Yam

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.

yam, common name for some members of the Dioscoreaceae, a family of tropical and subtropical climbing herbs or shrubs with starchy rhizomes often cultivated for food. The largest genus, Dioscorea, is commercially important in East Asia and in tropical America. The thick rhizomes, often weighing 30 lb (13.6 kg) or more, are used for human consumption and for feeding livestock. A number of species of Dioscorea are cultivated for extraction of diosgenin, a female hormone precursor used in the manufacture of the contraceptive pill. In the United States, cultivation of yams for food is restricted to the South, but the wild yam (sometimes used medicinally) is indigenous farther north, and another species, the cinnamon vine, is cultivated as a decorative plant. The sweet potato, which belongs to the morning glory family, is sometimes erroneously called yam. The S African elephant's-foot (Testudinaria elephantipes), also called Hottentot bread and tortoise plant, is sometimes grown in greenhouses; its large rootstock was formerly eaten by the natives. Yams are classified in the division Magnoliophyta, class Liliopsida, order Liliales, family Dioscoreaceae.

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yam / yam/ • n. 1. the edible starchy tuber of a climbing plant, widely distributed in tropical and subtropical countries. 2. the plant (genus Dioscorea, family Dioscoreaceae) that yields this tuber. 3. a sweet potato. ORIGIN: late 16th cent.: from Portuguese inhame or obsolete Spanish iñame, probably of West African origin.

yam Tubers of about 10 cultivated species of Dioscorea, most of which are tropical, but 2 of which (D. opposita and D. japonica) are cultivated in temperate regions in Asia. In the USA the name is also applied to the sweet potato (Ipomoea batatas), a somewhat similar but unrelated edible tuber. The yam bean is a name given to the seed of several species of Leguminosae (e.g. Pachyrhizus erosus and Sphenostylis stenocarpa) which also produce edible tubers.

yam Tubers of perennial climbing plants of Dioscorea spp.: D. rotundala white yam, and D. cayenensis, yellow or Guinea yam, water, trifoliate, or Chinese yam. A major food in parts of Africa and also the Far East. A 150‐g portion is a source of vitamins B₁ and C; provides 5 g of dietary fibre; supplies 200 kcal (840 kJ). In the USA sweet potatoes are sometimes called yams.

yam Any of several species of herbaceous vines that grow in warm and tropical regions; also the large, tuberous roots of several tropical species, which are edible. The plant is an annual, with a long, climbing stem, lobed or unlobed leaves and small clusters of greenish, bell-shaped flowers. The sweet potato is also sometimes called a yam. Family Dioscoreaceae; genus Dioscorea.

yam (tuberous root of) species of Dioscorea. XVII (earlier in Eng. writings in various alien forms, e.g. inany, nname, igname). — Pg. inhame or Sp. †igñame (mod. ñame); ult. orig. unkn.