POTATO is the well known crop plant known botanically as Solanum tuberosurn and belonging to the family Solanaceae. Its value as food is due to the habit of developing underground a number of slender leafless branches which swell at the free ends into tubers, i.e., potatoes. Their stem-like nature is not only known by their origin, but by the fact that they bear leaf-buds the so-called "eyes," which later develop into ordinary shoots. The tubers are usually formed underground, but may appear on stems above ground. Darkness and low temperature appear to favour tuber-formation, as does also any checking of the growth of the shoots or fruits. It has been suggested that the formation of tubers is associated with the presence of certain fungi, but this is unproven; of the 1,200 species of Solanum less than a dozen have the property of forming tubers. When the young tubers are exposed to light they become green and are acrid to the taste and develop solanine, a poisonous chemical substance. The ripe tuber, when cut across, shows an outer cork skin or periderm, a thin cortical layer bounded internally by a ring of vascular bundles, and a thick medulla divided into an external and an internal portion. The internal medulla is usually star-shaped and is poorer in starch and in solid matter generally and is richer in water.
The "eye" is usually a group of buds which lie in a slight de pression in the tuber, the depression representing the aril of a scale leaf. The line above the depression, the "eye-brow" or "eye-yoke," meets the junction of scale leaf and stem; the eye is thus an axillary branch with undeveloped internodes. Examination shows that the buds are arranged spirally on the tuber, in con sonance with the ordinary spiral phyllotaxy of leaves. (See LEAF.) The end attached to the stalk is the "stem end" or "heel" of the tuber; and the free end is the "crown" or "rose end" or "seed end." The terminal bud, i.e., the eye at the seed end is usually the strongest and develops the strongest shoot. The eyes may be deep set or level with the surface producing a smooth potato.
Composition of Potato.—The composition of the tuber naturally varies with the variety and in part with the conditions of growth. Potatoes contain about 75% of water, 12-15% of
starch, and from 2-21% of protein material, together with a small amount of ash. The normal potato contains only a small quantity of sugar (about 0.3%), but exposure to frost markedly increases the amount: the sweetness of "frosted" potatoes is familiar. If the tuber is analysed in part it is found that the dry matter decreases as we pass inwards In one type analysed the percentage of dry matter in the cortex, outer medulla, and inner medulla respectively, was 22.20, 19-41 and 14.92. The desirable qualities of a potato, in England and the United States, are a capacity to develop mealiness on boiling and this largely depends on the amount of starch. In France, however, potatoes are rarely boiled but usually cooked in fat, hence there is a demand for a different type of potato with a firm yellow flesh, becoming not mealy but "soapy" on boiling ; such a potato is usually low in starch and high in nitrogen content. Where potatoes are grown for the manufacture of spirit, as in Germany, the content of starch is of the first importance.