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TEXTBOOK OF BOTANY 



other forms of bacteria may reduce the nitrates and nitrites to gaseous 

 nitrogen. This is denitrification. 



Other groups of bacteria bring about the transformation of free nitro- 

 gen of the air to amino compounds which accumulate in the organisms. 

 This is known as nitrogen- fixation. Nitrogen-fixing bacteria are of two 



PLANT 



AMINO 



ACIDS. 



SULPHATES 





FREE 

 rNlTRATEs\2t^ilIM:l^£QWN ITR06EN\ 



RN03 / i °^ ^^^ 



TLANT^ 



ANIMAL^ 



vRESIDUESy 







f AMMONIA! 

 NH 3 



(^Bacteria) 



Fig. 233. Diagram of the nitrogen cycle in nature. Broken arrows indicate escape 

 of ammonia and other volatile nitrogen compounds to the air. 



kinds: those living symbiotically in legume roots, and those living free 

 in the soil. Upon their death the nitrogen compounds become available. 



All the processes having to do with nitrogen in its relation to green 

 plants may be grouped together into what has generally been knoMoi as 

 the nitrogen cycle (Fig. 233). 



Sulfur bacteria. We have seen earlier that sulfur is a part of the mole- 

 cule of some amino acids and proteins. Green plants in general secure 



