THE ATMOSPHERE 79 
3. Nitrogen (see p. 99). 
The Interdependence of Plant and Animal Life.—In the 
preceding section we have drawn attention to the influ- 
ence of the life and death of organic material upon the 
constitution of the atmosphere. A large part of organic 
matter is used as food. Food is eaten by animals for two 
purposes : 
1. By its further elaboration it is made into new tissue, 
which either replaces the waste of the old or adds to 
growth. 
2. By its destruction during the processes of respira- 
tion it maintains the heat. of the body, and supplies to 
every organ the energy necessary for it to perform its 
functions. 
Animals all their lives consume food, but the food which 
they eat they do not make. The real maker of food, and 
the only maker, is the green plant, which manufactures 
from two bodies present everywhere—carbonic acid gas 
and water—the primary food-stuff out of which all others 
are elaborated. Animals all their lives are putting car- 
bonie acid gas back into the air, and from this body food 
is again made by the vegetation. And so the cycle ever 
goes on, food being continually re-formed out of the 
elements of its destruction. Without the green plants 
there could be no animals, because food would not exist. 
If animals alone existed, their respiration would, in the 
course of time, make the air unbreathable, because its 
oxygen could not be replenished. Without the food- 
consuming animals the supply of carbonic acid gas would 
sooner or later be used up, and the green plant would 
vanish. The interdependence of plants and animals is 
complete. 
