NUTRITION 71 
"raw materials/' With a very few exceptions among the 
bacteria, plants that are not green (such as toad-stools, 
molds, and other fungi), and most animals cannot do this. 
Therefore we are wholly dependent upon green plants for 
the food supply of the world. Animals and non-green 
plants live, either directly or indirectly, upon green plants. 
The bodies of all living things are constantly giving off 
inorganic compounds (such, for example, as the carbon 
dioxide given off in respiration, and the water in trans- 
piration) , and after death all bodies are (by the action of 
bacteria) gradually broken up into inorganic substances. 
Thus, we see, there is a kind of perpetual cycle, or cir- 
culation, from one realm to the other, as indicated in the 
diagram (Fig. 52). 
69. Kinds of Foods. When we examine the bodies of 
plants we find that the foods elaborated are stored in 
various organs or tissues. These foods all belong to one 
of three classes of substances, viz., carbohydrates, re- 
presented by starch, sugars', and cellulose; proteins, 
represented by protoplasm itself, the gluten of wheat, 
and other substances ; and fats, represented by the various 
oils, such, for example, as olive and cotton-seed oil. 
70. Chemical Composition of These Foods. A chemical 
examination of carbohydrates reveals the fact that they 
are composed of the elements, carbon, hydrogen, and 
oxygen, the two latter occurring in the same proportion 
as in water, namely, two parts of hydrogen to one of water 
(H 2 O). Fats contain the same elements, only in different 
proportions, while proteins, in addition to carbon, oxygen, 
and hydrogen, always contain nitrogen. Other sub- 
stances almost universally found in plants are calcium, 
potassium, magnesium, phosphorus, sulphur, iron, and 
