PRODUCTS OF METABOLISM. 127 



be disastrous to organic life on earth if there were no means to compensate 

 for the loss of nitrogen in circulation. Imagine what would happen if 

 there were no such compensation! Part of the nitrate in the soil is 

 destroyed, the nitrogen gas escapes into the air and is as indifferent as the 

 nitrogen of the atmosphere, lost to organic life forever. More nitrate 

 would be produced from decaying organic matter and would be destroyed. 

 After a certain time, this continuous 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 continuously. Finally, the plants would 

 cease to grow because the nitrogen in the soil would be exhausted. 



ny clroqen^Su Iphicte 



frofe/, 



FIG. 51. Sulphur cycle. (Original.) 



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 protoplasm. 

 Thus, organic nitrogen is produced from nitrogen gas and the constancy 

 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 containing 

 a certain amount of sulphur (Fig. 51). These bodies are either di- 

 gested by higher animals or broken down by putrefaction to the final 



