PLANT LIFE 153 



at once by plants as food ; it is necessary that 

 they should first go through a transformation 

 in which their chemical composition becomes 

 changed. It is as the agents of this change 

 or transformation that bacteria play their 

 greatest role in the world of life. In short, the 

 final destiny of all living substance is, sooner 

 or later, directly or indirectly, to become 

 food for bacteria. 



Through the activity of an enzyme produced 

 by certain bacteria a simple excretory product 

 may be converted into carbonate of ammonia. 

 Now, while green plants or algae can derive 

 some nitrogen from ammoniacal compounds, 

 it is a well-established fact that nitrogen may 

 be obtained by plants much more readily from 

 nitrates. There are then still other bacteria — 

 nitrifying bacteria — which oxidise the am- 

 moniacal nitrogen into the more available 

 form of nitrites and nitrates ; one group 

 attacks the ammonium compounds, changing 

 them to nitrites, another group completes the 

 oxidation into nitrates. Denitrifying bacteria 

 reverse this action, and reduce nitrates to 

 nitrites, nitrites to ammonia, and ammonia to 

 free nitrogen. 



There is thus a never-ending cycle : through 

 the agency of chlorophyll in sunlight there is a 

 progressive complexity in the living organic 

 matter which is built up, vegetable proteins 



