats, as in the Pacific Northwest, but the nitrogen is taken up as NH4+, not NO3-, ions (Kranabetter et al. 2015) as is a feature of the organic nutrient ecomonomy of ECM communities (Phillips et al. 2013). ECM, ERM, and their seed-plant associates together, breaking down soil organic matter to get at the nutrients it contains, nitrogen in particular, in a way that AM cannot and competing with microbial decomposers for nitrogen, so forming the soil/humus conditions that they all prefer (see also Read 1993; Phillips et al. 2013; Shah et al. 2016). Indeed, interactions between the nitrogen metabolism of the plant and their ECM/ERM associates is a recurring theme in this whole literature (e.g. Koele et al. 2012).

Furthermore, ECM fungal hyphae in particular may have melanin, a substance particularly resistant to degradation (e.g. Butler & Day 1998: white rot fungi may be able to decompose it; Bardgett et al. 2014; Clemmensen et al. 2014), in their hyphae (e.g. Fernandez et al. 2013; Peter et al. 2016), and so carbon in ECM hyphae turns over more slowly than that in AM hyphae (Phillips et al. 2013). Substantial amounts of mycelium may be produced every year, whether as individual hyphae, as in ascomycete ECM, or in more massive rhizomorphs, as in many basidiomycete ECM (Visser 1999; Agerer 2001; Hobbie & Agerer 2010; Koide et al. 2013; Águeda et al. 2014), although the actual contribution of ECM fungi to soil organic matter is unclear (Ekblad et al. 2013: few systems studied; Soudzilovskaia et al. 2015). Much of this hyphal biomass ultimately comes from the host plant, the ECM fungi utilizing up to 20% of the photosynthetic C it produces (S. E. Smith & Read 2008; Koide et al. 2013; Högberg et al. 2010; Phillips et al. 2013; Ekblad et al. 2013 and literature), and Litton et al. (2007) note the total below-ground carbon flux in forests is 25-63% of their gross primary productivity. These figures are similar to the amount of productivity devoted to fine root production and turnover, and so the total figure of plant-soil biomass changes considerably if both are incorporated into the community carbon budget, as needs to be done (Philips et al. 2013; Bargett et al. 2014; McCormack et al. 2015; Laliberté 2016). As Schmidt et al. (2011) noted in a review that did not focus on ECM, soils contain at least three times as much carbon as soil organic matter as there is in the above-ground biomass. Fungi with long distance rhizomorphs may contribute around 15 times more biomass to the soil than do those with short-distance rhizomorphs (Koide et al. 2013), indeed, the rhizomorphs may live longer than the roots with which they were initially associated (references in Ekblad et al. 2013). Fernandez and Koide (2014) found that amounts of melanin, along with those of nitrogen, determined the rate of hyphal breakdown, suggesting that from this point of view melanin was an analogue of lignin, being decay-resistant; fungal rhizomorphs were likely to decay more slowly than individual hyphae (Koide et al. 2013 and references). However, hyphae of the common and widespread ECM (it can also be a saprophyte - Meyer 1964) ascomycete Cenococcum humile, which have much melanin, persisted up to ten times longer than the hyphae and rhizomorphs of other ECM fungi (Fernandez et al. 2013). (Interestingly, somewhat carbonized sclerotia of Cenococcum or similar fungi were mistaken for carbonaceous spherules produced by intense fires following a supposed bolide impact that triggered the Younger Dryas period - Scott et al. 2010.) The hyphae of Cenococcum are notably common in dry and nutrient-poor mor humus in boreal and temperate forests, thus mor in Fagus sylvatica forests has both a very high density of beech rootlets and substantial amounts of mycorrhizae, about half of which is Cenococcum (Meyer 1964). In Swedish boreal forests, much root biomass was in the soil to about 20 cm down, the depth of soil in younger forests, but older soils were deeper and in the deeper portions ECM and EM were major contributors to the biomass. In younger soils, although the fungal biomass might be considerable, it decayed faster than that in the older, deeper soils (Clemmensen et al. 2013). Dark septate endophytes also have melanin, and may on occasion form ECM or ERM associations (Lukesová et al. 2015). All in all, dead and rather decay-resitant fungal mycelium makes up a substantial component of the total soil carbon in ECM forests, and the source of this mycelial carbon is largely the ECM trees (Clemmensen et al. 2013; Koide et al. 2013).

The combined result of plant decay and fungal activity in ECM plants is often soil with acid conditions and persistent litter with a high C:N ratio; this will tend to constrain mineralization by other than ECM plants, as will the seasonal/cold and sometimes dry climates that many ECM/ERM plants favour (Read 1991; Augusto et al. 2014). ECM/ERM communities are often found on rather extreme soils, including serpentines (Branco & Ree 2010), that are either poor in nutrients (e.g. Michelsen et al. 1998), and/or rich in organic materials and/or without much other vegetation (e.g. Read 1993). In such communities soil pH is, or becomes, low, and sometimes massive amounts of mor humus commonly accumulates, especially in cooler climates; podzolization may also occur (van Schöll et al. 2008). In general, the acid, nutrient-poor conditions and high water tables that develop are not conducive to the activity of many potential decomposers of humus - a self-reinforcing cycle - and there is CO2 sequestration.

There is one more aspect of ECM activities which affects global carbon balances. ECM fungi in particular - AM fungi less so, although they, too, are active - facilitate subsurface weathering of rocks, especially when basaltic, so sequestering additional CO2 in the process (e.g. Landeweert et al. 2001; van Schöll et al. 2008: Al moves from the rock to the humus layer?; Taylor et al. 2009, 2011, 2012; Comas et al. 2012; Quirk et al. 2012, 2014, c.f. Porada et al. 2016 for remarkably high estimates of Ordovician bryophyte and lichen rock weathering). Siderophores and low molecular weight organic chelators like oxalic acid produced both by ECM fungi and their bacterial associates all increase the breakdown of silicate minerals and weathering of rocks, both basaltic and granitic (e.g. Knoll & James 1987; Frey-Klett et al. 2007; L. L. Taylor et al. 2009, 2011, 2012; Comas et al. 2012). These low molecular weight organic acids can mobilize cations such as Ca++ and Mg++, increase phosphorus availability, etc. (Taylor et al. 2012); siderophores chelate iron and oxalate forms complexes with aluminium ions, detoxifying the aluminium but also increasing the weathering of aluminium-containing minerals in rocks (Landeweert et al. 2001; Hoffland et al. 2001; van Schöll et al. 2008). CO2 produced by the respiration of the fungus-plant association is used up in this weathering as it reacts with water, CaCO3 and silicate minerals, and carbon is ultimately carried out to sea (e.g. Berner 1997; Beerling 2005a; L. L. Taylor et al. 2009). Quirk et al. (2014) showed that plants allocated more carbon to ECM than to VAM, and that ECM caused correspondingly higher rates of calcium silicate dissolution from basalt, a rate proportionally reduced when atmospheric CO2 concentrations were low. It has been suggested that the evolution of the ECM habit "represents the most profound alteration in root functioning to occur in plant history..." (Taylor et al. 2011: p. 369; c.f. in part Boyce & Lee 2011), and it is not for nothing that ECM fungi have been dubbed "rock-eating fungi" (Jongmans et al. 1997: p. 682).

Thinking about EM and ECM emphasizes the below-ground dimension of plants, i.e. the large proportion of the plant's metabolic capabilities that go to support both its roots and their fungal associates and the important part that roots and mycorrhizae together play in carbon sequestration and the like. It also emphasizes how difficult it is to understand what is going on below ground, and how little is known about biomass accumulation there and its longevity (e.g. Jones et al. 2004; Bardgett et al. 2014; Laliberté 2016), an issue that needs to be remembered in the following discussion.

Age: So what are the major fungal clades involved and when did ECM-plant interactions develop?

ECM associations have formed perhaps 78-82 times in fungi, especially in ascomycetes and basiodiomycetes, but also in Zygomycota, and more origins are likely to be discovered especially in tropical and south temperate areas (Tedersoo & Smith 2013; see Wurzburger et al. 2016 for clades). Dates of the origins of these fungal ECM clades, including those associated with Pinaceae, are split about equally between Late Cretaceous (e.g. Amanita) and Cenozoic (e.g. Hebelomateae, Tricholomatineae) (Ryberg & Matheny 2012; Tedersoo et al. 2014a and references; Sánchez-García & Matheny 2017: movement on to Pinus even then). Bonito et al. (2013) looked at the evolution of truffles (ascomycetes), and they suggested that the age of the clade that included Helvellaceae and Tuberaceae, all ECM fungi, was (184.7-)160.8(-137.4) m. years. The m.r.c.a. of Tuberaceae was dated to some (179.1-)156.9(-134.5) m.y.a., and Bonito et al. (2013) thought that its host was likely to have been an angiosperm. Kohler et al. (2015) suggest that ECM associations with plants developed in the last ca 175 m.y., and a clade of ECM, brown rot, and one white rot fungus is dated to ca 115 m.y. ago. Augusto et al. (2014) dated confirmed ECM symbioses in both angiosperms and gymnosperms as mid-Cretaceous some 115 m.y.a., probable ECM symbioses in gymnosperms may have occurred over 200 m.y.a. in the Late Triassic and possible ECM symbioses in the Permian, over 250 m.y.a., although the authors warn about extrapolating from the ecophysiological proclivities of modern gymnosperms to those of early gymnosperms. Clade size varies greattly. Thus the ascomycete Cenococcum geophilum, the commonest ECM fungus, is the only member of the 19,000+ species of the Dothidiomycetes with this life style (Peter et al. 2016).

The Plants. ECM plants are clumped phylogenetically (e.g. Alexander & Lee 2005; L. L. Taylor et al. 2009, 2011; Maherali et al. 2016), and the ecologically most important ECM species are found in four main clades, Pinaceae: 1 origin, Fagales: 1 origin, Dipterocarpaceae/Sarcolaenaceae/Cistaceae: ?1 origin, and Fabaceae-Detarioideae: ?origins. Over 40 families of seed plants are involved (Hibbett et al. 2000; Hibbett & Matheny 2009; Bruns & Schefferson 2004, B. Wang & Qiu 2006; S. E. Smith & Read 2008; Tedersoo et al. 2010b, 2014a; Koele et al. 2012 in part; Wurzburger et al. 2016 for ECM/ERM seed plant orders). For the evolution of ECM associations from vesicular-arbuscular mycorrhizae, see Maherali et al. (2016); reversals from the ECM condition are uncommon and are unknown in Pinaceae, mycorrhizal associations in general being rather stable (see also Kiers & van der Heijden 2006). ERM are restricted to Ericaceae. See below under individual groups for suggestions of dates.

some ECM/ERM forestsPinaceae (red in map) are the major component of boreal/subarctic forests; there are also Salicaceae and Betulaceae, also ECM, while ERM Ericaceae may be common in the understory (e.g. Read 1991; Villareal et al. 2004; Vrålstad et al. 2002; Vrålstad 2004; Kranabetter & MacKenzie 2010; Gauthier et al. 2015). Estimates of the area occupied by these forests range from 12 x 106 km2, or ca 17% of the land surface of the earth (Moore 1996; Lindahl et al. 2002), to 9.2 x 106 km2, 73% of the conifer forests of the world (Kuusela 1992), or 30% of the world's forest (Gauthier et al. 2015). However, () suggested that the core area of boreal forests in Eurasia alone was 12 x 106 km2, so the global total may be as much as 17.1 x 106 km2 (see also Melillo et al. 1993: estimates for boreal woodland + forest are 18.5 x 106 km2).

Botkin and Simpson (1990) estimated above-ground biomass and carbon for boreal forests in North America to be 4.2±1.0 kg/m2 and 1.9±0.4 kg/m2 respectively which, when extrapolated to a total forest area of 5,172,427 km2 gave biomass and carbon figures of 22.5±5 x 1015 g and 9.7±2 x 1015 g respectively. Moore (1996) estimated living above-ground biomass ("phytomass") in North American boreal forests as 12 x 1015 g, to which could be added 76 x 1015 g in soils (including dead and fallen trees) and 135 x 1015 g in peatlands. Extrapolating to a total area of 10.6 x 106 km2 (the average of the first two figures in the preceding paragraph), this would then give a figure of some 420 x 1015 g of carbon in soils and peatland combined. Carbon burial figures are estimated at 49.3 Tg C y-1 (Chmura et al. 2011: area 13.7 x 106 km2), while soil C storage estimates in Averill et al. (2014) are (49.7-)61.4(-73.1) kg C m-2, NPP figures in the latter being (292-)319(-346) kg C m-2 yr-1. Estimates of the current carbon pool for forests in Russia, Canada, and Alaska, roughly boreal forest, are 88 x 1015 g (vegetation) plus 471 x 1015 g (soils), the area under forest cover being 13.7 x 106 km2 (Dixon et al. 1994). Figures for the carbon in boreal forests in Carvalhais et al. (2014: tables S1 and S2) are some 505 PgC, ca 34.2 kgC m-2, and a mean turnover time of (45.4-)53.3(-73.4) years. Another way of putting it is that boreal forests contain at least 32% of global carbon stock, sequestering around 20% of total forest carbon (Pan et al. 2011; Gauthier et al. 2015).

Buffam et al. (2014) noted the importance of both peat-containing wetlands and lakes in long-term carbon storage in rather mixed Wisconsin-Michigan forests of the Northern Highlands Lake District; at 33% of the area, they represented over 80% of the fixed carbon storage, and so were major players in long-term carbon sequestration there. Clemmensen et al. (2013), working on Swedish conifer forests, emphasized that in older, less disturbed forests much carbon came from roots and in particular from their fungal associates, and the latter also contributed substantially to total respiration, which affects rock weathering (see also Högberg et al. 2010; Tedersoo et al. 2012, also below). In younger forests cord-forming basidiomycete ECM were commoner, mycelial biomass turnover was fairly rapid, and little carbon was stored, in older forests ERM, with their decay-resistent melanized hyphae, were associated with increased carbon sequestration (Clemmensen et al. 2014; see also Fernandez et al. 2013). Indeed, the factors affecting carbon storage are complex. Pinaceae, for example, in boreal forests differ in the kinds of litter that they produce. There is massive accumulation of non-flammable litter under genera like Tsuga and Picea while the litter of Pinus is much more flammable and accumulates much less readily (Cornwell et al. 2015 and references). Furthermore, brown-rot fungi are commonly associated with Pinaceae, and these decompose wood less thoroughly than white-rot fungi, being unable to break down lignin (Riley et al. 2014; Floudas et al. 2015 and references), but there is no sharp distinction between these two types of fungi. Interestingly, bacteria related to Rhizobium dominate in coniferous forests where nitrogen fixing plants are vanishingly uncommon (VanInseberghe et al. (2015).

Age: The age of ECM Pinaceae is uncertain. They may be some 350-200 m.y. old (see Eckert & Hall 2006), although the earliest fossils identified as Pinaceae are from Upper Jurassic deposits ca 155 m.y.o. (Rothwell et al. 2012). Other estimates of the age of crown-group Pinaceae range from (271-)153(-136) m.y. (Gernandt et al. 2008; Magallón et al. 2013) to late Cretaceous or even younger (Willyard et al. 2007; Crisp & Cook 2011). Crown and stem ages of 100 and 263 m.y.a. respectively were suggested by Quirk et al. (2012), of course, establishment of ECM associations can be any time between these estimates. Crown-group Pinus has been dated to 131-129 m.y.a. using fossil evidence (Ryberg et al. 2012), although they may may be as old as 237 m.y.a. (He et al. 2012). Pseudolarix was widely distributed in the northern hemisphere at latitudes above 400 in the Early Cretaceous (Barremian, 115 m.y.a.).

Fagales, particularly Nothofagaceae and Fagaceae, are common in forests of temperate areas in eastern and western Eurasia and North America, in southern temperate regions, and on hills and mountains in Central America and Malesia; see below for the fossil history of this clade, which is quite well documented. They often grow in association with ECM Pinaceae, as in eastern North America (e.g. Abrams 1996). Indeed, in most of these forests ECM and AM trees are overall about equal, although ECM trees predominate in the north and also in the southeast (Phillips et al. 2013).

Fagaceae frequently dominate north temperate vegetation. White oak (Quercus alba) alone represents (12-)19-26(-49)% of witness trees, i.e. trees that were probably present before Europeans arrived, in the oak-dominated forests of eastern North America (81% in some southern Illinois forests). White oak, which grows with up to three more ECM species, two of which are usually other Fagaceae, makes up anything from (36-)50-80(-± 100)% of all trees (Abrams 2003). Six of the 30 species of Quercus growing in those forests are notable dominants (Abrams 1996). Fagaceae, again mostly Quercus, are abundant in western North America, and in California the black oak, Quercus kelloggii, is particularly widespread and has the greatest timber volume of any oak (Waddell & Barrett 2005). Oak trunks may get buried in sediment in flood plains, and the mean age of carbon storage in such conditions is ca 1,960 years, individual trunks being up to 14,000 years old (Guyette et al. 2008), while in mixed ECM temperate forests the half-life of conifer wood was about 20 years, but buried wood persisted for up to 1,400 years (Hyatt & Namian 2001).

The ECM American chestnut, Castanea dentata, was previously the dominant large tree in some 800,000 km2 of forest in eastern North America, but it now persists largely as suckers after its devastation by chestnut blight in the first half of last century (Thompson 2012). However, Faison and Foster (2014) qualify earlier literature reports, i.a. noting that some of the dominance of chestnut may be quite recent, the result of coppicing after being cut down by early Europeans. In any event, it has been replaced by mixed oak or oak-hickory forests (Abrams 1996; see e.g. van der Gevel et al. 2012 for the future), so the forests remain dominated by ECM trees.

Age: The development of ECM associations is likely to be an apomorphy for Fagales. Sauquet et al. (2011) give a range of crown-group ages for the clade, (124.8-)120.2-67.3(-48.9) m.y.a., and these include the ranges of dates suggested in eight other studies. Stem-group ages for Fagales, i.e. the earliest that the ECM association could have developed here, are 125-84(-73) m.y. (e.g. H. Wang et al. 2009; Xiang et al. 2014; Tank et al. 2015) Palynological evidence suggests that Fagaceae were diverse 42-40 m.y.a. in western Greenland (Grímsson et al. 2015). Judging by pollen and wood fossils in particular, Nothofagaceae have been very important in southern latitudes (Australia, Patagonia, Antarctica) since the mid-Campanian ca 78 m.y.a. (Cantrill & Poole 2012). Nothofagus is thought to have dominated in mid-Eocene forest on Wilkes Land, the eastern Antarctic, ca 66o S (Contreras et al. 2013) and it made up a major component of Antarctic vegetation and biomass through the Palaeogene, perhaps persisting there in tundra-like vegetation until the Pliocene (Cantrill & Poole 2012).

Plants with distinctive pollen assignable to the Normapolles complex, comparable with pollen of a subset of Fagales, but not Nothofagaceae or Fagaceae, were both diverse and ecologically prominent in the northern hemisphere from the Turonian-Campanian 94-80 m.y.a. (but c.f. Batten 1981, 1989; Clarke et al. 2011 for cautionary comments, e.g. on pollen identification). Molecular estimates for the age of this clade range from (50-)41, 37(-28) m.y. (Bell et al. 2010) to (96.9-)93.4(-88.2) m.y. (Xiang et al. 2014: c.f. topology). Normapolles pollen has been found in much of the area 20-45oN Cretaceous palaeolatitudes from eastern North America to west central Asia (Kedves 1989; Vakhrameev 1991; Sims et al. 1999; Friis et al. 2003a, esp. 2006a, 2010b and references). Elsewhere in the Northern hemisphere Aquilapollenites and Wodehouseia pollen, of uncertain affinities (Farabee 1993: Aquilapollenites variously linked with Santalales, Apiaceae and Caprifoliaceae-Morinoideae) predominated, and in tropical Gondwanan areas pollen of Arecaceae was common. If Normapolles pollen is correctly identified, it would suggest that Normapolles communities were ECM communities. A temperate Gondwanan pollen province was characterized by Nothofagites pollen, probably from the fagalean Nothofagus, another ECM plant (e.g. Pacltová 1981 for a review; Kedves & Diniz 1983; Friis et al. 2006b, 2010b; Nichols & Johnson 2008). However, there has been little discussion about any ECM-associated activities of these plants.

Dipterocarpaceae-Dipterocarpoideae Dipterocarpaceae (map: Pakairaimaea, close to Cistaceae - blue; Monotoideae - green; Dipterocarpoideae - red: from Trop. Afr. Fl. Pl. Ecol. Distr. 1. 2003; Gottwald & Parameswaran 1966; Ashton 1982) dominate large areas of Southeast Asian l.t.r.f.. Common in Malesia, and in particular Borneo, they form extensive areas of peatlands but are also abundant away from peatlands, but in both cases the soil is rather humus-rich. In Lambir forest, Sarawak, dipterocarps make up only 7.4% of the species but 41.6% of the basal area (918.41 m2); the figures for Shorea alone are 4.7%, 21%, and 467.8 m2, Dryobalanops aromatica and Dipterocarpus globosus between them accounted for 13.2% of the basal area, and seven dipterocarps (out of the ten most dominant species) accounted for 23.1% (Davies et al. 2005). Dipterocarps are also important elements in drier and more open woodlands. Shorea robusta (sal) is a gregarious tree that grows in monsoon areas from Pakistan to China, especially in the India-Assam-Myanmar area. Sal forests occupy 115,000 to 120,000 km2 (11.5 x 106 ha) and make up ca 15% of Indian forests (Tewari 1995). In Africa Monotes is a significant component of the rather dry forests and woodland otherwise dominated by ECM Fabaceae-Detarioideae (see below).

Dipterocarpaceae occupy about 56% of all tropical peatland areas, close to 250,000 km2, about 6.2% of peatlands globally, and they dominate on huge peat lenses (Page et al. 2011, 2012; Richards 1996; Rieley " Page 2016); note, these figures are not corrected to take account of the recent realization that there are ca 145,500 km2 of peat with ca 30.6 petagrams/30.6 X 1015 g. carbon in the Cuvette Centrale in the Congo (Dargie et al. 2017), with Uapaca paludosa, alone of the four common species mentioned, probably an ECM plant. This peat contains an estimated 68.5 Gt carbon, some 77% of the tropical and 11-19% of the global totals for peatlands (Page et al. 2011: above-ground biomass not included). These figures are over twice the total carbon storage in all other forests in Malaysia and Indonesia (Brown et al. 1993; Page et al. 2012), which also include a substantial component of other ECM trees. (Other estimates are 84 Gt C in tropical peat - Rydin & Jeglum 2013 and references; also Immirizi & Maltby 1992; Rieley et al. 1996, etc., including estimates of pre-human peatland areas). Carbon in waters draining from disturbed dipterocarp peat swamps may be as much as ca 4,180 years old (Moore et al. 2013), indicating that carbon storage there can be quite long term. Peat deposits started to form in the late Pleistocene 40,000 y.a., and now the peat may be 25 m deep (Page et al. 2004, 2012; see Raes et al. 2014 for dipterocarps on the Sunda Shelf during glacial maxima).

Somewhat unusually for ECM forests, Malesian dipterocarp forests are very diverse on a global scale (e.g. Lee et al. 2002) and from this point of view represent one extreme of the ecological spectrum of ECM plants. They have high above-ground wood productivity, even when compared with west Amazonian forests largely similar in soil, precipitation, etc., the dipterocarps being taller trees gaining in diameter faster, and there is also higher solar radiation in Borneo (Banin et al. 2014). For Malesian dipterocarps, see also papers in Osaki and Tsuji 2016). Amazonian peatlands sequester perhaps 9.7 Gt carbon, but on a per area basis their carbon accumulation is only a little over half that in Malesia; unfortunately, the mycorrhizal status of the plants in these peatlands seems to be largely unknown (Lähteenoja 2011; Lähteenoja et al. 2011; Rieley & Page 2016).

Of other close relatives of Dipterocarpaceae that are ECM plants, Cistus in particular dominates in the shrubby Maquis vegetation in the Mediterranean region, while species of Sarcolaeanaceae are often reported to be common in Madagscar (Cavaco 1952).

Age: A substantial clade in Malvales, [[Pakaraimaea + Cistaceae] [Sarcolaenaceae + Dipterocarpaceae]], most/all members of which are ECM, can be dated to as little as (25-)23(-21) m.y.a. (Wikström et al. 2001) or over 88 m.y.a., the split of Dipterocarpaceae and Sarcolaenaceae (Ducousso et al. 2004). Suggestions that ECM associations in Dipterocarpaceae developed before the break-up of Gondwana over 130 m.y.a. (Henkel et al. 2002; Ducousso et al. 2004; Moyersoen 2006; Alexander 2006; see also Sato et al. 2016) are overly optimistic; later Cretaceous or early Caenozoic ages are more likely. There are massive amounts of dipterocarp resin from India in the Early Eocene around 52-50 m.y.a. (Rust et al. 2010).

Fabaceae are often not thought of as being ECM plants, but Old World Detarioideae, which dominate millions of square kilometres in Africa (blue, two maps above), are ECM plants (e.g. Read 1991; Onguene & Kuyper 2001). Species in some 36 of the ca 82 genera included in Detarieae are reported to be at least locally dominant (e.g. Letouzey 1968; Mackinder 2005), and 11 of these dominants are in a rather small clade (Macrolobieae/the Berlinia clade) with 16 genera, of which 10 are known to be ECM plants (see also Wieringa & Gervais 2003). A few species of Detarioideae dominate ca 3.27-3.75 x 106 km2 of Miombo forests in the Zambezian region (estimates from White 1983; see also Newbery et al. 2006). The ECM detarioid Isoberlinia is a major component of Sudanian Woodland (White 1983) which forms an interrupted band south of the Sahara from Mali to Uganda (White 1983; upper band of blue in the map above). This forest is biogeographically closest to Miombo woodlands among other African vegetation (Linder et al. 2012).

Detarioideae like Cynometra are endomycorrhizal (AM), and it, too, can be a dominant tree in tropical African rainforests (Eggeling 1947; Makana et al. 2011). Interestingly, litter type and amount, soil nitrate, etc., of the caesalpinioid AM Mora excelsa which can dominate forests in Trinidad (see Brookshire & Thomas 2013: it also has endophytes), are like those of several ECM-dominated communities. A few other Fabaceae, both ECM and AM, may also dominate locally (see also Peh et al. 2011b - check).

Detarioideae represent 20-90% of the trees in Miombo forests, 30-96% of the basal area, and with biomass estimates in the range of 35-97 Mg ha-1 (Högberg & Piearce 1986; Frost 1996). Figures for the carbon dynamics of tropical savannas and grasslands together in Carvalhais et al. (2014: Tables S1 + S2) are around 328 Pg total C, a carbon density of ca 17.7 kgC m-2, and a mean turnover time of (12.2-)16(-22.1) years.

Detarioideae are ecologically important elsewhere in Africa. Gilbertiodendron dominates large areas of the eastern Congo Ituri rainforest (Torti et al. 2001; Makana et al. 2011), one study finding that it made up 20% of the above-ground biomass there (Bastin et al. 2015). Microberlinia dominates Guineo-Congolian forests in Cameroon, and other Detarioideae dominate parts of the forest that grows inland from the coast from Sierra Leone to western Gabon, and again in the periphery of the Zaire basin (White 1983). A caesalpinioid Biafran forest subtype has been recognised that includes this sub-coastal forest, and of the 34 important genera recorded from it, 28 are Detarieae, and 11 of these are described as being characteristically gregarious (Letouzey 1968). Other ECM plants in the woodlands and savannas of Africa and Madagascar include Monotes (Dipterocarpaceae), Uapaca (Phyllanthaceae), Asteropeiaceae and Sarcolaeanaceae (Tedersoo et al. 2011 and references)

In the New World, Aldina (Faboideae) and the coppicing Dicymbe (Detarioideae), both ECM plants, dominate forests in the Pakaraima Mountains in the central Guiana Shield region (McGuire 2007b; M. E. Smith et al. 2011). Dicymbe has a remarkably high basal area of 38.4-52.5 m2 per hectare, around 25(-40) m2 being more normal figures (Henkel 2003).

None of the 42 common Amazonian species mentioned by Pitman et al. (2001) is known to be an ECM plant. However, of the 20 most abundant Amazonian trees, plants 10 cm d.b.h. or more, listed by ter Steege et al. (2013) as being "hyperdominants", the ECM Eperua falcata (see Peh et al. 2011b), along with E. leucantha (mycorrhizal status?), Detarioideae, are notable as being 50% more abundant (usually far more) than any other non-palm on the list. 5 of the 20 species with most above-ground woody biomass are Fabaceae (2 known to be ECM; representing 4.96% of the total biomass) as are 6/top 20 species ranked by productivity (Fauset et al. 2015); most of the others are probably AM plants (Béreau & Garbaye 1994). Peltogyne (Detarioideae: ?mycorrhizal status) is a rare Amazonian monodominant, and it occupies ca 53% of the basal area of trees 10 cm or more in d.b.h. on Maraca Island, Roraima, a figure that increases the larger the trees (Nascimento et al. 1997). For further discussion about Amazonian "hyperdominance", see above.

Age: There are suggestions that ECM associations in Fabaceae-Amherstieae (= Detarioideae) developed before the break-up of Gondwana over 130 m.y.a. (Henkel et al. 2002; Moyersoen 2006), but an early Caenozoic date is more likely. Indeed, the crown age of this clade has been estimated to be ca 29.2 m.y. (Lavin et al. 2005) or ca 53.8 m.y., or as little as ca 17.3. m.y. (Bruneau et al. 2008a).

ERM Ericaceae, a clade including the old Epacridaceae, Prionotaceae, Empetraceae and Vacciniaceae, are characteristic and often very common in heathlands world-wide (green in map above, inc. tundra: end-papers in Specht 1979a; Read 1996: also Grubbiaceae and Diapensiaceae), including those in alpine and arctic tundra (Specht 1979b; Chapin & Körner 1995; Jonasson & Michelsen 1996; Michelsen et al. 1998), in montane shrubberies especially in the northern Andes (especially Vaccinieae), parts of the eastern Himalayan-Yunnan region and Malesia (Vaccinieae, Rhodoreae), and in heathlands of southern Africa (Ericeae) and Australia (Styphelioideae). Tundra vegetation occupies some 8% of the global land surface (Read 1991; Chapin & Körner 1995; Gardes & Dahlberg 1996; Camill et al. 2001; Kranabetter & MacKenzie 2010), or around 9.7 x 106 km2 (Melillo et al. 1993: alpine tundra included).

Age: ERM associations in Ericaceae have been dated to ca 90 m.y.o. (van der Heijen et al. 2015 and references), but the fossil Paleoenkianthus on which this age is based may not even be stem Ericaceae, and in any event Enkianthus itself does not have ERM; the origin of ERM here can perhaps be dated to around 77-65 m.y. (Wagstaff et al. 2010) or around 77 m.y. (Z.-Y. Liu et al. 2014), with a stem group age of around 91 m.y. ( Liu et al. 2014).

Tundra, Boreal Forests and Permafrost. ECM/ERM plants are abundant in boreal forests and tundra in particular, indeed, all boreal tree species have ECM associations (S. E. Smith & Read 2008), and most tundra ECM fungi are found also in the adjacent boreal forest (Hewittt et al. 2017). Tundra-type habitats are often dominated by Ericaceae, Vaccinium and Empetrum being two of the seven prominent biomass accumulators there (Chapin & Körner 1995; Gardes & Dahlberg 1996; Kranabetter & MacKenzie 2010), and Read (1993) characterized the tundra by the prevalence of ERM. Ericaceae represent 30-87% of the above-ground biomass and 40-83% of the net annual above ground primary productivity in tundra (figures from Bliss 1979), and ECM and ERM plants together made up more than 95% of the vascular plant biomass in some heath tundra sites (Michelsen et al. 1998; see also Sistla et al. 2013: 6/10 species listed in Table 2). Three of the other major biomass accumulators, Salix, Betula and Dryas, are also ECM plants, while two Cyperaceae (Eriophorum, Carex: see below) are the others (see also Timling & Taylor 2012 for mycorrhizal diversity). ECM Polygonum (Bistorta) viviparum is a perennial herbaceous tundra plant that is both a pioneer and a prominent component of established vegatation (e.g. Gardes & Dahlberg 1996; Michelsen et al. 1998; Brevik et al. 2010). The mycorrhizal status of Diapensiaceae (Ericales) needs clarification; the family is not immediately related to Ericaceae, and only a single species, the circumpolar Diapensia lapponica, grows in the tundra.


The northern polar permafrost encompasses the tundra region and also much of the boreal conifer forest above 60o N if areas of patchy permafrost are included, although permafrost is widespread only above 70o N in western Asia and even further north in much of Europe. The permafrost area includes extensive bogs dominated by Sphagnum and Cyperaceae (e.g. Camill et al. 2001). The map opposite is based on that by Brown et al. (2002, q.v. for detail; c.f. Tarnocai et al. 2009) - note that quite extensive areas with some, but less than 50%, permafrost are not included. Tarnocai et al. (2009) estimated permafrost to occupy about 18 x 106 km2, about 16% of the global soil area.

Very large amounts of carbon are in long-term storage in the extensive peat deposits found in tundra, boreal forests, and heathlands. Estimates in MacDonald et al. (2006) are that northern peatlands stored 188-455 Pg carbon; Yu et al. (2010) thought that there were around 547 gigatons of carbon there, with an additional 15 gt in Patagonian peatlands; or there may be some 16 Gt C in southern peats and as much as 621 Gt C in northern peats (Rydin & Jeglum 2013 and references; see also Gorham 1991; Immirizi & Maltby 1992). Figures for carbon storage in the tundra are about 158 PgC (Carvalhais et al. 2014: tables S1 and S2) and for that of the permafrost areas as a whole 1,035±150 PgC, but add to that another ca 272 Pg stored below 3 m in the Yedoma region, Russia, and in delatic deposits, with around 822 Pg of this being perpetually frozen (esp. Hugelius et al. 2014; Schuur et al. 2015; Olefeldt et al. 2016), 1035 pG being about one third the global total (Jobbágy & Jackson 2000: to 3 m depth). Ecosystem turnover times for carbon are much longer in these cooler climates: Times for tropical, temperate and boreal forests, and tundra are ca 14.2, 23.5, 53.3, and 65.2 years respectively, and above 75o N the mean turnover time is ca 255 years (Carvelhais et al. 2014). Note, however, that carbon residence time can also be very long in both dipterocarp and mangrove peats, while subsoil radiocarbon ages range from 1,000-10,000 years or more (Schmidt et al. 2011).

Soils contain much of the carbon in tundra ecosystems (e.g. Gorham 1991), and although the net primary productivity of Ericaceae there may be high, they are only one the major sequesterers of carbon in peat soil, thus mosses, especially Sphagnum, are important contributors to peat (Turetsky et al. 2008). Indeed, carbon cycling in these environments needs more study. Comparing ECM and AM plants in subarctic alpine Sweden, Soudzilovskaia et al. (2015) found much variation, although there was substantial soil C from fungal hyphae of ECM plants, and soils in areas dominated by ECM plants showed evidence of slower C cycling than AM-dominated areas. Of course most of the carbon in the thick, carbon-rich sediments in the Yedoma reghion, deltaic deposits, etc., is currently immobile, while some 20% of the circumpolar permafrost has thermokarst areas with a distinctive topology that are susceptible to rapid carbon release on further warming (see Tesi et al. 2016 for rapid C release in northern Siberia with warming at the end of the Younger Dryas ca 11,600 y.a.), and these areas store 30%, or perhaps 50% if Yedoma lake thermokarst is included, of the global carbon storage (Oldefeldt et al. 2016).

Mosses are very important components of the tundra and boreal forests and represent a substantial proportion of the biomass (Gorham 1991; Chapin & Körner 1995). Particularly in more boggy areas Spagnum is the main moss, and it decomposes more slowly than the other bryophytes and vascular plants there (Verhoeven & Liefveld 1997: secondary metabolites; Lang et al. 2009; Lindo et al. 2013; Sistla et al. 2013; Rydin & Jeglum 2013), although mosses in general tend to decompose slowly (Turetsky et al. 2008). Sphagnum-dominated poor fens in northern Alberta may not be very productive, but respiration tends to be low, the plants start photosynthesizing early in the year, etc., so net carbon production may be higher than in, for example, Carex-dominated rich fens with their shorter growing seasons (Flanagan 2014; see also Ragoebarsing et al. 2005; Larmola et al. 2010; Hájek 2014; Bragina et al. 2104). Interestingly, the ERM ascomycete Oidiodendron maius is saprotrophic and can break down Sphagnum peat (Kohler et al. 2015).

The ecological role of Cyperaceae is poorly understood (Barrett 2013). Members of the family are often particularly common in rich fens in wet tundra habitats in the Arctic (ca 8% of the land surface), and include Eriophorum and Carex, two of the seven major contributors to the biomass there (Chapin & Körner 1995; see also Flanagan 2014 and above); Carex itself is the most speciose genus in the Arctic (Elven et al. 2011. Roots of cyperaceous plants may penetrate into the mineral soil below the shallow layer of soil dominated by roots of ECM plants (Read 1993). Cyperaceae-dominated communities were notably extensive during the last glacial maximum north of 550 N (Bigelow et al. 2003). Even today about 16% of all species growing in Quebec and Labrador north of 54o N are Cyperaceae, and 13% are Carex (Poaceae are next at 11%), and they can be major components of the plant cover, especially in wetter habitats (Cayouette 2008; Escudero et al. 2012). Habitats in alpine and other extreme conditions may alsobe dominated by single species of ECM Cyperaceae. Thus there are some 450,000 (or 1.5 x 106) km2 of meadows between 3,000 and 5960 m altitude on the Tibetan plateau that are dominated by the ECM Kobresia pygmaea (= Carex parvula), and there is around 18.1 x 1016 g of carbon in the soil (Miehe et al. 2008, 2014; Qiu 2016, see also Zhou 2001). ECM Kobresia, now in the Core Unispicate Clade of Carex (Global Carex Group 2015), are widespread and sometimes dominant in other alpine, Arctic and tundra habitats (e.g. Gardes & Dahlberg 1996; Muhlmann & Peintner 2008; Newsham et al. 2009; Gao & Yang 2010).

Age: For the most part the clades discussed above are small and young. Of the seven major angiosperm biomass accumulators in the tundra, Dryas (Rosaceae) diverged from other Dryadoideae (83.2-)63.1(-44.7) m.y.a. (Chin et al. 2014) or ca 38 m.y.a. (Y. Xiang et al. 2016), but ages for the other six, Vaccinium, Empetrum (Ericaceae), Eriophorum, Carex (Cyperaceae), almost 70 species of Salix (Salicaceae), and Rubus (Rosaceae) (Chapin & Körner 1995) are unknown, the species involved for the most part being members of large genera. Dating the most prominent bryophyte in today's Arctic, Sphagnum, is problematic. Sphagnum-like fossils are known from Ordovician rocks ca 455 m.y.o. (Cardona-Correa et al. 2016), Carpenter et al. (2015) found spores of Stereisporites, linked with Sphagnum, to be very common in fire-prone heathlands in Central Australia 75-65.5 m.y.a., and Daly et al. (2011) suggest that Sphagnum-type mosses were components of peats produced by mire vegetation in northern Alaska ca 60 m.y.a. that later were converted to coal. However, the clade to which extant species of peat-forming Sphagnum belong has been dated to as late as the mid-Miocene ca 14 m.y.a. (Shaw et al. 2010a; Shaw & Devos 2014).

Another way of thinking about the age of tundra - and boreal - vegetation is to think about how long the climate that these vegetation types prefer has been around. The cold climate that characterises today's Arctic and Antarctic communities started to develop perhaps as much as 47 m.y.a. in the Eocene, cooling being accentuated at the beginning of the Oligocene (e.g. Wolfe 1978; Millar 2011; Pagani et al. 2005). Estimates are of a 30o C or more reduction in the mean annual temperature in the far north since the end of the Eocene (Jahren 2007), temperatures dropping 8.2±3.1oC in just 400,000 years at the beginning of the Oligocene some 33.5 m.y.a. in central North America (Zanazzi et al. 2007). Arctic ice started developing ca 7 m.y.a. (Zachos et al. 2001), becoming widespread only in the early Pleistocene 2.4-2.2 m.y.a. (Brigham-Grette et al. 2013; Knies et al. 2014), 10 m.y. later than the development of major ice sheets in the Southern Hemisphere (e.g. Zachos et al. 2001, 2008; Retallack 2009; Millar 2011; Crisp & Cook 2011). The Antarctic ice sheet appeared ca 33.5 m.y.a. at the Eocene-Oligocene boundary and persisted through much of the Oligocene (Zachos et al. 2001; Coxall et al. 2005; Eldrett et al. 2009 and references), although cooling in the Wilkes Land shelf area had begun by the middle Eocene (Pross et al. 2102) and there may have been short-lived periods of glaciation even ca 42 m.y.a. (Tripati et al. 2005) or perhaps still earlier (Bowman et al. 2014; Ladant & Donnadieu 2016). Along with cooling, the climate also became seasonal, as is evident in extratropical floras by the end of the Eocene or somewhat earlier (e.g. Wing 1987; Eldrett et al. 2009).

Seagrasses, Mangroves, and Tidal Saltmarshes.

Sea-grasses, mangroves and tidal salt marshes all have carbon burial rates well over 100 g C M2 y-1, considerably more than twenty times that in tropical, boreal or temperate forests, which are usually substantially less than 10 g C M2 y-1 (Mcleod et al. 2011; Chmura 2011). All three systems are accretionary, in that they also capture much sediment in which the carbon they produce, but also allochthonous carbon, is stored, and sediments can reach 10 m or more thick (Chapman 1974; McKee et al. 2007; Mcleod et al. 2011; Chmura 2011; Fourqueran et al. 2012; Marani et al. 2013). C storage can be for thousands of years, well over ten times as long as that in l.t.r.f. (Chambers et al. 2001; Mcleod et al. 2011).

Seagrasses Sea-grasses are the only fully marine angiosperms, aside from some true grasses growing in estuaries (see below). There are about 55 species, all members of Alismatales, so they are unrelated to true grasses; a few are Hydrocharitaceae and the rest belong to the [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]] clade. Sea-grasses often form monodominant stands, individual clones of some species being very long-lived. The map here - perhaps too optimistic - comes partly from Green and Short (2003), although there are substantial differences with individual family maps, particularly those for Zosteraceae and Ruppiaceae. This is partly because there is some disagreement over what a sea-grass is, but partly because Green and Short (2003) show very little data both for South America in general and places like the Solomon Islands in particular.

Estimates of the area occupied by sea-grass communities range from 22.8 x 106 (Waycott et al. 2009) to 30 x 106 (Duarte et al. 2005) to 60 x 106 ha (); although the last is an old estimate, it is relevant here where the emphasis is on conditions immediately before human activities became transformative. Although the amount of carbon in an individual sea-grass plant (= ramet) itself is small, that stored in the "soil", which can form mats up to 11 m thick, as in the Mediterranean, is very great (Fourqueran et al. 2012), larger than that of most forests and comparable with mangrove storage, and carbon can be sequestered for 4,000 years or more in the anoxic sea-grass beds (Orem et al. 1999; Serrano et al. 2011, 2013).

The gross primary productivity of sea-grasses has been estimated at 1903 g C m2y-1, rather like that of mangroves, their global primary productivity is 628 Tg C y-1, while their net ecosystem production (1211 g C m2 y-1 and globally 400 Tg C y-1) is substantially higher than that of mangroves because of their relatively low respiration rates. Sea-grasses are responsible for about 1.13% of all marine primary productivity, yet they bury as much as an estimated 27-44 Tg C y-1, some 12% of the total C storage in the marine ecosystem (Duarte et al. 2005: area 30 x 106 ha; Duarte 2011), although they occupy less than 0.2% of the area of the oceans. Indeed, this burial estimate may be only half the actual amount (Fourqueran et al. 2012). When thinking about sea-grass communities as carbon sinks, an estimate of 169-186 g C m-2 yr-1 seems reasonable - net community production of ca 120 g plus 41-66 g of allochthonous C (Kennedy et al. 2010: highest areal estimate above). Estimates in Mcleod et al. (2011) vary - they suggest a carbon burial rate of (100-)138(-176) g C m-2 y-1 (range 45-190), total carbon burial of 48-112 Tg C y-1, for a sea-grass area of 17.7-60.0 x 106 ha. Other estimates of global carbon storage by sea-grasses range from 4.2-8.4 or 9.8-19.8 Pg C, depending on the assumptions made, somewhat over 0.5% of the global total (Fourqueran et al. 2012; see Charpy-Roubaud & Soumia 1990 for estimates of benthic algal productivity). A substantial amount of sea-grass carbon moves into other marine ecosystems, including the deep sea (Suchanek et al. 1985).

To summarize. The sea-grass ecosystem is very productive, supports a considerable amount of diversity, does not suffer from much herbivory, captures much sediment, and stores much carbon, both autochthonous and allochthonous (Orth et al. 2006; Kennedy et al. 2010 for summaries).

Age: Of the two main seagrass clades, the stem age of [Thalassia + Enhalus + Halophila] in Hydrocharitaceae has been estimated to be 47.8-38 m.y.o. (Iles et al. 2015) - other estimates are dramatically older. Within the [[Posidoniaceae [Ruppiaceae + Cymodoceaceae]] [Zosteraceae + Potamogetonaceae]] clade, the first split has been dated to ca 73 m.y. and its stem age is dated to less than 82 m.y. (Janssen & Bremer 2004).

Fossils of Thalssocharis bosquetii ca 72 m.y.o. from the early Maastrichtian of western Europe have been identified as those of a seagrass, although to what clade they should be assigned is unclear - they seem to completely lack intravaginal squamules (van der Ham et al. 2017). Fossils from the late Middle Eocene in Florida have been identified as Hydrocharitaceae (Thalassia) and Cymodoceaceae (Thalassodendron, Cymodocea), indeed, some have been referred to extant species, and records from the Eocene in the Old World suggest a considerable age for the seagrass community (Lumbert et al. 1984; Ivany et al. 1990). Sea grasses may have originated in the eastern Tethys in the Late Cretaceous, and some taxa recorded from the New World by the Eocene are now known only in the Old World (Ivany et al. 1990). It is unclear how many times adaptation to the marine habitat has evolved (three times or so), since individual species in sea-grass families and other Alismatales can tolerate a range of salinities (e.g. Barbour 1970).


The mangrove ecosystem is very productive and also has high carbon flux rates (see Feller et al. 2010 for a good summary). Mangroves occupy 13.7-15.2 million hectares, and they store 4-20 PgC globally (Bouillon et al. 2008; Donato et al. 2011 and references; 16.7 m ha in Valiela et al. 2001). Other estimates are that they bury 17.0-23.6 Tg C y-1, their gross primary productivity is 2087 g C m2y-1, global primary productivity is 417 Tg C y-1, but with a rather lower net ecosystem production (221 g C m2 y-1, globally 44 Tg C y-1) because of a relatively high respiration rate, at least when compared with the sea grass community (Duarte et al. 2005: area 0.2 x 1012 m2). Estimates in Mcleod et al. (2011) are a carbon burial rate of (187-)226(-265) g C m-2 y-1 (range 20-949), total carbon burial of 25.7-40.3 Tg C y-1, area 13.8-15.2 x 106 ha. Mangrove peat can become very thick, and carbon in Caribbean peat has been dated to around 7,500 years old (McKee et al. 2007).

Spalding et al. (2010) estimated mangrove net primary productivity to be 140-168 tg y-1, of which 10(-30)% was incorporated into sediments where it made up 15% of the organic carbon accumulating in marine sediments globally. 10% of terrestrially-derived dissolved organic carbon in the oceans comes from mangroves (Dittmar et al. 2006). Similarly, 10% of refractory organic carbon in marine sediments is mangrove in origin, which equals the amount of carbon in atmospheric CO2 (Spalding et al. 2010).

Mangroves can be divided into two groups, the much more speciose eastern group, from east Africa to the western Pacific, which includes ca 40 species, ca 14 of which are Rhizophoraceae, and the western group, from west Africa to the Americas, with only eight species, three of which are Rhizophoraceae (for their evolution, see Ricklefs et al. 2006). Plaziat et al. (2001) suggested that the separation of the two groups occurred ca 20 m.y. ago. Depending on how species limits are drawn, no dominant mangrove species is common in both areas (Tomlinson 1986). For salt and water balance in mangroves, see Reef and Lovelock (2015) and other papers in Ann. Bot. 115(3). 2015.

Age: There have been a number of independent adaptations to the mangrove habitat (Tomlinson 1986; Spalding et al. 2010), and Rhizophoraceae-Rhizophoreae and Arecaceae-Nypa are particularly important mangrove plants. By the Eocene, ca 50 m.y.a., many mangrove genera are known from the fossil record, and several are known from both the Old and the New World (Plaziat et al. 2001; Ricklefs et al. 2006 for some dates; but see Martínez-Millán 2010 for Pelliciera). Thus Nypa, today found only in the Indo-Malesian area, is known from the Upper Cretaceous ca 70 m.y.a. and by the early Palaeocene ca 55 m.y.a. was found in both the Old and New Worlds (see Arecaceae). Fossil hypocotyls identified as Ceriops and preserved with good anatomical detail have been found in the Lower Eocene London Clay (Wilkinson 1981; but c.f. Collinson & van Bergen 2004). Rhizophora is known from the Caribbean in the late Eocene (Graham 2006) and Rhizophoreae from the Early Eocene 55-48.5 m.y.a. in western Tasmania, Australia (Pole 2007).

Estuarine productivity is difficult to estimate. Estimates of carbon burial in salt marshes (2.2-40 x 106 ha in extent) are given by Mcleod et al. (2011): the rate of burial is (194-)218(-242) g C m-2 y-1 (range 18-1713) and total carbon burial is 4.3-96.8 Tg C y-1. Duarte et al. (2005) estimated that salt marshes occupied ca 40 x 106 ha with a gross primary productivity of some 3595 g C m2y-1 and global primary productivity of 1438 Tg C y-1, while their net ecosystem production was 1585 g C m2 y-1 and globally 634 Tg C y-1, substantially higher than either mangrove or seagrasses. They estimated C burial to be 60.4-70.0 Tg C y-1.

Age: Pseudoasterophyllites, ca 97 m.y.o. from the European Cenomanian, is possibly the earliest halophyte, and was described as growing in supratidal salt marshes; since morphologically, it tends to link Chloranthaceae and Ceratophyllum (Kvacek et al. 2016) it is not immediately related to any other extant halophytic group. Extant angiosperms that live in estauries include a variety of more or less salt tolerant members of Alismatales, Poaceae such as Spartina (= Sporobolus), some species of Juncus, Limonium, Atriplex, a few Asteraceae, etc. - overall, quite a number of species, but no major clades - so it is difficult to provide useful estimates of the age of estaurine vegetation.


The ecosystem functions emphasized here are carbon sequestration and to a lesser extent net primary productivity, but the two are not necessarily linked (Lähteenoja 2011). Carbon estimates can be of above-ground biomass, or in soils and peats, and in peats in particular below-ground C can represent over half the total forest carbon pool (Dixon et al. 1994), with sequestration times being relatively long term. On the other hand, in many speciose tropical lowland rainforests productivity is high, standing carbon biomass is high, but below ground biomass is relatively low, carbon sequestration times are short, and biomass turnover is relatively fast (e.g. Dixon et al. 1994; Peh et al. 2011a). There are suggestions that the total standing biomass of the trees in forests is invariant with respect to species number and composition or latitude (Enquist & Niklas 2001; Enquist et al. 2007; but c.f. e.g. Dixon et al. 1994), but above- and below-ground biomass, nutrient cycling and productivity all vary considerably.

Communities in which ECM clades dominate often grow under relatively extreme environmental conditions, whether of substrate or climate (Read 1991; Augusto et al. 2014). ECM Pinaceae (also other Pinales, which are endomycorrhizal), successfully compete with angiosperms, but not in the most productive environments (Brodribb et al. 2012). The marine and estuarine environments inhabited by mangroves and sea-grasses are physiologically extreme for angiosperms.

It should be noted that the although ECM Fagaceae, for example, may seem to be fairly invarint in their ecology, ecophysiological boundaries between different mycorrhizal "types" are not that clearcut (e.g. Martijena 1998; . Not all monodominant legumes are ECM (Torti & Coley 1999; Torti et al. 2001). However, like ECM legumes, the AM Mora excelsa has low foliar and litter nitrogen contents are low, there is foliar resorbtion of nitrogen, soil nitrate concentrations are low, litter decomposes slowly and accumulates (Brookshire & Thomas 2013). Two other species of AM Mora may also be monodominants, as is the AM Prioria copaifera, Pentaclethra macroloba (also clonal), Cynometra alexandri (Torti et al. 1997; Henkel 2003; Makana et al. 2011; Peh et al. 2011b; Menge & Chazdon 2015, etc.). Again, communities dominated by these legumes may have some of the features of those dominated by ECM plants, and this is also true of communities dominated by the AM Piranhea mexicanum, in Malpighiales-Picrodendraceae (Martijena 1998: as Celaenodendron).

Answering questions like, "How many times did ECM associations/seagrass communities develop?, When did they evolve?, When did they become common?", is central to our understanding of the long-term effects on the biosphere of the various associations under discussion (see also Eastwood et al. 2011; Maherali et al. 2016: discussion on evolution of different types of mycorrhizal associations). In these associations, one or a few clades largely dominate important aspects of community/ecosystem functioning, sometimes over very large area - relatively few clades of ECM plants occupy perhaps 50% of the earth's forested areas (L. L. Taylor et al. 2011). The dominance of a relatively few groups of plants in often rather unproductive terrestrial environments and particularly marked as one proceeds polewards, or in marine environments, may reflect the relative rarity of successful adaptations to more extreme conditions. Although the physiological/ecological traits under discussion have evolved several times, all have a strong phylogenetic signal. The clusters of origins of C4 photosynthesis in the PACMAD clade of Poaceae, and again in Cyperaceae and in Amaranthaceae (e.g. Kadereit et al. 2012), and the separate origins of adaptations to life growing completely submerged in the sea in Alismatales, suggest further complexities underlying the evolution of these traits. Similarly, the ECM habit has originated several times in the N-fixing clade.

However, understanding ecophysiological relationships earlier in the Caenozoic, let alone in the Mesozoic, presents major challenges. Three immediate issues arise when thinking of the possible changes of carbon sequestration over time. 1. I have already noted that the definitions of these vegetation types are imprecise. Thus the area of grassland mentioned above depends on how "grassland" is defined, similarly, there is no consensus over the definition and extent of vegetation types in the forest/savanna transition (Timberlake et al. 2010; Torello-Raventos et al. 2013; Bond 2016a, c.f. DeWitt et al. 2016 and Bond 2016b). Similarly, although I have separated mangrove- and sea grass-dominated communities, members of both are halophytes, that is, plants tolerating at least 200mM salt, and both intergrade with estuarine and inland halophytic vegetation, the former including abundant Poales, especially Poaceae, the latter often dominated by Caryophyllales (see also Veldkornet et al. 2015). In both these other vegetation types C4 plants are common (Flowers et al. 2010).

2. Estimates of the amount of above- and below-ground carbon and similar measures for current ecosystems are estimates for ecosystems that have often been more or less profoundly modified by the activities of humans. The effects of climate change on productivity and carbon sequestration that are mediated by changes in temperature, precipitation, atmospheric CO2 concentration, etc., are of course ubiquitous (e.g. H. Chen & Luo 2015 and references). The focus here is on pre-agricultural vegetation, yet as soon as humans started using fire, they began to cause major vegetational changes - and when fires are prevented, there are also major changes in the vegetation, although fires are likely to have been central to the existence of various grass-dominated communities for the last 10 m.y. or so (e.g. Retallack 2001; Bond 2016a). Indeed, the Late Quaternary megafaunal extinctions, largely caused by human hunting although perhaps exacerbated by climatic fluctuations, have occurred both on land and in marine communities (Valentine & Duffy 2006; Lorenzen et al. 2013; Cooper et al. 2015; Saltré et al. 2015) and have had substantial effects on community composition, nutrient cycling, and biome limits (Gill 2013). Overall, the areas occupied by different vegetation types have changed greatly over the last 10,000 years, hence, in part, the differences in some of the area estimates (Dixon et al. 1994). Longleaf pine savanna has decreased from ca 90 million to less than 2 million acres, and the area occupied by mangroves has also decreased greatly because of cutting (e.g. Spalding et al. 2010). Stands of thermophilous Abies alba in the Mediterranean have disappeared because of human activities (Tinner et al. 2013). Even in "primary" forests, human activities can have an impact on biomass estimates. Thus there has been ca 1/3 loss in biomass in primary - but obviously not untouched - forests in Peninsula Malaysia over a single decade late last century (Kerridge et al. 1987; Dixon et al. 1994 and references), the above-ground carbon in forest throughout the whole Indo-East Malesian area is substantially below its potential value because of human activities (Brown et al. 1993), and forest plots in Amazonia that provide the data used to understand Amazonian forests are notably common in areas that are likely to have been affected by the activities of pre-contact humans (McMichael et al. 2016).

Estuarine and salt marsh vegetation have been very considerably affected by human activity. Their floristic composition has changed and their extent reduced because of agricultural runoff, drainage, grazing and reduced sedimentation (e.g. Chapman 1984; Brush & Hilgartner 2000; Kirwan et al. 2011; Mariotti & Fagherazzi 2013). However, land clearance and the resultant increase in sediment in rivers can also facilitate the development of estuarine salt marshes (e.g. Kirwan et al. 2011; Chmura 2011), while agricultural nutrients in runoff negatively affect subaquatic estuarine vegetation (e.g. Brush & Hilgartner 2000). Mangroves have also been severely impacted by humans, and towards the beginning of the last century there may have been 22.0-25.5 x 106 hectares (estimated, from Valiela et al. 2001, current figures corrected by those multiyear records that exist, also with the lower current estimate of Spalding et al. 2010), but now there is at most 3/5 of this area. However, loss of mangrove vegetation since the beginning of the century may be rather less than supposed, even if what will happen in the immediate future remains unclear (Richards & Friess 2015).

However, depending on the nature and intensity of human activities, not all ECM plants, for instance, will be negatively affected. Oak has increased in eastern North American forests since Europeans arrived there (Abrams 1996), while grazing pressure associated with the spread of the Tibetan empire in the seventh century CE facilitated the development of the some 450,000 km2 of ECM Kobresia pygmaea (= Carex parvula)-dominated community of Tibetan plateau (Miehe et al. 2008, see also Zhou 2001). In other cases the basic vegetation type may be unaffected, despite human activities. If ECM ecosystems include members of different clades of ECM plants there may be ecological complementarity and stasis (c.f. Cadotte et al. 2012); Salicaceae, Betulaceae and Pinaceae, all ECM plants (or mostly so), may interact in this way in Boreal forests. Castanea dentata, an ECM plant and previously dominant in much of the eastern U.S.A., has suffered ecological death there, but perhaps with little overall effect, since it has been replaced by ECM oak-hickory forests (Abrams 1996). Although the species of trees growing in eastern deciduous forests in the eastern North America have changed considerably - and continue to change - in response to logging pressure, changing fire regimes, etc., since the advent of Europeans, the dominant species have remained ECM plants, even if their relative abundance has changed (Abrams 1996, 2003). However, with the suppression of fires in the last three hundred years or so replacement of oak-pine forests by largely non-ECM species does seem to be under way (Abrams 1996, 2003). The extent to which the ECM-dominated Mediterrananean Maquis vegetation reflects human activity is unclear, but again, the major components of the different successional stages are all ECM plants (Comandini et al. 2006). ECM trees remain important in Californian forests, even if tree size has changed and the ECM species are different (McIntyre et al. 2015).

3. The last issue is that adding a temporal component to thinking about ecosphere effects of the vegetation makes life very difficult. How have ecosystems with small groups of species that have a disproportionately great influence on current global ecology behaved over time? Grasslands, peat swamps, mangrove vegetation and the like are not fixed and invariant elements of the biosphere; their extents, and the roles that individual species play in them, have changed over even quite short periods, and as we think about the longer term, change has been ubiquitous - and with climate change, it continues (Williams et al. 2007; Maguire et al. 2015). Grasslands, C4 grasslands in particular, are clearly a novel biome, but what about other past communities with the same or similar species or the same genera as the communities of today? Becklin et al. (2016) discuss how the ecophysiology of individual species may change in response to Pleistocene and Holocene climate changes. The relative recency of the development of latitudinal gradients of diversity is just one aspect of this problem (see above). Jackson and Williams (2004: see also Williams & Jackson 2007; Veloz et al. 2012; Donoghue & Edwards 2014; Maguire et al. 2015; Sluiter et al. 2016: S.E. Australia peat swamps) discuss "no-analog communities" of the past that have species combinations unlike those of any current communities, often associated with "no-analog climates". Thus, to say that the accuracy of the use of nearest living relatives as a guide to the past "decreases with an increasing age of a palaeoflora" (Uhl 2006: p. 95; see also Wing 1987, etc.) rather understates the problem. Little et al. (2010) noted that there was no evidence that such iconic temperature indicators as leaf teeth in fact indicated temperature - latitude, perhaps, but temperature, no. Indeed, they found that many "climatic indicators" were in fact more or less linked with phylogeny, and conversely, that phylogeny could at times obscure the climate relations of traits. General patterns of associations in communities that have been stable for around 300 million years seem to have changed since the beginning of the Holocene (Lyons et al. 2016). Similar changes may have occurred in the relationships between plants and the insects that eat them, thus plant-sawfly associations may have become reorganised around the KTR (Schnweider 2016).

Not all ECM/ERM communities are notably productive, but those in the boreal zone in particular (Dixon et al. 1994) sequester considerable amounts of carbon in their soils, while the accumulation of raw humus on the forest floor in tropical ECM forests has long been noted (e.g. Alexander 1989a). The rate of nutrient turnover in mono- or oligo-dominant ECM vegetation types varies. It can be very high, particularly in the tropics (Torti et al. 2001), and ECM-dominated woody vegetation there is not always species-poor, as White (1983) noted for Miombo vegetation and Beard (1946) for the Mora-dominated forests of Trinidad. The dipterocarp-dominated forests at Bukit Lambir, Sarawak, are among the most species-rich tropical forests anywhere (Lee et al. 2002). Interestingly, Mora in particular behaves like some conifers (Enright & Ogden 1995; Aiba et al. 2007) and is almost an add-on to the vegetation, communities with and without Mora being otherwise similar; rather similarly, emergent dipterocarps may form a separate stratum above the rest of the forest (see Ashton & Hall 1992). Bornean Dipterocarps show very high levels of above-ground wood production when compared with other species in the same community, and also when compared with forests in the western Amazon (Ecuador, Peru) that are similar in soils, precipitation, etc.; there dipterocarps were more productive by as half as much again than their non-dipterocarp counterparts (Banin et al. 2014). This high productivity was despite a much lower amount of phosphorus in the soil in the dipterocarp forests and a C:N ratio about 50% higher (Banin et al. 2014). Litle is known about underground carbon storage and ECM activities in the Amazonian forests.

The importance of ECM fungi for angiosperm evolution is not just because they facilitate the nutrient and water supply of their associates and make life difficult for non-ECM plants and for free-living microbes, but also because of their direct and indirect effects on the biosphere - on soil, on weathering, on carbon sequestration, and hence on the earth's climate. ECM plants increase mineral weathering, and rainfall, derived in part from transpiration, also allows more silicate weathering, and this weathering is a principal sink for atmospheric carbon dioxide (Boyce et al. 2010; Berner 1997). An increase in atmospheric CO2 removed by the rock weathering has been linked to the decrease in atmospheric CO2 concentration during the Caenozoic (Pagani et al. 2009; L. L. Taylor et al. 2009, 2011; Quirk et al. 2014). In drier years, there may even be competition between ECM and lignin-decomposing fungi for water, leading to a reduction in the rate of wood decomposition (Koide & Wu 2003). Finally, carbon is buried in sediments - at least medium-term sequestration - much more easily than in AM forests, particularly those in the tropics where carbon turnover is very fast (Tedersoo et al. 2012; see Schmidt et al. 2011 for the very long C turnover times in subsoils, mycorrhizae not mentioned). All these biogeochemical effects of ECM plants are as much caused by the activity of fungi and bacteria associated with the plant as by any activities of the plant itself, all three forming a functional whole (e.g. Landeweert et al. 2001; L. L. Taylor et al. 2009; Bonfante & Anca 2009).

General diversity and community/ecosystem stability in the face of environmental change are connected (e.g. Hautier et al. 2015 and references). How this happens is a matter of discussion. Petchey and Gaston (2002a, b) suggested that if functional diversity/functional traits in the community are to be conserved, a large proportion of species in that community will also have to be preserved; there is little redundancy in functional diversity. Even if dominant species can maintain ecosystem functioning in the face of the loss of rare species, at least for a time (e.g. M. D. Smith & Knapp 2003), phylogenetic diversity may still improve ecosystem functioning, although this may also depend on rainfall, temperature, levels of soil nutrients and CO2, etc. (see e.g. Chapin et al. 1997; Zavaleta et al. 2003; Maestre et al. 2012; Cadotte et al. 2012). Experiments measuring biomass production find that productivity and overall diversity become more closely linked over time (Reich et al. 2012); Isbell et al. (2011) found that 84% of the grassland species studied promoted ecosystem functioning at least under some conditions even in the limited periods during which their experiments were carried out. As conditions change, different species may assume importance - and conditions have never been fixed for long throughout the whole Caenozoic period. Some species may even be quite flexible in the ecological roles they play (Aizen et al. 2012), although other studies suggest more conservatism (Maherali & Klironomos 2007; Stouffer et al. 2012). However, little of the work on community/ecosystem functioning has emphasized the kinds of communities that are the focus here.

Even since the beginning of the Pleistocene ca 2.6 m.y.a. there have been great changes in community composition and location, many plant communities being quite novel; the present is at best an imperfect guide to the past (e.g. Meseguer et al. 2014b), even over the short term. Prior to 3.3 m.y.a., boreal forests with ECM taxa like pine, spruce, larch and birch grew in the east Siberian-North American-Greenland area from 60-80o N, although especially since 2.7 m.y.a. the conifers, etc., have been replaced by tundra (Brigham-Grette et al. 2013). The association of Picea, Betula and Alnus, characteristic of Recent boreal forest, was first recorded in North America a mere 7,000 years ago (Williams et al. 2004). Mapping of post-glaciation forest changes in North America shows that some species have been fairly constant in abundance, if not in location, but they are mixed with other species that as it were appear from nowhere and come to be abundant over wide areas (e.g. Webb 1988; Williams et al. 2004; see also Jahn 1991). Many forest communities found in North America are Holocene in age, communities coming and going even in recent times (Curtis 1959; Williams et al. 2004 and references), and the same is true elsewhere in the world (Torres et al. 2013). Boreal peatlands are post-Pleistocene in age (MacDonald et al. 2006), even if some tropical peatlands are somewhat older (Page et al. 2004). The composition of tundra vegetation changed considerably from glacial to interglacial periods, and more carbon accumulated in the latter (Brubaker et al. 1995). Changes were complex (Lindo et al. 2013), and mosses were in places replaced by angiosperms with their rather more labile leaf litter, plant biomass showing an overall increase (Sistla et al. 2013). The changing relative proportions of forbs and graminoids in Arctic tundra and steppe over the last 50,000 years are detailed in Willerslev et al. (2014). As permafrost thaws, peat accumulation, especially by Sphagnum, but also by spruce, etc., may increase along with above-ground primary productivity (Camill et al. 2001).

The floristic composition of vegetation seems to have become much more "modern" in the latter part of the Caenozoic, and in North America forest composition may be little changed over the last 15 million years or so (Graham 1999; Hawkins et al. 2014). However, although biomes may have been fairly stable over the medium term, this is partly the result of how they are delimited, and individual species and their abundance may vary substantially within the one biome (Williams et al. 2004); communities, and biomes themselves, have certainly not been stable over this period (Moncrieff et al. 2016). The grassland and savanna biomes that are now such a prominent feature of vegetation can be dated to the Pliocene, within the last 10 m.y. or so, and especially within the last 3 m.y. (e.g. R. Sage et al. 2012; Pennington et al. 2006b; Simon et al. 2009; Simon & Pennington 2012). The boreal biome, now dominated by ECM taxa, is dated to 10-4 m.y.a. (references in Fine & Ree 2006), while diversification of Sphagnum, now a prominent and ecologically important component of boreal forest and tundra vegetation, is dated to the middle Miocene ca 14 m.y.a. (Shaw et al. 2010a; Shaw & Devos 2014; Johnson et al. 2015). Betula, now conspicuous in northern forests, has probably diversified within the last 10 m.y. (Xing et al. 2014) - yet fossils that are remarkably like Sphagnum are known from Ordovician rocks ca 455 m.y.o. (Cardona-Correa et al. 2016), while the palynomorph Stereisporites, linked to Sphagnum, was very common in fire-prone heathlands in Central Australia 75-65.5 m.y.a. (Carpenter et al. 2015). What was the ecological role of these plants? Sea grass communities may be quite old, mid Eocene or older, the sea grasses living then being assigned to extant genera and even species. However, sirenian grazers of New World seagrasses, very diverse through the Miocene, declined in the Pliocene (Domning 2001), and Plio-Pleistocene New World sea grasses may have grown in waters to 20-30 m deep, compared with ca 10 m today (Budd et al. 1996). Again, aspects of the ecology of early sea grass communities may be rather different from apparently similar communities today.

In the Palaeocene and Eocene in particular species that today have different climatic preferences grew together, and in the late Eocene mixed deciduous broad-leaved and evergreen and deciduous conifer forests grew within both the Arctic and Antarctic circles (e.g. Collinson 1990; Jahren 2007; Harrington et al. 2011; Collinson et al. 2012; Pross et al. 2012; Cantrill & Poole 2012). A unique biome in which angiosperm and AM gymnosperm trees were mixed developed in Eocene South America below 24oS (Jaramillo & Cárdenas 2013), while much more recently during the warmest part of the Miocene 17-15 m.y.a. in western North America there were associations of species unlike any extant (Millar 2011). The problem of no-analog communities can only increase with age (see also Cantrill & Poole 2012; Moncrieff et al. 2016). However, Bouchal et al. (2014) suggest that vegetation similar to that of the modern chaparral and nemoral conifer forest of the Coastal Ranges was to be found in the Late Eocene Front Range in west North America.

The current latitudinal diversity curves, with diversity commonly - but by no means always - peaking near the Equator, are a phenomenon of a post-Eocene globe with strong and seasonal N-S temperature gradients (see above), and even today, estimates of both living carbon biomass and dead and below-ground biomass are highest, not in l.t.r.f., but in some temperate and warm temperate forests, as well as mangroves and peat swamps (Keith et al. 2009; Pan et al. 2013: living biomass; c.f. in part Carvelhais et al. 2014). In the Mesozoic and early Caenozoic these curves were almost flat or they even peaked in more temperate areas (e.g. Wolfe 1987; Mannion et al. 2012, 2013; Archibald et al. 2012; see above). As temperatures dropped, particularly in the early Oligocene, vegetation with more local facies developed, seasonality became evident, etc.. Currently tall trees (80+ m tall: Tng et al. 2012 for records) tend to grow in thermally equable climates, so the distribution of such trees in the early Caenozoic is an interesting unknown (Larjavaara 2013); a single tall tree can sequester a considerable amount of carbon.

Pinus seems to have been a mid-latitude (30-50o N) plant in the Cretaceous, but Pinaceae in general may have been negatively affected by fires fuelled by angiosperm shrubs and ferns that Belcher and Hudspith (2016) suggested had a major effect on the vegetation from the middle Cretaceous onwards. In the warm Palaeocene and Eocene Pinus retreated to high latitudes (Millar 1998; Daly et al. 2011: also Cupressaceae there, conflict between evidence from pollen and macrofossils; see also Augusto et al. 2014), although it also grew near the equator. In high latitude (65-80o N) Eocene floras Pinaceae could be quite common, and with other ECM plants they made up about 2/5ths of the species on Canadian islands 75-80o N (McIver & Basinger 1999). With the climatic deterioration of the Late Eocene-Oligocene, Pinus moved back to mid latitudes while persisting at higher latitudes (C. I. Miller 1993, 1998), and Late Miocene-Pliocene mountain building would favour Pinaceae (LePage 2003). In the Oligocene Pinus moved into western Malesia (e.g. Muller 1972).

OUT OF PLACE The proportion of wind-pollinated trees and shrubs are higher in humid areas away from the tropics (Regal 1982; Ollerton et al. 2011); temperate wind-pollinated trees and shrubs tend to be ECM plants, as in about half the examples mentioned by Regal (1982).

The decline of atmospheric CO2 over the last 120 m.y. may in part be connected with the origin of clades of ECM plants (L. L. Taylor 2009, 2011; Quirk et al. 2012, 2014), although other contributing factors may include reduced volcanism from subduction along continental margins (McKenzie et al. 2016). If the present and past are connected, Normapolles and Nothofagites plants, along with most Fagales, were ECM; fossil remains of these plants are abundant in Late Cretaceous and early Cainozoic rocks (Friis et al. 2011 for a summary), and they may have had a transformative effect on the environment. Given the ages of Fagales and Pinaceae, ECM seed plants "may have [been found] over a larger area and for a much longer time period in northern temperate zones than in the tropics" (Tedersoo et al. 2012: p. 4167). ECM plants may even have progressively supplanted AM plants at weathering hotspots from some time in the Cretaceous (Taylor et al. 2011), indeed, ECM associations may be "the most profound alteration in root functioning to occur in plant evolutionary history" (ibid., p. 369). However, age uncertainties make life particularly difficult here; from the ages given above, the fungal ECM habit is at least sometimes likely to have originated rather later than the ECM seed plant clades on which the fungi are now found (see also Ryberg & Matheny 2012; Bruns et al. 1998; Horton & Bruns 2001).

The related ideas of keystone species and ecosystem engineers, species that directly or indirectly disproportionately control the resources needed by other organisms (Wright & Jones 2006), may be helpful here (e.g. Leighton & Leighton 1983; Terborgh 1986; Watson 2001; Watson & Herring 2012; Mouquet et al. 2012b; Sultan 2015: habitat construction). The clades we are talking about have a disproportionate effect on the community, ecosystem or even biosphere relative to their species numbers (see Power et al. 1996). Thus Brodribb et al. (2012; see also Coomes & Bellingham 2011) thought of conifers in general as being ecosystem engineers because of their major effect on the environnment. The clades under discussion have major effects at ecological scales from the local community up to the globe and are associated with major biomes or ecosystems (Pennington et al. 2004; see other papers in Proc. Royal Soc. B 359(1450). 2004), and it is at this level that the ecological interactions play out. It is increasingly a matter of comment that a number of clades seem to be more or less restricted to biomes (e.g. Schrire et al. 2004; Pennington et al. 2009; Dick & Pennington 2011; de Nova et al. 2012; Moncrieff et al. 2016), and in such cases ideas of phylogenetic, biome or niche conservatism are invoked: Clades retain niche-related traits or, more generally, have conserved ecological roles (e.g. Wiens & Donoghue 2004; Crisp et al. 2009; Crisp & Cook 2012; B. T. Smith et al. 2012; Kooyman et al. 2014). The ages of the ECM clades mentioned above are usually much more than 10 m.y., and so their evident ecological conservatism is relatively ancient (Tedersoo et al. 2014a).

However, whether keystone clades or ecosystem engineers, they are not sharply distinguishable from all other clades in terms of their effects on the environment; for demolition of the simple idea that there are keystone species - species "important for something", see Hurlbert (1997). The term "phylogenetic conservatism" has been used in various ways in the literature on phylogenetic community ecology (Mouquet et al. 2012a), while niche conservatism seems to be little more than the recognition that some ecological features are associated with clades more than might have been expected, but this is true of subsets of many groups of characters.

Estimates are that tropical ecosystems store 47% terrestrial carbon and have 59% terrestrial primary productivity (10% of both in the Amazon Basin alone: Tian et al. 2010). Similarly, of global terrestrial net primary production, that in l.t.r.f. is around 36% (19.1/53.2 x 1015 gC) of the total, that of grassland + savanna ca 19% (9.7/53.2 x 1015 gC, of which over half comes from tropical savanna), contributions of boreal forest, temperate coniferous forest, etc., being less than 6% each (Melillo et al. 1993). On a per area basis the inequalities between biomes are clearer, thus soil carbon storage, at ca 11.7 kg C m-2 and NPP, at ca 956 kg C m-2 yr1 in tropical forests, compares with figures of 14.5 and 576 respectively for grassland and ca 61.4 and ca 319 for boreal forests (Averill et al. 2014).

Overall, the implications of these asymmetries in relationships between animals, plants, and the environment, are complex, but species numbers are clearly just one way of thinking about seed plant evolution. By focusing on the construction and maintenance of the ecological scaffolding of community structure over evolutionary time and in a phylogenetic context, angiosperms with dense venation, C4 grasses and ectomycorrhizal plants represent pillars, and ants, bumble bees, fruit bats and the like, arches and spandrels. These groups appear to have had a major role in constructing and maintaining the environment, while the bulk of the tens of thousands of euasterid species make up the paintings in the spandrels (apologies to Gould & Lewontin 1979). These paintings are forever changing as individual species go extinct, for instance because of the breakdown of plant/pollinator relationships, while other relationships are evolving. Plant communities come and go, and the relation between present, past and future is unclear (e.g. Torres et al. 2012). Over time the whole biosphere has changed as groups of plants with different eco-physiological capabilities assume prominence, and such changes help shape the background ecological context for the diversification of seed plants and of the animals associated with them.

10. In Conclusion.

Innovations in reproductive biology are thought to characterise the evolution of new plant groups, allowing increases in diversity in part by greater subdivision of the environment (e.g. Niklas et al. 1983). Gorelick (2001) summarized some twenty hypotheses that have been advanced to explain diversification/success of the angiosperms (see also Crepet & Niklas 2009), many having to do with flowers, and all told some 120 or more hypotheses have been advanced to explain the patterns of plant species richness that are such a distinctive feature of the environment (Palmer 1994).

A focus has been on understanding speciation within individual very speciose clades (e.g. Davies et al. 2004c), and much literature emphasizes the acquisition of "key innovations", apomorphic features whose advantages - sometimes more or less assumed - allowed a subsequent increase in the overall speciation/diversification rate of the clade in which they arose (e.g. Marazzi & Sanderson 2010), and there are related ideas like "ecological opportunity" (Yoder et al. 2010b) that have much the same flavour. Thus clades with latex (Farrell et al. 1991; see also Powell et al. 1999; Agrawal & Konno 2009: survey of laticiferous plants and latex; Konno 2011: chemistry), nectar spurs (Hodges & Arnold 1995; Hodges 1997; Kay et al. 2006), monosymmetric flowers (Donoghue et al. 1998; Neal et al. 1998; Endress 2001; Sargent 2004; Kay & Sargent 2009; c.f. in part Kay et al. 2006), hummingbird pollination (Schmidt-Lebuhn et al. 2007), animal pollination (Eriksson & Bremer 1992; Kay et al. 2006b), self sterility (Ferrer & Good 2012), or the climbing habit (Gianoli 2004), are often more diverse in terms of extant species than their sister clades lacking these distinctive features (see also Ferrer et al. 2014 for vegetative and reproductive features associated with high diversification rates). Interestingly, Malpighiales and Ericales, disproportionately common among the small trees of the understory of tropical rain forests (Davis et al. 2005a), include taxa with many kinds of flowers and fruits. Neither clade can be well characterised either morphologically or chemically and they seem to lack innovations, whether key nor not, although the latter are sometimes evident in smaller clades within these two major groups.

It is a challenge to think about the evolution of the morphological and other novelties that are the focus here. Key innovations that cause the more or less immediate diversification of the clade in which they arise may be individually less important than we might like to think, and identifying key innovations is far more than simply linking a feature to a node on the tree (e.g. Sims & McConway 2003; Davies et al. 2004a; Donoghue 2005; Erkens 2007; Crepet & Niklas 2009; Marazzi & Sanderson 2010; c.f. Endress 2011a). Overall, determining that an innovation might be a key innovation is a difficult process, and speciation and the evolution of a key innovation are not necessarily to be linked (e.g. Cracraft 1990; Sanderson 1998; Galis 2001; Ree 2005b; Maddison et al. 2007). Indeed, Hedges et al. (2015) suggest that speciation and diversification are largely dominated by random events, while adaptive changes are something else again. Understanding the not-so-simple idea of persistence is also important (Leslie et al. 2013).

Key innovations are rarely simple features, rather, they may involve a complex suite of changes that occur over a protracted period, as with angiosperm flowers and vessels (e.g. Horn et al. 2012: Euphorbia subgenus Chamaesyce; Schranz et al. 2012; Donoghue 2005; Donoghue & Sanderson 2015; see also Stebbins 1951). Furthermore, the importance of some changes may be less in the changes themselves, but subsequent changes that they make possible and/or their importance in ecological conditions that may develop long after their origin. Thus Donoghue and Sanderson (2015) and Bouchenak-Khelladi et al. (2015) look at diversification and its relation to where characters that might be involved in that diversification actually change on the tree. Similarly, Edwards and Donoghue (2006) suggest that several key elements of the cactus ecological niche were established before the evolution of the cactus life form and subsequent diversification of Cactaceae (Ogburn 2007; Ogburn & Edwards 2009; Nyffeler & Eggli 2010). The evolution of C4 photosynthesis (e.g. Strömberg 2011) and the effects of genome duplications (e.g. Tank et al. 2015) both fit this model.

The increase in speciation that results from the acquisition of a key innovation has to be distinguished from simple radiation of a clade when it moves into in a new area, thus much speciation in Guatteria may have occurred only subsequent to its entry into South America (Erkens et al. 2007). CYC-like genes are widely involved in symmetry changes, especially in core eudicots (X. Yang et al. 2012; Preston & Hileman 2012), but direct links between gene and diversification remain to be established. Howarth and Donoghue (2004, esp. 2005) note possible connections between changes in such genes and changes in floral form in Dipsacales. Here crown-group Caprifoliaceae-Valerianoideae (315 spp.) are estimated to be 60-55 m.y.o. (Bell & Donoghue 2005a), but diversification in the Andean paramo, which has resulted in ca 1/7th of the species currently recognized in the clade, happened only after their arrival in South America less than 5 m.y.a. (Bell & Donoghue 2005b; Moore & Donoghue 2007, see also Viburnum) and is not obviously associated with the evolution of particular floral (or other) key innovations (see also Richardson et al. 2001). Similarly, rapid diversification of Andean species of Lupinus - where most species of the genus are now found - began only some 1.76-1.19 m.y.a. and was probably driven by the ecological opportunities available in the high altitude habitats there (Hughes & Eastwood 2006; Drummond 2008; Drummond et al. 2012); bumble bees, also recent immigrants to South America, may have been an important factor in this diversification (Hines 2008). Finally, in Halenia (Gentianaceae) with its "key innovation" of five nectar spurs, diversification and acquisition of these spurs are not simply linked (von Hagen & Kadereit 2003, see also Gentianella, etc.). Of course, factor(s) that enable a clade to move into new ecological space within which much diversification subsequently occurs may be part of the definition of a key innovation (Sargent 2004; Marazzi & Sanderson 2010). Finally, deciding that a clade is notably speciose is not a simple task (e.g. Sanderson & Wojciechoswki 1996; Donoghue & Sanderson 2015).

In many very speciose clades, including angiosperms as a whole, patterns of clade numbers do not suggest an immediate diversification after the acquisition of putative key innovations (see also above). Diversification rates of early diverging clades of angiosperms are low, clades with higher rates came later after what might be thought of as "typical" angiosperm flowers, i.e. those of Pentapetalae and of monocots, slowly assembled (see e.g. Sanderson & Donoghue 1994; Magallón & Sanderson 2001). Friis et al. (2006b) emphasized that such early clades have long fossil records yet include only a few extant species, and they also differ from other angiosperms in ecophysiological features. Thus ANA grade angiosperms have low veinlet densities rather like those of gymnosperms and ferns, hence transpiration rates and photosynthetic capacity are rather low (Brodribb et al. 2007; Boyce et al. 2009; Feild et al. 2009a; Brodribb & Feild 2010); their xylem is functionally not very different from that of vessel-less Pinales (e.g. Sperry et al. 2007; Feild & Thomas 2012; Plavcová & Jansen 2015). Similarly, distinctions between the nature and arrangement of floral parts that are obvious in say, Pentapetalae are less evident in members of the ANA grade, endosperm formation is variable, etc. (e.g. Buzgo et al. 2004; Endress 2005c; M. L. Taylor et al. 2008; Friedman 2008b). Flowers may become important in facilitating diversification only with the evolution of bees (Cappellari et al. 2013). Characters that seem to facilitate diversification but that evolve well before the diversification they are supposed to facilitate can be thought of as exaptions (Gould & Vrba 1982; de Queiroz 2002; see also 2015), and this may be appropriate for several of the characters considered to be key innovations of angiosperms or of major clades within it.

Parallelism and convergence, homoplasy, are everywhere one looks, even in early land plant evolution (e.g. Boyce 2010; Endress & Matthews 2012). Nearly all angiosperm characters are highly homoplastic, arising in parallel and being lost many times and characterising both large and small clades. Parallelisms occur even at the amino acid level as in C4 photosynthesis (e.g. Bläsing et al. 2000; Christin et al. 2007b, 2008b, 2009a; Brown et al. 2011). Particular features are rarely consistently key innovations. The evolution of extra-floral nectaries may be deemed a key innovation in some Senna (Fabaceae) but the loss of such nectaries may equally be a key innovation in related taxa (Marazzi & Sanderson 2010). Similarly, Weber and Agrawal (2014) found that the acquisition of defensive metabolites increased diversification in 4/6 of the clades on which they focussed, although overall families with extra-floral nectaries showed a doubling of diversification rates. Some wind-pollinated clades are speciose, although most are not. Understanding developmental/regulatory pathways is important. The frequent reaquisition of woodiness in clades that have become herbaceous may be because elements of the cambial regulatory program remain untouched (Groover 2005; see also Blein et al. 2010: vegetative development). There are elements of common developmental mechanisms involved in independent acquisitions of monosymmetry (e.g. Feng et al. 2006: Fabaceae and Plantaginaceae; Zhang et al. 2010, Malpighiaceae), duplication of CYC genes being involved (see also Damerval & Manuel 2003; Rosin & Kramer 2009; Preston et al. 2011b). Irish (2009) suggests that petals may have evolved several times because of the independent cooption of underlying gene regulatory networks, and, as has been mentioned, the acquisition of CAM and C4 photosynthesis, and nitrogen fixation, and the development of ectomycorrhizal associations are similar.

Heterochrony (e.g. the male gametophyte of flowering plants - see Takhtajan 1976), heterotopy (e.g. Baum & Donoghue 2002), and homeosis (e.g. Mathews & Kramer 2012) are all part of this mix. As Preston et al. (2011b) put it as they summarized aspects of the developmental evolution of angiosperm flowers, "reduce, reuse, and recycle" has been the order of the day, and it seems that old dogs can indeed be taught new tricks (Rosin & Kraemer 2009; Mathews & Kramer 2012). We have tended to think of evolution as the modification of pre-existing form: "Are petals in x really modified stamens?". Now we have the tools to think more about the evolution of novelty, elements of developmental pathways that merge and form new combinations; Mathews and Kramer (2012) review floral and in particular ovule development across seed plants from this point of view, and this is true of vegetative features, too, as in the distinctive leaves of the Inverted Repeat Loss Clade of Fabaceae.

The often rather sporadic distributions of secondary metabolites has long been difficult to understand. But as with cambia, the ability to synthesise a particular secondary metabolite having been acquired, it may be switched off easily, but not lost, and so the metabolite can be "reacquired" (e.g. Grayer et al. 1999; Wink 2003, 2008, 2013; Liscombe et al. 2005; Albach et al. 2005c; Agrawal et al. 2012). However, in other situations pathways may degenerate, and change becomes irreversible (Zufall & Rauscher 2004). Associations between plant and fungus/microbe in both mycorrhizal and endophytic associations may also go some way towards understanding the rather unpredictable pattern of distribution of many secondary metabolites (Wink 2008; Lamit et al. 2009); endophytes may synthesize metabolites normally ascribed to the plant. Genes can be transferred via grafts in host-parasite connections, chloroplasts can move via grafts between organisms - and perhaps genes may be transferred from live pollen that lands on the stigma, germinates, but does little else (Christin et al. 2012: confirmation needed!).

The idea of evolutionary "tendencies" persists (e.g. Endress & Matthews 2012), similar discussions having recurred periodically in the phylogenetic literature (e.g. Cantino 1985; Sanderson 1991). Indeed, some phenotypes may be the result of parallel mutations that occur only because of a previous change in the larger clade (see Shubin et al. 2009 on deep homology), and Marazzi et al. (2012) attempt to locate such an evolutionary precursor - in this case, extrafloral nectaries in Fabaceae - on the tree. The ability of a plant to form an association with nitrogen-fixing bacteria is a good example (see Fabales: e.g. Soltis et al. 1995b), the clustered origins of C4 photosynthesis in grasses, sedges and core Caryophyllales invite a similar explanation (see e.g. McKown et al. 2004; Christin et al. 2011a, 2013; Grass Phylogeny Working Group II 2011), as do the origins of various symmetry types in angiosperm flowers (Irish 2009; Preston et al. 2009). Recent work on the independent acquisition of the ability of the plant to synthesizie caffeins i.a. shows how difficult it is to distinguish between convergence and parallelism (Huang et al. 2016).

The rise to dominance of the angiosperms and the diversification of particular angiosperm clades also involves other organisms - plants, animals, fungi, bacteria - as well as changes in the environment itself in which angiosperms are also involved, and it is a thoroughly ecological process (e.g. Thompson 1998; Harmon et al. 2009). Lavin et al. (2004) and Schrire et al. (2005) suggest that it is more profitable to think of diversification and distribution of Fabaceae in terms of vicariance of biomes rather than of the classic geographical areas. The area that a clade inhabits, especially if it is non-contiguous, may affect diversification (Vamosi & Vamosi 2010, 2011; see also Marazzi & Sanderson 2010 above). Under such circumstances diversity may be limited by ecological factors (e.g. Vamosi & Vamosi 2010), although if the assumption that clades increase steadily in diversity with time is unreasonable (Rabosky 2009; Vamosi & Vamosi 2010), so is any implicit assumption that the environment does not change.

The evolution of the first angiosperms took place under ecological conditions rather different from those under which they later prospered; then continents were drifting apart, there were high atmospheric carbon dioxide concentrations and rising sea levels, and ever-wet tropical humid climates were rather restricted. Even if "basal" clades that are now species poor were more diverse in their early history (Magallón & Castillo 2009; Friis et al. 2011 and references), conditions then were unlike those of today. Diverse angiosperm-dominated vegetation is largely of Caenozoic age, so the early Caenozoic environment - warm, not seasonal, few fires, the beginning of the diversification of some pollinators, herbivores and frugivores - provide the context for thinking about its evolution. Subsequent diversification occurred as temperatures and atmospheric CO2 concentrations were dropping, seasonality was increasing, and, especially within the last 10 m.y, fires were increasing. Climate has been been changing throughout the history of crown-group angiosperms, spurred in part by angiosperms themselves and their association with ECM fungi (e.g. Knoll & James 1987; Volk 1989; Boyce et al. 2010).

Thus establishing an immediate connection between acquisition of an apomorphy or group of apomorphies and diversification is difficult (e.g. Galis 2001). The evolution of flowers, vascular systems, and just about all aspects of plants seems a complex, protracted, and clade-dependent process involving both intrinsic and extrinsic factors, and often with a time lag between origin and effect. As Feild and Arens (2007: 21) noted, "Among basal angiosperms, the initial transitions to higher-light environments are characterized by a high degree of lineage-dependent, functional experimentation, in which fine-tuned performances were assembled piece-by-piece."

Diversification may well depend "on the fortuitous combinations of a large repertoire of traits" (Feild & Arens 2005: p. 402) rather than on any particular key innovation (see also Crepet & Niklas 2009 and references; Magallón & Castillo 2009). Overall angiosperm success is in considerable part the result of diversification of individual angiosperm clades with particular combinations of characters that are responding to various ecological/environmental contexts. Angiosperms show bursts of diversification in separate clades, especially in a number of asterids and monocots (e.g. Magallón & Sanderson 2001; Sims & McConway 2003; Crepet & Niklas 2009). Thus two thirds (16,360 spp.) of Asteraceae are members of the chemically very distinct Asteroideae. Over four out of five Orchidaceae are Epidendroideae (21,600 spp.) in which the epiphytic habitat predominates, and much diversification occurred in the "higher epidendroids" some (64-)59-42(-36)/(49-)39-34(-22) m.y.a. (Ramírez et al. 2007; Gustafsson et al. 2010). Exactly where monosymmetry is an apomorphy in Asterales as a whole is unclear - and so on. Phylogenetic niche conservatism - another way of saying ecological traits that do not reverse or reverse only little - or adaptations to "major ecological niches" mean that some groups will follow these niches when there is an opportunity (Donoghue 2008; especially Lavin et al. 2004; Schrire et al 2005; Marazzi & Sanderson 2010); adaptation to such niches may not occur very frequently. Extinction, although difficult to document, plays an important role. Thus as recently as ca 30 m.y.a. there were stem-group hummingbirds in Europe (Mayr 2004, 2009) and Cyclanthaceae are known from European Eocene deposits (S. Y. Smith et al. 2008). Both groups are now iconically New World, and hummingbirds are involved in the pollination of (well) over 5,000 species of flowering plants there (see also Abrahamczyk & Kessler 2014).

I finish by returning to a point that has been made frequently in this section. Species number is only one estimate of success in evolution, and there is a weak negative correlation between diversity and biomass produced (Wing & Boucher 1998). Biomass production, primary productivity, etc., provide ecological estimates of success, and are to be seen in the context of the eco-physiological evolution of angiosperms and the environmental changes that resulted. Venation density, vascular evolution and other ecophysiologically important features like the evolution of ectomycorrhizal associations and C4 photosynthesis have helped shape the evolution of biomes within which angiosperm diversification has occurred. Here and elsewhere we have to grapple with the evolutionary implications of clade size:ecological importance asymmetries.

AMBORELLALES Melikian, A. V. Bobrov & Zaytzeva  Main Tree.

Phloem loading via intermediary cells [specialized companion cells with numerous plasmodesmata; raffinose, etc., involved]; nodes 1:1; plant dioecious; P spiral; hypanthium +; nectar from base of P?; A sessile, middle layer of anther wall from both secondary parietal cells [ wall type]; pollen anaulcerate [pore-like, operculum endexinous, margin poorly defined], ektexine cupulate [distinctive undulate, columella-less exine]; stigma with uniseriate multicellular papillae; ovule 1/carpel, outer integument annular [cap-shaped], nucellar cap 0; embryo sac bipolar, 9-nucleate, with three synergids, antipodal cells die very early, polar nuclei in chalazal region; fruit a drupelet; endosperm triploid, develops in chalazal half. - 1 family, 1 genus, 1 species.

Note: Boldface denotes possible apomorphies, (....) denotes a feature common in the clade, exact status uncertain, [....] includes explanatory material. Note that the particular node to which many characters, particularly the more cryptic ones, should be assigned is unclear. This is partly because homoplasy is very common, in addition, basic information for all too many characters is very incomplete, frequently coming from taxa well embedded in the clade of interest and so making the position of any putative apomorphy uncertain. Then there are the not-so-trivial issues of how character states are delimited and ancestral states are reconstructed (see above). Of course, putting apomorphies here in particular is a distinctly dubious proposition, given both the position of the clade and the black hole of ignorance immediately basal to angiosperms.

Includes Amborellaceae.

Synonymy: Amborellineae Shipunov

AMBORELLACEAE Pichon, nom. cons. Back to Amborellales


Shrub or small tree; alkaloids?; stem cork cambium?; axial parenchyma apotracheal-diffuse; (some pits in tracheary elements lacking membranes); pericycle with hippocrepiform sclereids; mucilage cells 0; petiole bundles arcuate; (stomata anomocytic); ?tooth morphology; inflorescence cymose; flowers small [<7 mm across]; P spiral, 5-8(-15), basally slightly connate, with a single trace; staminate flowers: A 6-25, outer adnate to the base of P, vascular bundle branched near thecae; pistillode 0; carpellate flowers: staminodes 1-2; G 3-7, whorled; ovule ± median, pendulous, hemianatropous, sessile, micropyle endostomal; P persistent, stone largely mesocarpial in origin, surface sculpted; seed coat tanniniferous, also endotesta lignified, collapsed, exotegmen lignified; embryonic suspensor triangular, >2-seriate; germination hypogeal, seedlings/young plants sympodial; n = 13; horizontal transfer of atp1 gene.

1[list]/1: Amborella trichopoda. New Caledonia. [Photo - Leaves, Flower.]

Evolution: Divergence & Distribution. Assuming that New Caledonia finally became emergent only some 37 m.y.a. (Grandcolas et al. 2008; Cluzel et al. 2012; Swenson et al. 2014, 2015 for references, but c.f. Condamine et al. 2016: metapopulations on ephemeral islands?), proto-Amborella must have been hanging out somewhere else for a very long time.

Pollination Biology. Both insects and wind are effective pollinators, i.e. the plants are ambophilous (Thien et al. 2003; see Culley et al. 2002 for ambophily Gottsberger 2016). Stigmatic exudate may join all the stigmas of a single flower together, so pollen landing on a single stigma can pollinate ovules in more than one carpel, i.e. there is an extragynoecial compitum (Williams 2009).

Genes & Genomes. The mitochondrial genome of Amborella contains genes from a number of land plants, including at least three different mosses, and such "foreign" genes may also migrate to the nucleus (Bergthorsson et al. 2004; c.f. Goremykin et al. 2009: methodological problems?). The mitochondrial genome of Amborella is about seven times normal size and has acquired about three genomes worth of green algal DNA, two genomes worth of moss DNA, and one genome worth of angiosperm DNA (Rice et al. 2013). The algal DNA is similar to that of the trebouxiophyte Coccomyza, a component of lichens, perhaps suggesting that wounding of Amborella plants somehow facilitated the uptake of mitochondrial DNA from associated epiphytes. The angiosperm DNA came from Fagales, Oxalidales, Santalales, Ricinus (Malpighiales-Euphorbiaceae) and Bambusa (Poales-Poaceae) (Rice et al. 2013). Mitochondrial genomes like that of Amborella are as yet unknown from other angiosperms, although sampling is still poor, and one wonders what is distinctive about Amborella that might cause it alone to be such "a graveyard of foreign genes" (Rice et al. 2013: p. 70).

Chemistry, Morphology, etc. For absence of aluminium accumulation, see Thien et al. (2003). Amborella lacks reaction wood, its stems tending to sprawl, especially when young (but see Roussel & Clair 2015); in terms of architectural models (Hallé et al. 1978) the plant conforms to Troll's model. Some pits of the tracheary elements lack membranes, so technically they are vessels; open conduits are made up of only two such cells (Feild et al. 2000b), but Carlquist (2012a: p. 107; see also 2012c) thought that these were artefacts, noting that "intact porose pits in end walls of Amborella can be found". Stomatal morphology is quite variable, although the brachyparacytic configuration is common (Carlquist & Schneider 2001). The leaves are described as being spiral at first (Cronquist 1981; Takhtajan 1997), but c.f. Posluszny and Tomlinson (2003).

The perianth is spiral and undifferentiated. There is little agreement about pollen morphology: Sampson (2000) and Hesse (2001) suggest that the pollen is not really tectate (see also J. A. Doyle 2000, 2001, esp. 2009; Doyle and Endress 2000), and the aperture is difficult to categorise, as well as not always being present. Williams (2008, 2009) describes pollen tube development and fertilization. The ovule has been described as being orthotropous, anatropous, or intermediate (Tobe et al. 2000); for the embryo sac of Amborella, see above. The drupe of Amborella differs from a drupe in the strict sense in that the bulk of the woody layer is mesocarpial in origin, unlike the drupes of Laurales, etc., where the woody layer is endocarpial (Bobrov et al. 2005). The nature of the "resinous" cavities in the mesocarp is unclear; although not observed by Bobrov et al. (2005), they were conspicuous in material that I saw and are unlikely to be an artefact caused by re-expansion of dried fruits prior to study. The seed coat appears to have thin, unlignified walls, as might be expected in such a fruit, although some lignification has been reported (Tobe et al. 2000).

Additional information is taken from Bailey and Swamy (1948: general), Metcalfe (1987: anatomy), Philipson (1993), Sampson (1993: pollen), Yamada et al. (2001a: ovules), and Field et al. (2003: ecophysiology); for endosperm, see above. Chemistry?

Previous Relationships. Amborellaceae were included in Laurales by Cronquist (1981) and Takhtajan (1997).