The tubers of certain plants constitute, after the seeds, one of the most important classes of energy-yielding foods. The white Irish potato is by far the most important of these in the temperate zones. In the Orient several kinds of tubers are widely used as human food. An examination of the potato has revealed the special dietary properties of this tuber to be just what we should expect from its function as a storage organ for reserve food in the plant (26). The function of the potato is two-fold, reproduction of the species and a source of food supply for the young potato plantlet while it is developing a root and leaf system, which make it independent of the food in the old tuber. The "eyes" of the potato represent groups of cells which are analogous to the germ of the seed. These are the locations from which the potato sprouts when the conditions are favorable. There is underneath the skin of the tuber a layer of cells which are alive and which respire during the life of the tuber. The interior of the potato consists almost entirely of water, starch, protein, and to some extent of mineral salts. The cellular structures in the interior are gorged with starch. This portion is therefore analogous to the endosperm of the seed. Both are comparable to a mixture of purified protein, carbohydrate and salts, which, as we have previously seen, are not capable of supporting life. This portion like the portion of the rice kernel remaining after polishing, is almost devoid of the dietary essentials, fat-soluble A and water-soluble B, and accordingly cannot meet the nutritive needs of an animal, even though it may have an appropriate chemical composition.

From the dietary standpoint the potato is to be classed with the seeds because it consists largely of reserve materials and of relatively little vegetative tissues. It is very likely that when it is steamed and the thin, paper-like skin removed without the loss of the cellular layer just underneath, it will contain relatively more fat-soluble A than certain of the cereal grains. Although it has not been subjected to experimental test, it is probable that a potato pared in the ordinary way and the parings discarded, is changed in its dietary properties in much the same manner as is the rice kernel during polishing. In the latter case the bran layer and the germ are both rubbed off, leaving the endosperm without the small quota of cellular elements it possessed in the natural state. The protein of the potato is not quite so valuable for the support of growth as is that of the cereal grains when it serves as the sole source of this dietary factor (26). It is when raw an excellent anti-scorbutic food.