CIRCULATION OF NITROGEN 153 



which takes place in two stages, each effected by a special 

 set of bacteria, is called nitrification. 



Nitrates are the form in which green plants mostly 

 absorb their nitrogen supply through their roots, so 

 that the whole series of bacterial activities described 

 results in providing green plants with exactly the kind 

 of food they require, and at the same time gradually 

 remove dead bodies and organic debris from the surface 

 of the earth and make room for fresh life. 



During the progress of conversion of complex nitro- 

 genous compounds into simple ones a great deal of 

 nitrogen is lost to the air in the form of the free gas, 

 but certain soil bacteria (e.g. Azotobacter, Clostridium) 

 are able to fix this and incorporate it into the substance 

 of their bodies. Other bacteria (Pseudomonas) which 

 live in tubercles that are formed on the roots of certain 

 plants such as those of the pea family (Leguminosce) are 

 also able to absorb free nitrogen from the air, and the 

 green plant in which the bacterium lives gets the benefit 

 of this, eventually breaking up and absorbing the 

 products of the dead bodies of these tubercle bacteria. 

 Thus a certain amount of the nitrogen lost to the air 

 during the process of decomposition is again brought 

 into the form of living protoplasm in these ways. 



Circulation of Nitrogen and Carbon in Nature. In 

 all these ways the nitrogen contained in the proteins 

 of the protoplasm and other nitrogenous substances of 

 plants and animals is converted during their decay 

 into a form in which it can be used again by green 

 plants, so that a constant circulation of nitrogen is always 

 going on in nature, through the agency on the one hand 

 of various kinds of bacteria which break down the 

 complex substances into simple ones, and on the 

 other of green plants which build them up again into 



