METABOLISM OF COLORLESS PLANTS 47 



directly feed on plants so that the ultimate source of the 

 carbon of animals is likewise the carbon dioxide of the atmos- 

 phere. Although both plants and animals by their respir- 

 atory process are continually returning to the outer world 

 some of this carbon as carbon dioxide, it is evident that 

 relatively enormous amounts of carbon are nevertheless being 

 taken out of circulation and locked up in the bodies of the 

 plants and animals. For example, it has been estimated 

 that about one half the weight of a dried tree trunk is con- 

 tributed by carbon. 



The same general segregation is going on in regard to 

 nitrogen. The green plants take it in the form of nitrates, 

 for instance, and store it away in the proteins; and again 

 animals get their nitrogen from plant proteins, so that the 

 ultimate source of the animal nitrogen is the same. In a 

 somewhat similar manner we might trace the fate of the 

 other chemical elements necessary for protoplasm, but that 

 of carbon and nitrogen is particularly striking and instructive, 

 and is sufficient to illustrate the fact that although both 

 green plants and animals are continually taking elements 

 from and returning them to their environment, nevertheless 

 more is taken away than is returned. (Figs. 15, 16.) 



The agents which restore to the inorganic world the ele- 

 ments removed by green plants and animals are the colorless 

 plants, chief among which are the Bacteria. As we know, 

 when an animal or plant dies, decay sets in almost immedi- 

 ately; that is, the complex chemical compounds are slowly 

 but surely reduced to simpler and simpler forms until ' dust ' 

 remains. Although undoubtedly many of these compounds 

 would automatically, so to speak, tend to simplify, never- 

 theless this is not only hastened, but chiefly carried out by 

 organisms of decay such as the Bacteria. Through enzymes, 

 or ferments, which they form, FERMENTATION occurs. The 



