EMBRYOPSIDA Pirani & Prado

Gametophyte dominant, independent, multicellular, initially ±globular; showing gravitropism; acquisition of phenylalanine lysase [PAL], phenylpropanoid metabolism [lignans +, flavonoids + (absorbtion of UV radiation)], xyloglucans in primary cell wall with distinctive side chains; 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, interphase microtubules form hoop-like system; metaphase spindle anastral, predictive preprophase band of microtubules [where cell plate will join parental cell wall], phragmoplast + [cell wall deposition spreading from around the spindle fibres], plasmodesmata +; antheridia and archegonia jacketed, surficial; blepharoplast +, centrioles develop de novo, bicentriole pair coaxial, separate at midpoint, centrioles rotate, associated with basal bodies of cilia, multilayered structure + [4 layers: L1, L4, tubules; L2, L3, short vertical lamellae] (0), spline + [tubules from L1 encircling spermatid], basal body 200-250 nm long, associated with amorphous electron-dense material, microtubules in basal end lacking symmetry, stellate array of filaments in transition zone extended, axonemal cap 0 [microtubules disorganized at apex of cilium]; male gametes [spermatozoids] with a left-handed coil, cilia 2, lateral; oogamy; sporophyte multicellular, cuticle +, plane of first cell division horizontal [with respect to long axis of archegonium/embryo sac], early embryo developing towards the archegonial neck [from epibasal cell, 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, sporopollenin + laid down in association with trilamellar layers [white-line centred lamellae; tripartite lamellae], >1000 spores/sporangium; nuclear genome size <1.4 pg, main telomere sequence motif TTTAGGG, LEAFY and KNOX1 and KNOX2 genes present, ethylene involved in cell elongation; chloroplast genome with close association between trnLUAA and trnFGAA genes, precursor for starch synthesis in plastid.

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

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


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

[Anthocerophyta + Polysporangiophyta]: xyloglucans in the primary cell wall with fucosylated subunits; gametophyte leafless; archegonia embedded/sunken [on;y neck protruding]; sporophyte long-lived, chlorophyllous; cell walls with xylans.


Sporophyte dominant, branched, branching apical, dichotomous, potentially indeterminate; vascular tissue +; stomata on stem; sporangia several, each opening independently; spore walls not multilamellate [?here].


Sporophyte with photosynthetic red light response; (condensed or nonhydrolyzable tannins/proanthocyanidins +); plant homoiohydrous [water content of protoplasm relatively stable]; control of leaf hydration passive; plant endohydrous [physiologically important free water inside plant]; (condensed or nonhydrolyzable tannins/proanthocyanidins +); xylans in secondary walls of vascular and mechanical tissue; root hairs +; lignins +; tracheids +, in both protoxylem and metaxylem, G- and S-types; sieve cells + [nucleus degenerating]; endodermis +; leaves/sporophylls spirally arranged, blades with mean venation density ca 1.8 mm/mm2 [to 5 mm/mm2], 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; root lateral with respect to the longitudinal axis of the embryo [plant homorhizic].


Sporophyte endomycorrhizal [with Glomeromycota]; root cap +, protoxylem exarch, lateral roots +, endogenous; stem apex multicellular, G-type tracheids +, with scalariform-bordered pits; leaves with apical/marginal growth, venation development basipetal, growth determinate; sporangium dehiscence by a single longitudinal slit; cells polyplastidic, MTOCs diffuse, perinuclear, migratory; blepharoplasts +, paired, with electron-dense material, centrioles on periphery, male gametes multiciliate; chloroplast long single copy ca 30kb inversion [from psbM to ycf2]; LITTLE ZIPPER proteins.


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


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


Plant evergreen; nicotinic acid metabolised to trigonelline, (cyanogenesis via tyrosine pathway); primary cell walls rich in xyloglucans and/or glucomannans, 25-30% pectin [Type I walls]; lignins particularly with guaiacyl and p-hydroxyphenyl [G + H] units [sinapyl units uncommon, no Maüle reaction]; root stele with xylem and phloem originating on alternate radii, cork cambium deep seated; stem apical meristem complex [with quiescent centre, etc.], mitochondrial density in SAM 1.6-6.2[mean]/μm2 [interface-specific mitochondrial 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; megasporangium indehiscent; ovules with parietal tissue 2+ cells across, 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, plane of first cleavage of zygote transverse, shoot apex developing away from micropyle [i.e. away from archegonial neck; from hypobasal cell, endoscopic], suspensor +, short-minute, embryonic axis straight [shoot and root at opposite ends; plant allorhizic], cotyledons 2; embryo ± dormant; plastid transmission maternal; ycf2 gene in inverted repeat, whole nuclear genome duplication [ζ - 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 trans- nad2i542g2 and coxIIi3 introns present.


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 apical meristem intermediate-open; stele di- to pentarch [oligarch], pith relatively inconspicuous, 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, 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; 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 with increasing CO2 concentration; lamina formed from the primordial leaf apex, margins toothed, development of venation acropetal, overall growth ± diffuse, secondary veins pinnate, fine venation hierarchical-reticulate, (1.7-)4.1(-5.7) mm/mm2, vein endings free; flowers perfect, pedicellate, ± haplomorphic, protogynous; parts free, numbers variable, development centripetal; P +, ?insertion, members each with a single trace, outer members not sharply differentiated from the others, not enclosing the floral bud; A many, filament not sharply distinguished from anther, stout, broad, with a single trace, anther introrse, tetrasporangiate, sporangia in two groups of two [dithecal], each theca dehiscing longitudinally by a common slit, ± embedded in the filament, walls with at least outer secondary parietal cells dividing, endothecium +, cells elongated at right angles to long axis of anther; tapetal cells binucleate; microspore mother cells in a block, microsporogenesis successive, walls developing by centripetal furrowing; pollen subspherical, tectum continuous or microperforate, ektexine columellate, endexine lamellate only in the apertural regions, thin, compact, intine in apertural areas thick, pollenkitt +; nectary 0; carpels present, superior, free, several, ascidiate [postgenital occlusion by secretion], stylulus at most short [shorter than ovary], hollow, cavity not lined by distinct epidermal layer, stigma ± decurrent, carinal, dry, 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 [crassinucellate], nucellar cap?; megasporocyte single, hypodermal, functional megaspore lacking cuticle; female gametophyte lacking chlorophyll, not photosynthesising, four-celled [one module, nucleus of egg cell sister to one of the polar nuclei]; ovule not increasing in size between pollination and fertilization; pollen grains land on stigma, bicellular at dispersal, mature male gametophyte tricellular, germinating in less than 3 hours, pollen tube elongated, unbranched, growing between cells, growth rate (20-)80-20,000 µm/hour, apex of pectins, wall with callose, lumen with callose plugs, penetration of ovules via micropyle [porogamous], whole process takes ca 18 hours, distance to first ovule 1.1-2.1 mm; male gametes lacking cell walls, ciliae 0, siphonogamy; double fertilization +, ovules aborting unless fertilized; P deciduous in fruit; mature seed much larger than fertilized ovule, small [], dry [no sarcotesta], exotestal; endosperm +, cellular, development heteropolar [first division oblique, micropylar end initially with a single large cell, divisions uniseriate, chalazal cell smaller, divisions in several planes], copious, oily and/or proteinaceous, embryo short [<¼ length of seed]; dark reversal Pfr → Pr; Arabidopsis-type telomeres [(TTTAGGG)n]; nuclear genome very small [1C = <1.4 pg, 1 pg = 109 base pairs], whole nuclear genome duplication [ε/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, 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]]]]: 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]; tectum reticulate; anther wall with outer secondary parietal cell layer dividing; 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 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).


Plant herbaceous, perennial, rhizomatous, growth sympodial; non-hydrolyzable tannins [(ent-)epicatechin-4] +, neolignans, benzylisoquinoline alkaloids 0, hemicelluloses as xylans; root apical meristem?; epidermis developed from outer layer of cortex; trichoblasts in atrichoblast [larger cell]/trichoblast cell pairs, the former further from apical meristem, in vertical files; 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 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; stomata 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, colleters + ["intravaginal squamules"]; inflorescence terminal, racemose; flowers 3-merous [6-radiate to the pollinator], polysymmetric, pentacyclic; P = 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 [primordia often associated, and/or A vascularized from tepal trace], anther and filament more or less sharply distinguished, anthers subbasifixed, endothecium from outer secondary parietal cell layer, inner secondary parietal cell layer dividing; pollen reticulations coarse in the middle, finer at ends of grain, infratectal layer granular; G [3], with congenital intercarpellary fusion, opposite outer tepals [thus median member abaxial], placentation axile; ovule with 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, 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 +); cotyledon with a closed sheath, unifacial [hyperphyllar], both assimilating and haustorial; no dark reversion Pfr → Pr; duplication producing monocot LOFSEP and FUL3 genes [latter duplication of AP1/FUL gene], PHYE gene lost. (Some synapomorphies - almost whatever the immediate sister taxa to monocots might be - are in bold.)

[ALISMATALES [PETROSAVIALES [[DIOSCOREALES + PANDANALES] [LILIALES [ASPARAGALES + COMMELINIDS]]]]]: ethereal oils 0; raphides + (druses 0); leaf blade vernation supervolute-curved or variants, (margins with teeth, teeth spiny); endothecium develops directly from undivided outer secondary parietal cells; tectum reticulate with finer sculpture at the ends of the grain, endexine 0; (septal nectaries + [intercarpellary fusion postgenital]).

[PETROSAVIALES [[DIOSCOREALES + PANDANALES] [LILIALES [ASPARAGALES + COMMELINIDS]]]]: cyanogenic glycosides uncommon; starch grains simple, amylophobic; leaf blade developing basipetally from hyperphyll/hypophyll junction; epidermis with bulliform cells [?level]; stomata anomocytic, (cuticular waxes as parallel platelets); colleters 0.

[[DIOSCOREALES + PANDANALES] [LILIALES [ASPARAGALES + COMMELINIDS]]]]: nucellar cap 0; endosperm nuclear [but variation in most orders].

Age. The age of this node was estimated at (127-)118, 105(-94) m.y. (Bell et al. 2010) but only around 83.6 or 83.1 m.y. by Xue et al. (2012); around 120 or 114.5 m.y.a. by S. Chen et al. (2013) and about 119.8 m.y.a. by Magallón et al. (2015).

Evolution. Divergence & Distribution. Thadeo et al. (2015) found that the anatomy of fleshy fruits in taxa scattered in this clade was quite similar, and they entertained notions of a long-conserved developmental pathway for these fruits, or perhaps the differences between caposular and baccate fruits were so slight that transitions between the two might be relatively simple.

The optimisation of nuclear endosperm to this node of the tree (Tobe & Kadokawa 2010) may well not hold up; variation in the patterns of endosperm development is great in many orders.

Plant-Animal Interactions. Larvae, and sometimes also adults, of the Chrysomelidae-Criocerinae are scattered as herbivores on plants throughout this clade, being perhaps especially common on commelinids (e.g. Schmitt 1988; Gómez-Zurita et al. 2007).

[DIOSCOREALES + PANDANALES]: root hairs from unmodified rhizodermal cells, exodermal cells not dimorphic; outer integument 2(-3) cells across.

Age. The divergence of these two orders is dated to about 131.4 m.y.a. by Tank et al. (2015: Table S2: stem age of Dioscoreaceae), ca 134.4 and 119.6 m.y. by Magallón and Castillo (2009); estimates were (130-)121(-119) m.y. in Merckx et al. (2008a), 123-96 m.y. in Mennes et al. (2013, 2015), ca 110.5 m.y.a. in Magallón et al. (2015), and (129-)119, 116(-110) m.y. in Hertweck et al. (2015). Although Janssen and Bremer (2004) showed the two as successive branches in the tree, they gave stem ages for both of ca 124 m.y. ago.

Phylogeny. For discussion on the relationships of Dioscoreales and Pandanales, see the Petrosaviales page.

DIOSCOREALES Martius  Main Tree.

Steroidal saponins +{?}; vascular bundles in rings; vessels also in stem and leaf; flowers or inflorescence with glandular hairs; styles free early in ontogeny, branches well developed, adaxially grooved; T persistent in fruit; ovules many/carpel; endotegmen tanniniferous; embryo at most short. - ?5 families, 21 genera, 1037 species.

Age. Crown group Dioscoreales are dated to ca 123 m.y. (Janssen & Bremer 2004); estimates were (126-)116(-113) m.y. in Merckx et al. (2008a), (120-)116(-111) m.y. in Merckx et al. (2010a; see also Viruel et al. 2015), 116-85 m.y.a. in Mennes et al. (2013, 2015), about 95.2 m.y.a. in Magallón et al. (2015) and (123-)115, 110(-104) m.y. in Hertweck et al. (2015).

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

WARNING. Since the basic phylogenetic structure of Dioscoreales remains unclear, ages for clades below need to be read in the context of phylogenies.

Chemistry, Morphology, etc. For morphology and anatomy, see Ayensu (1972), for seed coat, see Bouman (1995) and Oganezova (2000b), for pollen morphology and development see Caddick et al. (1998) and Schols et al. (2005a: Nartheciaceae and Dioscoreaceae). For much information on morphology in the order, see Caddick et al. (2000a: floral morphology and development, 2000b: general); prolongations of the anther connective are homoplasious.

Phylogeny. Nartheciaceae are rather consistently placed with the other Dioscoreales, albeit sometimes with only moderate support (e.g. Chase et al. 2000a; Caddick et al. 2002a; Tamura et al. 2004a: 97% bootstrap, Nartheciaceae well sampled, but otherwise only Dioscorea, Tacca and three Pandanales; Janssen & Bremer 2004: one gene, very good sampling; Chase et al. 2006; Givnish et al. 2006: see also Goldblatt 1995). However, Davis et al. (2004) found them to associate with Pandanales, although support was weak (<70%) and they lack the 6bp atpA deletion of many members of that clade, while in mitochondrial genome analyses Burmannia was sister to [Pandanales + Narthecium] yet Thismia was in Dioscoreales (G. Petersen et al. 2006b).

Evidence is mounting that the myco-heterotrophic members of Dioscoreales do not come close to forming a clade. Merckx et al. (2006: good sampling, no outgroup to Dioscoreales), using both mitochondrial and nuclear genes, found substantially different relationships within Dioscoreales from those depicted in the tree given by Caddick et al. (2002a: see also /APweb/ version 6 [November] and earlier, classification as in Caddick et al. 2002b). However, as Merckx et al. (2006) note, the relationships found by Caddick et al. (2002a) were dominated by chloroplast data, and since Burmanniaceae s.l. are largely myco-heterotrophic they have much diverged plastid sequences. Indeed, Geomitra, apparently Thismiaceae without any doubt, nevertheless came out with Burmanniaceae in some analyses (Caddick et al. 2002a). The positions of Dioscoreaceae and [Taccaceae + Thismiaceae] are sometimes reversed (Yokoyama et al. 2008: support slight). The situation is yet more complex, since Merckx and Bidartondo (2008) and Merckx et al. (2009a) suggest that Thismiaceae s. str. may be paraphyletic, Afrothismia being sister to [Taccaceae + rest of Thismiaceae]. Merckx et al. (2010a; see also Merckx & Smets 2014) confirmed the paraphyly of Thismiaceae, and also suggested that Trichopus could be sister to [Taccaceae + Thismiaceae]; Stenomeris was well embedded in Dioscorea. However, Hertweck et al. (2015) found Burmannia and Thismia to be sister taxa with very long branches, but sampling was poor since this was not the main point of their paper. In another study where broader relationships were also not the point of the paper, Viruel et al. (2015) found the relationships [Burmanniaceae [Taccaceae + Dioscoreaceae s.l.]].

Classification. The classification below follows that suggested by Merckx et al. (2006), although for relationships within Dioscoreaceae, see Caddick et al. (2002a, b). However, if the tree suggested by Mercx et al. (2010a) holds up, Afrothismia and Trichopodaceae will have to be split off, or... Trias-Blasi et al. (2015) recognised a Burmanniaceae that includes Thismiaceae and a Dioscoreaceae that includes Thismiaceae, but, as they noted, phylogenetic relationships here need attention.

Includes Burmanniaceae, Dioscoreaceae, Nartheciaceae, Taccaceae, Thismiaceae.

Synonymy: Burmanniales Martius, Nartheciales Reveal & Zomlefer, Taccales Dumortier, Tamales Dumortier

NARTHECIACEAE Bjurzon   Back to Dioscoreales


Chelidonic acid +, flavonols 0; air spaces in root cortex; fibers intermixed in phloem; (sieve tube plastids with large polygonal crystal - Narthecium); endodermal cells evenly thickened [distribution around here?]; raphides 0, usu. druses +, but prismatic crystals in bundle sheath; cuticular wax with parallel platelets; leaves spiral (two-ranked, isobifacial [oriented edge on to the stem]); bracteole 0 (+); T basally connate; A adnate basally; pollen orbicules with a circular perforation; septal nectaries +/0; G to half inferior, partly ascidiate, fusion congenital to postgenital, compitum +/0, (stylulus long); ovules ana-campylotropous, integuments lack cuticle [check], integumentary obturator + [Aletris]; antipodal cells not multinucleate; seeds obliquely stacked, (with appendages), shape various; tegmen flattened, persistent, or testa cells flattened, (esp. outer anticlinal walls of exotegmen thickened - Lophiola); endosperm helobial, [how much?, walls?]; embryo size?; n = (12) 13 (21, 22), 0.7-1.4 µm long; cotyledon bifacial or not, ?collar rhizoids.

4-5[list]/41: Aletris (30). Interrupted N. temperate, Venezuela and Guiana and scattered in W. Malesia (map: from Hultén & Fries 1986; Jessop 1979; Fl. China 24. 2000; Fl. N. Am. 26: 2002). [Photo - Inflorescence]

Age. Crown-group Nartheciaceae are dated to ca 76 m.y.a. (Janssen & Bremer 2004) and (92-)41(-10) m.y. by Merckx et al. (2010a).

Chemistry, Morphology, etc. Metanarthecium may have abaxial prophylls, in Lophiola they can be lateral. There are reports that the ovules of Narthecium may be unitegmic (Remizowa et al. 2006a). In general, variation in floral development is considerable, and the ovary may even be secondarily superior in Metanarthecium (Remizowa et al. 2008a; Fuse et al. 2012).

Additional information: For endodermal thickening, see Zomlefer (1997a), for leaf anatomy, see Luque Arias et al. (2006), for pedicel anatomy, see Gatin (1920), for inflorescence and flower development, see Remizowa et al. (2006a, b), for pollen morphology, see Merckx et al. (2008b), for embryology, see Ono (1929) and Cave (1968), for seed coat, see Takhtajan (1985: as Hypoxidaceae).

Phylogeny. Aletris, with its spiral and bifacial leaves, was sister to the three other genera examined (Tamura et al. 2004a), with Metanarthecium having weak support as sister to the rest of the family in Fuse et al. (2012). Although genera like Lophiola and Narthecium have two-ranked and isobifacial leaves, such leaves are unlikely to be an apomorphy for the family.

Previous Relationships. The relationships of Nartheciaceae, with their rather ordinary-looking monocot flowers, have long been problematic. Cronquist (1981) did not even mention them, but they would probably have been included in his highly heterogeneous Liliaceae, Dahlgren et al. (1985; see also Takhtajan 1997) placed them - along with representatives of Tofieldiaceae (here Alismatales) and Petrosaviaceae (Petrosaviales) - in Melianthaceae (Liliales), while Tamura (1998) placed them in Petrosaviaceae, along with genera here included in Tofieldiaceae.

Synonymy: Lophiolaceae Nakai

[[Taccaceae + Thismiaceae] [Burmanniaceae + Dioscoreaceae]]: plants geophytes; stem with endodermis; T tube well developed, broad, [?vasculature]; A incurved [± pendulous, anthers extrorse], inserted towards the mid/upper part of the tepal tube; ovary inferior, style short; exo- and endotesta tanniniferous.

Age. Divergence times at this node are (118-)109(-98) m.y. (Merckx et al. 2010a: Burmanniaceae sister to rest); other estimates are (112-)96, 93(-87) m.y. (Wikström et al. 2001), ca 115 and 127 m.y. (Magallón & Castillo 2009) and (111-)92, 83(-68) m.y. (Bell et al. 2010: c.f. topology). The node is dated to ca 116 m.y. by Janssen and Bremer (2004) and ca 79.9 m.y.a. by Magallón et al. (2015). The age of a clade including Afrothismia, Thismiaceae, and Trichopodaceae is some (109-)95(-79) m.y. (Merckx et al. 2010a).

Evolution. Divergence & Distribution. Myco-heterotrophy has evolved at least three, perhaps six or ten times in this clade (Merckx et al. 2006, 2008a, 2009, 2010a), but the exact number depends very much on relationships within the group. Glomeromycota are the fungi involved (Franke et al. 2006). The myco-heterotrophic habit may have evolved some time before the beginning of the Palaeocene (83-)63(-45) m.y.a. when Afrothismia gesnerioides diverged from the rest of the Taccaceae-Thismiaceae clade (Merckx et al. 2010a).

The early evolution of myco-heterotrophism here and elsewhere in this clade raises the issue of when more or less closed tropical rainforest developed (Merckx et al. 2008a, 2010a) - see also the parasitic Rafflesiaceae

Endress (2011a) thought that the inferior ovary in Dioscoreales might be a key innovation.

Chemistry, Morphology, etc. Cell walls in the endosperm are often thick, but are usually not pitted. The embryos of Taccaceae and Diocoreaceae are similar in their more or less lateral cotyledon (Solms Laubach 1878).

Phylogeny. Remember, relationships in this clade remain unclear!

[Taccaceae + Thismiaceae]: T whorls differentiated, but both well developed and petal-like; septal nectaries 0; placentation parietal, stigmas broad.

Age. The age of this node is some (92-)79(-68) m.y. (Merckx et al. 2010a: excl. Afrothismia).

TACCACEAE Dumortier, nom. cons.   Back to Dioscoreales


Plant with stem tubers or rhizomes; (velamen +); secondary thickening + [?]; vessels 0; hairs with multicellular stalk row, a head, and then another cell row; petiole bundles in ring; leaves basal, blade margins entire to ± deeply pinnately or palmately divided, fine venation reticulate, petiole +, base somewhat sheathing; inflorescence scapose, umbellate, of groups of cincinni, inflorescence bracts petal-like, floral bracts long, filiform, among the flowers; flowers medium in size; T with median member of outer whorl adaxial, tube short; A adnate to P at base, connective broad, not prolonged, forming a hood around anther; middle layer of anther wall from outer secondary parietal cells [?type]; G opposite C/inner T, stylar canal with secretion, stigma often petal-like; nucellar cells laterally anticlinally expanded, nucellar cap 0; megaspore mother cells several; fruit a berry (capsule); seed ribbed; (endotesta crystalliferous), exotegmen ± thick-walled and elongated, esp. radially; embryo short to minute, cotyledon ± lateral; n = 15; cotyledon ?bifacial, sheath lobed.

1/12. Pantropical, esp. Malesian-Pacific (map: from Drenth 1976; Australia's Virtual Herbarium, i.2015). [Photos - Collection.]

Age. Crown-group Taccaceae are (60-)35(-15) m.y. (Merckx et al. 2010a).

Evolution. Pollination Biology. The dark purple flowers of most species of Tacca and the long, dangling bracts suggest some sort of fly pollination. The inflorescence as a whole can be strongly monosymmetric, as in Tacca integrifolia; there two large and conspicuous white inflorescence bracts are held above the dark purple flowers and less conspicuous lower inflorescence bract and the whitish dangling filiform bracts.

Chemistry, Morphology, etc. Tacca seems to lack the distinctive vasculature of Dioscoreaceae, in which it used to be included. It is unclear if the midrib is distinct or multistranded (Inamadar et al. 1983).

The flowers are drawn with the odd member of the outer whorl in the adaxial position by Ronse de Craene (2010). The cells around the raphal bundle can be thin-walled; perhaps they are attractive to animals?

For general information, see Limpricht (1928) and Kubitzki (1998b), for xylem anatomy, see Carlquist (2012a).

THISMIACEAE J. Agardh nom. cons.   Back to Dioscoreales


Plants echlorophyllous, myco-heterotrophic, associated with glomeromycotes; saponins?; roots coralloid, vermiform or tuberous, root hairs 0; stomata 0; leaves reduced to scales, (borne immediately below the flower); inflorescences (branched), cymose [Haplothismia], or flowers single; (flowers monosymmetric); T with single trace, (whorls not differentiated - Haplothismia), (apex of inner P whorl members long and slender, often connate and the whole like a "mitre", contorted - Thismia, etc., outer P lobes then small or even 0), annulus or ring of short projections at mouth of tube + (0 - Haplothismia), T tube well developed and expanded below attachment of A at apex, with intrastaminal lobes; A (3, opposite inner T, Oxygyne), ± connate, (adnate to stigma), thecae with prolonged connective, filaments variously ornamented; tapetal cells uninucleate; pollen grains (tricellular), porate; placentae as separate columns, often ± free from walls, ± apical, or parietal, style short, connate at least early in ontogeny, branches slightly dorsi-ventrally flattened, (undivided), stigmas capitate to elongated; ovules lacking parietal tissue, funicles long [?all]; (embryo sac bisporic); fruit irregularly dehiscent, T basally circumscissile (not - Haplothismia); seeds minute, testal cells ± spiral, tegmic cells compressed; endosperm helobial (cellular - Thismia), thick-walled, with starch when young, not persisting; embryo undifferentiated; ?seedling; n = 6-9, chromosomes 1-4 µm long.

5 [list]/55: Thismia (45). Widely scattered, mostly (sub)tropical (map: from Jonker 1938; van Steenis & van Balgooy 1966; Maas et al. 1986; Larsen & Averyanov 2007; Dauby et al. 2008; Ho et al. 2010). [Photo - Thismia.]

Age. The age of crown Thismiaceae is some (85-)68(-49) m.y. (Merckx et al. 2010a: excl. Afrothismia).

Evolution. Divergence & Distribution. Haplothismia is different in a number of respects from the other genera; where it ends up in the phylogeny may affect familial apomorphies.

The number of specoes in Thismiaceae is unclear, since new species are described every few months.

The immediate relatives of Thismia americana, collected near Chicago just before WW I, not seen after 1916, and now apparently extinct, were thought to be Antipodean, a remarkable disjunction (Thorne 1972, 1992). However, the recently-described Thismia huangi, from Taiwan, is also morphologically similar to the Antipodean species, although in molecular analyses they are not so close (Merckx & Smets 2014); either way, biogeographic scenarios need to be rethought.

Pollination Biology and Seed Dispersal. Self-pollination or apomixis may be common (e.g. Maas et al. 1986), although (sapro)myophily is also a likely means of cross pollination (Woodward et al. 2007; Mar & Saunders 2015). There are no nectaries (Caddick et al. 2000).

Splash-cup seed dispersal is likely in at least some taxa (Mar & Saunders 2015).

Bacterial/Fungal Associations. Glomeromycote fungi form quite specific associations with species of Thismiaceae (Merckx et al. 2012).

Chemistry, Morphology, etc. The stems may be endogenous in origin (Pfeiffer 1914); the underground parts of Tiputinia are quite massive. Inflorescence morphology of species of Thismia described by Larsen and Averyanov (2007) is not easy to understand.

Additional information is taken from Dahlgren et al. (1985), Rübsamen (1986), Maas-van der Kamer (1999) and Tsukaya et al. (2007: chromosome number and size).

Phylogeny. For the delimitation of Thismiaceae, see above. Relationships within Thismiaceae s. str. are unclear, although Thismia itself may also be paraphyletic; Oxygyne may be sister to the rest of the family (Merckx et al. 2009a; Merckx & Smets 2014).

Afrothismia Kosteltsky

Plants echlorophyllous, myco-heterotrophic, associated with glomeromycotes; rhizome with aggregations of small bulbils along its length [basally swollen roots each with a terminal rootlet]; inflorescence cymose; flowers often monosymmetric, plane of symmetry oblique; T whorls not differentiated, (with basal retrorse projections), tube bent (straight, with mushroom-like corona), usually divided into two chambers by an annulus or constriction; connectives expanded; pollen porate; anthers adnate to the stigma by their connectives; placenta swollen, basal; fruit circumscissile at the top, T tube basally circumscissile.

1/16. West and East Africa.

Age. The age of crown-group Afrothismia is some (83-)63(-45) m.y. (Merckx et al. 2010a).

Evolution. Divergence & Distribution. Merckx and Bidartondo (2008) described what they called delayed co-speciation (the delay is 65-170 m.y.!) of a group of Afrothismia on/with their Glomus fungal symbiont, which seems a little odd.

Chemistry, Morphology, etc. The inflorescence of Afrothismia hydra appears to be cymose (Imhof & Sainge 2008). The plane of symmetry of the flowers is not easy to interpret, thus Afrothismia amietii apparently has two adjacent tepals much longer than the other four; see also the floral diagram in Maas-van de Kamer (2003). The seedling is also not easily describable (Imhof & Sainge 2008).

Some information is taken from Cheek (2003b), Maas-van de Kamer (2003) and Sainge et al. (2013 and references).

Classification. Most Thismiaceae s.l. are small to minute plants with small and inconspicuous flowers and are not often found in flower - and judging by the continuous stream of new species and even genera that are being described, extremely poorly known on even an alpha taxonomic level (see Franke 2007: comments on distributions).

[Burmanniaceae + Dioscoreaceae]: fruit winged, dehiscing laterally.

Age. Note that Janssen and Bremer (2004) included Geomitra (Thismiaceae) in their Burmanniaceae, so ages are comprimised; ca 72.8 m.y.a. is the age in Magallón et al. (2015).

BURMANNIACEAE Blume, nom. cons.   Back to Dioscoreales


Plants (echlorophyylous, myco-heterotrophic, associated with glomeromycotes - esp. some Burmannia), roots ± fleshy, or root tubers +; saponins?; (root hairs 0); root stele di- to pentarch, (not medullated); (stem with vascular bundles in a single ring); raphides 0; (stomata 0), (cuticular waxes as platelets transversely arranged in parallel series); leaves (two-ranked), usu. reduced to scales; (bracteoles lateral - Burmannia); (flowers monosymmetric), T (moderately large), valvate, outer larger (hardly), (3-lobed), (with marginal wings), enclosing the inner, (inner 0), T tube well developed and expanded below attachment of A; A 3, opposite inner T, connective broad, thecae widely separate, lateral, transversely dehiscent; pollen often tricellular, monoporate, sulcate or inaperturate, smooth [psilate]; anthers ± associated with style; placentation ± parietal, (axile - Burmannia), (paired glands [modified septal nectaries] at apex of each placenta), style long, (branches with paired long-filiform receptive[?] appendages), stigmas ± capitate (with a long, thin, apical process); ovules many/carpel, parietal tissue none, chalazal tissue quite conspicuous, persistent; (antipodal cells persist - Gymnosiphon); fruit transversely (Burmannia) or septicidally and/or loculicdally dehiscent (indehiscent), T persistent, (circumscissile at the middle of the tube); seeds minute; testal cells ± spiral, ± elongated, tegmic cells compressed, (tanniniferous); endosperm usu helobial, [cell formation in chalazal endosperm chamber first], thick-walled, with starch when young, ± 0 when mature, chalazal endosperm semi-haustorial?; embryo undifferentiated; ?seedling; n = 6 (7) 8, 12, 16, ... 88, much and high polyploidy, chromosomes 0.7-5.9 µm long.

9 [list]/95: Burmannia (63), Gymnosiphon (30). Largely tropical, esp. American (map: from Jonker 1938; Maas et al. 1986). [Photo - Flower, Campylosiphon, Hexapterella.]

Age. Crown-group Burmanniaceae are dated to ca 93 m.y. (Janssen & Bremer 2004: three genera sampled); dates in Merckx et al. (2008a), at 96.4 m.y., are similar, while Merckx et al. (2010a) suggests somewhat younger ages of (99-)75(-52) m. years.

Evolution. Divergence & Distribution. The rather wide geographical range of the family is thought to have been achieved largely by migration. Diversification rates were notably high in the Cretaceous and again in the Eocene (Merckx et al. 2008a).

There have been perhaps eight losses of chlorophyll in Burmanniaceae, assuming that loss is irreversible - if Burmannia, the only genus with some autotrophic species, is well embedded in the family (Merckx et al. 2006, 2008a, but c.f. Merckx et al. 2010a).

Bacterial/Fungal Associations. Glomeromycote fungi are involved in myco-heterotrophy here (Hynson & Bruns 2010). At least some of the chlorophyllous species of Burmannia can grow well under high light conditions without any association with arbuscular mycorrhizal glomeromycotes (Merckx et al. 2010b).

Chemistry, Morphology, etc. Although roots of Burmanniaceae are often described as lacking root hairs (e.g. Maas-van der Kamer 1998), as might befit their close association with fungi, root hairs are shown in Burmannia (von Guttenberg 1968). In echlorophyllous taxa vessels may be restricted to the roots, in others there are vessels in the leaves.

Rübsamen (1986) suggests that Burmnanniaceae lack nectaries. In a floral diagram (Eichler 1874) the stigmas are shown as being commissural.

Information is taken from Johow (1889 and references: anatomy, embryology), Aoyama et al. (2014 and references: chromosome numbers), and Dahlgren et al. (1985), Rübsamen (1986) and Maas-van der Kamer (1998), all general.

Phylogeny. Merckx et al. (2008a) provide a detailed phylogeny of the family that has quite good support. Relationships are [[Campylosiphon + Burmannia reducta (myco-heterotrophic)] [Dictyostega [[Hexapterella + Gymnosiphon] [Aptera + Burmannia]]]]. However, in a subsequent analysis Burmannia and [Campylosiphon + Burmannia reducta] were successively sister to the rest of the clade,

DIOSCOREACEAE R. Brown, nom. cons.   Back to Dioscoreales

Plant rhizomatous; lianes or vines, climbing by twining; (saponins 0), norditerpenes, flavonols +; in the stem small common and larger cauline bundles alternating, vascular bundles in two circles [not Trichopus], phloem internal to metaxylem; vessels in cauline bundles interrupted at nodes by tracheids, sieve tubes similarly interrupted; vessel elements with scalariform perforation plates, in petiole but not blade; stomatal ontogeny irregular; hairs glandular; (prophylls lateral); leaf ?insertion, with petiole and blade, blade vernation conduplicate, fine venation reticulate, (vein endings free), petiole pulvinate at both ends, leaf base not sheathing; inflorescences axillary; microsporogenesis simultaneous [tetrads tetrahedral]; carpels plicate, filled with secretion [Stenomeris, Trichopus?], compitum +, stigma wet; ovules with bi(endo-)stomal micropyle, (outer integument ³3 cells across); seeds winged; endotestal cells elongated, thick-walled, with crystals (0), exotegmen thickened; embryo broad, cotyledon ± lateral.

4[list]/870. Largely tropical. Three groups below.

Age. Crown-group Dioscoreaceae are dated to ca 80 m.y.a. (Janssen & Bremer 2004), (91.4-)77.2(-63.7) m.y.a. (Viruel et al. 2015: c.f. topology) or some (54-)26(-9) m.y. (Merckx et al. 2010a: c.f. topology).

1. Stenomeris Planchon


Underground stem thickened; ?chemistry; tannin cells 0; petiole bundles in arc; hairs with two-celled gland heads; T tube well developed [= torus] and expanded below attachment of A; anther connective much prolonged, joining stigmatic head, filaments flattened; microsporocytes markedly elongated; G half inferior, style bowling-pin shaped, styles 3, bifid, stigma punctate; seed coat with phlobaphene, tegmen collapsed; n = ?; seedling?

1/2. W. Malesia, not Java and islands east (Map: from Caddick & Wilkin 1998).

Synonymy: Stenomeridaceae J. Agardh

[Trichopus + Dioscorea]: anthers ± erect, adnate to base of the tepal tube; pollen with orbicules; ovules (1) 2/carpel, superposed, hypostase +; endosperm walls thickened.

2. Trichopus Gaertner


Stem ± climbing; flavones +; endodermoid layer fibrous; hairs with many transversely elongated cells in series in gland heads [Trichopus s. str.]; petiole bundles in arc; petiole with basal pulvinus only (Avetra s. str. - 0); stamens concave, surrounding style, anther connective (very) broad, connective prolonged apically; microsporocytes markedly elongated; pollen spinulose, (4-5-pantoporate); nectary 0; stigma bilobed; ovule (1/carpel), parietal tissue none, nucellar cap 0, lateral nucellar cells +, obturator +; fruit ± indehiscent, winged [samara], or semi-berry; seed not winged; endotesta not thickened, exotegmic cells elongated, with reticulate thickenings; endosperm ruminate, walls massively thickened, embryo minute; n = 14, chromosomes 1.5-2.7 µm long; germination?

1/2. Madagascar, Peninsula India, Ceylon, Peninsula Malaysia (Map: from ).

Age. The age of the crown group is some (42-)19(-2) m.y. (Merckx et al. 2010a), the stem age in the topology there is ca 90 m.y. ago.

Synonymy: Avetraceae Takhtajan, Trichopodaceae Hutchinson, nom. cons.

3. Dioscorea L.


(Plant with ± hypocotylar tubers), (herbaceous, not climbing); steroidal saponins, chelidonic acid, (mannans) +; (velamen +); (secondary thickening +); sieve tube plastids also with protein crystals and starch grains; petiole bundles in ring (not D. hemicrypta); (stomatal morphology odd); gland heads many celled; leaves two-ranked (opposite), blade (palmately compound), (midrib +), vernation flat to curved or conduplicate, (lobed), petiole bifacial [?all], (base with paired evascular processes); serial buds common; plant dioecious, inflorescences two or more together, (bracteoles lateral); flowers small, T free or connate, with a single trace; staminate flowers: A (1) 3 [opposite outer T], 6, (free), (connate); (pollen disulcate); pistillode +; carpellate flowers: staminodes +; stylar canal with secretion, stigma bilobed or not; ovule with outer integument 4-5 cells across, nucellar cap ca 3 cells across, supra-chalazalal tissue ± massive, hypostase +; (fruit baccate - Tamus; samaroid - Rajania); (seeds not winged); testa (multi-layered), (with much phlobaphene), exotegmen sclerotic [with branched protrusions of the cell walls], endotesta crystaliferous, exotegmen usu. thick-walled, endotegmen tanniniferous; endosperm usu. (very) thick-walled, embryo small to medium; n = (7, 8), 9, 10, 12 [up to 14-ploid], chromosomes 0.3-2.7 µm long (Epipetrum - chromosomes 1.9-2.9 µm long); cotyledon flattened and photosynthetic or not, second leaf a scale leaf.

1/350-?800. Largely tropical, also warm temperate, esp. seasonal (map: see Meusel et al. 1965; Fl. N. Am. 26: 2002; FloraBase 2004). [Photo - Inflorescence, Flower, Fruits.]

Age. Crown-group Dioscorea is dated to (49.1-)48.3(-47.6) m.y.a., in the Eocene (Viruel et al. 2015).

Synonymy: Tamaceae Berchtold & J. Presl, Tamnaceae J. Kickx f.

Evolution. Divergence & Distribution. For an evaluation of the fossil record, see Iles (2015) and Viruel et al. (2015).

For the biogeographic history of Diocorea, see Viruel et al. (2015: many dates); most diversification is Oligocene or later, and Madagascar was colonized from Asia. Schols et al. (2005b) outline pollen evolution, while Martí and Ortiz (1963) discuss aspects of cytological evolution.

Ecology & Physiology. Dioscoreaceae are a major clade of mostly rather herbaceous twining climbers that have some sort of rhizome/"tuber"/underground rootstock.

Economic Importance. In addition to being important sources of starch, tubers of Dioscorea spp. can contain very large amounts of steroidal saponins that provide the precursors of drugs like testosterone, progesterone, estrone, cortisone, and the like that are now synthesized artifically.

Chemistry, Morphology, etc. Dioscorea batatas has storage mannans in its vegetative tissues (Meier & Reid 1982). The cork in the tubers of Dioscorea is subepidermal and there may be secondary thickening; the exact morphological nature of the tuber is in some dispute. The vascular bundles in the stem may be arranged in a ring. For midrib anatomy, see Edeoga and Ikem (2001); three vascular bundles may enter the leaf (Periasamy & Muruganathan 1985). Leaflets of compound leaves are initiated in basipetal pairs and may be represent localised activity in the marginal blastozone (Periasamy & Muruganathan 1985: apparently not; Gunawardena & Dengler 2006), and the very apex of the leaflets differentiates early; there is a late-developing adaxial petiolar meristem, as in palms, Acorus and Araceae. Prophylls are at least sometimes lateral.

The flowers of Dioscorea are shown with the median member of the outer whorl in the adaxial position (Spichiger et al. 2004). The thickness of the parietal layer of the ovule is taken from Torshilova et al. (2003); Nagaraja Rao (1953) described it as being massive, and it could be interpreted as being 6-8 cells across. Seed coat anatomy would repay attention (Huber 1998). Huber (1998) noted that the mechanical layer of the seed coat was the exotegmen, but Nagaraja Rao (1953) drew the endotesta of Dioscorea oppositifolia as made up of small, heavily U-thickened and crystal-bearing cells.

The pollen of Avetra s. str. is pantoporate. Trichopus s. str. lacks the distinctive vascular bundles of Avetra (the two are a single genus here) and other Dioscoreaceae, and the position of its inflorescence is not clear. For the microsporogenesis of Avetra, see Caddick et al. (1998, 2000b).

For additional information, see Burkill (1960), Conran and Clifford (1985), Huber (1998), and Viruel et al. (2010), all general, for steroidal saponins, see Sautour et al. (2007) and references, for anatomy, Ayensu (1972) and Behnke (1990b: nodal anastomoses), for ovules, Igersheim et al. (2001), for ovules and seeds, Nagaraja Rao (1955: Trichopus), and Torshilova and Titova (2010), and for chromosome numbers, etc., Viruel et al. (2008).

Phylogeny. Much more detailed information is provided by Caddick et al. (2002b); the positions of Stenomeris and Trichopus are unclear (Viruel et al. 2015 and references). For the circumscription of Dioscorea and relationships within the genus, see Bharathan et al. (2001), Caddick et al. (2002a), Wilkin et al. (2005) and especially Viruel et al. (2015): Section Stenophora is sister to the rest of the genus, which has implications for genus apomorphies since that section is the only clade with tricolpate pollen.

Classification. The three genera have usually been placed in separate (albeit more or less closely related) families.

Dioscorea s. str. was divided into 22 sections by Huber (1998), and all were fully described, but, assuming their monophyly holds up, having 22 genera would not seem a desirable option.