Yams are any of the 10 economically important species of Dioscorea, a genus in the monocotyledonous family Dioscoriaceae. These species, all tropical in their origin, are cultivated for their edible tubers (enlarged, fleshy, usually underground storage stems). In the United States, the name yam is often misapplied to the sweet potato (Ipomea batatas).
Yams are herbaceous plants whose stems twine up and around bushes, trees, or poles. Depending on the species of yam, stems twine either clockwise or counterclockwise. The stems bear stalked, palmately veined leaves that are simple and entire, although a few species have three-lobed leaves. All yams have a dioecious lifestyle, which means that the staminate and pistillate flowers are borne on separate plants. The flowers are inconspicuous, being only 1/8 in (2-4 mm) long and whitish or greenish. The fruits produced from the flowers are three-angled and contain winged seeds. Some cultivars of yam, however, rarely flower or set seed.
In commonly cultivated yams, the tubers lie underground and are one (rarely two or three) per plant. These tubers resemble huge, elongated potatoes, typically growing 2-6 ft long (0.6-2 m) and weighing 11-33 lb (5-15 kg). A thin skin protects their outer surface, and on the inside they are filled with starch which can be white or yellow depending on the species. One cultivated yam (Dioscorea bulbifera) bears small tubers along the aerial stems in the leaf axils (the angle between the stem and leaf stalk).
There are two centers of yam cultivation worldwide. The first is the high rainfall region of western Africa, from the Ivory Coast to Cameroon. Here the most important species are the white yam (Dioscorea rotundata) and the yellow yam (D. cayenensis), named for the color of their tuber's flesh. The second center is Vietnam, Cambodia, Laos, and neighboring regions where the most commonly cultivated species is the Asiatic yam (D. alata). Secondary areas of yam cultivation are the West Indies, Pacific islands, and southeastern United States (from Louisiana to Georgia). Most yam species originated in Asia and Africa; only one, the cush-cush yam (D. trifida), is native to the New World.
The world production of yams amounts to about 22 million tons (20 million metric tons) per year, of which two-thirds comes from tropical West Africa. Yams are to tropical West Africans what wheaten bread is to North Americans and Europeans. In tropical west-Africa, many social and religious festivals are associated with planting and harvesting yams.
Yams are propagated from cuttings of the tuber. Because the plants climb, they are provided with poles or trellises for support. It generally takes seven to 10 months before the tubers can be harvested, and this must be done by hand because mechanical harvesters tend to damage the tubers. Yams store better than most tropical tuber crops and this is one reason why they are widely grown. Before eating, yams are usually peeled and then either boiled, roasted, or fried. In Africa yams are usually prepared as fufu or four-fou, made from peeling, cutting, and boiling the tuber, and then pounding it into a gelatinous dough. It is served with soups or stews or cooked raw in palm oil. Nutritionally, the yams are equivalent to the common potato, containing 80-90% carbohydrates, 5-8% protein, and about 3.5% minerals. Yam production is now declining because cassava (Manihot utilissima) and sweet potatoes (Ipomea batatas)—sources of starch that are easier to cultivate—are increasingly being used. Yams are not fed to livestock because they are more expensive than other kinds of animal feed.
Yams are a source of steroids and alkaloids-chemicals that are extremely active physiologically in vertebrate animals. The most important yam steroid is diosgenin used in the production of birth-control pills. Alkaloids from yams have been used to kill fish and to poison darts and arrows for hunting. Some yams are poisonous to humans because of their high alkaloid content, and their tubers must be boiled before eating to remove the toxins.