yam

Any of a group of climbing plants cultivated in tropical regions; the starchy tubers (underground stems) are eaten as a vegetable. The Mexican yam (D. composita) contains a chemical that is used in the contraceptive pill. (Genus Dioscorea, family Dioscoreaceae.)

The elephant's foot yam, a wild variety that produces edible tubers of up to 661 lb/300 kg each, was found not to be extinct after 40 years, by UK botanists from the Millennium Seed Bank in May 2001. It is found only in the Northern Cape Plateau, South Africa.