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Key economic species, in which numerous cultivars have been developed, include D. rotundata (white yam) and D. cayenensis (yellow yam), which originated in Africa, and D. alata (water yam), which originated in Southeast Asia. There are also species native to the New World tropics of Central and South America. Several Mexican species have tubers that are not edible but are high in saponins, so they are harvested for use in manufacturing hormone drugs. Several species, including D. villosus, grow in North America, although these have only slender tubers or rhizomes, not widely used as food. A few species, such as D. discolor, are grown as ornamentals.
Yam vines often have cordate leaves that may be alternate, opposite, or whorled. The flowers, arranged in spikes or racemes, are small and generally unisexual. The fruits are 3-angled or winged capsules (in some species, berries), containing winged seeds. The roots of most species are rhizamotous or produce tubers. Yam tubers can grow up to 1.5 m (4.9 feet) long and weigh up to 70 kg (154 pounds). The vegetable has a rough skin which is difficult to peel, but which softens after heating. The skins vary in color from dark brown to light pink. The majority of the vegetable is composed of a much softer substance known as the "meat,” which ranges in color from white or yellow to purple or pink in mature yams.
Yams are versatile vegetables, composed mostly of starches, with significant amounts of vitamin C. However, many species of yams must be cooked to be safely eaten, because raw yams contain various natural compounds, including phenols, tannins, hydrogen cyanide, oxalate, amylase inhibitor activity and trypsin inhibitor, that can cause illness or lead to nutritional deficiencies if consumed. (In addition, contact with uncooked yam fluids can cause skin irritations.) Common yam cooking methods include boiling, frying and roasting.
Yams have a long history of cultivation, possibly dating as far back as 8000 B.C., and have various ritual uses in African cultures. Yams are still important for survival in Africa, and are a primary agricultural commodity in Sub-Saharan Africa, particularly West Africa. Of the total 2010 global production 47.7 million metric tons commercially harvested from 4.8 million hectares, Nigeria accounted for 61% of the harvest, with Ghana, Côte d’Ivoire, and Benin also among top producers.
Although the plants are affected by various pathogens and pests, the tubers, once harvested, can be stored four to six months without refrigeration, which makes them a valuable resource for the yearly period of food scarcity at the beginning of the wet season.
(Bailey 1976, Sadik 1988, Shanthakumari et al. 2008, Wikipedia 2011.)