This section is from the book "The Potato: A Compilation of Information from Every Available Source", by Eugene H. Grubb, W. S. Guilford. Also available from Amazon: The Potato: A Compilation Of Information From Every Available Source.
The potato (Solarium tuberosum) belongs to the Solanum or Nightshade family. (Sola-men is a Latin word meaning soothing or quieting.) In Bailey's "Encyclopedia of Horticulture," published by the Macmillan Company, New York, it is technically described as follows:
Solanum, giving name to the family Solana-ceae, is a vast genus of temperate and tropical herbs, shrubs and even trees, but is comparatively poorly represented in temperate North America. Dunal, the latest monographer, in 1852 recognized 901 species, and many species have been described since that time. The genus finds its greatest extension in tropical America. Of the vast number of species barely 25 are of much account horticulturally, and half that number will comprise all the species that are popularly well known. One of these is the Potato, Solanum tuberosum, one of the leading food plants of the human race. The genus seems to abound in plants with toxic properties, although its bad reputation in this respect is probably exaggerated.
As a genus, Solanum is not easily separated from other genera, but some of its most desig-native characters are as follows: Leaves alternate: inflorescence mostly sympodial and therefore superaxillary or opposite the leaves: corolla gamope-talous and rotate or shallow-campanulate; plaited in the bud, the limb angled or shallow lobed; stamens usually 5, inserted on the throat of the corolla, the anthers narrower or elongated and connivent and mostly opening by an apical pore or slit: ovary usually 2-loculed, ripening into a berry which is sometimes enclosed in the persistent calyx. The flowers are white, purple or yellow. The species are herbs in temperate climates, but in warm countries many of them are shrubby and some are small trees. Many of them are climbers. Two species bear underground tubers beside the tuberosum. The tuberosum is described as follows:
Low, weak-stemmed, much-branched perennial with tender, herbaceous tops, and perpetuating itself asexually by means of thickened or tuberous underground stems, glabrous or pubescent-hirsute: leaves are unequally pinnate; the 5-9 oblong-ovate leaflets are interposed with much smaller ones: flowers are lilac or white, in long-stemmed dichotomous clusters, the corolla prominently lobed: the fruit is a small globular yellow berry, usually not produced in the highly developed modern varieties. It is a native of the temperate Andes of Chile and adjacent regions. There is a form with yellow-blotched leaves (known as var. variegatum) sometimes cultivated for ornament.'"
A study of the structure and composition of the potato is very interesting. Dr. C. F. Langworthy, in " Farmers' Bulletin 295 " of the United States Department of Agriculture, gives a very full and comprehensive description. It follows:
The potato tuber is in reality a modified stem, being shortened and thickened as a storehouse for material held in reserve for the early growth of new plants. The outer skin of the tuber consists of a thin, grayish brown corky substance and corresponds roughly to the bark of an overground stem. If a crosswise section of a raw potato is held up to the light three distinct parts besides the skin may be seen. The outermost one is known as the cortical layer and may be from 0.12 to 0.5 inch in thickness. This layer is slightly colored, the tint varying with the kind, and turns green if exposed to the light for some time, thus showing its relation to the tender green layer beneath the bark of overground stems. It is denser than the other parts of the potato and contains many fibro-vas-cular bundles, especially on the inner edge where a marked ring of them plainly separates this layer from the next. The interior or flesh of the tuber is made up of two layers known as the outer and inner medullary areas. The outer one forms the main bulk of a well-developed potato and contains the greater part of the food ingredients. The inner medullary area, sometimes called the core, appears in a cross section of the tuber to spread irregular arms up into the outer, so that its outline roughly suggests a star. It contains slightly more cellulose and less water and nutrients than the outer medullary portion. These four parts of the tuber are shown in the illustration.
As in all other plant forms, the framework of the tuber is made up of cellulose, a carbohydrate or group of carbohydrates familiar in many forms, as, for instance, the fibre of cotton or linen or the bran of wheat. In food and feeding stuff analyses it is usually designated crude fibre. Cellulose forms the walls of a network of cells, which in turn form the body of the tuber. These cells vary in shape and size in different sections of the tuber according to the part they play in its life. In the flesh they serve mainly for storage, and in them lie the starch grains.
The interior of the tuber is more or less permeated by water in which are dissolved nearly all the soluble ingredients, including the various soluble carbohydrates. In this connection it is well to recall that the carbohydrates (cellulose, starch, the different kinds of sugars, etc.) are all closely related, and that under the influence of certain acids, heat, or other agency an insoluble form, such as starch, may be changed into a soluble form, or vice versa.
Cultural varieties of a given plant often have very different habits, appearance and quality, and it is natural that the amounts and proportions of water, carbohydrates, fats, protein and mineral matters which the potato contains should vary with the variety as well as with the character of the soil, the climate, and other conditions under which it grows. Moreover, since the needs of the potato plant vary at different stages of its development, it will provide for them by varying the ingredients stored in the tubers and elsewhere. Taking into account all these factors, it might seem impossible to make any general statements about the chemical composition of the potato, but it may be said that the variations are in degree rather than in kind, and so many analyses and studies have been made, both in this country and in Europe, that the average or general characteristics of the potato are now well established. The following table shows the composition of raw and cooked potatoes and represents the average of many American analyses. For comparison the composition of white bread is also given.