Greek = two houses (pronounced dye-ee-shous). A plant species in which the male and female sexes are separated in different plants. See also: Hermaphrodite. Dioscorea alata

This is the Asian yam, also known as the white yam, the greater yam, the winged yam, and the water yam. See Dioscorea spp., for a description of the genus . This yam was of major importance to the seafaring Polynesians who took it to most of the tropical islands of the Old World. It is propagated vegetatively, because most cultivar never produce fertile seed, and some are completely sterile. Not recommended for amateur breeders.

This is a crop with extinct wild progenitors. Dioscorea bulbifera

This is the aerial yam, also known as the potato yam. It is of minor importance as a food crop but was probably important in ancient times. It is the only species that occurs wild in both Africa and Asia. See Dioscorea spp., for a description of the genus . Not recommended for amateur breeders . Dioscorea cayenensis

This is the yellow yam, also known as the twelve-month yam, and the yellow guinea yam. In spite of its name, this is a West African species that still occurs wild. It was taken to the New World with the slave trade. It is widely grown in West Africa but it is not as important as Dioscorea rotundata. Not recommended for amateur breeders . See Dioscorea spp., for a description of the genus . Dioscorea rotundata

This is the white yam, also known as the Guinea yam, and the eight-months yam. It originated in West Africa and is the most important species agriculturally. Many clones exist but most of them set fertile seed so rarely that breeding is all but impossible. Not recommended for amateur breeders . See Dioscorea spp., for a description of the genus . Dioscorea spp.

These are the true yams, not to be confused with sweet potatoes, which are called yams in the southern USA. Although generally considered a Monocotyledons , this genus has many features of Dicotyledons , including reticulate veining in the leaves and occasional seeds with two cotyledons, in which only one cotyledon develops. The genus is very old geologically, and it occurs, and has been domesticated, in both the Old and the New Worlds. The principle species are described under their specific names.

The true yams are monoecious. Most cultivar are propagated vegetatively because they produce fertile seed rarely or not at all. This makes breeding extremely difficult, and these crops are not recommended for amateur breeders . There are few pests and diseases of yams and, because all the cultivated clones are ancient, they are a useful demonstration of both the durability and the efficacy of horizontal resistance.

Wild yams were recently in danger of extinction due to the demand for natural diosgenin in the manufacture of oral contraceptives. However, the development of synthetic diosgenins has eliminated this threat.

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