Yam (genus Dioscorea), plant of the yam family with a flowering vine and a large tuber, similar to a sweet potato but of a different botanical family. The name “yam” often refers to the yellow- or white-fleshed tuber of edible species (e.g., D. alata and D. rotundata). The tuber is actually an enlarged underground portion of the stem, weighing as much as 100 lb (45 kg), in which the plant stores starch, water, and some sugar. Some varieties contain toxic substances that can only be eliminated by cooking, and cortisone and other drugs are derived from saponin compounds found in certain wild yams. Yams grow in warm, moist climates, and are an important food crop in West Africa, India, Southeast Asia, and the Caribbean area.