YAM  , a

term usually applied to the tubers of various species of Dioscorea . These are plants with thick tubers (generall. 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 gene-rally 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, three-winged 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 roo parts there are of water 82.6, starch 13.1, albumen 2.4, fat o•2, woody fibre c•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 1 nat. size . Root much reduced . 902 weigh more than II, 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 too 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 . (189o) .