YAM, Doscorea, a genus of plants of the natural order dioscoreacew, distinguished by an inferior ovary and membranous winged fruit. The species are mostly tropical, natives of the East and West Indies, etc. They have tuberous roots and herbaceous twining stems. The great fleshy roots of some of them are very much used as an article of food, in the same way that potatoes are in more temperate climates. They contahr much starch, and generally become somewhat mealy and pleasant to the taste when boiled. This, however, is not the ease with all; the roots of D. triphylla, D. deemonurn, D. MOSti, and several other species with ternate leaves are very nauseous even when boiled, and are poisonous. The tubers of all the yams contain an acrid substance, which, however, is dissipated by boiling, except in the species withcompound leaves. The WINGED YAM (D. elate) is an article of food in daily use in the South Sea islands. The roots are LI to 3 ft,. long, and often 30 lbs. in weight, with a brownish. or black skin, juicy and reddish within. They vary exceedingly in form. The stem, which is winged, twines up tall poles which are imovided for it by the cultivator; the leaves are between heart-shaped and arrow-shaped. Two or three small tubers are generally found in the axils of the leaves. It is supposed that this species may be the original of most, or per. Imps all, of the yams cultivated in the tropical parts of Asia, Africa, and America—as The common yam of the West Indies (D. saliva), which has a round stein and heart shaped leaves; D. bulbifera, in which the tubers in the axils of the leaves attain the size
apples; the prickly yam (D. a,culeata), which has a prickly stem, and a fasciculated, tuberous root; D. globosa, the most esteemed yam of India, which has very fragrant flowers, and roots white internally; D. rubella, another Indian kind, with tubers some times 3 ft. long, tinged with red below the skin; etc. The species are not well ascer tained. Yams aro progagated by means of their tubers; the small axiliary tubers, or the small tubers produced at the base of the stem around the neck of the large tuber, being used for this purpose.—A species of yarn (D, batatas) has recently been brought from the temperate parts of China, where it appears to have been long in cultivation, and is found to succeed well in France. It is hardy enough to endure the climate even of Scotland without injury; but the heat of the summer is not sufficiently great and long-continued for its profitable growth, so that, in general, the plant merely lives, without producing a large tuber. The root is of very fine quality, and attains a very considerable size. The stem requires the support of a pole, round which it twines; the leaves are more elongated and acuminated than those of the West Indian yams; the root strikes perpendicularly down into the ground, and forms its tuber often at a very con siderable depth, which is sometimes inconvenient to the cultivator; but this is prevented by putting a slate under it.