25 per cent of dry
matter, the larger portion being starch. There is but little nitrogenous
material in the potato, only 2.25 per cent, of which about half is in
the form of proteids. There are ten parts of non-nitrogenous substance
to every one part of nitrogenous; or, in other words, the potato has a
wide nutritive ratio, and as an article of diet needs to be supplemented
with foods rich in protein. The mineral matter, cellular tissue, and
fat in potatoes are small in amount, as are also the organic acids.
Mechanically considered, the potato is composed of three parts,--outer
skin, inner skin, and flesh. The layer immediately beneath the outer
skin is slightly colored, and is designated the fibro-vascular layer.
The outer and inner skins combined make up about 10 per cent of the
weight of the potato.

[Illustration: FIG. 9.--TRANSVERSE SECTION OF POTATO.
(After COWDEN and BUSSARD.) _a_, skin; _b_, cortical
layer; _c_, outer medullary layer; _d_, inner medullary layer.]

A large portion of the protein of the potato is albumin, which is
soluble in water. When potatoes are peeled, cut in small pieces, and
soaked in water for several hours before boiling, 80 per cent of the
crude protein, or total nitrogenous material, is extracted, rendering
the product less valuable as food. When potatoes are placed directly in
boiling water, the losses of nitrogenous compounds are reduced to about
7 per cent, and, when the skins are not removed, to 1 per cent.
Digestion experiments show that 92 per cent of the starch and 72 per
cent of the protein are digested.[12] Compared with other foods,
potatoes are often a cheap source of non-nitrogenous nutrients. If used
in excessive amounts, however, they have a tendency to make the ration
unbalanced and too bulky.

MECHANICAL COMPOSITION OF THE POTATO

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|Per Cent
Unpeeled potatoes | 100.0
Outer, or true skin | 2.5
Inner skin, or fibro-vascular layer[A] | 8.5
Flesh | 89.0
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CHEMICAL COMPOSITION OF THE POTATO

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| | | | CARBOHYDRATES
|-----|-------|---|------------------------
|Water| Crude |Fat|Nitrogen-free-|