Dioscorea villosa
Dioscorea alata
colic root, wild yam, yam root
greater yam, water yam, white yam
longitudinally grooved or sometimes narrowly winged, usually terete in cross section, 1–7 m, ± rigid proximally, or flexible, glabrous or rarely with sparse pubescence, wings when present less than 1 mm and stems polygonal.
twining clockwise, climbing up to 20 m, broadly winged, 4-angular, producing bulbils up to 4 cm in diam. in leaf axils, wings often purplish.
alternate, subopposite, subverticillate, or in verticels of 3–7 proximally, due to suppression of proximal internodes (esp.
alternate proximally, opposite and ultimately decussate distally, 6–16 × 4–13 cm;
petiole ca. as long as blade, winged, base clasping, basal lobes stipulate, growing as extensions of wings, less than 1 mm wide;
blade 5–7-veined, ovate, glabrous, base typically sagittate, margins entire, apex acute to acuminate.
perianth greenish white, appearing darker in some specimens due to presence of irregularly distributed tannin crystals, rotate-campanulate to funnelform, 1–2(–3) mm diam.;
tepals ± glandular, ovate-elliptic, margins hyaline, apex rounded or acute;
stamens in 2 subequal whorls, erect;
anthers ca. ½ length of filaments, thecae distinct, widely spreading.
perianth greenish white, rotate-campanulate, 2–4 mm wide;
tepals as in staminate flowers;
staminodes 6, differentiated into anthers and filaments, less than 1/2 length of fertile stamens.
perianth lightly colored;
tepals broadly ovate, ca. 1 mm, fleshy;
staminodes 6, smaller than fertile stamens.
greenish gold, ovoid to obovoid to obreniform, 1–3 × 1–3.5 cm, varying continously in size, occasionally ± glaucous.
not reflexed at maturity, broadly ovate, ca. 2 × 3.5 cm.
generally 2 per locule, rarely 1, 5–18 mm.
winged all around.
woodland understory), always alternate distally, 3–13 × 2–13 cm, ca. as long as wide;
petiole ridged or narrowly winged, 3–14 cm, glabrous or puberulent at pulvinus, base not clasping;
blade green to ± glaucous, (7–)9–11-veined, ovate-cordate, abaxial surface sometimes ± glandular, or sparsely or sometimes densely pubescent to glabrous, base with sinus rounded, acute, or ± truncate basally, margins entire or repand, apex acute to acuminate, occasionally mucronate.
inflorescences solitary in leaf axils, rarely terminal, spicate or branched;
cymes sessile, bearing 1–3 sessile flowers, braceolate, internodes between cymes 1–8 mm, bracteoles ca. 1 mm;
rachis 2–30 cm, secondary axes to 15 cm, robust plants occasionally branched to third order, axes subtended by linear-lanceolate bracts 1–3 mm.
inflorescences axillary, 1–2 per axil, paniculate, fasciculate;
panicles bearing flowers singly, bracteolate, in a zigzag pattern along rachis, internodes less than 2 mm;
rachis to 25 cm, secondary axes 1–3(–6), fasciculate, less than 3 cm, each subtended by deltate-ovate bracteole shorter than 1 mm.
inflorescences solitary, 4–18-flowered, 4–20 cm, internodes 6–12 mm.
inflorescences solitary, 4–8(–20)-flowered, 6–35 cm, internodes ca. 1 cm. Staminate flowers: perianth whitish, cup-shaped;
tepals ca. 1 mm, connate at base in 2 subequal whorls, outer widely ovate, inner narrower, apex obtuse to rounded in both;
fertile stamens 6 in 2 equal whorls;
anthers longer than filaments, thecae distinct, not spreading.
= 20, 36, 54, 60.
= 30, 40, 50, 60, 70, 80.
Dioscorea villosa
Dioscorea alata
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.)
Dioscorea alata is the most extensively cultivated of the edible yams. There are hundreds of cultivars worldwide (L. Degras 1993; D. G. Coursey 1967), varying in the color and consistency of the flesh. The bulbils may also be eaten, but the large tubers are favored. Typically the plants are propagated vegetatively. The species is not known to exist in the wild, and its putative origin is from Southeast Asia. It has escaped from cultivation in the southeastern United States, recorded here from Florida and Georgia, although it is likely to be found elsewhere in the Gulf states. The plants seldom flower in the flora area; I observed no staminate flowers at all, and very few pistillate ones. Although the fruits do set seed, the seeds are frequently unviable.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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