EMBRYOPSIDA Pirani & Prado

Gametophyte dominant, independent, multicellular, thalloid, with single-celled apical meristem, showing gravitropism; rhizoids +, unicellular; acquisition of phenylalanine lysase [PAL], phenylpropanoid metabolism [lignans +, flavonoids + (absorbtion of UV radiation)], xyloglucans +; plant [protoplasm dessication tolerant], ectohydrous [free water outside plant physiologically important]; cuticle +; cell wall also with (1->3),(1->4)-ß-D-MLGs [Mixed-Linkage Glucans]; chloroplasts per cell, lacking pyrenoids; 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; blepharoplast, bicentriole pair develops de novo in spermatogenous cell, associated with basal bodies of cilia [= flagellum], multilayered structure [4 layers: L1, L4, tubules; L2, L3, short vertical lamellae] + 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 dependent on gametophyte, 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, with at least transient apical cell [?level], sporangium +, single, 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, initially laid down in association with several trilamellar layers [white-line centred layers, i.e. walls multilamellate]; nuclear genome size <1.4 pg, LEAFY gene present, ethylene involved in cell elongation; chloroplast genome with close association between trnLUAA and trnFGAA 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.

STOMATOPHYTES

Abscisic acid, ?D-methionine +; sporangium with tapetal layer, columella + [developing from endothecial cells], seta developing from basal meristem [between epibasal and hypobasal 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; spores trilete; polar transport of auxins and class 1 KNOX genes expressed in the sporangium alone; shoot meristem patterning gene families expressed; MIKC, MI*K*C* and class 1 and 2 KNOX genes, post-transcriptional editing of chloroplast genes; gain of three group II mitochondrial introns.

[Anthocerophyta + Polysporangiophyta]: archegonia embedded/sunken in the gametophyte; sporophyte long-lived, chlorophyllous; sporophyte-gametophyte junction interdigitate, sporophyte cells showing rhizoid-like behaviour.

POLYSPORANGIOPHYTA†

Sporophyte branched, branching apical, dichotomous; sporangia several, each opening independently; spore walls not multilamellate [?here].

EXTANT TRACHEOPHYTA / VASCULAR PLANTS

Photosynthetic red light response; plant homoiohydrous [water content of protoplasm relatively stable]; control of leaf hydration passive; (condensed or nonhydrolyzable tannins/proanthocyanidins +); sporophyte soon independent, dominant, with basipetal polar auxin transport; lignins +; vascular tissue +sieve cells + [nucleus degenerating], tracheids +, in both protoxylem and metaxylem, plant endohydrous [physiologically important free water inside plant]; endodermis +; 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, derived from periclinal divisions of several epidermal cells, wall multilayered [eusporangium]; columella 0; tapetum glandular; gametophytes exosporic, green, photosynthetic; basal body 350-550 nm long, stellate array in transition region initially joining microtubule triplets; placenta with single layer of transfer cells in both sporophytic and gametophytic generations, root lateral with respect to the longitudinal axis of the embryo [plant homorhizic].

[MONILOPHYTA + LIGNOPHYTA]

Sporophyte branching ± indeterminate; root apex multicellular, root cap +, lateral roots +, endogenous; endomycorrhizal associations + [with Glomeromycota]; G-type 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; 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.

LIGNOPHYTA†

Sporophyte 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].

SEED PLANTS†

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

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 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; 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 +; cork cambium superficial; 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.; axillary buds exogenous, (none; not associated with all leaves); prophylls two, lateral; leaves with petiole and lamina, development basipetal, blade simple; plant heterosporous, sporangia borne on sporophylls, sporophylls spiral; microsporophylls aggregated in indeterminate cones/strobili; grains monosulcate, aperture in ana- position [distal], primexine + [involved in exine pattern formation and deposition of sporopollenin from tapetum], exine and intine homogeneous; megasporangium indehiscent; ovules unitegmic, parietal tissue 2+ cells across, megaspore tetrad linear, functional megaspore single, chalazal, sporopollenin 0; gametophytes dependent on sporophyte, apical cell 0, rhizoids 0; male gametophyte development initially endosporic, tube developing from distal end of grain, gametes two, developing after pollination, with cell walls; female gametophyte initially syncytial, walls then surrounding individual nuclei; 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], 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?], 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; 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, hypodermis suberised and with Casparian strip [= 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; protogynous; 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], sporangium pairs dehiscing longitudinally by a common slit, ± embedded in the filament, walls with at least outer secondary parietal cells dividing, 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 lamellate only in the apertural regions, thin, compact; nectary 0; carpels present, superior, free, several, ascidiate, with postgenital occlusion by secretion, stylulus at most short [shorter than ovary], hollow, cavity not lined by distinct epidermal layer, stigma ± decurrent, carinal, stigma wet, 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, chalazal, lacking cuticle; female gametophyte 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, cilia 0, siphonogamy; double fertilization +, ovules aborting unless fertilized; P deciduous in fruit; mature seed much larger than ovule when fertilized, small [], dry [no sarcotesta], exotestal; 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; dark reversal Pfr → Pr; Arabidopsis-type telomeres [(TTTAGGG)n]; nuclear genome size <1.4 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 [extragynoecial compitum 0]; 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).

MONOCOTYLEDONS / MONOCOTYLEDONEAE / LILIANAE Takhtajan

Plant herbaceous, perennial, rhizomatous, growth sympodial; non-hydrolyzable tannins [(ent-)epicatechin-4] +, neolignans, benzylisoquinoline alkaloids 0, hemicelluloses as xylans; root apical meristem?; root 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 in root [uncommon] superficial; stele oligo- to polyarch, medullated [with prominent pith], lateral roots arise opposite phloem poles; primary thickening meristem +; vascular bundles in stem scattered, (amphivasal), vascular cambium 0 [bundles closed]; tension wood 0; vessel elements in root 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, 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. 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 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 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), 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: (....) denotes a feature common in the clade, exact status uncertain, [....] includes explanatory material. 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 (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. For a discussion on the relationships of Diocoreales, see the Petrosaviales page.

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.

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). In mitochondrial genome analyses Burmannia was sister to [Pandanales + Narthecium], while Thismia was in Dioscoreales (G. Petersen et al. 2006b). Some genes, at least, show accelerated evolution (G. Petersen et al. 2006b; Merckx et al. 2006). 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 and this was not the main point of their paper.

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...



Includes Burmanniaceae, Dioscoreaceae, Nartheciaceae, Taccaceae, Thismiaceae.

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

NARTHECIACEAE Bjurzon   Back to Dioscoreales

Nartheciaceae

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]]: 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).

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, which are still somewhat unclear. 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).

[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

Taccaceae

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

Thismiaceae

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 immediate relatives of Thismia americana, collected near Chicago just before WWI, 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.

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.

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

Pollination Biology. 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). There are no nectaries (Caddick et al. 2000).

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).

Age. The age of a clade including Afrothismia, Thismiaceae, and Trichopodaceae is some (109-)95(-79) m.y. (Merckx et al. 2010a).

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).

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

Burmanniaceae

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) 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 and expanded below attachment of A; A connective much prolonged, joining stigmatic head; microsporocytes markedly elongated; G half inferior, style bowling-pin shaped, stigma punctate; seed coat with phlobaphene, tegmen collapsed; n = ?; seedling?

1/2. W. Malesia.

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

<i>Trichopus</i>

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.

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..

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

3. Dioscorea L.

<i>Dioscorea</i>

(Plant with ± hypocotylar tubers); 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; 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.]

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

Evolution. Divergence & Distribution. For pollen evolution, see Schols et al. (2005b).

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). For the circumscription of Dioscorea and relationships within the genus, see Bharathan et al. (2001), Caddick et al. (2002a), and Wilkin et al. (2005). Section Stenophora is sister to the rest of the genus, which has implications for genus apomorphies; that section is the only clde 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.