SOME GENERAL BIOLOGIC PROCESSES 



529 



starch, set free in the air the oxygen again, and thus this circle is 

 completed. A study of the diagrams will help to fix this in your 

 mind. 



Nitrogen Circle. Nitrogen, also, is absolutely essential to all 

 living tissue and protoplasm as well as all proteid food. Plants 

 obtain nitrogen compounds from the soil, mainly as soluble ni- 

 trates. They use them in making their living tissues, which in 

 turn furnish to animals their only source of nitrogenous food. 



Here again one would be justified in supposing that the nitro- 

 gen was out of reach of future use. If this were so, life would long 

 since have ceased, as ordinary soil contains only enough nitrogen 



CYCLE. 



mr 



ffforo s Y/VTME 3 is 



FIG. 161. Chart showing interdependence of plants and animals for 

 oxygen and carbon dioxide. 



compounds to last about thirty years, if none were replaced. 



All waste excreted from animals contains nitrogen compounds, 

 and in the course of nature this should get back to the soil as 

 natural manures. Whenever a plant or animal dies, decay takes 

 place, and much of its nitrogen is thus returned by the action of 

 certain decay bacteria. However, neither manures nor decay 

 would give back enough, especially as man disposes of all his 

 sewage by washing it into rivers or ocean where it cannot get 

 back to the soil from which it came. 



Furthermore, much nitrogen is set free into the air by decay 

 and oxidation in such a way that plants cannot use it, except 

 it be combined with other elements. So there would be a serious 



