1911 Encyclopædia Britannica/Yam
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
Yam (Dioscorea Batatas). Branch about
12 nat. size. Root much reduced.
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, 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 100 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. saliva 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 weigh more than 112 ℔. 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 conveners. 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).