GENERAL CHEMISTRY OF BACTERIA ' 55 
proteins and carbohydrates essential for animal food. These pro- 
ducts of the synthetic activity of the plants are utilized by the animal 
kingdom for food; directly by the herbivora, indirectly by the carni- 
vora. These substances are either broken down within the digestive 
tract of the animal body and reconstructed to form the tissues and 
supply energy to the animal, or eliminated as excreta. The excreta 
of animals are not sufficiently simple in composition, as a rule, to be 
used directly by plants, and the tissues of dead animals and plants 
are of little value in their complex state for plant foods. Further 
cleavage, both of the excreta of animals and the dead bodies of plants 
and animals, is necessary to make the elements contained within 
them utilizable by plants, and this cleavage is brought about by bac- 
terial activity. Various saprophytic bacteria act successively upon 
these complex organic compounds, changing them, chiefly by hydro- 
lytic cleavage, into stable, fully mineralized salts, w^hich are directly 
utilizable in this state by the chlorophyll-bearing plants. There is, 
therefore, a constant rotation of the various elements which enter into 
the composition of animal and plant tissues between the plant and 
animal kingdoms respectively by means of an anabolic or constructive 
process in the one (plants), and a catabolic or destructive process in the 
other (animals). The cycle as outlined, however, is not a continuous 
one, for there are important gaps in the process of cleavage and in the 
process of synthesis which if left unbridged by the bacteria would 
eventually arrest all vital activity both of plants and animals, and all 
life would then inevitably cease on this planet. These gaps between 
the animal and vegetable kingdoms are filled by the analytical activity 
of bacteria. 
A small group of bacteria, on the other hand, is also important 
from the synthetical point of view. A certain amount of nitrogen is 
lost in the animal and vegetable kingdoms by various natural agencies, 
and this supply of nitrogen must be made good from sources which 
are not directly available either to plants or to animals. Approxi- 
mately 80 per cent of the atmosphere is made up of nitrogen, and a 
certain group of bacteria, "the nitrogen-fixation" bacteria so-called, 
which are found chiefly on the nodules or roots of leguminous plants, 
are able to draw upon this great reservoir of atmospheric nitrogen 
and synthesize it into nitrogen-containing compounds which plants 
can utilize directly. 
Another type of bacterial activity of importance is the oxidation 
of ammonia, the final step in the degradation of protein, into nitrites 
and nitrates. This is carried on by the nitrifying bacteria of the soil. 
Contrary to the generally accepted idea, therefore, the activities of 
the majority of bacteria are not in opposition to the activities of man, 
animals and plants; bacteria are indispensable agents in the economy 
of Nature. 
