Flora of North America species comparison

Dioscorea villosa is a highly polymorphic species, exhibiting complex patterns of variation across its geographic range. Characters that have been used previously to delineate taxonomic boundaries within this complex—pubescence, glaucousness, rhizome thickness and shape, length of internodes within the inflorescence, arrangement of proximal leaves, geometry of the stem, and fruit and seed size/shape—fail when individuals from all parts of the range and specimens representing both apical and basal portions of single stems can be examined. At its morphological extremes, D. villosa comprises 1) small vines with tightly congested inflorescences, winged stems, and variously pubescent leaves, occurring in bogs and branch swamps; and 2) robust plants, rigid at the base, the proximal leaves verticillate with large, glaucous blades, from the axils of which arise lax spikes or panicles, inhabiting rocky, upland woods and steep talus slopes. As one ascends from the Atlantic Coastal Plain through the Appalachians, continuing westward to the Great Lakes region, south to the Ozarks, and east to the branch swamps of Georgia, particular morphologies are associated with particular ecological conditions, independent of geography. As well as the morphological extremes, every intermediate condition of leaf, stem, and inflorescence architecture can be found, in all combinations, and variation may be encountered even within individual plants. What sort of genetic structure underlies these patterns of morphological diversity remains an open question. That there is a significant degree of genetic variability within the complex is evident from the chromosome counts thus far reported. Further research is needed to shed light on patterns of gene flow in the complex, and garden studies would be instructive as to the limits of individual plasticity. At present, I can find no natural gaps in the variation between the plants that have been called (albeit ambiguously; see H. H. Bartlett 1910) D. villosa and those called D. quaternata, and therefore I am treating the complex as a single species.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

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).

(Discussion copyrighted by Flora of North America; reprinted with permission.)