Herbs twining or woody vines, rarely erect, small herbs. Rootstock rhizomatous or tuberous. Stem twining to left or right, pubescent or glabrous, sometimes prickly. Leaves alternate or opposite, petiolate, simple or palmately compound, basal veins 3--13, interstitial veins reticulate; leaflets of palmately compound leaves often ovate or lanceolate. Flowers usually unisexual (when plants dioecious, rarely monoecious), sometimes bisexual, solitary, clustered, or in cymules, these in a spike, raceme, or thyrse, these sometimes grouped into panicles. Male flowers: perianth lobes 6, in 2 whorls, basally connate or free; stamens 6, sometimes 3 reduced to staminodes or absent, inserted on perianth or receptacle; ovary rudimentary or absent. Female flowers: similar to male ones; staminodes 3, 6, or absent; ovary inferior, 3-loculed, ovules usually 2 per locule (more than 2 in a few small genera), placentation axile; styles 3, free. Fruit a capsule, berry, or samara. Seeds with a membranous wing or not; endosperm present; embryo small.
Vines [herbs], geophytes perennating from fleshy rhizomes or tubers. Stems renewed annually or occasionally persisting for more than a single growing season, twining-climbing, procumbent, or seldom erect; vascular bundles commonly arranged in 2 concentric circles. Leaves alternate, opposite, or whorled at basal nodes; blades simple or occasionally digitately compound, typically cordate, reticulate-veined; principal veins arcuate, ascending from base to apex; margins entire or sometimes palmately lobed; stipules absent, rarely stipulate, never with tendrils; petioles typically with distinct pulvinus at each end. Inflorescences axillary, solitary or in fascicles of 2–7, branched and paniculate, or unbranched and spicate or racemose, ultimate inflorescence unit cymose, sessile or pedunculate, often reduced to 1 flower. Flowers unisexual, staminate and pistillate flowers on different plants, rarely staminate and pistillate flowers on same plant or bisexual; perianth epigynous, rotate, campanulate, or funnelform; tepals 6, petaloid, in 2 similar whorls of 3, connate at least basally; stamens 6, in 2 whorls of 3, inner whorl sometimes sterile or absent; filaments distinct [connate basally or completely into tube], inserted at base of perianth; anthers 2-locular, tetrasporangiate, dehiscing by longitudinal slits, thecae distinct or sometimes connate, rarely divided by a long-appendiculate connective; pistils 3; ovary (1–)3-locular, ovules anatropous, 2(–many) per locule, placentation axile or parietal; styles 3, ± distinct. Fruits capsular, rarely baccate or samarate, dehiscence loculicidal. Seeds flattened or globose, winged or unwinged, embryo small, endosperm copious, starchless.
Fls trimerous, regular, epigynous, unisexual (the plants dioecious) or seldom perfect; tep in 2 similar sets of 3, petaloid or chaffy, mostly connate at base; both septal and tepalar nectaries commonly present; stamens usually 6 in 2 cycles, or the inner set staminodial or obsolete; filaments borne on the base of the perianth; ovary inferior, trilocular with axile placentation; styles or stigmas distinct; ovules mostly 2 per locule; fr mostly capsular; embryo small, with subterminal plumule and broad, lateral cotyledon, axially embedded in the very hard endosperm; twining-climbing or seldom erect herbs from a fleshy-thickened rhizome, or much more often from a large basal "tuber" of complex structure; lvs alternate or sometimes opposite or whorled, generally with a distinct blade and petiole, the blade broad, entire or less often palmately lobed or cleft or even compound, commonly with 3–13 "parallel," curved-convergent main veins and a network of smaller veins; vessels generally present in all vegetative organs; fls small, variously in racemes, spikes, or panicles. 6 genera, all but Dioscorea small.
Female inflorescences: spikes looser, longer than the male ones, solitary or paired or sometimes more numerous, in the leaf axils
Stamens 6, or 3 with or without 3 staminodes
Filaments free or shortly connate; anthers 2-locular
Bracteole generally present in oblique position
Flowers placed singly along the axis or in short few-flowered lateral cymules
Male flowers sessile or shortly pedicelled
Flowers small, inconspicuous, actinomorphic
Inflorescence spicate, racemose or paniculate
Plants dioecious; exceptionally on the same inflorescence are clustered male and female flowers6
Leaves moving, following the conditions of lighting
Petiole generally twisted and sometimes jointed at the base or with more or less leathery auricles
Leaves alternate or opposite (sometimes both on the same plant), often cordate, entire or lobulate, more or less digitately nerved or palmately compound, acumen often large and glandulose
Male inflorescences: spikes generally several in the leaf axils, sometimes clustered in racemes or compound panicles
Perianth campanulate or spreading, 6-lobed, lobes 2-seriate, often connate at the base
Fruits (in the tropical African species) 3-valved capsules
Placentation axile: 2 anatropous ovules in each loculus
Climbers (at least the West African species), spiny or not, annual or perennial with tubers annually renewed or perennial
Tubers toxic or edible, often protected by thorny roots
Aerial tubers (bulbils) present or absent
Perianth segments biseriate, usually united basally
Leaves alternate or opposite, often ovate-cordate, but sometimes with 3–7 digitate leaflets
Flowers bisexual or unisexual, the latter usually dioecious
Twining herbs with annual stems arising from tubers or rhizomes, rarely stems self-supporting
Ovary inferior, rarely semi-inferior or superior, (1)3-locular
Al-Shehbaz, I. A. and B. G. Schubert. 1989. The Dioscoreaceae in the southeastern United States. J. Arnold Arbor. 70: 57–95. Ayensu, E. S. 1972. Dioscoreales. In: C. R. Metcalfe, ed. 1960+. Anatomy of the Monocotyledons. 8+ vols. Oxford. Vol. 6. Bouman, F. 1995. Seed structure and systematics in Dioscoreales. In: P. J. Rudall et al., eds. 1995. Monocotyledons: Systematics and Evolution. 2 vols. Kew. Vol. 1, pp. 139–156. Burkill, I. H. 1960. The organography and evolution of the Dioscoreaceae, the family of yams. J. Linn. Soc., Bot. 56: 319–407. Caddick, L. R. et al. 1998. Microsporogenesis and pollen morphology in Dioscoreales and allied taxa. Grana 37: 321–336. Caddick, L. R. et al. 2000. Yams and their allies: Systematics of Dioscoreales. In: K. L. Wilson and D. A. Morrison, eds. 2000. Monocots: Systematics and Evolution. Melbourne. Pp. 475–487. Coursey, D. G. 1967. Yams: An Account of the Nature, Origins, Cultivation and Utilisation of the Useful Members of the Dioscoreaceae. London. Huber, H. 1998. Dioscoreaceae. In: K. Kubitzky et al., eds. 1990+. The Families and Genera of Vascular Plants. 4+ vols. Berlin etc. Vol. 3, pp. 216–235. Knuth, R. 1924. Dioscoreaceae. In: H. G. A. Engler, ed. 1900–1953. Das Pflanzenreich…. 107 vols. Berlin. Vol. 87 [IV,43], pp. 1–387. Queva, C. 1894. Recherches sur l’Anatomie de l’Appareil Vegetatif des Taccacées & Dioscorées. Lille.