104 The Living Plant 



leased again by respiration or decay. A quantity, rather small, 

 of the earth's supply of carbon dioxide and water is therefore 

 always locked up in plant and animal substance; but though the 

 quantity is approximately constant the precise molecules are 

 constantly changing, and with the changes go those transforma- 

 tions of energy which are the principal manifestation of life. And 

 if the question be asked, why are not more of the carbon dioxide 

 and water of nature locked up in plant and animal substance, 

 that is, why are there not more and larger plants and animals on 

 earth, I think the answer is easy. There do already exist upon the 

 earth all of the plants and animals, and as big ones, as the physical 

 conditions permit. As to plants, every spot on the earth that 

 can maintain plant life at all is bearing all the plants it can sup- 

 port, and these plants are just as big as the physical conditions 

 permit them to grow. As to animals, they are dependent upon 

 plants for their food, and it is evident that there is available for 

 their use only the surplus of food produced by plants over that 

 which these need for themselves, and animals are just as abun- 

 dant and big as that surplus can support. 



Thus, these apparently very complicated processes of photo- 

 synthesis and respiration, like many another and probably like all 

 of the physiological processes in plants and in animals, can be 

 reduced to a basis of pure physics and chemistry. And we shall 

 learn later, in our chapters on Irritability and on Growth, that 

 we have a good explanation of the orderly sequence and regular 

 connection of the processes in their linking up together through 

 their interactions as stimuli. Is there then, nothing in the plant 

 except the interactions of chemistry and physics? Let the remain- 

 ing pages of this book give their testimony before we attempt the 

 answer. 



