1 86 



NUTRITION AND METABOLISM 



loss of nitrogen would become quite noticeable in the growth of plants; 

 there would be a scarcity of nitrogen in soil, since part of it is lost continu- 

 ously. Finally, the plants would cease to grow because the nitrogen in 

 the soil would be exhausted. 



The compensation for this destruction of available nitrogen is found 

 in the nitrogen-fixing bacteria, which, either living in symbiosis with 

 leguminous plants or growing independently in the soil, have the power 

 to use the atmospheric nitrogen for the formation of their own proto- 

 plasm. Thus, organic nitrogen is produced from nitrogen gas and the 

 continuance of organic life is guaranteed. 



uLp/i 



ur 



Or. 



yaniSms 



Fig. 104. — Sulphur cycle. 



Sulphur Cycle. — Little more can be said about sulphur, since the 

 rotation is quite similar to that of nitrogen. Plants will take sulphur 

 usually in the form of sulphates and make protein compounds contain- 

 ing a certain amount of sulphur (Fig. 104). These bodies are either 

 digested by higher animals or broken down by putrefaction to the 

 final product, hydrogen sulphide, which is oxidized by the sulphur 

 bacteria first to sulphur, then later to sulphates. 



Phosphorus Cycle. — ^The cycle of phosphorus has not been worked 

 out completely, but from the discussion in the last pages, it is plainly 

 seen that a simple cycle very much like the ones above must exist. It 



