EXTANT SEED PLANTS

Plant woody, 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 rich in guaiacyl units; true roots present, apex multicellular, xylem exarch, branching endogenous; arbuscular mycorrhizae +; shoot apical meristem multicellular, interface specific plasmodesmatal network; stem with ectophloic eustele, endodermis 0, xylem endarch, branching exogenous; vascular tissue in t.s. discontinuous by interfascicular regions; vascular cambium + [xylem ("wood") differentiating internally, phloem externally]; wood homoxylous, tracheids +; tracheid/tracheid pits circular, bordered; sieve tube/cell plastids with starch grains; phloem fibers +; stem cork cambium superficial, root cork cambium deep seated; nodes ?; stomata ?; leaf vascular bundles collateral; leaves megaphyllous [determinancy evolved first, then ad/abaxial symmetry], spiral, simple, axillary buds +[?], prophylls [including bracteoles] two, lateral, veins -5 mm/mm² [mean for all non-angiosperms 1.8]; plant heterosporous, sporangia eusporangiate, on sporophylls, sporophylls aggregated in indeterminate cones/strobili; true pollen [microspores, i.e. no distal pore for release of gametes] +, grains mono[ana]sulcate, exine and intine homogeneous; ovules unitegmic, crassinucellate, megaspore tetrad tetrahedral, only one megaspore develops, megasporangium indehiscent; male gametophyte development first endo- then exosporic, tube developing from distal end of grain, to ca 2 mm from receptive surface to egg, gametes two, with cell walls, with many flagellae; female gametophyte endosporic, initially syncytial, walls then surrounding individual nuclei; seeds "large", first cell wall of zygote transverse, embryo straight, endoscopic [suspensor +], short-minute, with morphological dormancy, white, cotyledons 2; plastid transmission maternal; two copies of LEAFY gene, PHY gene duplication [N/O//A/C and P//BE lines], mitochondrial nad1 intron 2 and coxIIi3 intron present.

MAGNOLIOPHYTA

Plant woody, evergreen; lignans, O-methyl flavonols, dihydroflavonols, triterpenoid oleanane, non-hydrolysable tannins, quercetin and/or kaempferol +, apigenin and/or luteolin scattered, [cyanogenesis in ANITA grade?], lignins derived from both coniferyl and sinapyl alcohols, containing syringaldehyde [in positive Maüle reaction, syringyl:guaiacyl ratio less than 2-2.5:1], and hemicelluloses as xyloglucans; root apical meristem intermediate-open; root vascular tissue oligarch [di- to pentarch], lateral roots arise opposite or immediately to the side of [when diarch] xylem poles; origin of epidermis with no clear pattern [probably from inner layer of root cap], trichoblasts [differentiated root hair-forming cells] 0; stem with 2-layered tunica-corpus construction; wood fibers and wood parenchyma +; reaction wood ?, with gelatinous fibres; starch grains simple; primary cell wall mostly with pectic polysaccharides; tracheids +; sieve tubes eunucleate, with a sieve plate and cytoplasm with P-proteins, companion cells from same mother cell that gave rise to the sieve tube; nodes unilacunar [1:?]; stomata with ends of guard cells level with pore, paracytic, outer stomatal ledges producing vestibule; leaves with petiole and lamina [the latter formed from the primordial leaf apex], development of venation acropetal, 2ndary veins pinnate, fine venation reticulate, veins (1.7-)4.1(-5.7) mm/mm², endings free; flowers perfect, polysymmetric, parts spiral [esp. the A], free, development in general centripetal, numbers unstable; P not sharply differentiated, outer members not enclosing the rest of the bud, smaller than inner members; A many, with a single trace, introrse, filaments stout, anther ± embedded in the filament, tetrasporangiate, dithecal, with at least outer secondary parietal cells dividing, each theca dehiscing longitudinally by action of hypodermal endothecium, endothecial cells elongated at right angles to long axis of anther; tapetum glandular, binucleate; microspore mother cells in a block, microsporogenesis successive, walls developing by centripetal furrowing; pollen subspherical, binucleate at dispersal, trinucleate eventually, tectum continuous or microperforate, ektexine columellar, endexine thin, compact, lamellate only in the apertural regions; nectary 0; G free, several, ascidiate, with postgenital occlusion by secretion, few [?1] ovules/carpel, ovules marginal, anatropous, bitegmic, [outer integument often largely subdermal in origin, inner integument dermal], micropyle endostomal, integuments 2-3 cells thick, nucellus at apex of ovule 1-3 cells thick, megasporocyte single, megaspore lacking sporopollenin and cuticle, chalazal, female gametophyte four-celled [one-modular, nucleus of egg cell sister to one of the polar nuclei], stylulus short, hollow, cavity not lined by distinct epidermal layer, stigma ± decurrent, dry [not secretory]; P deciduous in fruit; seed exotestal; pollen germinating in less than 3 hours, siphonogamy, tube elongated, growing at 80-600 µm/hour, with callose plugs and callose-based walls, penetrating between cells, penetration of ovules within ca 18 hours, distance to first ovule 1.1.-2.1 mm; tube moves between nucellar cells, double fertilisation +, endosperm diploid, cellular [first division oblique, micropylar end initially with a single large cell, chalazal end more actively dividing], copious, oily and/or proteinaceous, embryo cellular ab initio, minute; germination hypogeal, seedlings/young plants sympodial; Arabidopsis-type telomeres [(TTTAGGG)_n]; whole genome duplication, 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 PHYA + C/PHYB + E gene pairs.

Evolution. Possible apomorphies for flowering plants are in bold. Note that the actual level to which many of these features, particularly the more cryptic ones, should be assigned is unclear, because some taxa basal to the [magnoliid + monocot + eudicot] group have been surprisingly little studied, there is considerable variation between families in particular for several of these characters, and also because details of relationships among gymnosperms will affect the level at which some of these characters are pegged. For example, if reticulate-perforate pollen is optimized to the next node on the tree (see Friis et al. 2009 for a discussion), it effectively makes the pollen morphology of the common ancestor of all angiosperms ambiguous... For other features such a a nucellus only one (Nymphaeales) to three cells thick above the embryo sac and a stylar canal lacking an epidermal layer, although plesiomorphous for basal grade angiosperms (Williams 2009), where on the tree a thicker nucellus and a stylar epidermal layer are acquired has not yet been indicated.

NYMPHAEALES [AUSTROBAILEYALES [[CHLORANTHALES + MAGNOLIIDS] [MONOCOTS [CERATOPHYLLALES + EUDICOTS]]]]: vessels + [one position], elements with elongated scalariform perforation plates; axial parenchyma diffuse or diffuse-in-aggregate; tectum reticulate-perforate [here?]; ?genome duplication; "DEAER" motif in AP3 and PI genes lost, gaps in these genes.

PETROSAVIALES [[DIOSCOREALES + PANDANALES] [LILIALES [ASPARAGALES + COMMELINIDS]]]: Starch grains simple, amylophobic; stomata anomocytic, (cuticular waxes as parallel platelets); colleters 0; endosperm nuclear.

[[DIOSCOREALES + PANDANALES] [LILIALES [ASPARAGALES + COMMELINIDS]]]]: nucellar cap 0.

Evolution. Larvae, and sometimes also adults, of the Chrysomelidae-Criocerinae are scattered through this clade, being perhaps especially common on commelinids (e.g. Schmitt 1988).

[DIOSCOREALES + PANDANALES]: ?

Evolution. The first split in this clade is dated to ca 124 million years before present (Janssen & Bremer 2004) or ca 134.4 and 119.6 million years for relaxed and constrained penalized likelihood datings respectively (Magallón & Castillo 2009).

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

DIOSCOREALES J. D. Hooker  Main Tree, Synapomorphies.

Steroidal saponins +; vascular bundles in rings; vessels also in stem and leaf; flowers or inflorescence with glandular hairs; style at most short, style branches well developed; T persistent in fruit; endotegmen tanniniferous; embryo at most short. - 5 families, 21 genera, 1037 species.

Evolution. Stem-group Dioscoreales are dated to ca 124 million years before present, crown group Dioscoreales to ca 123 million years before present (Janssen & Bremer 2004); comparable figures in Magallón and Castillo (2009) are ca 134.4 (stem) and 101.4 (crown) and 119.6 (stem) and 115.1 (crown) million years for relaxed and constrained penalized likelihood datings respectively.

Myco-heterotrophy is likely to have evolved several times in the clade (Merckx et al. 2006, 2009), and Glomeromycota seem to be the fungi involved (Franke et al. 2006).

Chemistry, Morphology, etc. The tepals of Narthecium ossifragum have three traces, those of Dioscoreaceae are single trace.

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 Dioscoreales, albeit sometimes with only moderate support (e.g. Chase et al. 2000a; Caddick et al. 2002a; Tamura et al. 2004 [97% bootstrap, good sampling of Nartheciaceae, but otherwise only Dioscorea and Tacca and three members of Pandanales]; Janssen & Bremer 2004 [one gene, but 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.

Recent studies by Merckx et al. (2006, 2009a) strongly suggest that the myco-heterotrophic members of Dioscoreales do not come close to forming a clade. Merckx et al. (2006), using mitochondrial and nuclear genes and with very good sampling of these sapromyctotrophic taxa, but no outgroup to Dioscoreales, found substantially different relationships within Dioscoreales from those depicted in the tree given by Caddick et al. (2002a: see also /APweb/ version 7 [November] and earlier which followed the classification in Caddick et al. 2002b). However, as Merckx et al. (2006) note, the relationships found by Caddick et al. (2002a) were dominated by data from the chloroplast, and since Burmanniaceae s.l. are largely mycoheterotrophic they have much diverged plastid sequences. These myco-heterotrophic taxa have indeed caused problems. Geomitra, apparently Thismiaceae without any doubt, nevertheless came out with some Burmanniaceae in some analyses (Caddick et al. 2002a), while in mitochondrial genome analyses members of Burmanniaceae s.l. were separated, Burmannia being sister to [Pandanales + Narthecium], while Thismia was in Dioscoreales (G. Petersen et al. 2006b). In the study of the relationships of Burmanniales by Neyland (2002), Burmannioideae were well supported, Thismia was sister, but with less support, but Burmanniaceae s.l. did not link with other Dioscoreales. Some genes, at least, show accelerated evolution (G. Petersen et al. 2006b; Merckx et al. 2006). Note that the positions of Dioscoreaceae and [Taccaceae + Thismiaceae] are reversed in the tree presented by Yokoyama et al. (2008), but with little support. The situation promises to become 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]. More work is needed to elucidate the basic phylogenetic structure of Dioscoreales.

Classification. The classification here follows that suggested by Merckx et al. (2006), although for relationships within Dioscoreaceae in the sense recognized here, see Caddick et al. (2002a, b).

Includes Burmanniaceae, Dioscoreaceae, Nartheciaceae, Taccaceae, Thismiaceae.

Synonymy: Burmanniales Heinze, Nartheciales Reveal & Zomlefer, Taccales Dumortier, Tamales Dumortier - Dioscoreanae Reveal & Doweld - Burmanniidae Heintze

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); raphides 0, usu. druses +, but prismatic crystals in bundle sheath; cuticular wax with parallel platelets; leaves spiral (equitant and 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, many (unitegmic) ana-campylotropous ovules/carpel, integuments lack cuticle [check], integumentary obturator + [Aletris], (stylulus long); seeds obliquely stacked (with appendages), shape various; tegmen flattened, persistent; endosperm helobial, [how much?, walls?]; n = (12) 13 (21, 22), 0.7-1.4 µm long; cotyledon bifacial or not, collar ?rhizoids.

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

• Most Nartheciaceae can be recognised by their isobifacial, equitant leaves and racemose, bracteate inflorescences with small, rather undistinguished monocot flowers that have partly free tepals that persist under the capsular fruit.

Evolution. Stem-group Nartheciaceae are dated to ca 123 million years before present, the crown group diverged ca 76 million years before present (Janssen & Bremer 2004).

Chemistry, Morphology, etc. Metanarthecium may have abaxial prophylls. There are reports that the ovules of Narthecium may be unitegmic (Remizowa et al. 2006 for references); in general, variation in floral development is considerable (Remizowa et al. 2008a).

For inflorescence and flower development, see Remizowa et al. (2006a, b), leaf anatomy, see Luque Arias et al. (2006), for pollen morphology, see Merckx et al. (2008b).

Phylogeny. Aletris, with its spiral and bifacial leaves, was sister to the three other genera examined (Tamura et al. 2004). Although these latter all have equitant, isobifacial leaves, that set of features, although the "common" character of the family, are unlikely to be an apomorphy for it.

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, Dahgren et al. (1985) placed them - along with representatives of Tofieldiaceae and Petrosaviaceae - in Melianthaceae (Liliales here), and Tamura (1998) placed them in Petrosaviaceae (Petrosaviales), along with genera here placed in Tofieldiaceae (Alismatales).

[Taccaceae + Thismiaceae] [Burmanniaceae + Dioscoreaceae]: stem with endodermis; T tube broad, ovary inferior, style short, branched, stigmas ± bilobed; fruit winged; exo- and endotesta tanniniferous.

Chemistry, Morphology, etc. Cell walls in the endosperm are thick, but are usually not pitted.

Taccaceae + Thismiaceae: A inflexed in flower [more or less hanging down and the anthers then seemingly extrorse], septal nectaries 0, placentation parietal.

TACCACEAE Berchtold & J. Presl, nom. cons.   Back to Dioscoreales

Stem tubers or rhizomes, secondary thickening(?); hairs with multicellular stalk row, a head, and then another cell row; petiole bundles in ring; leaves basal, margins entire or ± deeply pinnately or palmately divided, basally somewhat sheathing; inflorescence scapose, umbellate, of groups of cincinni, bracteate, long filiform bracts among the flowers; flowers medium in size, T tube short, A adnate at base, connective broad, not prolonged, forming a hood, anther wall formation "dicot type", stylar canal with secretion, stigma often petaloid; fruit a berry (capsule); seed ribbed, (endotesta crystalliferous), exotegmen ± thick-walled and elongated, esp. radially; embryo short to minute; n = 15; cotyledon ?bifacial, sheath lobed.

1/12. Pantropical, esp. Malesian-Pacific (map: from Drenth 1976). [Photos - Collection.]

• Taccaceae are herbs with long-petiolate leaves and a scapose, umbellate inflorescence in which long, filiform bracts are mixed in with the flowers. These are medium-sized, dark purple, and with inferior ovaries; the ovary placentation is parietal.

Evolution. The dark purple flowers 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, where two large and conspicuous white inflorescence bracts are held above the dark purple flowers and dangling bracts.

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

THISMIACEAE Agardh nom. cons.   Back to Dioscoreales

Plants mycoheterotrophic; saponins?; roots coralloid, vermiform ot tuberous, root hairs 0; stomata 0; leaves reduced to scales; flowers often single or in (branched) inflorescences, (monosymmetrical); T whorls often differentiated, both well developed, tube well developed, (apex of inner T lobes connate into a "mitre", contorted - Thismia, etc., outer T lobes then small or even 0), annulus or ring of short projections at mouth of tube +/0 [Haplothismia], A inserted at apex of tube, with intrastaminal lobes, (3, opposite inner T, Oxygyne), filaments variously ornamented, connective well developed, prolonged, (adnate to stigma), tapetal cells uninuclear, pollen porate, many tenuinucellate ovules/carpel, placentae as separate columns, often ± free, ± apical, (embryo sac bisporic), style connate early in ontogeny, stigma capitate/(ob)pyramidal; fruit not winged, irregularly dehiscent, T basally circumscissile; seeds minute, testal cells ± spiral, tegmic cells compressed; endosperm helobial (cellular - Thismia), thick-walled, with starch when young, not persisting; embryo minute, undifferentiated; seedling not easily describable [Imhof & Sainge 2008]; n = 6-9, chromosomes 1-4 µm long.

5/45: Thismia (35), Afrothismia (12). 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). [Photo - Thismia.]

• Thismiaceae are small, echlorophyllous herbs that may be recognised by their distinctive flowers. These are usually single and terminal, the outer tepals are often rather different from the inner, there is usually an annulus surrounding the broad throat, the stamens are inflexed, or at least form a cone in the centre of the flower, and the ovary is inferior. The flowers can look like a little jelly fish, or a group of small fungi, or... Some are very small, and several new species and even genera are being described.

Evolution. The immediate relatives of Thismia americana, once collected near Chicago but apparently now extinct, are Antipodean (Thorne 1992). Merckx and Bidartondo (2008) describe what they called delayed cospeciation (the delay is 65-170 million years) of a group of Afrothismia on/with their Glomus fungal symbiont.

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).

Chemistry, Morphology, etc. The stems may be endogeneous in origin (Pfeiffer 1914). The inflorescence of Afrothismia hydra seems to be cymose (Imhof & Sainge 2008), although inflorescences of species of Thismia described by Larsen and Averyanov (2007) are not so easy to interpret. Complicating the issue is the nature of the organism: Burmanniaceae s.l. are often very small-flowered and inconspicuous, not often found in flower, and judging by the continuous stream of new species and genera that are being described, extremely poorly known on even an alpha taxonomic level (see Franke 2007 for intering comments on distributions).

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. As mentioned above, Merckx et al. (2009a) suggest that Thismiaceae s. str. may be paraphyletic, Afrothismia being sister to [Taccaceae + rest of Thismiaceae]. Relationships between other Thismiaceae are unclear, although Thismia itself may be paraphyletic (Merckx et al. 2009a).

Afrothismia is indeed a distinctive genus: Rhizome with aggregations of small bulbils, swollen roots each with a terminal rootlet, along its length; inflorescence cymose; flowers often monosymmetric, plane of symmetry?; tepal tube is usually divided into two chambers by an annulus or constriction, lobes not differentiated, often with basal retrorse projections; stamens are strongly incurved, anthers being adnate to the stigma by their expanded connectives (e.g. Cheek 2003); fruit a pyxidium, i.e. it is circumscissile at the top, T basally circumscissile. The plane of symmetry of the flowers is not easy to interpret, thus Afrothismia amietii has two adjacent tepals much longer than the other four; see also the floral diagram in Maas-van de Kamer (2003). Some information is taken from Cheek (2003) and Maas-van de Kamer (2003).

Classification. Most species are very small and have inconspicuous flowers, many are uncommon, and new species and even genera are steadily being described.

Burmanniaceae + Dioscoreaceae: ?

BURMANNIACEAE Blume, nom. cons.   Back to Dioscoreales

Plants mycoheterotrophic, (chlorophyllous - some Burmannia), roots ± fleshy, or root tubers +; saponins?; raphides 0; cuticular waxes with parallel platelets; leaves (two-ranked), usu. reduced to scales; (bracteoles lateral - Burmannia); (flowers monosymmetric), T (moderately large), valvate, outer larger, enclosing the inner, (lobed), (inner 0), tube [hypanthium] well developed; A 3, opposite inner T, thecae separated by connective, transversely dehiscent; pollen monoporate, sulcate or inaperturate, psilate, placentation ± parietal, (axile - Burmannia), many tenuinucellate ovules/carpel, (paired glands [modified septal nectaries] at apex of each placenta), style long, (branches with paired long-filiform receptive[?] appendages), stigma infundibular; fruit winged, transversely (Burmannia) or longitudinally dehiscing down the sides, or indehiscent, T persistent, (circumscissile at the middle of the tube); seeds minute, testal cells ± spiral, tegmic cells compressed; endosperm usu helobial, thick-walled, with starch when young, not persisting, [cell formation in chalazal endosperm chamber first]; embryo minute, undifferentiated; ?seedling; n = 6 (7) 8, much and high polyploidy, chromosomes 1-2 µm long.

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

• Burmanniaceae are rather small, often echlorophyllous herbs that may be recognised by their radially symmetrical, often more or less blue flowers with an inferior ovary that is often winged. The outer tepals are rather larger than the inner and there are only three stamens, borne opposite the inner tepals.

Evolution. Stem group Burmanniaceae are dated to ca 116 million years before present, crown group Burmanniaceae to ca 93 million years before present (Janssen & Bremer 2004: three genera sampled; similar dates in Merckx et al. 2008a). It has been suggested that the rather wide geographical range of the family has been achieved largely by migration, with diversification rates being notably high in the Eocene (Merckx et al. 2008a).

There has been more than one loss of chlorophyll in Burmanniaceae (Merckx et al. 2006).

Chemistry, Morphology, etc. The root stele is di- to pentarch and may lack a pith. Although roots of Burmanniaceae are often described as lacking root hairs, 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. The floral diagram in Eichler (1874) suggests that the stigmas are commissural.

Information is taken from Dahlgren et al. (1985), Rübsamen (1986) and Maas-van der Kamer (1999).

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

Climbers; 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 lamina; stomatal ontogeny irregular; hairs glandular; (prophylls lateral); leaves not sheathing, conduplicate, petiolate, with midrib and reticulate fine venation, (vein endings free), petiole pulvinate at both ends; inflorescences axillary; microsporogenesis simultaneous [tetrads tetrahedral]; carpels plicate, filled with secretion [Stenomeris, Trichopus?], compitum +, stigma wet; ovules (outer integument ³3 cells across), micropyle bistomal; seeds winged; endotestal cells elongated, thick-walled, with crystals (0), exotegmen thickened; endosperm nuclear, embryo broad.

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

1. Stenomeris

Underground stem thickened; tannin cells 0; petiole bundles in arc; hairs with two-celled gland heads; T tube [hypanthium] well developed; A inserted in mouth of T tube, reflexed; microsporocytes markedly elongated; G half inferior; tegmen collapsed; n = ?; seedling?

1/2. W. Malesia.

Synonymy: Stenomeridaceae J. Agardh

Trichopus + Dioscorea: plant monoecious to dioecious; 2 ovules/carpel; endosperm walls thickened.

2. Trichopus

Rhizomatous, 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); staminate flowers: anther connective broad; microsporocytes markedly elongated; pollen spinulose; carpellate flowers: ovules tenuinucellate, ?micropyle; fruit dry, ± indehiscent, winged [samara], or semi-berry; seed not winged, with irregular intrusions; endotesta not thickened, exotegmic cells elongated, with reticulate thickenings; endosperm ruminate; embryo minute; n = 14, chromosomes 1.5-2.7 µm long; germination?

1/2. Madagascar, Peninsula India, Ceylon, Peninsula Malaysia.

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

3. Dioscorea

Tubers +; chelidonic acid +; (secondary thickening +); sieve tube plastids also with starch grains; petiole bundles in ring (not D. hemicrypta!); gland heads many celled; leaves two-ranked, also opposite, flat to curved or conduplicate, (lobed; palmately compound; midrib +; paired basal evascular processes); plant dioecious, inflorescences two or more together, (bracteoles lateral); flowers small, T free or connate, with a single trace; staminate flowers: A (1) 3 (6), adnate to T or free (connate), (extrorse; prolonged connective); pollen often disulcate; pistillode +; carpellate flowers: staminodes +; stylar canal with secretion, stigma usually not bilobed; fruit capsular, baccate, or samaroid; (seeds not winged), ?phlobaphene, testa multi-layered, endotesta crystalliferous, exotegmen sclerotic; embryo small; n = (7, 8), 9, 10 [up to 14-ploid], 12, chromosomes 0.3-2.7 µm long (Epipetrum - chromosomes 1.9-2.9 µm long); cotyledon flattened and photosynthetic or not.

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 Martynov, Tamnaceae J. Kickx f.

• Dioscoreaceae are more or less herbaceous plants with often rather small and not very brightly coloured flowers; the ovary is inferior and often ridged or winged in fruit. Many are climbers recognised by their spiral, petiolate leaves with pulvini at both ends of the petiole; the leaf base does not surround the stem. The lamina has several strong longitudinal veins and transverse secondary veins, or reticulate fine venation.

Evolution. Stem group Dioscoreaceae are dated to ca 116 million years before present, crown group Dioscoreaceae to ca 80 million years before present (Janssen & Bremer 2004).

Chemistry, Morphology, etc. 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). Compound leaves in this group result from the localised activity of the marginal blastozone (Gunawardena & Dengler 2006). Lateral prophylls occur at least sometimes. The flowers of Dioscorea are shown with the median member of the outer whorl in the adaxial position (Spichiger et al. 2004). For chromosome numbers, etc., see Viruel et al. (2008).

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 microsporogenesis of Avetra, see Caddick et al. (1998, 2000b), and for general information, see Conran and Clifford (1986) and Huber (1998).

For general information, see Burkill (1960), Conran and Clifford (1985), Huber (1998), and Viruel et al. (2010), for anatomical information, Ayensu (1972) and Behnke (1990b: nodal anastomoses, see sieve tube interruption above), for ovules, Igersheim et al. (2001), and for pollen evolution, Schols et al. (2005b).

Phylogeny. Descriptions are provided for all three genera since they have usually been placed in separate (albeit more or less closely related) families. 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).

Classification. The phylogeny of Dioscorea and genera immediately related to it suggests that taking a broad view of Dioscorea is desirable - see Caddick et al. (2002a, b) and Wilkin et al. (2005). Dioscorea s. str. was divided into 22 sections by Huber (1998), and all were fully described, but having 22 genera would not seem a desirable option.