divided into subdivisions, as carbohydrates, pectose substances or
jellies, fats, organic acids, essential oils, and mixed compounds. In
plants the carbohydrates predominate, while in animal tissue the fats
are the chief non-nitrogenous constituents.

7. Carbohydrates.--This term is applied to a class of compounds
similar in general composition, but differing widely in structural
composition and physical properties. Carbohydrates make up the bulk of
vegetable foods and, except in milk, are found only in traces in animal
foods. They are all represented by the general formula CH_2n_2n, there
being twice as many hydrogen as oxygen atoms, the hydrogen and oxygen
being present in the same proportion as in water. As a class, the
carbohydrates are neutral bodies, and, when burned, form carbon dioxide
and water.

[Illustration: FIG. 2.--CELLULAR STRUCTURE OF PLANT CELL.]

8. Cellulose is the basis of the cell structure of plants, and is
found in various physical forms in food materials.[3] Sometimes it is
hard and dense, resisting digestive action and mechanically inclosing
other nutrients and thus preventing their being available as food. In
the earlier stages of plant growth a part of the cellulose is in
chemical combination with water, forming hydrated cellulose, a portion
of which undergoes digestion and produces heat and energy in the body.
Ordinarily, however, cellulose adds but little in the way of nutritive
value, although it is often beneficial mechanically and imparts bulk to
some foods otherwise too concentrated. The mechanical action of
cellulose on the digestion of food is discussed in Chapter XV.
Cellulose usually makes up a very small part of human food, less than 1
per cent. In refined white flour there is less than .05 of a per cent;
in oatmeal and cereal products from .5 to 1 per cent, depending upon the
extent to which the hulls are removed, and in vegetable foods from .1 to
1 per cent. The cellulose content of foods is included in the crude
fiber of the chemist's report.

9. Starch occurs widely distributed in nature, particularly in the
seeds, roots, and tubers of some plants. It is formed in the leaves of
plants as a result of the joint action of chlorophyll and protoplasm,
and is generally held by plant physiologists to be the first
carbohydrate produced in the plant cell. Starch is composed of a number
of overlapping layers separated by starch cellulose; between these
layers the true starch or am