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The sweet potato is only distantly related to the true potato (Solanum tuberosum). Although some varieties are referred to as yams, they are not true yams, which are a distinct set of species of the genus Dioscorea, in the monocot family Dioscoreaceae, native to Asia and Africa.
The sweet potato plant, which is often cultivated as an annual from the easily rooted vegetative cuttings, is a trailing vine with alternate leaves up to 30 cm (12 inches) long, which range from heart-shaped to lobed, depending on the cultivar. Flower petals are fused into a radially symmetrical funnel-shaped corolla, pink to purple in color. (Various Ipomoea species are cultivated as ornamental morning glories, and although the flowers of I. batatas are not as showy as some, it is the source of ornamental as well as edible cultivars, including some with variegated foliage.) Tubers are long and tapered, with a smooth skin whose color ranges between purple, red, brown, and beige. The flesh ranges from beige through yellow, orange, and purple, and may be dry and mealy (as in the yellowish types referred to as yams) or moist and smooth.
The tubers are an important source of carbohydrates in many developing nations, and are also a regional favorite in the southern U.S. Sweet potatoes are high in fiber and are a good source of vitamins B, C, and beta-carotene, as well as the amino acids lysine and threonine. Young leaves and shoots are also edible, and the entire plant is used as animal feed.
Total global commercial production of sweet potatoes in 2010 was 106.6 million metric tons, harvested from 8.1 million hectares. China is the leading producer of sweet potatoes accounting for around 72% of the global total, due in part to the use of sweet potatoes (rather than corn) to make ethanol. African nations including Uganda, Nigeria, and Tanzania, as well as Indonesia, Vietnam, India, and Japan also rank among the top ten producers. U.S. researchers are also exploring the use of sweet potatoes for biofuel, and developing new cultivars for this purpose using traditional and genetic engineering techniques.
(Biofuelscenter.org 2011, Sadik 1988, USDA 2012, Wikipedia 2012)