42 THE NITROGEN CYCLE 



Nitrogen, like carbon dioxide, is found free in the atmo- 

 sphere, but it is in the elemental condition, and this is 

 chemically extremely inactive. As we have stated, in the 

 "nitrogen cycle" inactive atmospheric nitrogen is converted 

 into combined nitrogen through the action of "nitrifying" 

 bacteria. Others, the "denitrifying" bacteria, break down 

 combined nitrogen back into the inert elemental form. 

 Combined nitrogen can be transformed into protoplasm by 

 plants. 



The object of the diagram on page 41 is to show that 

 these cycles are not to be considered as separate but are 

 all part of one big "protoplasm cycle." The water cycle, the 

 carbon dioxide cycle, and the nitrogen cycle are all mutually 

 dependent; if one ceased, protoplasm could no longer be 

 formed and the others would cease also. 



Water is always available for living protoplasm, carbon 

 dioxide is available through the mediation of plant life, while 

 nitrogen is only available through the action of nitrogen- 

 fixing bacteria and subsequently that of plants. 



One important point is that nitrifying bacteria and plants 

 are absolutely essential to the protoplasm cycle, whereas 

 animals are only incidental; we can consider the whole of 

 animal life merely as one of the ways in which living proto- 

 plasm is converted into dead organic matter. 



