EMBRYOPSIDA Pirani & Prado (crown group)
Gametophyte dominant, independent, multicellular, thalloid, with single-celled apical meristem, showing gravitropism; flavonoids + [absorbtion of UV radiation]; protoplasm dessication tolerant [plant poikilohydric]; cuticle +; cell walls with (1->4)-ß-D-glucans [xyloglucans], lignin +; rhizoids unicellular; several chloroplasts per cell; glycolate metabolism in leaf peroxisomes [glyoxysomes]; centrioles in vegetative cells 0, metaphase spindle anastral, predictive preprophase band of microtubules, phragmoplast + [cell wall deposition spreading from around the spindle fibres], plasmodesmata +; antheridia and archegonia jacketed, stalked; spermatogenous cells monoplastidic, centrioles develop de novo, associated with basal bodies of flagellae, multilayered structure +, proximal end of basal bodies lacking symmetry, stellate pattern associated with doublet tubules of transition zone; spermatozoids with a left-handed coil; male gametes with 2 lateral flagellae; oogamy; diploid embryo initially surrounded by haploid gametophytic tissue, plane of first division horizontal [with respect to long axis of archegonium/embryo sac], suspensor/foot +, cell walls with nacreous thickenings; sporophyte multicellular, sporangium +, single, with polar transport of auxin, dehiscence longitudinal; meiosis sporic, monoplastidic, microtubule organizing centre associated with plastid, cytokinesis simultaneous, preceding nuclear division, sporocytes 4-lobed, with a quadripolar microtubule system; spores in tetrads, sporopollenin in the spore wall, wall with several trilamellar layers [white-line centred layers, i.e. walls multilamellate]; spores trilete; close association between the trnLUAA and trnFGAA genes on the chloroplast genome.
Note that many of the bolded characters in the characterization above are apomorphies in the streptophyte clade along the lineage leading to the embryophytes rather than being apomorphies of the crown embryophytes.
Abscisic acid, ?D-methionine +; sporangium with seta, seta developing from basal meristem [between epibasal and hypobasal cells], sporangial columella + [developing from endothecial cells]; stomata +, anomocytic, cell lineage that produces them with symmetric divisions [perigenous]; underlying similarities in the development of conducting tissue and in rhizoids/root hairs; polar transport of auxins and class 1 KNOX genes expressed in the sporangium alone; MIKC, MI*K*C* and class 1 and 2 KNOX genes, post-transcriptional editing of chloroplast genes; gain of three group II mitochondrial introns.
[Hornworts + Polysporangiophyta]: archegonia embedded/sunken in the gametophyte; sporophyte long-lived, chlorophyllous, nutritionally largely independent of the gametophyte; sporophyte-gametophyte junction interdigitate, sporophyte cells showing rhizoid-like behaviour.
Sporophyte well developed, branched, free living, sporangia several; spore walls not multilamellate [?here]; apical meristem +.
EXTANT TRACHEOPHYTA / VASCULAR PLANTS
Photosynthetic red light response; water content of protoplasm relatively stable [plant homoiohydric]; control of leaf hydration passive; (condensed or nonhydrolyzable tannins/proanthocyanidins +); vascular tissue +, sieve cells + [nucleus degenerating], tracheids +, in both protoxylem and metaxylem; endodermis +; root xylem exarch [development centripetal]; stem with an apical cell; branching dichotomous; leaves spirally arranged, blades with mean venation density 1.8 mm/mm2 [to 5 mm/mm2]; sporangia adaxial on the sporophyll, sporangia derived from periclinal divisions of several epidermal cells, wall multilayered [eusporangium]; columella 0; tapetum glandular; stellate pattern split between doublet and triplet regions of transition zone; placenta with single layer of transfer cells in both sporophytic and gametophytic generations, embryonic axis not straight [root lateral with respect to the longitudinal axis; plant homorhizic].[MONILOPHYTA + LIGNOPHYTA]
Branching ± monopodial; lateral roots +, endogenous, root apex multicellular, root cap +; tracheids with scalariform-bordered pits; leaves with apical/marginal growth, venation development basipetal, growth determinate; sporangia borne in pairs and grouped in terminal trusses, dehiscence longitudinal, a single slit; cells polyplastidic, microtubule organizing centres not associated with plastids, diffuse, perinuclear; male gametes multiflagellate, basal bodies staggered, blepharoplasts paired; chloroplast long single copy ca 30kb inversion [from psbM to ycf2].
Plant woody; lateral root origin from the pericycle; branching lateral, meristems axillary; cork cambium + [producing cork abaxially], vascular cambium bifacial [producing phloem abaxially and xylem adaxially].
EXTANT SEED PLANTS / SPERMATOPHYTA
Plant evergreen; nicotinic acid metabolised to trigonelline, (cyanogenesis via tyrosine pathway); primary cell walls rich in xyloglucans and/or glucomannans, 25-30% pectin [Type I walls]; lignins derived from (some) sinapyl and particularly coniferyl alcohols [hence with p-hydroxyphenyl and guaiacyl lignin units, so no Maüle reaction]; root with xylem and phloem originating on alternate radii, vascular tissue not medullated, cork cambium deep seated; arbuscular mycorrhizae +; shoot apical meristem interface specific plasmodesmatal network; stem with vascular cylinder around central pith [eustele], phloem abaxial [ectophloic], endodermis 0, xylem endarch [development centrifugal]; 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 +; stem cork cambium superficial; branches exogenous; leaves with single trace from vascular sympodium [nodes 1:1]; stomatal pore with active opening in response to leaf hydration, control by abscisic acid, metabolic regulation of water use efficiency, etc.; leaves with petiole and lamina, development basipetal, blade simple; axillary buds +, (not associated with all leaves); prophylls two, lateral; plant heterosporous, sporangia borne on sporophylls; microsporophylls aggregated in indeterminate cones/strobili; true pollen +, grains mono[ana]sulcate, exine and intine homogeneous; ovules unitegmic, parietal tissue 2+ cells across, megaspore tetrad linear, functional megaspore single, chalazal, lacking sporopollenin, megasporangium indehiscent; pollen grains landing on ovule; male gametophyte development first endo- then exosporic, tube developing from distal end of grain, gametes two, developing after pollination, with cell walls; female gametophyte endosporic, initially syncytial, walls then surrounding individual nuclei; seeds "large" [ca 8 mm3], but not much bigger than ovule, with morphological dormancy; embryo cellular ab initio, endoscopic, plane of first cleavage of zygote transverse, suspensor +, short-minute, embryonic axis straight [shoot and root at opposite ends; plant allorhizic], white, cotyledons 2; plastid transmission maternal; ycf2 gene in inverted repeat, whole nuclear genome duplication [zeta duplication], two copies of LEAFY gene, PHY gene duplications [three - [BP [A/N + C/O]] - copies], nrDNA with 5.8S and 5S rDNA in separate clusters; mitochondrial nad1 intron 2 and coxIIi3 intron and trans-spliced introns present.
ANGIOSPERMAE / MAGNOLIOPHYTA
Lignans, O-methyl flavonols, dihydroflavonols, triterpenoid oleanane, apigenin and/or luteolin scattered, [cyanogenesis in ANITA grade?], S [syringyl] lignin units common [positive Maüle reaction - syringyl:guaiacyl ratio more than 2-2.5:1], and hemicelluloses as xyloglucans; root apical meristem intermediate-open; root vascular tissue oligarch [di- to pentarch], lateral roots arise opposite or immediately to the side of [when diarch] xylem poles; origin of epidermis with no clear pattern [probably from inner layer of root cap], trichoblasts [differentiated root hair-forming cells] 0, exodermis +; shoot apex with tunica-corpus construction, tunica 2-layered; reaction wood ?, associated gelatinous fibres [g-fibres] with innermost layer of secondary cell wall rich in cellulose and poor in lignin; 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, cytoplasm not occluding pores of sieve plate, companion cell and sieve tube from same mother cell; sugar transport in phloem passive; nodes 1:?; stomata brachyparacytic [ends of subsidiary cells level with ends of pore], outer stomatal ledges producing vestibule, reduction in stomatal conductance to increasing CO2 concentration; lamina formed from the primordial leaf apex, margins toothed, development of venation acropetal, overall growth ± diffuse, venation hierarchical-reticulate, secondary veins pinnate, veins (1.7-)4.1(-5.7) mm/mm2, endings free; most/all leaves with axillary buds; flowers perfect, pedicellate, ± haplomorphic, parts spiral [esp. the A], free, numbers unstable, development in general centripetal; P +, 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], ± embedded in the filament, with at least outer secondary parietal cells dividing, each theca dehiscing longitudinally, endothecium +, endothecial cells elongated at right angles to long axis of anther; (tapetum glandular), cells binucleate; microspore mother cells in a block, microsporogenesis successive, walls developing by centripetal furrowing; pollen subspherical, tectum continuous or microperforate, ektexine columellate, endexine thin, compact, lamellate only in the apertural regions; nectary 0; carpels present, superior, free, several, ascidiate, with postgenital occlusion by secretion, stylulus short, hollow, cavity not lined by distinct epidermal layer, stigma ± decurrent, carinal, dry [not secretory]; 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 [crassinucellate], nucellar cap?; megasporocyte single, hypodermal, functional megaspore lacking cuticle; female gametophyte four-celled [one module, nucleus of egg cell sister to one of the polar nuclei]; supra-stylar extra-gynoecial compitum +; ovule not increasing in size between pollination and fertilization; pollen grains landing on stigma, bicellular at dispersal, mature male gametophyte tricellular, germinating in less than 3 hours, pollination siphonogamous, tube elongated, growing 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, flagellae 0, double fertilization +, ovules aborting unless fertilized; P deciduous in fruit; seed exotestal, becoming much larger than ovule at time of fertilization; endosperm diploid, cellular, heteropolar [micropylar and chalazal domains develop differently, 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; embryogenesis cellular; dark reversal Pfr -> Pr; Arabidopsis-type telomeres [(TTTAGGG)n]; 2C genome size 1-8.2 pg [1 pg = 109 base pairs], whole nuclear genome duplication [epsilon duplication]; protoplasm dessication tolerant [plant poikilohydric]; 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, paleo AP3 and PI genes [paralogous B-class genes] +, with "DEAER" motif, SEP3/LOFSEP and three copies of the PHY gene, [PHYB [PHYA + PHYC]].
[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]]]]: vessel elements with scalariform perforation plates in primary xylem; essential oils in specialized cells [lamina and P ± pellucid-punctate]; tension wood +; tectum reticulate; anther wall with outer secondary parietal cell layer dividing; carpels plicate; 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 [possible position]; pollen tube growth intra-gynoecial; embryo sac bipolar, 8 nucleate, 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).
[CERATOPHYLLALES + EUDICOTS]: ethereal oils 0.
EUDICOTS: (Myricetin, delphinidin +), asarone 0 [unknown in some groups, + in some asterids]; root epidermis derived from root cap [?Buxaceae, etc.]; (vessels with simple perforation plates in primary xylem); nodes 3:3; stomata anomocytic; flowers (dimerous), cyclic; K/outer P members with three traces, ("C" +, with a single trace); A few, (polyandry widespread, initial primordia 5, 10, or ring, ± centrifugal), filaments fairly slender, anthers basifixed; microsporogenesis simultaneous, pollen tricolpate, apertures in pairs at six points of the young tetrad [Fischer's rule], cleavage centripetal, wall with endexine; G with complete postgenital fusion, stylulus/style solid [?here]; seed coat?
[PROTEALES [TROCHODENDRALES [BUXALES + CORE EUDICOTS]]]: (axial/receptacular nectary +).
[TROCHODENDRALES [BUXALES + CORE EUDICOTS]]: benzylisoquinoline alkaloids 0; euAP3 + TM6 genes [duplication of paleoAP3 gene: B class], mitochondrial rps2 gene lost.
[BUXALES + CORE EUDICOTS]: ?
CORE EUDICOTS / GUNNERIDAE Back to Main Tree
(Ellagic and gallic acids +); leaf margins serrate; compitum + [one place]; micropyle?; whole nuclear genome duplication [palaeohexaploidy, gamma triplication], PI-dB motif +, small deletion in the 18S ribosomal DNA common.
Age. The crown group age of core eudicots is some 116-115 m.y. (Anderson et al. 2005). Other crown group ages are 132-119(-89) m.y. in Soltis et al. (2008: a variety of estimates), 114.4 or 115.1 m.y. in Magallón and Castillo (2009), (113-)110(-105) m.y. (Moore et al. 2010: 95% HPD), (139-)127, 119(-109) or (127-)121, 117(-107) m.y. (Bell et al. 2010 for details), (116-)112(-107) m.y. (N. Zhang et al. 2012) and (118.3-)112.9-111.7(-105.3) m.y. (Magallón et al. 2013: with temporal constraints); Schneider et al. (2004) offer a range of dates up to over 180 m.y.a.. Wikström et al. (2003) suggested a crown group age of (131-)127, 116(-111) m.y., and ca 118.1. m.y. is the age in Naumann et al. (2013).
The age of stem core eudicots has been estimated as some 120-116 m.y. (Anderson et al. 2005). Other stem group ages are 121.2 or 121.9 m.y. (relaxed and constrained penalized likelihood: Magallón & Castillo 2009), while Wikström et al. (2003) suggested a stem age about (140-)135, 123(118) m.y. - c.f. sister group.
Evolution. Divergence & Distribution. Vekemans et al. (2012) emphasized that the gamma genome duplication event (see below) occurred ca 7 m.y. before the divergence of Gunnerales or other core eudicots; there was a lag between duplication and divergence. However, if one tries to understand any link between this event and diversification, the lag may be somewhat greater, since although Gunnerales may be chemically like other core eudicots, they are not so close florally, and in any event they hardly represent a particularly speciose clade (Mayrose et al. 2011; Schranz et al. 2012 discuss lags between genome duplication/polyploidy and diversification).
Indeed, the floral morphology of Gunnerales is more like that of Buxales, etc., than that of other core eudicots. That is, it is more like taxa on more basal branches in the eudicots and so it is apparently plesiomorphic (e.g. D. Soltis et al. 2003a; Doust & Stevens 2005; Kubitzki 2006a). Wanntorp and Ronse De Craene (2005) and Ronse De Craene and Wanntorp (2006) also note that the morphology of Gunnerales flowers cannot be directly related to that of the pentamerous core eudicots, the floral morphology of the former being shaped by the exigencies of wind pollination (see also Ronse de Craene & Brockington 2013). Wanntorp and Ronse De Craene (2005) observed that three successive floral whorls may be opposite each other, and this is a feature e.g. of Ranunculales like Berberidaceae and of Sabiaceae, etc., but not of core eudicots. Indeed, the flowers cannot readily be derived from those of other core eudicots given our current knowledge of floral morphology and development, even if the duplication of a number of genes important in determining the identity of floral parts (AP3, AP1, SEP, AG) may be connected with this gamma event (Jiao et al. 2012). For discussion of the "typical" core eudicot flower, see the Dilleniales page.
Chemically Gunnerales do seem similar to core eudicots, e.g. they have ellagic acid (e.g. Soltis et al. 2005b), and general molecular data link them closely with core eudicots. Some perhaps important gene duplications may have occurred somewhat earlier (see the Trochodendrales page), and it is not known if they have the euAP3 gene, etc.
For general information on core eudicot diversification, see Magallón et al. (1999); most of the estimates of percentage diversity of clades are taken from this work. The diversification rates of many of the clades are higher than those in other angiosperms (Magallón & Sanderson 2001).
Pollination Biology & Seed Dispersal. Compitum presence can perhaps be pegged as a key innovation at this node, given that it is found in Gunneraceae, the rosids, and the extended asterid clade, although not in Myrothamnaceae or Dilleniaceae (c.f. Endress 2011).
Genes & Genomes. Severin et al. (2011) offered a spread of 240-130 m.y. for the palaeohexploidy event, probably in an ancestor of Pentapetalae. Vekemans et al. (2012) narrowed the estimate for the age of this genome triplication at (121.9-)120.4(-118.9) or (122.7-)120.05(-117.4) m.y., depending on the method used, while Jiao et al. (2012) dated duplications associated with the gamma event to around 117 m.y.a.
The importance of the possible palaeohexaploidy event (the gamma triplication) now placed at this node was first suggested in work on Vitales (Jaillon, Aury et al. 2007). It was then proposed that there had been gene duplication, possibly because of hybridization, within the Vitis lineage itself, apparently bringing the Vitis genome more into line with that of other rosids (Velasco et al. 2007). Nevertheless, Freeling et al. (2008: the Carica papaya genome was included in their study) suggested that most rosids, i.e. the node [Vitales + rosids s. str], were indeed palaeohexaploids in which two of the three genomes involved had lost notably more ancestral genes than had the other, the Atg event. Evidence from collinearity between genomes then suggested that this palaeohexaploidy event had occurred in the anestor of the aterid Coffea (Cenci et al. 2010), while Jiao et al. (2012) placed the event before the split of the asterids and rosids but after the divergence of Ranunculales - see also Tang et al. (2008a, b), Diaz-Riquelme et (al. 2009), Barker et al. (2009), Abrouk et al. (2010) and Severin et al. (2011) for this genome triplication, whether or not also involving asterids. In any event, evidence of large genome duplications may be lost, as in Fragaria vasca (Rosaceae) (Shulaev et al. 2010).
The most recent work estimates that palaeohexaploidy happened in the immediate ancestor of this node (Jiao et al. 2012; esp. Vekemans et al. 2012). Given earlier genome duplications (all angiosperms, all seed plants), by the time one gets to genera like Brassica, Arabidopsis, and Gossypium, the genome must have duplicated many, many times... (e.g. Paterson et al. 2012).
There are quite a number of gene duplications known in the general Dilleniales-Vitales area, perhaps a whole genome duplication is involved (e.g. Litt & Irish 2003; Kramer et al. 2004; Kim et al. 2004; Zahn et al. 2005b; Howarth & Donoghue 2006; especially Kramer & Zimmer 2006; Shan et al. 2007 - see immediately above!). Duplications include: euAP1 + euFUL + AGL79 genes [duplication of AP1/FUL or FUL-like gene], PLE + euAG [duplication of AG-like gene: C class], SEP1 + FBP6 genes [duplication of AGL2/3/4 gene]. The euAP1clade includes key regulators that have been implicated in the specification of perianth identity (Litt & Irish 2003). However, not all major core eudicot groups have been sampled for this gene, the situation in Santalales, for example, being unknown; there has been another duplication of this gene (and also of the AGL1/2/3 gene) perhaps immediately below the Pentapetalae node, but above the Ranunculales node. The roles such genes may play in many eudicot groups is unknown. Duplications of the CYC2 gene clade are widespread, and they are often associated with the evolution of monosymmetric flowers (Howarth et al. 2011 and references).
Chemistry, Morphology, etc. Wanntorp and Ronse De Craene (2005) and Ronse De Craene and Wanntorp (2006) note that the morphology of Gunnerales flowers cannot be directly related to that of the pentamerous core eudicots, the floral morphology of the former being shaped by the exigencies of wind pollination. Wanntorp and Ronse De Craene (2005) also note that three successive floral whorls may be opposite each other, and this is a feature e.g. of some Ranunculales like Berberidaceae, Sabiales, etc., but not of core eudicots. Indeed, the flowers cannot be considered as being readily derived from those of other core eudicots given our current knowledge of floral morphology and development, rather, they seem fundamentally similar to those scattered in other eudicot clades basal to Gunnerales. For discussion of the "typical" core eudicot flower, see the Dilleniales page.
Whether or not the rps2 and rps11 genes occur in Gunnerales is apparently unknown, but the first is absent in Buxales and Trochodendrales and the second in Buxales alone (Adams et al. 2002b); they are probably lost slightly lower down on the tree (as are benzylisoquinoline alkaloids).
Phylogeny. This clade is strongly supported, e.g. Chase et al. (1993), D. Soltis et al. (1997, 1999, 2003a), Hoot et al. (1998), Nandi et al. (1998), D. Soltis et al. (2003a: four genes), S. Kim et al. (2004), Zhu et al. (2007), and Qiu et al. (2010: support weak), although indistinguishable from other core eudicots in (e.g.) P. Soltis et al. (1999). However, in some earlier studies the exact position of Gunnerales was unclear (e.g. Chase et al. 1993; Morgan & Soltis 1993), but quite strong support for a position sister to the rest of the core eudicots was provided by Senters et al. (2000: Gunneraceae alone sampled), rather weaker support by Hilu et al. (2001). Zhu et al. (2007) found a weakly supported [Gunnera + Dillenia] clade sister to other core eudicots; see also Dilleniales.
Classification. Gunnerales were excluded from the core eudicots in earlier versions of this site (pre Version 7; c.f. A.P.G. I and II 1999, 2003) because of their apparently largely plesiomorphic floral morphology. However, in the interests of consistency between classification systems their limits have been adjusted to conform with those now generally used.
GUNNERALES Reveal Main Tree.
Ellagic acid +; vessel elements?; sieve tube plastids with protein crystalloids and starch; pith with sclerenchymatous diagrams; lamina margins with hydathodal teeth, secondary veins palmate; plants dioecious; inflorescences with terminal flowers; flowers small; P ?0; A latrorse; stigma at most weakly secretory; seed coat? - 2 families, 2 genera, 42-52 species.
Age. Estimates of the age of crown group Gunnerales are (123-)118, 108(-103) m.y. (Wikström et al. 2001), 90-55 m.y. (Anderson et al. 2005), (132-)102, 95(-64) m.y. (Bell et al. 2010), or ca 77 m.y. (Magallón et al. 2013).
Note: Possible apomorphies are in bold. However, the actual level at which many of these features, particularly the more cryptic ones, should be assigned is unclear. This is partly because many characters show considerable homoplasy, in addition, basic information for all too many 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 is the not-so-trivial issue of how ancestral states are reconstructed...
Chemistry, Morphology, etc. This is a rather surprising group. Gunneraceae and Myrothamnaceae look very different; one is an often gigantic mesophytic herb, the other a resurrection shrub of arid habitats. In the former, hydathodes are well developed and mucilage or possibly resinous lacquer is secreted, in the latter, hydathodes are poorly developed (but see Drennan et al. 2009) and the plant secretes resin. They do both have flowers without much of a perianth, although the plesiomorphic condition for the order may be to have some kind of perianth, but details of pollen (e.g. c.f. Zavada & Dilcher 1986; Wanntorp et al. 2004), etc., differ. González and Bello (2009) suggest possible apomorphies for the pair, including the presence of stipules, but see below for a possible interpretation of the stipules of Gunnera.
Classification. There is an option of including Myrothamnaceae in Gunneraceae in A.P.G. II (2003), both being small and monogeneric families, but they are so different in appearance that it seems best to keep them separate (see Wilkinson 2000 for a table of differences).
Includes Gunneraceae, Myrothamnaceae. 2 families, 2 genera, ca 45 species.
Synonymy: Myrothamnales Reveal - Gunnerineae Shipunov - Myrothamnanae Takhtajan
GUNNERACEAE Meisner, nom. cons. Back to Gunnerales
Perennial (annual) herbs, rhizomatous or stoloniferous; close association with the N-fixing Nostoc in stem and root; cork ?; root stele polyarch; vascular cambium 0 [?all taxa]; vessel elements with simple or few-barred scalariform perforation plates [stems] or scalariform, bars to ca 150 [stolons]; axis polystelic [erect stem] or vascular cylinder [stolons]; nodes multilacunar; stem with endodermis, also low glandular areas; petiole anatomy complex; cataphylls common; stolons with opposite scales; leaves spiral, colleters +; inflorescence usu. branched-racemose, (plant polygamous), bracteoles 0; P 2 (3), valvate; staminate flowers: (P 4, 0); A 1-2; pollen semitectate-reticulate; pistillode +; carpellate flowers: staminodes +; G , inferior, transverse and alternate with P, uni(bi)locular, stigmas dry; ovules 1(-2)/carpel, apical, pendulous, epitropous, micropyle endostomal, outer integument ca 3 cells across, inner integument ?2/ca 4 cells across; embryo sac tetrasporic, 16-celled [Peperomia-type]; fruit drupaceous (nut); seed coat?; endosperm with some starch; n = 17.
1[list]/40-50. Circum S. Pacific, Africa and Madagascar (map: see van Balgooy 1975; Trop. Afr. Fl. Pl. Ecol. Distr. 1. 2003; Wanntorp & Wanntorp 2003; fossil records [green] from the Late Cretaceous, see Jarzen & Dettmann 1989, also Osborne & Sprent 2002). [Photo - Leaf, Inflorescence]
Evolution. Divergence & Distribution. The pollen is distinctive and is known from the Early Cretaceous onwards (Wanntorp et al. 2004b) in all four continents of the Southern Hemisphere, as well as from North America, India and deposits in the Indian and south Atlantic Oceans (the latter three localities in the Palaeogene - see Jarzen & Dettmann 1989). Wanntorp and Wanntorp (2003) interpreted the distribution of Gunnera in the context of its phylogeny and assuming Gondwanan-age vicariance events (c.f. Beaulieu et al. 2013, not independent evidence for vicariance).
Bacterial/Fungal Associations. All taxa have an association with the cyanobacterium, Nostoc (Osborne & Sprent 2002 for the ecology of the interrelationship; Santi et al. 2013 for literature). Glands on the stem found immediately below the leaves secrete mucilage, and Nostoc enters the plant here as motile hormogonia. Johansson and Bergman (1994, and references) and in particular Khamar et al. (2010) describe the establishment of this association; there are different sugars in the mucilage that attracts Nostoc and in the gland tissue in which Nostoc grows (Khamar et al. 2010). Nostoc infects as hormogonia, but in the plant it loses its motility and produces many heterocysts; it may not photosynthesise at all there (Bergman 2002). Söderbäck and Bergman (1993) detail the physiology of the two partners; see also Adams et al. (2006).
Chemistry, Morphology, etc. There are stipule-like structures on the stem of many species (but this is not an apomorphy for the family) that are interpreted as being cataphylls by Wanntorp et al. (2003). Since they are at least sometimes opposite they may be prophylls; they range in shape from suboblong and entire to deeply laciniate with linear lobes. The lamina varies from 7 mm to 3 m across, and the teeth have a glandular apex that broadens distally; two higher order veins are also involved. The difference in anatomy between stems and stolons is striking; the roots are triarch to polyarch (Wilkinson 2000). Although Gunnera herteri, sister to the rest of the genus, has normal stem anatomy, it is an annual and its anatomy is conceivably derived. The difference in size, etc., between the inner and outer tepals is such that they are sometimes described as sepals and petals (e.g. Wanntorp & Ronse de Craene 2005; Ronse de Craene & Wanntorp 2006; González & Bello 2009). The rather uncommon perfect flowers then have two median sepals, two lateral petals, two stamens opposite the petals, and two carpels also opposite the petals (Ronse de Craene & Wanntorp 2006; González & Bello 2009), indeed, there is considerable infraspecific variation in floral morphology (González & Bello 2009).
Much information is taken from Wilkinson (1998: anatomy); for ovules, etc., see Schnegg (1902) and Warming (1913), for pollen, Wanntorp et al. (2004a), and for floral morphology and development see Rutishauser et al. (2004), Wanntorp and Ronse De Craene (2005) and Ronse De Craene and Wanntorp (2006); Wilkinson and Wanntorp (2006) summarize what is known about the family.
Phylogeny. For a phylogeny of Gunnera, see Wanntorp et al. (2001, also Wanntorp 2006: summary); the annual G. herteri is sister to the rest of the genus.
Previous Relationships. Gunneraceae have often been associated with Haloragaceae (e.g. cronquist 1981), also with an inferior ovary and reduced flowers, but in the latter the stamens are as many as the sepals, and opposite them, the gynoecium is multilocular, with one ovule/loculus, etc. - see Saxifragales. Gunneraceae were included in Saxifraganae: Rosidae by Takhtajan (1997). Fuller and Hickey (2005), examining details of leaf architecture, etc., suggested that Gunneraceae were best associated with the herbaceous Saxifragaceae, but this is probably because of habit/habitat-associated parallelisms.
MYROTHAMNACEAE Niedenzu, nom. cons. Back to Gunnerales
Aromatic-resinous shrubs, resurrection plants; essential oils +, gallotannins, myricetin, dihydro/chalcones +; cork?; vessel elements with reticulate perforations; pith star-shaped/tetragonal; nodes split-lateral; petiole bundle arcuate; individual epidermal cells resiniferous, midrib bundle lacking fibres, palisade tissue 0; plant glabrous; leaves opposite, basally forming a sheath, lamina vernation plicate, secondary veins palmate-flabellate, stipules 2, small, persisting on the petiolar sheath; spikes bracteate, with terminal flowers; staminate flowers: A 3-4, or (3-)4(-8) and connate, anthers valvate basally, connective produced; pollen in tetrads, triporate, intectate, with clavate projections themselves papillate; pistillode 0; carpellate flowers: staminodes 0; G 3-4, only basally connate, with 5 vascular bundles and surface oil cells, the odd member abaxial, styluli short, recurved, stigma decurrent, in two crests; ovules many/carpel, micropyle bistomal; embryo sac bisporic [chalazal dyad], eight-celled [Allium-type]; fruit follicular (and septicidal); exotestal cells with somewhat thickened outer walls; endosperm development?; n = 10.
1[list]/2. Africa and Madagascar (map: from Puff 1978b; Trop. Afr. Fl. Pl. Ecol. Distr. 1. 2003). [Photos - Collection]
Evolution. Ecology & Physiology. Myrothamnaceae are resurrection plants. The leaves may appear to dry out completely, but they quickly regain turgor, etc., when conditions improve. Moore et al. (2007) discussed the physiology of Myrothamnus, which shows surprising infraspecific variation; Bianchi et al. (1993) found much glucopyranosyl-ß-glycerol and some trehalose along with the more normal high concentrations of sucrose in the dry leaf (see also Farrant 2000).
Chemistry, Morphology, etc. The stems are narrowly winged and there is no axial parenchyma. There are four veins in the leaf sheaths - two bundles going directly to the midrib, and two commissural veins (Grundell 1933). Since carpellate flowers lack perianth and staminodes, it is not clear if the ovary is inferior. When flowers are terminal - on the main or lateral axes - there are four "Hochblätter" as well as bracts and bracteoles, and the four carpels are opposite the "Hochblätter" (Jäger-Zürn 1966) which might even be interpreted as perianth or tepals (Wanntorp and Ronse De Craene 2005) and so the ovary would be superior; see also Puff (1978a, 1978b). The carpellate flowers have been described as being zygomorphic (Moore et al. 2007).
For more information, see Carlquist (1990a: leaf anatomy) and Kubitzki (1993b: general).
Previous relationships. Myrothamnaceae were included in Hamamelididae by Takhtajan (1997).