Yam is the common name for some plant species in the genus Dioscorea (family Dioscoreaceae) that form edible tubers (some other species in the genus being toxic). Yams are perennial herbaceous vines native to Africa, Asia, and the Americas and cultivated for the consumption of their starchy tubers in many temperate and tropical regions. The tubers themselves, also called "yams", come in a variety of forms owing to numerous cultivars and related species.
Description
A monocot related to lilies and grasses, yams are vigorous herbaceous, perennially growing vines from a tuber. Some 870 species of yams are known, a few of which are widely grown for their edible tuber but others of which are toxic (such as D. communis). Yam plants can grow up to 15 metres (50 feet) in length and 7 to 15 centimetres (3 to 6 inches) high. The tuber may grow into the soil up to 1.5 m (5 ft) deep. The plant disperses by seed. The edible tuber has a rough skin that is difficult to peel but readily softened by cooking. The skins vary in color from dark brown to light pink. The majority, or meat, of the vegetable is composed of a much softer substance ranging in color from white or yellow to purple or pink in mature yams.
Etymology The name "yam" appears to derive from Portuguese inhame or Canarian Spanish ñame, which derived from Fula, one of the West African languages during trade. However, in Portuguese, this name commonly refers to the taro plant (Colocasia esculenta) from the genus Colocasia, as opposed to Dioscorea. The main derivations borrow from verbs meaning "to eat". True yams have various common names across multiple world regions. In some places, other (unrelated) root vegetables are sometimes referred to as "yams", including:
In the United States, sweet potatoes (Ipomoea batatas), especially those with orange flesh, are often referred to as "yams" In Australia, the tubers of the Microseris walteri, or yam daisy, were a staple food of Aboriginal Australians in some regions. In New Zealand, oca (Oxalis tuberosa) is typically referred to as "yam". In Malaysia and Singapore, taro (Colocasia esculenta) is referred to as "yam". In Africa, South and Southeast Asia as well as the tropical Pacific islands Amorphophallus paeoniifolius is grown and known as "elephant foot yam".
Distribution and habitat Yams are native to Africa, Asia, and the Americas. Three species were independently domesticated on those continents: D. rotundata (Africa), D. alata (Asia), and D. trifida (South America).
Ecology Some yams are invasive plants, often considered a noxious weed outside cultivated areas.
Cultivation
Yams are cultivated for the consumption of their starchy tubers in many temperate and tropical regions, especially in West Africa, South America and the Caribbean, Asia, and Oceania. About 95% of yam crops are grown in Africa. A yam crop begins when whole seed tubers or tuber portions are planted into mounds or ridges, at the beginning of the rainy season. The crop yield depends on how and where the sets are planted, sizes of mounds, interplant spacing, provision of stakes for the resultant plants, yam species, and tuber sizes desired at harvest. Small-scale farmers in West and Central Africa often intercrop yams with cereals and vegetables. The seed yams are perishable and bulky to transport. Farmers who do not buy new seed yams usually set aside up to 30% of their harvest for planting the next year. Yam crops face pressure from a range of insect pests and fungal and viral diseases, as well as nematodes. Their growth and dormant phases correspond respectively to the wet season and the dry season. For maximum yield, the yams require a humid tropical environment, with an annual rainfall over 1,500 millimetres (59 in) distributed uniformly throughout the growing season. White, yellow, and water yams typically produce a single large tuber per year, generally weighing 5 to 10 kilograms (11 to 22 pounds). Yams suffer from relatively few pests and diseases. There is an anthracnose caused by Colletotrichum gloeosporioides which is widely distributed around the world's growing regions. Winch et al., 1984 finds C. gloeosporioides afflicts a large number of Dioscorea spp. Despite the high labor requirements and production costs, consumer demand for yam is high in certain subregions of Africa, making yam cultivation quite profitable to certain farmers.
Major cultivated species Many cultivated species of Dioscorea yams are found throughout the humid tropics. The most economically important are discussed below. Non-Dioscorea tubers that were historically important in Africa include Plectranthus rotundifolius (the Hausa potato) and P. esculentus (the Livingstone potato); these two tuber crops have now been largely displaced by the introduction of cassava.
D. cayennensis The yellow yam, D. cayenensis, and the white yam, D. cayenensis subsp. rotundata, are native to Africa. They are the most important cultivated yams. In the past, the white yam was considered to be a separate species, D. rotundata, but most taxonomists now regard it as a subspecies of yellow yam. Over 200 varieties between them are cultivated. White yam tuber is roughly cylindrical in shape, the skin is smooth and brown, and the flesh is usually white and firm. Yellow yam has yellow flesh, caused by the presence of carotenoids. It looks similar to the white yam in outer appearance; its tuber skin is usually a bit firmer and less extensively grooved. The yellow yam has a longer period of vegetation and a shorter dormancy than white yam. The Kokoro variety is important in making dried yam chips. They are large plants; the vines can be as long as 10 to 12 m (33 to 39 ft). The tubers most often weigh about 2.5 to 5 kg (6 to 11 lb) each, but can weigh as much as 25 kg (55 lb). After 7 to 12 months' growth, the tubers are harvested. In Africa, most are pounded into a paste to make the traditional dish of "pounded yam", known as Iyan.
D. alata
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