Yam
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Yam, common name for any of several members of a genus of perennial herbs. Members of the genus are monocots and are native to tropical regions throughout the world. Yams are cultivated for their edible tubers, which grow up to 2.4 m (8 ft) long and weigh up to 45 kg (100 lb). The inconspicuous white or greenish-yellow flowers, arranged in spikes or racemes, have a six-part calyx (outer flower whorl), a six-lobed corolla (inner floral whorl), six stamens, and a solitary pistil. The fruit of the yam consists of a membranous, three-winged capsule.

The true yam is about 20 percent starch and as a food is very similar to a potato. True yams (Dioscorea spp., Dioscoreaceae) are the third most important tropical "root" crop after cassava (Manihot esculenta, Euphorbiaceae) and sweet potato (Ipomoea batatas, Convolvulaceae). This is especially true in West Africa, parts of Central America and the Caribbean, the Pacific Islands, and Southeast Asia. Other important starchy "root" crops are taro and dasheen (Colocasia esculenta, Araceae), the source of Polynesian poi, arrowroot (Maranta arundinacea, Marantaceae), and achira (Canna edulis, Cannaceae). The most commonly cultivated edible yams are D. rotundata and D. cayensis in Africa, D. alata and D. esculenta in Asia, and D. trifida in the New World, although many cultivated varieties are known. They are climbing perennial vines with shiny, heart-shaped leaves, arising from large underground stems that are technically called tubers rather than roots. Like a potato, the tubers can be propagated by planting sections containing the "eyes" or buds; however, harvesting them is a laborious task because the tubers are deeply buried. Dioscorea is a large genus with more than 600 species. One African species, called elephant's foot or Hottentot's bread (D. elephantipes) produces a huge underground tuber weighing up to 700 pounds (318 kg). The woody, above-ground part of this huge tuber resembles the shell of a tortoise--hence the common name of "turtleback plant." Like other caudiciform xerophytes (desert plants with enlarged stems), the vine relies on carbohydrates and moisture stored in its stem during extended periods of drought. During severe drought conditions the gigantic tubers are cooked and eaten as famine food by Hottentots.  Another unusual African yam is the air potato (Dioscorea bulbifera). It has small or no subterranean tubers, but instead develops large, liver-shaped aerial tubers up to four pounds (2 kg) each. In New Guinea and Melanesia special ceremonial yams weighing over 120 pounds (54 kg) are grown to reflect the grower's status in the community. The yams are used for gifts and ritualized exchanges. A yam festival is held at harvest time during which the tubers are covered by elaborate woven masks. Although yam is rich in starch and easy to grate the starch is difficult to extract from the grating due its viscous and fibrous nature.

Yams have been domesticated independently in many different parts of the world. The water yam is a species from south-eastern Asia that is commonly cultivated. The Chinese yam, or Chinese potato, also known as cinnamon vine, is cultivated as an ornamental climbing vine but is also an important food plant; its thick tubers reach a length of about 1 m (3 ft). The air potato is native to southern Asia; it bears white flowers and aerial tubers that attain a weight of several kilograms. The elephants foot of South Africa is an important ornamental or emergency food. Cush cush or yampi is commonly cultivated in the tropical Americas. The name "yam" is commonly but incorrectly applied to varieties of the genus containing the sweet potatoes.

Yams are the source of the chemical diosgenin, a precursor of progesterone, cortisone, and other medically important steroids. The collection of plants for the production of diosgenin is licensed in several parts of the world.

Scientific classification: Yams make up the genus Dioscorea of the family Dioscoreaceae. The water yam is classified as Dioscorea alata, the Chinese yam, or Chinese potato, as Dioscorea batatas, the air potato as Dioscorea bulbifera, the elephants foot as Dioscorea elephantipes, and cush cush, or yampi, as Dioscorea trifida.