40 



ANIMAL BIOLOGY 



returning to the outer world some of this carbon as carbon dioxide, 

 it is evident that relatively enormous amounts of carbon are nev- 

 ertheless 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 contributed 

 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 proto- 

 plasm, but that of carbon and nitrogen is particularly striking and 



Carbohydrates, 

 Proteins, Fats, 

 in Green Plants 



Dead 

 Organisms 



Living 

 Animals 



Fermentation 

 and Animal 

 Respiration 



Bacterial 

 Decay 



Atmosphere 



Intermediate 

 Decomposition 

 Products 



Fig. 15. 



Fermentation 

 and Plant 

 Living^^Respiration 

 Green Plants\ I 



^-co 2 



The Carbon Cycle. A schematic representation of the circulation 

 of carbon in nature. 



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 elements 

 removed by green plants and animals are the colorless plants, such 

 as the Bacteria, Molds, Yeasts, etc. As we know, when an animal 

 or plant dies, decay sets in almost immediately; that is, the com- 

 plex chemical compounds are slowly but surely reduced to simpler 



