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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.