Whatever your supermarket may claim, the “yams” they have for sale are almost certainly not yams. The yam that am is an African tuber not at all related to the native American sweet potato, which is often marketed as a yam. The sweet potato (Ipomoea batatas) is a small thing compared to a true yam, which has orange flesh and a fairly smooth, potatolike skin. I. batatas does have a white-fleshed variety, but it is sold as boniato and not yam. The true yam (Dioscorea spp.) is large, has white flesh (D. rotundata) or yellow flesh (D. cayenensis), and appears only in U.S. markets that offer the full range of tropical tubers. Even these Linnaean names incorporate the confusion that surrounds this extremely valuable plant. By one theory, the yellow and white yams are part of the same “species complex, ” but, van Wyk points out, they do not have the same number of chromosomes.