262 NUTRITION AND METABOLISM 



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

 continuance of organic life is guaranteed. 



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. 116). 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 

 is probably much simpler because phosphorus does not enter as easily 

 into organic compounds as nitrogen. 



