True yams were domesticated three separate times on three continents
People in Africa, Asia and South America each tamed their own yam, independently. Today about 95 percent of the world's crop grows in Africa, where tubers of up to 25 kilograms are pounded into iyan. Meanwhile American shoppers buy orange sweet potatoes labelled yams that belong to a different family entirely.
True yams belong to Dioscorea, a genus of some 870 species of perennial climbing vines related to lilies and grasses; several are edible, others toxic. Vines can stretch 15 metres and tubers push as deep as 1.5 metres into the ground. The rough skin, dark brown to pale pink, is hard to peel raw but softens with cooking, while the flesh ranges from white or yellow to purple. The English word likely came through Portuguese or Canarian Spanish from Fula, a West African language, and traces to verbs meaning to eat.
The white and yellow yams of Africa are the most important cultivated kinds, with over 200 varieties between them; the yellow gets its colour from carotenoids. Farmers plant whole tubers or pieces in mounds at the start of the rains and harvest after seven to twelve months, often saving up to 30 percent of the crop as seed. The work is heavy, but strong demand makes it profitable in parts of Africa.
In Asia, the purple or water yam, first grown in Southeast Asia, now has the widest spread of any cultivated yam. Austronesian seafarers carried it and the lesser yam in their canoes as far as Madagascar and Polynesia. The frost-tolerant Chinese yam was tried in Europe as a potato substitute during the nineteenth-century blight, and in Japan it is a supermarket staple, unusually eaten raw and grated as tororo. South America's cush-cush yam, from the Guyana region, is the leading New World species.
Not every yam is welcome. The air potato, which bears potato-sized bulbils on its leaves, reached Florida in 1905 and now smothers native plants, regrowing from bulbils even after cutting or burning. Elsewhere the name wanders: New Zealanders call oca a yam, and in Malaysia and Singapore the word means taro.
Source: Yam (vegetable)
