378 PHYSIOLOGY 
to be composed chiefly of proteins (probably more complex and labile 
than in the non-living state), it is evident that protein foods are of the 
highest importance indeed indispensable for nutrition, since it is 
the protoplasm which grows, wastes, and needs repair. Proteins are, 
as it were, the highest type of foods; they represent the final stage of 
food making. 
Inasmuch as the carbohydrates contain only carbon, hydrogen, and 
oxygen, while proteins contain in addition nitrogen and sulfur and in 
many cases phosphorus also, it is plain that they cannot be formed from 
carbohydrates alone. A strict carbohydrate diet is as unsuitable for 
plants as it is for animals. Some materials must be supplied from which 
nitrogen, sulfur, and phosphorus can be obtained. 
Source of nitrogen. As the air contains 78 per cent of nitrogen, the 
atmosphere would appear to be a natural source of this element. But 
though the nitrogen is everywhere dissolved in the water of the plant, 
and can enter and leave it freely, no plants are known to be able to use 
it in this uncombined form, except certain bacteria, some of which live 
in the soil and in some waters. Certain soil species enter the roots of 
various plants, especially the Leguminosae, causing them to form tuber- 
cles. (See below, p. 379.) Almost all plants, therefore, must get com- 
bined nitrogen. This is found in soils as nitrates of various bases, e.g. 
calcium, magnesium, potassium, and sodium; and when a soil is deficient 
in nitrogen, such compounds are important constituents of the fertilizers, 
natural and artificial, which are added to it. The nitrates in the soil 
result mainly from the decay of organic matter in it. The later steps 
in the process are controlled by certain bacteria in the soil which bring 
about the oxidation of ammonia to nitrites, whereupon others oxidize 
the nitrites to nitrates. The very fertility of arable soils, therefore, 
depends on the microscopic organisms living in them, which prepare the 
way for the larger plants. 
Loss of N. The soil of cultivated areas is constantly losing its com- 
bined nitrogen by solution and drainage, and this loss is only partially 
made good by the ammonia and nitrous and nitric acids washed into it 
from the air by rains. Under natural conditions the dying vegetation 
ultimately returns its constituents to soil and air ; but crops are carried 
off, their nitrogen with them. Gardens and fields, therefore, require 
replacement of this nitrogen sooner or later. When they lie fallow, 
certain bacteria of the soil, associated with algae and perhaps with 
other plants, slowly increase the nitrogen content of the soil by fixing 
