YAM, a term usually applied to the tubers of various species of Dioscorea. These are plants with thick tubers (generally a development of the base of the stem), from which protrude long, slender, annual climbing stems, bearing alternate or opposite, entire or lobed leaves and unisexual flowers in long clusters. The flowers are generally small and individually inconspicuous, though collectively showy. Each consists of a greenish bell-shaped or flat perianth of six pieces, enclosing six or fewer stamens in the male flowers, and surmounting a three-celled, threewinged ovary in the female flowers. The ovary ripens into a membranous capsule, bursting by three valves to liberate numerous flattish or globose seeds. The species are natives of the warmer regions of both hemispheres.
According to Professor Church's analysis of the Chinese yam, it contains more nitrogenous matter, but less starch, than potatoes: in I oo parts there are of water 82.6, starch 13-1, albumen 2.4, fat 0.2, woody fibre 0.4 and mineral matter 1.3 parts.
D. sativa and D. alata are the species most widely diffused in tropical and subtropical countries. D. aculeata, grown ' in India, Cochin China and the South Sea Islands, is one of the best varieties. D. Batatas, the Chinese yam, is hardy in Great Britain, but the great depth to which its enormous tubers descend renders its cultivation unprofitable. It has deeply penetrating, thick, club-shaped, fleshy roots, full of starch, which when cooked acquire a mild taste like that of a potato; they grow 3 ft. or upwards in length, and sometimes Yam (Dioscorea Batatas). Branch about 2 nat. size. Root much reduced.
weigh more than 12 lb. The plant grows freely in deep sandy soil, moderately enriched. The sets, consisting of pieces of the roots, may be planted in March or April, and require no other culture than the staking of the climbing stems. They should not be dug up before November, the chief increase in their size `taking place in autumn. They sometimes strike downwards 2 or 3 ft. into the soil, and must be carefully dug out, the upper slender part being reserved for propagation, and the lower fleshy portion eaten after having been allowed a few days to dry. The tubers of D. alata sometimes weigh 100 lb. Most of the yams contain an acrid principle, which is dissipated in cooking.
The only European Dioscorea is that known as D. pyrenaica, a native of the Pyrenees, a remarkable instance of a species growing at a long distance from all its congeners. True yams must not be confounded with the sweet potato, Ipomoea Batatas, as they sometimes are in London markets. The common black bryony (Tamus communis) of hedges in England is closely allied to the yams of the tropics, and has a similar root-stock, which is reputed to be poisonous.
For the history of the yam, and its cultivation and uses in India, see G. Watt, Dictionary of the Economic Products of India, iii. (1890).
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