CHAPTER II 



THE ENERGY RELATIONS OF PLANTS AND 

 ANIMALS 



A CHEMICAL reaction can usually be assigned to one of 

 two classes. Either it is exothermic, that is, attended by 

 the evolution of heat, or it is endothermic, in which case 

 heat must be supplied to cause its occurrence. When 

 heat is evolved the products of the reaction are generally 

 simpler and more stable than the original material. The 

 most important reactions of this class are the oxidations. 

 Heat, or other forms of energy applied from without, may 

 effect the synthesis of complex substances rich in fuel 

 value from initial material comparatively destitute of 

 potential energy. 



Broadly speaking, the constructive chemical processes 

 in nature are the work of the higher plants. Animals, as 

 well as those forms of plant life which lack pigment, carry 

 on for the most part reactions of a destructive character. 

 This makes evident the dependence of all other forms of 

 living matter upon the pigmented plants. It is through 

 the agency of light-waves, a form of kinetic energy, that 

 the synthetic reactions resulting in the formation of starch 

 and other energetic compounds are made possible. When 

 light is excluded from the green plant it has no advantage 

 over the animal, but pursues a similar course of decomposi- 

 tion. In fact, there is always going on in the plant, 

 even when its constructive activity is most marked, an 

 undercurrent of an opposite trend. Early writers com- 

 monly exaggerated the supposed contrast between the 

 chemical conduct of plants and that of animals. They 

 were disposed to deny that an animal could execute any 

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