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. Pood 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- 

 bonic 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. 



