YAM

any of several plant species of the genus Dioscorea (family Dioscoreaceae), native to warmer regions of both hemispheres. A number of species are cultivated for food in the tropics; in certain tropical cultures, notably of West Africa and New Guinea, the yam is the primary agricultural commodity and the focal point of elaborate ritual. True yams are botanically distinct from the sweet potato (q.v.), but moist-fleshed varieties of sweet potato are often called yams in the United States. D. bulbifera, the air-potato yam, is one of the few true yams cultivated for food in the United States. Yams have thick tubers (generally a development of the base of the stem), from which protrude long, slender, annual, climbing stems bearing leaves, which are either alternate or opposite and either entire or lobed and unisexual flowers in long clusters. The flowers are generally small and individually inconspicuous, though collectively showy. Each consists of a greenish, bell-shaped or flat perianth of six pieces, enclosing six or fewer stamens in the male flowers and surmounting a three-celled, three-winged ovary in the female flowers. The ovary ripens into a membranous capsule, bursting by three valves to liberate numerous flattish or globose seeds. Most yams contain an acrid principle that is dissipated in cooking. D. trifida and D. alata are the edible species most widely diffused in tropical and subtropical countries. The tubers of D. alata sometimes weigh 45 kg (100 pounds). D rotundata and D. cayenensis are the main yam species grown in West Africa. D. esculenta, grown on the subcontinent of India, in southern Vietnam, and in the South Pacific islands, is one of the tastiest yams. D. batatas, the Chinese yam, or cinnamon vine, is widely cultivated in East Asia. Hundreds of species of yams are known, and they vary widely in taste and appearance. Yams' flesh ranges in colour from white to yellow, pink, or purple. They vary in taste from sweet to bitter to tasteless. Yams are consumed as cooked starchy vegetables. They are often boiled and then mashed into a sticky paste or dough, but they may also be fried, roasted, or baked in the manner of potatoes.