358 PHYSIOLOGY 
terials. What they make is universally known as food for their colorless 
cells, for non-green plants, and for animals. Why should it not also be 
recognized as their own food? 
Food for plants is organic. Food for plants, then, is like food for 
animals, always organic, the product of living beings; and in the last 
analysis, all food is made by green plants, for they alone among living 
beings have the power of making it out of the simple compounds CO 2 
and H2O. 1 They make it only in the green cells by the aid of light; 
and they make so much that they feed not only themselves, but all other 
creatures. The lion may live exclusively on flesh, but the flesh was 
built up by the herbivorous animal from the herbage it grazed, and the 
herbage was nourished by the foods it could itself make in sunlight. 
Man grows plants and appropriates the leaves, the roots, the stems, the 
fruits, or the seeds, improved by his selection and loaded with surplus 
food, for his own nourishment; or he feeds the steer, the sheep, and the 
hog with grass or grain that he may later use their flesh for his food. 
What are the plant foods ? Having established a general meaning for 
the word food, the next question is: To what specific substances is it 
to be applied? Foods come from many sources and are of many kinds; 
and because they are so various, only the principal classes can be named, 
and a few examples briefly described. The four most important sorts 
are carbohydrates, fats, amides, and proteins. 
Carbohydrates. Some carbohydrates are directly made by green 
plants; but there are also many that are secondary products. The 
name is no longer used in chemical classification; it is rather convenient 
than exact, just as " cryptogam " among plants or " invertebrate " 
among animals. Here belong the sugars, the starches, and the celluloses, 
each probably comprehending an indefinite number of different in- 
dividuals. This is certain among the simpler sugars, whose composi- 
tion is known; but only hypothetical for starch and cellulose, whose 
complexity has hitherto baffled all analysis. 
All these substances have a composition like this : C n H2nO n , or CnH2(n-i)O(n-i), 
or C n H2( n -2) O( B -2), in which the value of is 6 or a multiple of 6 where known, 
but may run up to several hundred. Thus grape sugar and its allied hexoses all 
contain CeH^Oe; while cane sugar and its allies consist of Ciar^On. Starch and 
cellulose can be represented only as n (CgHioOs), with the value of n quite uncertain, 
but large. These empirical formulas, however, cannot convey any idea of the 
1 Certain bacteria also seem to be able to utilize these substances to form foods; but 
so far as is known the product is utterly trivial in amount, and the fact is entirely without 
significance, were it not for its exceptional character. 
