Vines, typically herbaceous; rhizomatous or tuberous. Stems twining clockwise or counter-clockwise, branched or not, smooth or winged, polygonal or terete, glabrous or sometimes bearing prickles; some species producing bulbils (branches modified into small aerial tubers) in leaf axils. Leaves initiated in spiral pattern, modified in some species and appearing opposite or whorled. Inflorescences: staminate nodes at ultimate flowering spicate or paniculate, individual flowers grouped in bracteolate cymes of (1–)3(–8) sessile or pedicellate flowers; pistillate solitary or rarely fasciculate, 1 flower per node, sessile or subsessile, unbranched, subtended by pair of minute bracts. Flowers unisexual, staminate and pistillate flowers on different plants or rarely staminate and pistillate flowers on same plant; tepals glandular or not, 1–3(–30) mm; staminate flowers with filaments distinct (connate basally only in D. polystachya), gynoecium rudimentary; pistillate flowers with staminodes present or sometimes absent, ovary inferior, usually 2–3 times length of perianth, style 3-branched, branches 2-fid; pedicel, when present, not articulate. Fruits capsular, 3-winged, dehiscence loculicidal, perianth and styles generally persistent. Seeds (1–)2 per locule, compressed, usually winged unilaterally, bilaterally, or with wings expanded circumferentially; tannins, saponins present. x = 9, 10.
Species ca. 600 (6 in the flora).
The most comprehensive treatment to date (R. Knuth 1924) divides Dioscorea into 60+ sections. The native North American species are assigned to sect. Macropoda Uline: stems twining counter-clockwise, staminate flowers aggregated into more or less sessile cymes and with six fertile stamens, tepals united only at the base, and capsules as broad as or broader than long, reflexed at maturity, with seeds winged all around. This section also includes disjunct species from the Balkan Peninsula, the Himalayas, temperate East Asia, and the Caucasus Mountains.
The taxonomy of Dioscorea is notoriously problematic. Many of the species are poorly known, and in the absence of comparative studies there has been an unchecked proliferation of names in the genus. That Dioscorea exhibits considerable diversity across its expansive geographic range is not contested, but a great many of the names in current use are very narrowly applied and lack any corroboration from field, laboratory, or herbarium studies. At present there is also no phylogenetic framework from which to interpret the variation that has been documented. Segregate genera have been erected, only to be subsumed again. A robust classification will ultimately emerge from rigorous systematic investigation, now in progress at research institutions around the world (L. R. Caddick et al. 2000; H. Huber 1998; P. Wilkin 1999; C. C. Xifreda 2000).
Dioscorea species are cultivated circumtropically, especially in West Africa and the West Indies, for their starchy tubers (yams), the cultivars having been derived from about ten species, including three of the four taxa naturalized in the flora. The rhizomes/tubers of many noncomestible species accumulate varying concentrations of steroidal saponins (F. W. Martin 1969), and Dioscorea species of Mexican, South African, and Asian origin have been utilized extensively in the industrial synthesis of cortisone and human sex hormones. Much lower saponin yields are obtained from the native North American species than from the species harvested commercially elsewhere. Saponin content varies as a function of plant age and time of harvest, as well as phylogenetic position (L. Degras 1993). The rhizomes of D. villosa are included in some Native American pharmacopeias and are used to ease the pain of childbirth (H. H. Smith 1928). An alcohol extract of the “root” was widely administered in nineteenth- century eclectic medicine as a remedy for colic (H. H. Bartlett 1910).
|1||Plants rhizomatous; bulbils never produced in leaf axils; leaf margins entire or repand; petiole base never clasping.||> 2|
|1||Plants tuberous; bulbils produced in leaf axils; leaf margins lobed or entire; petiole base sometimes clasping.||> 4|
|2||Rhizomes brownish, nodes not articulate; staminate inflorescences solitary in leaf axils; perianth of both staminate and pistillate flowers greenish white; stamens erect; thecae distinct, widely spreading.||Dioscorea villosa|
|2||Rhizomes yellow, nodes articulate, each bearing 1 dark, contrasting, ± deltate scale leaf or the dark vertical scar remaining once leaf falls; staminate inflorescences (1–)2–5 in leaf axils; perianth of both staminate and pistillate flowers orangish yellow; stamens inwardly curved; thecae connate.||Dioscorea floridana|
|3||Leaf margins 3–5-lobed.||> 4|
|3||Leaf margins entire.||> 5|
|4||Leaf blade hastate-cordate, margins 3-lobed, apex acute or mucronate; petiole base not clasping; staminate plants flowering annually.||Dioscorea polystachya|
|4||Leaf blade reniform, margins irregularly 3–5-lobed, apex conspicuously caudate; petiole base clasping; plants rarely flowering.||Dioscorea sansibarensis|
|5||Stems broadly winged, 4-angular; distal leaves decussate, blade base sagittate; seeds winged all around.||Dioscorea alata|
|5||Stems terete, usually unwinged; leaves alternate throughout, blade base orbicular; seeds unilaterally winged.||Dioscorea bulbifera|