It is interesting to reflect upon the reasons why the leaf of the plant should show such decided differences in its nutritive properties as contrasted with the seed. A consideration of the function of the two organs of the plant gives the clue to the cause. The seed is composed of a germ, which in most cases forms but a small part of the entire seed, together with a relatively large endosperm. The germ consists of living cells, which respire and which are capable of germination (multiplication) when the conditions are favorable. In the wheat kernel the germ constitutes but 1.5 per cent of the entire kernel. The endosperm, on the other hand, contains large amounts of reserve materials such as protein, starch, sugars, fats and mineral salts. These are not living matter but its byproducts. The endosperm is, therefore, in most respects comparable to a mixture of purified food substances. Experiments have abundantly demonstrated that the endosperm contains relatively little of the anti-neuritic substance, whereas the germ is rich in this dietary factor. The last statement perhaps needs some qualification in the light of recent investigations. Osborne and Mendel (5) state that when they carefully dissected the germ from the wheat kernel, and secured practically pure germ substance, the product contained no water-soluble B. The latter substance is certainly situated immediately adjacent to the germ, for the germ as obtained in the milling process contains a very large part of all that is found in the entire kernel. The dietary factor, fat-soluble A, is likewise largely localized in the germ, where it is relatively abundant as compared with other seed products. Since the germ in most cases forms such a small fraction of the seed, the seed itself is in general poor in this substance as compared with the leaf, which is in large proportion made up of vegetative tissue containing all the protoplasmic structures necessary for active metabolism.

The leaf of the plant, on the other hand, is rich in cells which during life are actively functioning, and which, with but few exceptions, contains but little reserve food material. It is the laboratory of the plant. The chlorophyll, its green pigment, enables it to utilize the energy of sunlight, and from the carbon dioxid absorbed from the air together with water and mineral salts derived from the soil, it builds up protein, starch, sugar and fat, which are used for the growth of new plant tissue, or for storage in the seed, tuber or other organ. The surface of the leaf is a mosaic of living cells. It contains all the complexes necessary for the nutrition of animal cells, and the leaf is qualitatively a complete food.