PLANTS AND ANIMALS 31 



for the foraging of the snail or be exterminated. If they 

 are destroyed the mollusc will be left to starve. 



Is the animal necessary to the plants? The gardener 

 will say that it is not and in the actual conditions of 

 nature the snail can well be spared. There are other 

 sources of carbon dioxid than the respiration of animals, 

 and water is abundant enough in the world. The soil 

 of the hotbed doubtless swarms with lowly and colorless 

 plants which are oxidizing the surplus organic matter 

 much as the animal does. Moreover, these organisms of 

 the soil, mainly bacteria, affect usually the dead fragments 

 detached from the green plants above and do not prey 

 upon their living leaves as the animal does so ruthlessly. 



Nitrogenous Compounds. So far we have limited our 

 discussion to the formation of fuels like starch or oils 

 which yield no other products than carbon dioxid and 

 water when they are completely oxidized. Such' com- 

 pounds are the chief source of energy for both plants and 

 animals. But where life is manifested there will always 

 be chemical compounds of another sort and, indeed, the 

 bodies which are now to be discussed seem to be more in- 

 timately connected with life itself than the standard 

 fuels we have been considering. The compounds in 

 question are the proteins. 



Proteins contain nitrogen. In a much smaller percent- 

 age they also contain sulphur. Phosphorus is present in 

 some but not in the majority of the proteins. It will be 

 plain that such compounds cannot be oxidized to carbon 

 dioxid and water exclusively for the nitrogen and the 

 sulphur must be represented in some form among the de- 

 composition products. Neither can we have any forma- 

 tion of proteins without a supply of nitrogen and sulphur. 

 At another time we may have occasion to emphasize the 

 fact that the distinguishing character of the proteins is 

 not so much the list of the elements which enter into them 

 as the complex fashion in which these elements are com- 

 bined. For the present we are concerned rather with the 

 exchanges of these elements which occur in nature. 



