EMBRYOPSIDA Pirani & Prado
Gametophyte dominant, independent, multicellular, initially ±globular, not motile, branched; showing gravitropism; acquisition of phenylalanine lysase* [PAL], flavonoid synthesis*, microbial terpene synthase-like genes +, triterpenoids produced by CYP716 enzymes, CYP73 and phenylpropanoid metabolism [development of phenolic network], 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, growth 3-dimensional*, 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]; plastid transmission maternal; nuclear genome [1C] <1.4 pg, main telomere sequence motif TTTAGGG, KNOX1 and KNOX2 [duplication] and LEAFY genes present, ethylene involved in cell elongation; chloroplast genome with close association between trnLUAA and trnFGAA genes [precursors for starch synthesis], tufA, minD, minE genes 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.
Sporophyte well developed, branched, branching dichotomous, potentially indeterminate; hydroids +; stomata on stem; sporangia several, terminal; spore walls not multilamellate [?here].
II. TRACHEOPHYTA / VASCULAR PLANTS
Sporophyte long lived, cells polyplastidic, 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]; PIN[auxin efflux facilitators]-mediated polar auxin transport; (condensed or nonhydrolyzable tannins/proanthocyanidins +); xyloglucans with side chains uncharged [?level], in secondary walls of vascular and mechanical tissue; lignins +; roots +, often ≤1 mm across, root hairs and root cap +; stem apex multicellular [several apical initials, no tunica], with cytohistochemical zonation, plasmodesmata formation based on cell lineage; vascular development acropetal, tracheids +, in both protoxylem and metaxylem, G- and S-types; sieve cells + [nucleus degenerating]; endodermis +; stomata numerous, involved in gas exchange; leaves +, vascularized, spirally arranged, blades with mean venation density ca 1.8 mm/mm2 [to 5 mm/mm2], all epidermal cells with chloroplasts; 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; archegonia embedded/sunken [only neck protruding]; 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].[MONILOPHYTA + LIGNOPHYTA]
Sporophyte growth ± monopodial, branching spiral; roots endomycorrhizal [with Glomeromycota], 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; nuclear genome size [1C] = 7.6-10 pg [mode]; 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].
SEED PLANTS† / SPERMATOPHYTA†
Growth of plant bipolar [roots with positive geotropic response]; 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].
EXTANT SEED PLANTS
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]; roots often ≥1 mm across, 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], 2C genome size (0.71-)1.99(-5.49) pg, two copies of LEAFY gene, PHY gene duplications [three - [BP [A/N + C/O]] - copies], 5.8S and 5S rDNA in separate clusters.
IID. ANGIOSPERMAE / MAGNOLIOPHYTA
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 randomly oriented, brachyparacytic [ends of subsidiary cells level with ends of pore], outer stomatal ledges producing vestibule, reduction in stomatal 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; T +, petal-like, 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, orbicules +, pollenkitt +; nectary 0; carpels present, superior, free, several, spiral, 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, 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 bicellular at dispersal, germinating in less than 3 hours, siphonogamy, pollen tube 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 gametophytes tricellular, gametes 2, lacking cell walls, ciliae 0, double fertilization +, ovules aborting unless fertilized; P deciduous in fruit; mature seed much larger than fertilized ovule, small [<5 mm long], dry [no sarcotesta], exotestal; endosperm +, ?diploid, 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 [2C] (0.57-)1.45(-3.71) [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.
[NYMPHAEALES [AUSTROBAILEYALES [[CHLORANTHALES + MAGNOLIIDS] [MONOCOTS [CERATOPHYLLALES + EUDICOTS]]]]]: wood fibres +; axial parenchyma diffuse or diffuse-in-aggregates; pollen monosulcate [anasulcate], tectum reticulate-perforate [here?]; ?genome duplication; "DEAER" motif in AP3 and PI genes lost, gaps in these genes.
[AUSTROBAILEYALES [[CHLORANTHALES + MAGNOLIIDS] [MONOCOTS [CERATOPHYLLALES + EUDICOTS]]]]: phloem loading passive, via symplast, plasmodesmata numerous; vessel elements with scalariform perforation plates in primary xylem; essential oils in specialized cells [lamina and P ± pellucid-punctate]; tension wood + [reaction wood: with gelatinous fibres, G-fibres, on adaxial side of branch/stem junction]; anther wall with outer secondary parietal cell layer dividing; tectum reticulate; nucellar cap + [character lost where in eudicots?]; 12BP [4 amino acids] deletion in P1 gene.
[[CHLORANTHALES + MAGNOLIIDS] [MONOCOTS [CERATOPHYLLALES + EUDICOTS]]] / MESANGIOSPERMAE: benzylisoquinoline alkaloids +; sesquiterpene synthase subfamily a [TPS-a] [?level], polyacetate derived anthraquinones + [?level]; outer epidermal walls of root elongation zone with cellulose fibrils oriented transverse to root axis; P more or less whorled, 3-merous [?here]; pollen tube growth intra-gynoecial; extragynoecial compitum 0; carpels plicate [?here]; embryo sac monosporic [spore chalazal], 8-celled, bipolar [Polygonum type], antipodal cells persisting; endosperm triploid.
[MONOCOTS [CERATOPHYLLALES + EUDICOTS]]: (extra-floral nectaries +); (veins in lamina often 7-17 mm/mm2 or more [mean for eudicots 8.0]); (stamens opposite [two whorls of] P); (pollen tube growth fast). - Back to Main Tree
Age. This node has been dated to 147-143 m.y.a. (Leebens-Mack et al. 2005), while Foster et al. (2016a: q.v. for details) estimated it to be ca 160 m.y.o., Chaw et al. (2004: 61 chloroplast genes, sampling poor) dated it to 150-140 m.y.a., Moore et al. (2010: 95% highest posterior density) estimated an age of (142-)135(-127) m.y., Clarke et al. (2011: other estimates) an age of (161-)137(-124) m.y.; 138-134 m.y. was the estimate in Mennes et al. (2013), 145 or 142.3 m.y. in Naumann et al. (2013), (158.5-)ca 143, 138.4(-130.5) m.y. in Xue et al. (2012), ca 135.8 m.y.a. in Magallón et al. (2015) and (143-)138, 131(-126) m.y. in Hertweck et al. (2015). Ages of ca 172.4 m.y. in Tank et al. (2015: Table S2), ca 213 m.y. in Z. Wu et al. (2014), (238-)214(-190) m.y. (Murat et al. 2017) and (394-)301(-208) m.y. in Zimmer et al. (2007) are much higher, while the ages of around (126.8-)125.1(-124.1) m.y. in Iles et al. (2014) are the lowest.
Fossil-based estimates are somewhat younger, ca 100 m.y. (Crepet et al. 2004: monocots sister to magnoliids) or at least 110 m.y. (e.g. Friis et al. 2010: see below).
Evolution. Genes & Genomes. x = 15 may be the base chromosome number for the clade (Murat et al. 2017).
Taxa in which DEF-like proteins cannot form heterodimers predominate in this clade (Melzer et al. 2014), and heterodimerization of B class protiens is perhaps derived in (some) PACMAD grasses (Bartlett et al. 2015).
Phylogeny. Relationships between the lineages immediately above the basal pectinations in the main tree, the ANA grade (Amborellales, Nymphaeales, Austrobaileyales), are slowly being clarified. For further information, see the mesangiosperm node. Chloranthales, eudicots, magnoliids, and Ceratophyllales are the other clades involved. There is, however, some evidence that Ceratophyllales are sister to eudicots.
MONOCOTYLEDONS / MONOCOTYLEDONEAE / LILIANAE Takhtajan - Back to Main Tree
Plant herbaceous, perennial, rhizomatous, growth sympodial; non-hydrolyzable tannins [(ent-)epicatechin-4] +, neolignans 0, CYP716 triterpenoid enzymes 0, benzylisoquinoline alkaloids 0, hemicelluloses as xylan, cell wall also with (1->3),(1->4)-ß-D-MLGs [Mixed-Linkage Glucans]; root epidermis developed from outer layer of cortex; endodermal cells with U-shaped thickenings; cork cambium [uncommon] superficial; stele oligo- to polyarch, medullated [with prominent pith], lateral roots arise opposite phloem poles; stem primary thickening meristem +; vascular development bidirectional, bundles scattered, (amphivasal), vascular cambium 0 [bundles closed]; tension wood 0; vessel elements in roots with scalariform and/or simple perforations; tracheids only in stems and leaves; sieve tube plastids with cuneate protein crystals alone; ?nodal anatomy [see below]; stomata oriented parallel to the long axis of the leaf, in lines; prophyll single, adaxial; leaf blade linear, main venation parallel, the veins joining successively from the outside at the apex and forming a fimbrial vein, transverse veinlets +, unbranched [leaf blade characters: ?level], vein/veinlet endings not free, margins entire, Vorläuferspitze +, base broad, ensheathing the stem, sheath open, petiole 0; inflorescence terminal, racemose; flowers 3-merous [6-radiate to the pollinator], polysymmetric, pentacyclic; P = 3 + 3 T, each with three traces, median T of outer whorl abaxial, aestivation open, members of whorls alternating, [pseudomonocyclic, each T member forming a sector of any tube]; stamens = and opposite each T member [A/T primordia often associated, and/or A vascularized from T trace], anther and filament more or less sharply distinguished, anthers subbasifixed, wall with two secondary parietal cell layers, inner producing the middle layer [monocot type]; pollen reticulations coarse in the middle, finer at ends of grain, infratectal layer granular; G , with congenital intercarpellary fusion, opposite outer tepals [thus median member abaxial], placentation axile; outer integument often largely dermal in origin, parietal tissue 1 cell across; antipodal cells persistent, proliferating; fruit a loculicidal capsule; seed small to medium sized [mean = 1.5 mg], testal; embryo long, cylindrical, cotyledon 1, apparently terminal [i.e. bend in embryo axis], with a closed sheath, unifacial [hyperphyllar], both assimilating and haustorial, plumule apparently lateral; primary root unbranched, not very well developed, stem-borne roots numerous, hypocotyl short, (collar rhizoids +); no dark reversion Pfr → Pr; 2C nuclear genome (0.7-)1.29(-2.35) pg, duplication producing monocot LOFSEP and FUL3 genes [latter duplication of AP1/FUL gene], PHYE gene lost. - 11 orders, ca 78 families, 60,100 species.
Note: In all node characterizations, boldface denotes a possible apomorphy, (....) denotes a feature the exact status of which in the clade is uncertain, [....] includes explanatory material; other text lists features found pretty much throughout the clade. 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).
Age. Molecular ages of crown-group monocots have been variously estimated at ca 200±20 m.y. (Savard et al. 1994), ca 189 m.y. (Z. Wu et al. 2014), and 160±16 m.y. (Goremykin et al. 1997). A number of estimates are centred on 140-130 m.y.a.: 135-131 m.y. (Leebens-Mack et al. 2005), 133.8-124 m.y. (Moore et al. 2007), (147-)134(-121) m.y. (Bremer 2000b: age used for dating monocot groups in general, Janssen & Bremer 2004), ca 133.2 m.y.a. (Magallón et al. 2015), (154.4-)137.1, 134.1(-123.4) m.y. (Magallón et al. 2013), 138-134 and 136-132 m.y. (Mennes et al. 2013, 2015 respectively), (143-)138, 131(-126) m.y. (Hertweck et al. 2015) and (142-)136(-130) m.y.(Givnish et al. 2016b). Magallón and Castillo (2009) suggest ca 177 m.y. or 127 m.y. for this split while Bell et al. (2010) estimate ages of (157-)146, 130(-109) m.y.; Moore et al. (2010) offer an age of (129-)122(-117) m. years. Other suggestions range from (174.3-)152.9(-134.1) m.y. (Eguchi & Tamura 2016; see also Tang et al. 2016), 181-123 m.y. (Barba-Montoya et al. 2018) and (191-)164, 156(-139) m.y.a. (Smith et al. 2010: c.f. Table S3, slightly younger), later Jurassic, to 228.6-128.3 m.y. (Nauheimer et al. 2012: Table S4), although most estimates there are in the 150-139 m.y. range. An age of (174-)169, 134(-119) m.y. was proposed by Zeng et al. (2014), Zhang et al. (2012) suggested an age of (142-)124(-108) m.y., and there are similar ages (ca 125.1 or 121.5 and 128.5-114.5 m.y.) in Xue et al. (2012) and Morris et al. (2018) respectively, as little as as 106.7 m.y. is the age in Naumann et al. (2013), (110.5-)104.2(-98) m.y. in Iles et al. (2014), 167-120 m.y.a. (Foster & Ho 2017). Estimates in Schneider et al. (2004) pretty much cover all possibilities.
An early fossil-based estimate of the age of stem monocots was only ca 98 m.y. and that of crown monocots ca 90 m.y. (Crepet et al. 2004). Fossil evidence suggested to Jud and Wing (2012) that monocots and eudicots were present ca 125-119 m.y.a. by the Early Aptian; if monocots and eudicots are sister taxa, their stem group ages will have to be at least as old as the tricolpate pollen that characterises eudicots.
Liliacidites pollen, boat-shaped, monosulcate, and with reticulate sculpture that becomes finer at the ends of the grain, can be assigned to monocots, Liliacidites sp. A being dated to 125-113 m.y.a. (e.g. Doyle 2014b; Iles et al. 2015). Leaves of Acaciaephyllum, considered to be monocots, are also well known in the fossil record (Doyle et al. 2008; Doyle & Upchurch 2014; Doyle 2014b). For these and other fossil monocots, see Gandolfo et al. (2000), Friis et al. (2006b, 2011), Doyle et al. (2008) and especially Iles et al. (2015). The monocot Sinoherba ningchengensis has recently been described from Lower Cretaceous deposits in Inner Mongolia ca 125 m.y. old. It has petiolate leaves with parallel venation, small flowers with a 2-3-seriate perianth, and its apically bilocular gynoecium has a single basal ovule; morphological phylogenetic analyses place it in a clade with Hydatellaceae, Cyclanthaceae and Najadaceae, the broader clade being made up of all the monocots in the analysis plus Nymphaeaceae (Z.-J. Liu et al. 2018: if a monocot, perhaps Alismatales?).
Evolution: Divergence & Distribution. For diversification rates in monocots, see Hertweck et al. (2015); note their caveats about ages and life forms. Tang et al. (2016: Fig. 1 ages?) looked at monocot diversification from a global point of view, suggesting i.a. that species numbers/genera tended to be associated with having larger ranges and living at lower elevations, but the story is complex..
If the aquatic Ceratophyllales are sister to monocots, synapomorphies like the herbaceous habit, absence of vascular cambium, etc., could be moved down a node, but evidence for such a relationship is not strong (see Jansen et al. 2007; Saarela et al. 2007; Moore et al. 2007, also the discussion under mesangiosperms). Ceratophyllales, very derived morphologically, are remarkable in both their vegetative and floral morphology. Nymphaeales are also aquatics that were also once believed to be close to monocots, and they now include the ex-monocot Hydatellaceae. Similarities between monocots and Nymphaeales and Ceratophyllales are likely to be convergences, and their common ancestors with other angiosperms are likely to have been plants with broad, petiolate leaves and a more or less woody stem with conventional lateral thickening meristems, cork and vascular cambia (e.g. Doyle 2013; see also early angiosperm evolution). However, Du et al. (2016) considered Nymphaeales, Acorales and Alismatales, and Ceratophyllales to be extant members of an early radiation of aquatic angiosperms that did not have any extant terrestrial relatives, which would make monocots polyphyletic; Asia was "one of the centers" for early diversification of this clade (ibid.: p. 342).
Monocots, whatever their relationships, appear to be so different from other angiosperms that relating their morphology, anatomy and development to that of broad-leaved angiosperms has been difficult (e.g. Zimmermann & Tomlinson 1972; Tomlinson 1995). Thus it has been suggested that vessels in monocots and those in other angiosperms evolved independently (Cheadle 1943a, 1953; c.f. Carlquist 2012a), while Meeuse (1975a: p. 435) observed "the question whether "the" monocots are "derived from the dicots" (or vice versa is utterly inane" as he toyed with the idea of some kind of polyphyly of the angiosperms. P. Soltis and Soltis (2016) also note difficulties in understanding the evolution of the numerous distinctive features that characterize monocots.
Note that over half the putative synapomorphies for monocots in Table 4.1 of Soltis et al. (2005b) may be best assigned elsewhere. Triterpenoids are not produced by CYP716 enzymes here, but other pathways are involved (Miettinen et al. 2017). The nature of the anther-filament junction has not been optimised in this part of the tree. For pollen and tapetum evolution, see Furness (2013), and for pollen evolution, see Luo et al. (2015). For the evolution of syncarpy and of septal nectaries, see Remizova et al. (2010b: associated with postgenital fusion of the carpels), Sokoloff et al. (2013: various trees, various definitions) and Tobe et al. (2018: placement equivocal). Septal nectary morphology (see e.g. Daumann 1970; Schmid 1985; van Heel 1988; Vogel 1998b; Smets et al. 2000; Rudall 2002; Remizowa et al. 2006a) is rather variable and is difficult to categorise when the carpels are more or less free. There are non-secreting slits in the ovary septae of Acorus, and if they are "really" septal nectaries, that feature becomes a synapomorphy (subsequently lost many, many times) of monocots as a whole.Both Tofieldiaceae, sister to other Alismatales, and Acoraceae have distinctive isobifacial leaves, but there is considerable variation in foliar morphology in Alismatales and it is likely that these leaves evolved in parallel in the two (c.f. also Luo et al. 2016); there is further discussion about monocot leaves below.
Ecology & Physiology. Monocot vegetative morphology, their ecology, and their physiology are all closely linked. It has long been noted that many of their distinctive features are compatible with an origin from aquatic or hydrophilous ancestors (e.g. Henslow 1893 and references: the style of comparison and suggested mechanisms are interesting!). The scattered vascular bundles in the stem, long linear and flexible leaves, absence of secondary thickening, clusters of adventitious roots rather than a single, branched taproot (see nature of substrate: mud), even the sympodial habit, etc., are all compatible with such an origin (see Mangin 1882 for "adventitious" roots in monocots; Schutten et al. 2005 and references for the biomechanics of living in water), and Carlquist (2012a) discussed variation in xylem anatomy in context of a more or less aquatic origin of the clade. Many members of the first two pectinations in the monocot tree, Acorales and Alismatales, are water or marsh plants or at least prefer to grow in damp conditions. Indeed, angiosperm aquatic herbs in general, unlike terrestrial herbs, often entirely lose the capacity to produce cambium, and reacquisition of a "normal" bifacial cambium in such plants is unknown (Groover 2005; Feild & Arens 2007).
In their analysis of major functional traits in vascular plants, Cornwell et al. (2014) noted that plants above the first node of the monocot branch were notably small, although Acoraceae do not differ from other monocots in this feature; palms and to a lesser extent bambusoid grasses show a marked increase in plant size. Most monocots, like Acoraceae themselves, are perennial, sympodial plants (Holttum 1955; see also Levichev 2013) that form tufts of leaves in part of each growth cycle and/or are geophytes, and their internode elongation is slight. The stem apex is under or at the surface of the ground except at flowering time. So-called "adventitious" roots develop from the growing stem, the older part of the stem decaying along with any roots, primary or stem-borne, it may have. The flowering stem commonly has elongated internodes, and the stem's response to gravity in the absence of secondary thickening is interesting. In some Poales (Cyperaceae, Juncaceae, Poaceae), at least, adjustments are made by an intercalary meristem at the base of in the internodal zone. For the role of the leaf sheath in supporting the stem under such conditions, see Kempe et al. (2013 and references).
The hydraulic systems of root and stem are not in direct contact because there is usually no secondary thickening (Carlquist 2009). Distinctive monocot-type secondary thickening (Rudall 1995b for records; see below) is very infrequent, although somewhat less so in Asparagales. Since the major monocot woody clades, Arecaceae and Poaceae-Bambusoideae, lack any secondary thickening (but c.f. Botánico & Angyalossy 2013), the xylem and phloem in their vascular tissue must be very old, but in both groups - perhaps the latter in particular - root pressures are extremely high, which may at least help in embolism repair after xylem cavitation develops (Davis 1961; Cao et al. 2012). It is unclear if high root pressures, perhaps associated with tolerance to cavitation, occur throughout the monocots (Cao et al. 2012), however, root pressures listed by Fisher et al. (1997) for vines and woody species show no differences between monocots and other angiosperms.
A number of monocots are plants of quite considerable size, some being giant herbs or large trees, and there is a period of establishment growth (e.g. Tomlinson & Esler 1972; Bell & Bryan 2008), between germination and the mature (flower-producing) stage. It usually occurs before stem elongation, and the apical meristem increases in size and there are often changes in leaf morphology. Burtt (1972) noted that during germination in monocots the plumule is frequently carried below the surface of the ground; a tube formed by the cotyledonary sheath (the "dropper") with the plumule and radicle/root area at the bottom grows out of the seed and so carries both away from the seed, and establishment growth proceeds while the meristem in underground. However, the stem may thicken in other ways, as in some palms where ground tissues in both stem and root remains undifferentiated for some time, with limited mitosis and/or cell expansion and/or formation of schizogenous lacunae occurring and the trunk markedly thickening and lengthening - sustained primary growth (Waterhouse & Quinn 1978, see also Arecaceae).
There is some variation of lignin composition within monocots and also in the rate of litter decomposition (see also nutrient cycling). Thus Cornwell et al. (2008) noted that "graminoid" (sedges and grasses) litter decomposed more slowly than that of forbs, and "monocot" lignin more slowly than that of other angiosperms, at about the same rate as that of gymnosperms. Wardle et al. (2002: no Poaceae or conifers) noted that monocot litter decomposed more slowly even than that of ferns. More work on lignin decomposition is in order.
Scattered in monocots are taxa with broad leaf blades that have reticulate venation (see Cameron & Dickison 1998, also below) and also fleshy fruits (excluding things like arillate, ant-dispersed seeds), adaptations to shady conditions. Givnish et al. (2005, 2006b) suggested that reticulate venation has arisen at least 26 times in monocots (and fleshy fruits 21 times), and have sometimes subsequently been lost. Both features showed very strong signs of tending to be gained (or lost) together, a phenomenon described as "concerted convergence" (see also Dahlgren & Clifford 1982; Patterson & Givnish 2002). Fleshy fruits are estimated to have evolved ca 110 m.y.a., perhaps because the canopy had become closed, while elaiosomes, which originated 24 or more times, appeared later especially at the end of the Eocene when ants became common (Dunn et al. 2007). For more on berries in monocots, see Rasmussen et al. (2006).
Deng et al. (2015: p. 563) thought that CAM "may have evolved just after the diversification of monocots, with CAM appearing prior to C4", although they were not more precise; their focus was on Orchidaceae.
Seed Dispersal. The evolution of fleshy fruits and fruits with myrmecochorous seeds is examined by Dunn et al. (2007). The latter have evolved at least 24 times in monocots, probably no earlier than later Eocene when ants first become abundant in the amber record. Fleshy fruits, also very polyphyletic in origin, evolved much earlier, 110+ m.y.a., perhaps connected with closing of the canopy; overall, monocots show increased reliance on animals for seed dispersal over time (Dunn et al. 2007).
Plant-Animal Interactions. Overall, herbivory in monocots is relatively low (Turcotte et al. 2014: see caveats), although leaf roll beetles are prominent herbivores of Zingiberales, brown butterflies of Poaceae, etc., q.v. for details. Caterpillars of Yponomeutoidea-Glyphipterigidae and -Ypsolophidae-Ochsenheimeriinae, ditrysian moths, are leaf miners or stem borers in monocots (Sohn et al. 2013), while Castniidae skipper butterflies eat a variety of monocots (Forbes 1956; see Powell et al. 1999 for some other groups that prefer monocots), as do members of the ca 500 species strong yponomeutoid moth Glyphipterygidae (Sohn et al. 2013), mostly leaf miners or stem borers. Larvae of the chrysomelid beetle group Galerucinae subribe Diabroticites are quite common on monocots, where they feed on roots (Eben 1999), indeed, Hispinae-Cassidinae (6000 species), sister to Galerucinae (10,000 species) are the major group of monocot-eating beetles (Jolivet & Hawkeswood 1995; Wilf et al. 2000; Chaboo 2007). Wilf et al. (2000) thought that these beetles initially ate aquatic members of Acorales and Alismatales, the association of commelinids with the Hispinae-Cassidinae being derived. However, Gómez-Zurita et al. (2007) thought that the two main clades of monocot-eating chrysomelid beetles included in their study were unrelated, and neither was close to the galerucines, there had been multiple colonizations of the monocots, and also that Chrysomeloidea-Chrysomelidae diversified 86-63 m.y.a., well after the origin of monocots. García-Robledo and Staines (2008) discuss problems in ascribing herbivory to particular insect groups when using fossil material.
The idea has been floated that monocots experience less herbivory in lowland tropical rainforests than do other angiosperms, in part because they are tough and in part because the leaves remain rolled up for a relatively long time (Grubb et al. 2008). Most monocots have raphides as their main crystalline form of calcium oxalate, and these may be involved in herbivore defence (e.g. see Araceae; Franceschi & Nakata 2005).
Bacterial/Fungal Associations. Monocots are practically never ectomycorrhizal, but mycoheterotrophy is disproportionally common here. This may be because there is no secondary thickening, a thick cortex, no primary root, etc. (Imhof 2010).
Vegetative Variation. The tunica-corpus construction in the stem apex of monocots is similar to that in other angiosperms, although a 1-layered tunica, as in maize, is somewhat more common (Stewart & Dermen 1979; Jouannic 2011 and references). The outer tunica layer can proliferate at the leaf margin as can be seen in some variegated leaves (Zonneveld 2007). Stem development is complex, and a primary thickening meristem is close to ubiquitous, but there is no vascular cambium, see e.g. Esau (1943), Rudall (1991a, a summary), de Menezes et al. (2005), Pizzolato (2009); this is quite variable in details of its origin and the tissues to which it gives rise. Endodermal initials in at least some cases produce radially-arranged cortical cells centrifugally, while derivatives of the pericycle (itself the very outside of the phloic tissue - see Esau 1943) initially produce the vascular system embedded in ground tissue centripetally (de Menezes et al. 2011; Cury et al. 2012); this meristematic activity "remains active throughout the life of the plant" (Cury et al. 2017: p. 42). In general, details of cauline vascular development are complex (Geeta 2016). Although de Menezes et al. (2011) suggested that there was no distinct primary thickening meristem in monocots, some of the argument here seems to be as much a matter of definitions than anything else.
Monocots show great variation in their basic leaf construction (e.g. Kaplan 1973), and seedlings, e.g. of Restionaceae, may hsve all sorts of leaf morphologies (e.g. Linder & Caddick 2001). One commonly thinks of the leaves of monocots as having broad, sheathing bases and elongated leaf blades with parallel venation that are about the same width, but this is perhaps because of the ubiquity of grasses (for the sheathing base of Zea, see Johnston et al. 2014a) and the fact that many commonly-cultivated bulbs have such leaves. The relation between the blade of a monocot leaf and that of a broad-leaved angiosperm is of the greatest interest. Leaves in general are often considered to be made up of a apical hyperphyll and basal hypophyll (e.g. ). In broad-leaved angiosperms the hyperphyll gives rise to the blade which has a marginal meristem (blastozone) and develops in an acropetal fashion - that is, the veins are first formed at the base of the blade. The hypophyll, on the other hand, gives rise to the petiole, a rather late-developing part of the leaf, leaf base, and stipules. In broad-leaved angiosperms like Arabidopsis development at the base of the blade, the junction of the hyperphyll and hypophyll, proceeds in two directions; cells are cut off both distally towards the apex and proximally towards the base (Ichihashi et al. 2011). In many monocots, most of the leaf is thought to be developed from the hypophyll alone and maturation of the blade proceeds basipetally as cell files are cut off from a transverse basal plate. Thus tissues in the apex of the blade of grass or palm leaf emerging from the sheath are mature while cells at the base are actively dividing and elongating. There is often a "Vorläuferspitze", a usually small abaxial unifacial conical or cylindrical protrusion at the apex of the mature leaf; this may represent the entire hyperphyll (e.g. Knoll 1948; Troll 1955; Bharathan 1996). In such leaves it is thought that the blade develops from the equivalent of the broad-leaved angiosperm leaf base; Croxdale (2000) mentions some broad-leaved angiosperms in which the polarity of growth is the same as that in monocots. In leaves in general, localization of PIN-FORMED1 auxin transport proteins is associated with provascular strand development, whether at the marginal meristem or the sheathing leaf base (Johnston et al. 2014a).
However, in Acorus (Kaplan 1970a) and most, but not all, Alismatales studied (e.g. Bloedel & Hirsch 1979) a bifacial blade may develop from the upper part of the leaf primordium. Such leaves are quite similar in development to those of broad-leaved angiosperms (Doyle 2013). Quite commonly there is still a small Vorläuferspitze, but they are visible only in cataphylls and/or early developed leaves in Alismataceae (Bloedel & Hirsch 1979), and they are also visible in Cyclanthaceae (Wilder 1986). How much hyperphyll a Vorläuferspitze represents varies, and Bharathan (1996) noted that it was to be found in some monocot leaves whose blade develops from the hyperphyll. Interestingly, Sinnott (1914) described both Acorus and Potamogeton as having 3:3 nodes. If some of the veins at the very base of the leaf develop acropetally and some develop basipetally (as also in Piper... Balfour 1958b), then using the one formula to describe nodal anatomy for all angiosperms would not seem to be very useful.
Many of the features that make up a "typical" monocot leaf seem to vary independently: Leaf base surrounding the stem/not; blade developed from the hypophyll/hyperphyll; Vorläuferspitze present/absent; venation parallel/reticulate; blade develops basipetally/acropetally; petiole and blade develop simultaneously/petiole tends to develop somewhat later (monocot states first: see e.g. Kaplan 1973; Bharathan 1996). Variation is especially great in Alismatales and Acorus, although relating this to monocot phylogeny is difficult. However, "typical" monocot leaves are unlikely to be an apomorphy of monocots (see also Doyle 2013). Geeta (2003) divided foliar features of angiosperms into eight separate characters, and in an analysis of these characters across 24 angiosperms did not quite recover a monophyletic monocots; as she summarized her findings "it is concluded that there is no entity, the "monocot leaf primordium"" (ibid.: p. 609). Leaf development in monocots much needs more broadly comparative studies (but see also Bharathan 1996; Geeta 2016).
Terete, unifacial blades with stomata all over the surface are scattered in monocots. These may result from the elaboration of the unifacial Vorläuferspitze (e.g. Arber 1925; Troll 1955; Troll & Meyer 1955; Kaplan 1973, 1975: comparison with Oxypolis, Acacia; Townsley & Sinha 2012). Monocots sometimes have laterally flattened and isobifacial leaves that are edge on to the stem, and they look like a bifacial dorsiventral blade that has folded and become connate adaxially, but Kaplan (1970a: Acorus) suggested that they were the adaxial elaboration of a midrib/costal region. Rudall and Buzgo (2002) suggest that this isobifacial leaf originates from an intermediate zone between hyperphyll and hypophyll. However, developmentally both these isobifacial and terete unifacial leaves represent the genetic abaxialization of the leaf, the genes normally expressed abaxially being the only genes expressed, at least at the leaf surface (Yamaguchi & Tsukaya 2010; Nakayama et al. 2013). Yamaguchi et al. (2010) show how in Juncus isobifacial leaves differed from terete leaves by the activity of the DL gene that elsewhere in monocots is involved in midrib development. In Maundia (Alismatales-Maundiaceae) the bundles on the abaxial side of the leaf are inverted, but the large central bundles and the adaxial bundles are normally oriented (Platonova et al. 2016).
Gifford and Foster [1988: Fig. 19-13] prefer to think of parallel monocot venation as being striate, emphasizing how the main veins join sequentially at the apex, although the different kinds of leaf venation that they show are variations on a parallel theme. Members of Zingiberales typically have a well-developed midrib from which numerous closely parallel veins leave, either proceeding straight to the margin, as in Musa, or taking a more arcuate path. Some kind of midrib/central vein is common, or there may be a few strong veins diverging from the base (Doyle et al. 2008, which see for further details of the venation of monocot leaves, etc.; Coiffard & Mohr 2015). Transverse veins joining the parallel veins are ubiquitous in monocots, and those in the broadly cordate blades of Stemona (Pandanales-Stemonaceae) are particularly conspicuous and elegant. In a number of monocots there are broad leaf blades, petioles, the venation is reticulate and with some free vein endings, and the stomata are unoriented (see Cameron & Dickison 1998). Net venation may have arisen at least 26 times in monocots (see above: Givnish et al. 2005, 2006b: see above). The plants involved are often vines/lianes (Smilax [Liliales], Dioscorea [Dioscoreales]) and/or plants which live in shady habits for at least parts of their lives (Trillium [Liliales]). Given the parallel evolution of leaf blades and uncertainties as to how monocot blades develop, the leaf blades of Hosta (Asparagales) and Orontium (Alismatales) are similar only in a functional sense (Troll 1955).
Truly compound leaves are rare in monocots - Zamioculcas, Anthurium and a few species of Dioscorea are examples - but cell death may result in the leaves appearing to be compound (a few Araceae) or having distinctive perforations (some Araceae and Aponogetonaceae). In palms, a process related to abscission causes the leaf blade to become dissected and appear compound (Nowak et al. 2007, 2008) while in Zingiberales like Musa and Ravenala the blade easily gets torn by the wind and then is pretty much functionally compound.
Ligules are scattered throughout the monocots and are born either at the base (e.g. Potamogetonaceae) or top (e.g. Poaceae) of the petiole or sheath. A ligule may mark the point of separation of the two parts of the leaf, and Zamioculcas has a ligule very near the base of the petiole, suggesting that the rest of the leaf is equivalent to the hyperphyll, i.e. it is like the lamina of a broad leaved angiosperm - and if it is such a marker, then the same would be true for other monocots with ligules. Genes expressed in ligule development in maize are also expressed elsewhere (base of leaf, branch points) where they mark developmental boundaries (Zhu et al. 2013; Johnston et al. 2014b). Ligules may be paired. Indeed, as has been pointed out by authors like Roth (1949) and Rudall and Buzgo (2002), the developmental origins of monocot ligules and at least some stipules of BLAs are not fundamentally different, both arising from adaxial cross meristems, a sort of intercalary meristem, in the transition zone between hyperphyll and hypophyll (see also Ichihashi et al. 2011). There are also developmental similarities between stipules and leaf sheaths (the latter as in Poaceae), and abaxializing factors may interfere with their development (Townsley & Sinha 2012). Smilax has paired tendrils near the base of the petiole, but such paired structures, whether tendril or ligule, are practically never called stipules because monocots are supposed not to have stipules.... Although I have not used the term "stipule" in the monocot characterisations, some structures there have at least as good a title to the name as some of the things called stipules in BLAs - or perhaps all should be called ligules (see also Colomb 1887).
Genes & Genomes. Salse et al. (2009; see also Murat et al. 2017) suggested that the common ancestor of monocots had five protochromosomes, however Ming et al. (2015) thought that there were seven such chromosomes. Puttick et al. (2015) estimate the ancestral genome size for monocots. For the τ/tau genome duplication event, see the [Asparagales + Commelinids] node. There is relatively little colinearity and synteny when monocots and rosids are compared, although these are extensive within each group (Tang et al. 2008).
The story of the evolution of the GC profile of protein-coding genes in the genome is complex, and Clément et al. (2014) suggest that the ancestral condition for monocots as a whole, not just grasses, is for the GC profile to be bimodal (see also McKain et al. 2016 for literature).
Lee et al. (2011: c.f. sampling and topology) found that genes involved in cell fate commitment, auxin metabolism, etc., tended to cluster at this node of the tree.
For the evolution of the IR/LSC junction in monocot chloroplasts, see R.-J. Wang et al. (2008), Mardanov et al. (2008), Yang (2010) and J. Luo et al. (2014); I have not integrated this with the tree here.
Chemistry, Morphology, etc. Some monocots (Amaryllidaceae, Araceae) have benzylisoqinoline alkaloids, but it is unclear if they are produced by the same biosynthetic pathway as these alkaloids in broad-leaved angiosperms (Waterman 1999). Little is known about cell wall polysaccharides like xylans and glucosomannans and how they interact with cellulose, lignin, etc., in cell wall development (Busse-Wicher et al. 2016). For the distribution of mixed-linkage glucans (MLGs) in both lignified and unlignified cell walls, readily detectable by immunogold labeling, see Trethewey et al. (2005). They are sometimes present in only very small amounts and may be localized according to the thickening of the cell wall, and the family-level sampling was a bit exiguous (see also Smith & Harris 1999); these glucans are also found in Equisetum (e.g. Fry et al. 2009). However the broader sampling of the Cellulose Synthase gene (CesA superfamily), suggests that there are three members involved in the synthesis of these MLGs - the evolution of two, CslH (sister to the eudicot CslB) and CslJ (sister to CslM), can be placed at this node, the other is restricted to some Poales - allows their distribution to be clarified; these MLGs are an apomorphy for monocots (Little et al. 2018).
Cork cambium in the roots is superficial in origin, developing just beneath the exodermis (Arber 1925), and although Morot (1885) described it as being cortical that may not be a fundamental difference, and the latter explicitly distinguished it from the pericylic initiation in the roots of other seed plants. In any event, a cork cambium in the roots of monocots seems to be rare. Cambial tissue in the stem, which gives rise to monocot-type of secondary thickening, as in a number of Asparagales (see above: Rudall 1995b for a summary), may represent a continuation of the activity of the primary thickening meristem (Carlquist 2012a) or whatever this tissue is. Vascular bundles in a number of monocots may have a sort of cambial layer, but its products never amount to much (Arber 1919). The roots of Dracaena produce secondary bundles (Carlquist 2012a). For stem anatomy in monocots and the establishment of linkages between cauline (e.g., developing inside the primary thickening meristem) and foliar vascular bundles, see Zimmermann and Tomlinson (1972) and Pizzolato (2009) and references.
Cheadle (e.g. 1942, 1943a, 1943b, 1944) and Wagner (1977) surveyed vessel types in the vegetative parts of monocots; although there appear to be large-scale patterns, these data need to be re-evaluated. Cheadle (e.g. 1944) noted that there could be substantial variation in vessel morphology between closely related (congeneric) species, and even between different organs on the same plant (see also Carlquist 2009). Such variation was used to establish his evolutionary trends (Cheadle 1955, 1964, 1969a, b, 1970; Cheadle & Tucker 1961; Cheadle & Kosakai 1971). However, Carlquist (2012a) found criteria for recognising vessels as distinct from tracheids to be difficult to formulate and questioned a number of these earlier reports of vessels, which makes life a bit difficult. Tomlinson and Fisher (2000) noted a correlation in climbing monocots between the presence of simple perforation plates in metaxylem vessels and absence of direct protoxylem/metaxylem continuty and of the presence of scalariform perforation plates and direct protoxylem/metaxylem continuty. Amphivasal vascular bundles are common in monocot stems, although they are absent in some groups (e.g. Jeffrey 1917; Arber 1925). Companion cells are lacking in the protophloem of at least some monocots (Ervin & Evert 1970; Platonova et al. 2016), but it is unclear how common this feature is (they are absent in Maundia, Potamogeton, Smilax, Typha). See Botha (2005) for distinctive thick-walled late-formed sieve tubes in some monocots.
Many monocots, although not members of the old Helobieae (here in Alismatales), have thin-walled bulliform cells in the adaxial epidermis and/or in other tissues that may cause the leaf to curl as they lose turgor (Löv 1926; c.f. Kellogg 2015). Paracytic (and tetracytic) stomata are common in monocots, and variations in how they develop may characterise major clades, although there is also much variation within them (see e.g. [Poales [Commelinales + Zingiberales]]: c.f. Tomlinson 1974a, q.v. for data, also Paliwal 1969; Pant & Kidwai 1965; esp. Rudall et al. 2017); more observations are still needed (Rudall 2000). Epidermal cells are often long, and stomata are usually oriented parallel to the long axis of the leaf and so also to the epidermal cells, but this stomatal orientation holds even when the epidermal cells are short and lack much in the way of orientation themslves, as in some Liliaceae and Pontederiaceae (Rudall et al. 2017).
Monocot leaf teeth, when present, are more or less spinose, never glandular. Colleter-like structures ("intravaginal squamules") may be a synapomorphy of monocots or of independent origins in Acorales and other Alismatales, within Araceae, for instance, they seem to be known only from very much phylogenetically embedded genera such as Philodendron, Cryptocoryne and Lagenandra (M. Carlsen, pers. comm.; see also Wilder 1975).
For racemose inflorescences in monocots, see Remizowa et al. (2011a) and Remizowa and Lock (2012), for bracts in early divergent monocots, see Remizowa et al. (2013a), for pollination and floral evolution, see Vogel (1981a: now somewhat dated).
Floral orientation in the monocots in part depends on the presence and position of the prophyll/bracteole, and also on the existence of other structures on the pedicel (see e.g. Eichler 1875; Engler 1888; Remizova et al. 2006b; Sokoloff et al. 2017). Stuetzel and Marx (2005) think that what appear to be axillary flowers are reduced racemes, hence the variability in bracteole position. Be that as it may, when the prophyll/bracteole is lateral, the floral orientation can be quite variable (although less so with respect to the bracteole - Remizowa et al. 2013b). Monosymmetric flowers are very frequently presented with the median sepal adaxial, i.e., the flowers are inverted; in taxa with a labellum, the labellum is the median tepal of the inner whorl (e.g. Rudall & Bateman 2004; Bukhari et al. 2017). This tepal may be a landing platform for the pollinator, being partly supported by the two adjacent tepals of the outermost perianth whorl when the flower is inverted; if the landing platform were a member of the outer whorl, there would not be the same support (see also Bukhari et al. 2017). In those Commelinaceae where the abaxial tepal is very small, the well developed inflorescence bract may serve the same purpose. Connected with this inverted monosymmetry is the suppression of at least the adaxial median stamen (Pattern I zygomorphy: Rudall & Bateman 2004); after inversion the flowers effectively have Pattern II zygomorphy, in which abaxial stamens are sterilized. Remarkably, although flowers on the one inflorescence of Crocosmia X crocosmiiflora (Iridaceae) were all monosymmetric, in some the odd member of the outer whorl was adaxial, and in others it was abaxial; patterning, etc. of the other floral organs was adjusted accordingly (pers. obs. vii.2009). (See also below for the arrangement of the carpels.)
Monocots and "dicots" were often distinguished in the past by the 3-merous flowers of the former and the predominantly 5-merous flowers of the latter, even as it was realised that some of the "primitive dicots" might have more or less 3-merous flowers. With our current knowledge of phylogeny and floral development, it seems that a 3-merous perianth is quite widespread near the base of the angiosperm tree and could even be a synapomorphy for a clade [[Chloranthaceae + magnoliids] [monocots [Ceratophyllaceae + eudicots]]] (Soltis et al. 2005b and literature cited). Lability in phyllotaxy is marked, thus flowers of Anemone tomentosa can have 5 quincuncial perianth members or six, arranged in two whorls of three (Kitazawa & Fujimoto 2018). The two perianth whorls in monocots are often similar and are then called tepals, although there is usually a slight difference between the members of the two whorls. Tepals range from small and greenish, as in Acorus, to very large and petal-like, as in Lilium, the two whorls sometimes being sharply differentiated, as in Alstroemeria. The stamens are individually opposite members of each whorl, stamen-tepal primordia being common, while in some Alismatales the outer whorl of stamens may even come to lie outside the inner perianth whorl, as in Juncaginaceae (see e.g. Dahlgren et al. 1985; Endress 1995b; Remizowa et al. 2010b and references). The individual perianth whorls do not completely encircle the floral apex (look at the base of a tulip, lily, or iris flower, for example). As a result, the tepals of each whorl, particularly the outer, may have open aestivation, although in Smilax, for example, the outer tepal members are valvate. Trimerous monocot flowers are rather highly stereotyped and usually pentacyclic; functionally, they are basically six-merous.
Dodsworth (2016 and references) thought that there was a connection between the development of this tepaloid perianth and the expansion of the activity of B-class genes to the outer perianth whorl, with the elaboration of the inner whorl in particular perhaps being linked with B gene duplication. However, in Asparagus the tepals, although fairly small, are petaloid, and here B-class genes are expressed only in the inner tepalline whorl (Park et al. 2003, 2004); looking at gene expression in the flowers of those Araceae (Alismatales) with small, greenish tepals would be interesting. Indeed, Iwamoto et al. (2018) looked at the floral development of members of the clade made up by the three families of petaloid Alismatales, the old Alismatales s. str., as possibly "key" (sic) to understanding the evolution of the trimerous pentacyclic monocot flower. Given that this clade is embedded within Alismatales, relating its floral morphology to that of the common ancestor of the monocots is hardly straightforward, but their comparison suggested to them that flowers of taxa like Egeria (Hydrocharitaceae) was getting close to - or even had - the ancestral monocot flower, the monocot floral Bauplan (Iwamoto et al. 2018). Of course, Hydrocharitaceae have sepals and petals, and the monocot flower with six stamens was derived from a flower with nine stamens.
The stamen-perianth member pairing, the fact that the bases of members of a perianth whorl do not completely surround the floral apex, and the trimerous organization of the flower are three features very common in monocots and are here considered to be apomorphies for monocots (c.f. Soltis et al. 2005b; Bateman et al. 2006b; see also Sauquet et al. 2017). These features are rather more scattered in the eudicot clades up to Gunnerales. Thus authors (e.g. Chen et al. 2007) have drawn attention to the occurrence of dimery and A-T pairing in the grade Proteales to Gunnerales, but where stamen-perianth pairing is to be placed on the tree is uncertain. Clades like Lauraceae may be interpreted as having flowers with these monocot features, but they are pretty much non-existent in Pentapetalae, although in taxa with antepetalous stamens, petal and stamen primordia can be confluent. However, the relevance of the odd more or less 3-merous flowers that do occur in Pentapetalae, e.g. Trihaloragis (Haloragaceae), for understanding monocot evolution is unclear. Note that in many commelinids the perianth whorls individually surround the floral apex and are quite often differentiated into a clearly smaller, sometimes more or less green outer "calyx", and a larger, coloured inner "corolla"; the androecium is inside the inner whorl.
There is some variation in the vasculature even of the stereotypical monocot flower, as Gatin (1920) described in her extensive survey of the flowers of Old Style Liliaceae. She found that a number of genera had only a single vascular bundle entering each tepal member; sometimes successively-dividing traces supplied different parts of the flowers, or each floral part was supplied by a separate trace, the gynoecium was variously vascularized, and so on. T-A primordia may develop quite quickly while the appearance of G primordia is delayed (Endress 1995b).
Gene expression in floral development varies somewhat from that in core eudicots like Arabidopsis. Thus B-class genes, usually expressed in petals in core eudicots, are also expressed in the outer petal-like tepal whorl of Tulipa (Kanno et al. 2003) and Dendrobium (Y. zu et al. 2006) (Tzeng & Yang 2001). Furthermore, even if expressed in both outer whorls, whether or not the B-function genes form obligate heterodimers varied (Zu et al. 2006). Moreover, Ochiai et al. (2004) found that DEF, a B-class gene, was not expressed in the sepals of two Commelinaceae they examined, and also not in the outer more or less petal-like tepal whorls of Asparagus or Lilium (Park et al. 2003, 2004; Tzeng & Yang 2001), while Almeida et al. (2013) found that in Canna ABC-type floral genes had very broad expression patterns across the various floral organs. A further wrinkle is that CYC-like genes in Zingiberales and Commelinales 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. In general the perianth part that is differentiated is the odd member (and adjacent members) of the inner whorl (see e.g. Reyes et al. 2016: fig. 4).
It is unclear how the anther wall develops in Acorus (Rudall & Furness 1997), although it inclines to the monocot "type" (Duvall 2001). Given the diversity of carpel development in monocots in, or near the base of, the basal pectinations in the monocot tree, whether or not the basic condition for monocot carpels is to be free or somewhat connate is unclear (e.g. Chen et al. 2004; Remizowa et al. 2006a).
Remizowa et al. (2006b) summarize variation in gynoecial morphology in some of these basal monocots. In the absence of bracteoles the median carpel is abaxial (Remizova et al. 2013b; Sokoloff et al. 2017), but this carpel is adaxial if there is a bracteole.
Zhao et al. (2016) suggest that in monocots the embryonic suspensor is difficult to differentiate from the rest of the embryo, clear homologues of the WOX8 gene, in other angiosperms a marker for suspensor cell fate, not being identifiable there. There has been much discussion about the evolution of the single cotyledon that characterizes the clade - by connation, or by suppression (see e.g. Haines & Lye 1979; Burger 1998)? Having a terminal cotyledon was initially thought to be a synapomorphy of the monocots, and Kaplan (1997: 1 ch. 4) noted that this is because the single cotyledon pushes the erstwhile terminal meristem to one side, so evicting it, similarly, the cotyledon of broad-leaved angiosperms that have only a single cotyledon is more or less terminal. Conversely, in Poaceae the whole embryo is well developed, primordia of foliage leaves being visible, so it is perhaps not surprising to find that the cotyledon is more obviously lateral there (see also Zhao et al. 2016). The relationship of the radicle to the suspensor seems to vary, and its point of origin is distinctly to one side in several Alismatales, at least (Yamashita 1976).
For monocots, in addition to references in the notes on the Characters page and under individual orders and families, there is much interesting information in Arber (1920, esp. 1925), Dahlgren et al. (1985) and Tillich (1998); Tomlinson (1970) outlined monocot morphology and anatomy, emphasizing the woody groups; Volumes III and IV of Families and Genera of Vascular Plants, edited and with useful outline classifications by Kubitzki (see especially 1998a, c), also contain a great deal of information. For the morphology of sieve tube plastids, see Behnke (1981a, 2000, 2001, esp. 2003), for dimorphism of the cells of the root epidermis and hypodermis, see Kauff et al. (2000), for rhizosheaths, known from Poaceae and other Poales, rare in broad-leaved angiosperms?), see McCulley (1995), for androecial variation, see Ronse Decraene and Smets (1995a), for endosperm development see above, also Floyd and Friedman (2000), etc., for incompatibility systems in monocots - quite common, many uncharacterized, but at least some gametophytic - see Sage et al. (2000), for the distribution of operculate pollen, see Furness and Rudall (2006b), for pollen variation in "basal" monocots, see Furness and Banks (2010), for the development of callose plugs in the pollen tube - quite often complete and regularly spaced in broad-leaved angiosperms, incomplete and irregularly spaced in monocots, see Mogami et al. (2006), for gynoecial morphology and evolution, see Remizowa et al. (2010b), for a summary of embryology, see Danilova et al. (1990a) and Rudall (1997), for antipodal cells, see Holloway and Friedman (2008), for early embryo development, see Zhao et al. (2016), for nuclear DNA content, see Bharathan et al. (1994), for seed and fruit morphology and anatomy, Takhtajan et al. (1985), for fleshy fruits, see Thadeo et al. (2015), for the evolution of seeds, see Danilova et al. (1990b: now somewhat dated), for seed size, see e.g. Moles et al. (2005a), and for seedling morphology, see Takhtajan et al. (1985: compilation), Kaplan (1997: 1 ch. 5) and Tillich (2007).
Phylogeny. For the immediate relatives of monocots, see the discussion at the mesangiosperm node. Chloranthales, magnoliids, and Ceratophyllales are the other clades close to the eudicots that may be sister to monocots.
Both molecular and much morphological data strongly support monocot monophyly. General relationships within monocots as outlined in molecular studies by Chase et al. (1995a, 1995b, 2000a, 2005), Tamura et al. (2004a, b), Chase (2004), Janssen and Bremer (2004: rbcL only, but 878 genera from 77 families), Graham et al. (2006: to 16 kb chloroplast DNA/taxon examined), Givnish et al. (2006b), Chase et al. (2006), Li and Zhou (2007), Soltis et al. (2011), Barrett et al. (2012b: 83 chloroplast genes, focus on commelinids), Iles et al. (2013), Ruhfel et al. (2014: chloroplast genomes), Foster et al. 2016a: position of Liliales has little support) and Givnish et al. (2016b: 75 plastid genes) are followed here; further comments may be found at various nodes within the monocot tree. Note that Lam et al. (2018) managed to get many mycoheterotrophic taxa, here placed in three orders, to cluster together in a parsimony analysis as sister to a [Tacca + Dioscorea] clade - as they note, long branch attraction to the max! Davis et al. (2013) tabulate support for various clades afforded by individual chloroplast genes.
In most studies Acorus is sister to all other monocots (e.g. Duvall et al. 1993a, b; Soltis et al. 2007a; Moore et al. 2010; Morton 2011; Hertweck et al. 2015; Ross et al. 2015; Luo et al. 2015). However, Stevenson et al. (2000) suggested a rather different set of relationships - [Acoraceae + most of Alismatales] [Araceae + all other monocots]]. Davis et al. (2001) found that a clade [Acoraceae + Alismatales (as delimited here)] was sister to other monocots, and this position was found in the morphological analysis of Doyle and Endress (2000). Davis et al. (2004) noted that this latter set of relationships was not found when rbcL sequences were analysed alone, but it appeared when mitochondrial atpA sequences were analysed, both alone and in combined analyses (see Davis et al. 2006: four genes, two nuclear and two chloroplast, matK also supports this relationship). Petersen et al. (2015: 2 chloroplast and 5 mitochondrial [atp1 very influential] genes, morphological data) found the relationships [Tofieldiaceae [Araceae [Acoraceae [Alismataceae ...]]]], although support for this position was not strong. Thadeo et al. (2015: characters from morphology and all three genomic compartments) obtained similar relationships. Finet et al. (2010) found that Acorus and Asparagales formed a clade sister to all other monocots, but this is probably a sampling problem; no Alismatales were included in the analyses. Mitochondrial genes showed a much higher rate of change in Acoraceae and many Alismatales, but not Tofieldiaceae and Araceae; Acoraceae linked with the fast-evolving group (G. Petersen 2006c). Some characters of floral development are also consistent with an Acoraceae-Alismatales relationship (e.g. see Buzgo 2001). Interestingly, there is a three-nucleotide deletion in the atpA gene in Acoraceae and Alismatales, although not in Cymodoceaceae or Tofieldiaceae in the latter (Davis et al. 2004).
Qiu et al. (2010) found Asparagales to be sister to all monocots other than Alismatales, although support for this position was not very strong and Petrosaviales were not included. However, when included, Petrosaviaceae (both genera) formed a clade sister to the remaining monocots (e.g. Chase et al. 2000a; Davis et al. 2004; Givnish et al. 2005: ndhF gene alone; Graham et al. 2006; Magallón et al. 2015; Lam et al. 2018). Although Petrosaviales are commonly placed as in the tree here, a recent study recovered a [Petrosaviales + commelinid] clade (Gitzendanner et al. 2018: plastome data).
Note that monocots were not found to be monophyletic in some morphological studies such as those by Hay and Mabberley (1991); Araceae were independently derived from broad-leaved angiosperms, perhaps from Nymphaeales. Some morphological cladistic studies have placed net-veined monocots as sister to all other monocots, suggesting that this leaf venation was plesiomorphic in the monocots (Stevenson & Loconte 1995, see also Dahlgren et al. 1985; Yeo 1989; Li & Zhou 2006, etc.); the broad-leaved angiosperm outgroups have similar foliar features. This harks back to some ideas of Lindley (1853), who thought that the monocots that had leaves with reticulate venation, which he called dictyogens, were intermediate between the exogens (dicots) and the endogens (other monocots). Morphological cladistic analyses of the net-veined taxa by themselves (Conran 1989) also suggested relationships which now seem rather unsatisfactory. The analysis of morphological characters alone in monocots has tended to produce trees with little resolution and little support for those branches that are resolved (e.g. Li & Zhou 2006: support only for Alismatales minus Aracaeae and for Zingiberales). Focussing on the single character of apocarpous gynoecium, Endress (1995b) suggested that Triuris (Pandanales-Triuridaceae) might be a rather basal monocot. Even in an analysis of fifteen chloroplast genomes, the five monocots included were not always monophyletic (Goremykin et al. 2005), while Duvall et al. (2006) discuss other studies in which monocots appear not to be monophyletic - the 18S gene is implicated in producing this topology. Acorales were sister (99% ML bootstrap!) to eudicots in the PHYC analysis of Hertweck et al. (2015).
For further discussion of relationships in the monocots, see especially the Petrosaviales and Arecales pages.
Classification. For a linear sequence of monocot genera, excluding Orchidaceae and Poaceae, see Trias-Blasi et al. (2015).
Thanks. I am grateful to Claudia Henriquez for much discussion on leaf development in monocots.
Synonymy: Acoranae Reveal, Alismatanae Takhtajan, Aranae Reveal, Arecanae Takhtajan, Bromelianae Reveal, Butomanae Reveal, Commelinanae Takhtajan, Cyclanthanae Reveal, Dioscoreanae Reveal & Doweld, Iridanae Doweld, JuncanaeMelanthianae Doweld, Myrtanae Takhtajan, Najadanae Reveal, Orchidanae Doweld, Pandananae Reveal, Petrosavianae Doweld, Poanae Doweld & Reveal, Pontederianae Reveal, Rapateanae Doweld, Triuridanae Reveal, Typhanae Reveal, Zingiberanae Reveal, Zosteranae Doweld - Alismatidae Takhtajan, Arecidae Takhtajan, Aridae Takhtajan, Bromeliidae C. Y. Wu, Burmanniidae Heintze, Commelinidae Takhtajan, Juncidae Doweld, Liliidae J. H. Schnaffner, Orchididae Heintze, Triuridae Reveal, Zingiberidae Cronquist - Aropsida Bartling, Bromeliopsida Brongniart, Crinopsida Horaninov, Hydrocharitopsida Bartling, Juncopsida Bartling, Liliopsida Batsch, Liriopsida Brongniart, Najadiopsida Hoffmannsegg & Link, Orchidopsida Bartling, Pandanopsida Brongniart, Phoenicopsida Brongniart
ACORALES Martius, see Main Tree.
Vessels 0; inflorescence scapose, a spadix with large associated inflorescence bract [spathe]; flowers dense, sessile, small [<5 mm across], weakly monosymmetric [abaxial member of outer T whorl precocious and larger]; tapetal cells (1-)2-4-nucleate; pollen tectum continuous; carpels ascidiate-plicate, syncarpy congenital; ovules straight, nucellar epidermis dividing periclinally; endosperm copious, perisperm +, derived from outer nucellar cells [= nucellar epidermis s. str.], not starchy; collar rhizoids +. - 1 family, 1 genus, 2-4 species.
Note: In all node characterizations, boldface denotes a possible apomorphy, (....) denotes a feature the exact status of which in the clade is uncertain, [....] includes explanatory material; other text lists features found pretty much throughout the clade. 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. Diversification rates in this clade are distinctly low (Hertweck et al. 2015; Tank et al. 2015) as are rates of molecular evolution of the plastome (Barrett et al. 2015b).
ACORACEAE Martinov Back to Acorales
Mycorrhizae 0; cauline endodermis +; leaves two-ranked, ventralized isobifacial [oriented edge on to the stem, bases equitant], intravaginal squamules + [?= colleters]; peduncle with two separate vascular systems; bracts 0, bracteoles 0; floral vasculature forms a basal complex; T ± hooded; anthers introrse [?level], thecae confluent apically on dehiscence, endothecial thickenings stellate; tapetum secretory, cells 2-4-nucleate; pollen sulcus lacking ectexine, endexine lamellate; dorsal carpellary bundle 0, ovary loculi with secretory trichomes, placentae apical, pendulous, style broad, massive, stylar canals with exudate; ovules several/carpel, outer integument 3-5 cells across, tips of integuments with multiseriate hairs, hypostase massive, with central column and radiating basal walls of nucellar cells, postament +; embryo sac with "ears", antipodal cells ± persistent, (dividing); fruit a berry; tegmen cells spirally thickened; P persistent; embryo suspensor ?unicellular, cylindrical; plastid accD gene lost; n = 9, 11, 12; first leaf terete, unifacial.
1[list]/2-4. Northern Hemisphere (map: from Hultén 1962; Fl. N. Am. 22: 2000), but perhaps naturalised in Europe and America (Mayo et al. 1997). [Photo - Habit.]
Age. Crown-group Acoraceae were dated to 19 ± 5.7 m.y. by Merckx et al. (2008a), 52-4 m.y. by Mennes et al. (2013, see also 2015) and (50-)30(-15) or (10-)9 m.y. by Hertweck et al. (2015).
Evolution: Divergence & Distribution. See Stockey (2006) for an evaluation of fossil remains of Acoraceae.
Genes & Genomes. There has been a great increase in the rate of synonymous substitutions in the mitochondrial genome, but not in that of the chloroplast genome (Mower et al. 2007; see also G. Petersen et al. 2006b).
Chemistry, Morphology, etc. The root stele is pentarch. Does Acorus have vessels? It seems to depend on one's definition, Carlquist (2012a, see also 2009) calling the plant functionally vesselless, the tracheids being "pre-vessel" in morphology - although derived. The primary root is described as being diarch by Buell (1935).
The abaxial tepal is large, bract-like, and encloses the young flower, indeed, it seems to have "merged" with the bract (Buzgo 2001), being depicted as an organ of "hybrid" nature (Bateman et al. 2006b); Ronse de Craene (2010) interprets it as a bract, one tepal being missing (if this is the correct interpretation, some apomorphies above will have to be changed). For possible septal nectaries, see above. The ovules are encased in mucilage secreted by the intra-ovarian trichomes.
Several details of embryogeny need confirmation. Buell (1938) suggested that cells of the nucellar epithelium divided periclinally, and cells of the inner layer produced - technically almost a nucellar cap of sorts - elongated radially. The polar nucleus divided before fertilization, one nucleus moving to the antipodal end of the cell, and a cell wall forming. Early development of the possibly diploid endosperm nucleus produced a file of four large cells. The suspensor was not described in detail, although Buell (1935) had mentioned that it was small and attached to the base of the embryo. The structure in Fig. 2A indicated as being the suspensor looks more like a slightly darkened file of cells in the root cap. Rudall and Furness (1997) unfortunately did not follow details of embryo development, but they illustrated the perisperm cells as being radially elongated and covered by a cuticle.
Additional information is taken from Grayum (1987), Bogner and Mayo (1998), and Bogner (2011), all general, Kaplan (1970a: leaf development), Carlquist and Schneider (1997) and Keating (2003a), both anatomy, Soukup et al. (2005: root development, intermediate), Buzgo and Endress (2000: floral morphology), Eyde et al. (1967: floral anatomy), Buell (1938: ovule), Floyd and Friedman (2000b: endosperm with uniseriate cells in micropylar chamber, etc.), and Tillich (1985: seedling). For a checklist and bibliography, see Govaerts and Frodin (2002) and World Checklist of Monocots.