Roots and tubers are much used as articles of diet and differ greatly in nutritive value. They are chiefly composed of starch or sugar. The most valuable of them is the potato. It contains about 80 per cent, of water, 18 per cent, of starch, approximately 2 per cent, of nitrogenous matter, and 1 per cent, of mineral matter. The amount of fat that it contains is a small fraction of 1 per cent. Less than one-half of the nitrogenous matter in it is protein. Extractives (asparagin, etc.) constitute the remainder. The potassium salts that it contains are important for the prevention of scurvy. Potatoes can be kept well and carried long distances, especially at sea. A new or young potato contains less starch than an old one and relatively more nitrogenous matter. Potatoes are often boiled and stewed. Cooked in this way they lose much of their nitrogenous matter and mineral constituents. To retain these they should be baked or roasted in their skins. An old potato that is baked and mealy is the most digestible. Puree or mashed potato is almost or quite as easily digested. Potatoes stewed, boiled, fried, or cooked otherwise in chunks, and swallowed in considerable masses, are least digestible. Two boiled potatoes of average size will remain in the stomach about two and one-half hours. More than 90 per cent, of the starch in them is absorbed from the intestine. Potatoes contain a relatively small amount ot cellulose. They must be used in the same manner as bread - that is, to supplement meat and fats. The sweet potato has a similar composition, but a larger proportion of carbohydrate. Carrots are rich in sugar. They contain almost no starch and relatively a large amount of cellulose. Nitrogenous matter and fat occur in them in very small amounts. They are not easy to digest, and much of their constituents is lost in the feces. More than one-fourth of their sugar, which is their chief nutritive ingredient, is lost. A given weight of carrots will stay in the stomach an hour longer than the same quantity of potatoes. Parsnips and beets are similar to carrots in that they are rich in sugar, containing from 10 to 15 per cent. of it. They contain so little nitrogenous matter that it need not be taken into account. Onions are chiefly valuable because of their pungent oil, which is relished as a condiment. They also contain a considerable percentage of carbohydrate.

Turnips have a small nutritive value, since they contain about 5 per cent, of a carbohydrate, which is neither starch nor sugar, but belongs to the group of bodies named pentoses. The changes that the latter undergo in digestion are not clearly understood.

Tapioca, arrow-root, and sago are almost pure starches. The first two are derived from roots of tropic plants; the third, from the pith of the sago palm. Like rice, they are nutritious, and are probably almost completely absorbed from the intestine. Tapioca, however, is not quickly disposed of by the stomach. If a large plateful of it is eaten, it will not disappear completely from the stomach in less than two and one-half hours.