95. Dioscorea, Linn.—The Yam.

Sex. Syst. Dioecia, Hexandria.

In tropical countries (East and West Indies, Africa, Polynesia) the tuberous roots of many species of Dioscorea or Yam [The term yam is frequently, but erroneously, applied to the tubers of Tacca and Arum (see ante, p. 157).] are used as food. Of seventeen species described by Roxburgh, [Fl. Indica, vol. iii. p. 797.] eleven are stated to be employed for food. The four following are cultivated in India, and are esteemed in the order in which they are enumerated: D. globosa, D. alata, D. purpurea, and D. rubella. Roxburgh also says that D. atropurpurea is extensively cultivated at Malacca, Pegu, and the eastern islands; and that D. fasciculata is cultivated to a considerable extent in the vicinity of Calcutta, not only for food, but to make starch of the roots. In the West Indies several species are used as food; the chief are D. aculeata, D. alata, D. bulbifera, and D. sativa. The tuberous roots sold in the London shops as West India yams are said to be those of Ds alata.

Yams are large fleshy roots, sometimes weighing from thirty to forty pounds each. Some of them are highly acrid in the fresh state, but become agreeable articles of food when cooked, owing to the dissipation or decomposition of the acrid principle.

The fresh root of Dioscorea sativa, from the West Indies, was analyzed by Süersen, [Quoted by L. Gmelin, Handb. d. Chemia, Bd. ii. S. 1334.] who obtained the following results: resin, 0.05; uncrystallizable sugar, 0.26; mucilage, 2.94; starch, 22.66; ligneous fibre, 6.51; nitrogenized matter, quantity undetermined; and water, 67.58. The fresh roots yield 0.52 of ashes, containing carbonate of lime and silica.

The following are the per centage quantities of yam-starch obtained by Dr. Sheir [Report on the Starch-producing Plants of the Colony of British Guiana, by John Sheir, LL. D. Demerara, 1847.—Dr. James Clark (Medicinal Facts and Observations, vol. vii. 1797) obtained from 4 lbs. of the roots of D. triphylla 5 oz. 2 dr. of starch, and from teh same quantity of D. bulbifera 8 oz. of starch.] from the fresh roots of several species of Dioscorea:—

Per Centage of Starch.
Common yam (D. sativa)24.47
Barbados yam (D. ___?)18.75
Guinea yam (D. aculeata)17.03
Buck yam (D. triphylla)16.07
Another sample15.63
A third (from a dark-coloured variety)14.83

I am indebted to Dr. Sheir for specimens of the starches of three species: viz., D. sativa, D. aculeata, and D. triphylla. They were prepared in the Colonial Laboratory at Demerara. They are beautifully white, inodorous, and tasteless. Examined by the microscope, the particles of the three starches present a general similarity of character. They are large, somewhat compressed, elliptical or ovate, or somewhat obtusely triangular. They may be compared in shape to the seed of the common scarlet runner bean (Phaseolus mulliflorus), and are surrounded by rings, which, when viewed on the middle of the flat side of the particle, appear to be but slightly curved. In the latter character they approximate to curcuma starch. Some of the particles present one or two slight nipple-like projections analogous to those of maranta starch. In polarized light, they present the usual crosses observed with most other starches. Their size is about 1/500th of an inch in length, and about 1/1000th an inch in breadth. [The following measurements, in parts of an English inch, of the particles of yam starch, were made for me by Mr, George Jackson:—

Particles.Guinea Yam.Common Yam.Buck Yam
The most prevalent-sized particles are those marked thus *.]

Yams are roasted and boiled, and eaten like potatoes. "They are dressed in various forms, being boiled in soups or broths, &c, made into pudding, or roasted in the fire." [Dr. Wright, Medicinal Plants growing in Jamaica, in the Memoir of his life, p. 208, 1828.] Some of them, however, are violently acrid, causing vomiting and diarrhoea, even after being carefully cooked. This is said to be the case with D. triphylla and daemona. Yet Dr. Wright and Dr. Sheir declare that the roots of D. triphylla are nearly equal to potatoes.

The Elements of Materia Medica and Therapeutics, Vol. II, 3th American ed., was written by Jonathan Pereira in 1854.