ENERGY RELATIONS OF PLANTS AND ANIMALS 21 



synthesis whatever. It is true that animal cells make no 

 useful application of the energy showered upon them by 

 the sun's rays. Nevertheless they do carry on synthetic 

 reactions, although to a limited extent. Since much energy 

 is released within such cells by the prevailing oxidative 

 changes it is not difficult to see that some portion of it may 

 be applied to promote endothermic reactions. When a 

 hydraulic ram supplies with water a house considerably 

 above the level of the stream which operates the device, 

 we understand that the result is made possible because a 

 great deal of water falls that a little may rise. The prin- 

 ciple of the conservation of energy is not violated here, 

 nor is it when animal cells erect from a portion of their 

 food molecular structures more complex and energetic 

 than anything in their current supply. The formation of 

 fat from sugar is a case in point. Weight for weight, the 

 fat is more highly endowed with potential energy than is 

 the sugar, but we must take into account the fact that the 

 quantity of sugar entering into this common transforma- 

 tion is much greater than the quantity of fat which can 

 be produced. The energy in the product is more concen- 

 trated, but not absolutely larger in amount than it was in 

 the sugar. 



There are many species of green plants which are uni- 

 cellular, just as there are many single-celled animal forms. 

 It is suggestive to consider the reciprocal relations of one 

 such plant and a solitary animal cell living beside it. A 

 constant and pressing need of the animal is oxygen. Now 

 oxygen is freely produced by plants when they avail 

 themselves of the energy of light to carry on constructive 

 processes. To this extent, then, the proximity of the 

 plant to the animal is advantageous to the latter. This 

 ceases to be true when light is succeeded by darkness. 

 The animal meanwhile is giving off oxidized products, of 

 which carbon dioxid is the most abundant. This, to- 

 gether with water, is the very material out of which the 

 plant can build its stores of starch and sugar. The out- 

 put of the animal includes also various compounds of 



