4 ELEMENTS OF WATEE BACTERIOLOGY 



gen to a more available form. This process of oxida- 

 tion is known as nitrification, and takes place in a suc- 

 cession of steps, the organic nitrogen being first con- 

 verted to the form of ammonium salts, and these in 

 turn to nitrites and nitrates, the oxygen used coming 

 from the air. Several groups of organisms are instru- 

 mental in bringing about this conversion. It is gen- 

 erally assumed that one group attacks the ammonium 

 compounds and changes them to nitrites; while another 

 group completes the oxidation to nitrates. In the 

 latter form nitrogen is readily taken up by green plants 

 to be built up into more complex albuminoid sub- 

 stances (organic nitrogen) through the constructive 

 power of chlorophyll. 



This never-ending cycle is illustrated in the accom- 

 panying figure, devised by Sedgwick (Sedgwick, 1889) to 

 illustrate the transformations of organic nitrogen in 

 nature, the increasing size and closeness of the spiral 

 on the left-hand side indicating the progressive com- 

 plexity of organic matter as built up by the chlorophyll 

 bodies of green plants in the sunlight, and the other 

 half of the figure the reverse process, carried out largely 

 by the bacteria. In nature there are many short 

 circuits, as, for instance, when dead organic matter 

 is used as food for animals and built up into the living 

 state again without being nitrified and acted upon 

 by green plants; but the complete cycle of organic 

 nitrogen is as indicated on the diagram. 



We have dwelt thus at length upon the general 

 relation between bacteria and organic decomposition 

 because in this relation will be found the master key 



