OUTLINES OF THE PHYSIOLOGY OF NUTRITION 22<> 



of quite another kind, by which the plant instead of 

 gaining substance loses it, but in doing so gains 

 energy for the work which it has to perform. This 

 process is the breathing or respiration of plants. So 

 long as plants are in active life, they, like animals, 

 are continually taking up oxygen from the air, and 

 giving off a corresponding amount of carbon dioxide. 

 The latter, as we know, is a product of combustion. 

 The oxygen which is absorbed is taken up by the 

 protoplasm, and ultimately combines with a portion of 

 its carbon. The product of oxidation, carbon dioxide, 

 is given off again into the air. The breathing of plants 

 is, in fact, just the same thing as our own breathing, 

 or that of other animals. We must be very careful 

 not to confuse respiration with carbon-assimilation. 

 In both processes an exchange of gases between the 

 plant and the atmosphere goes on. In assimilation, 

 however, the plant gains carbon at the expense of the 

 carbon dioxide of the air, while in respiration it loses 

 carbon at the expense of its own protoplasm. The 

 former is a constructive process, by which more com- 

 plex bodies are built up out of simpler ones; the 

 latter is a destructive process, by which complex sub- 

 stances are broken down into simpler ones. In 

 assimilation deoxidation takes place, while respiration 

 is a process of combustion or oxidation. 



In the case of green organs exposed to light, the 

 two processes go on together. In light of ordinary 

 intensity assimilation is much more vigorous than 

 respiration, so that carbon dioxide is decomposed much 

 more quickly than it is formed. In darkness, however, 



