142 PLANT-ANIMALS [CH. 



cent, of nitrogen gas which is contained in the 

 air. The ammonia may leak away also as every 

 dung-hill testifies or it may be fixed in the soil 

 by the agency of certain nitrifying micro-organisms. 

 These bacteria convert the ammonia into nitrates 

 and the nitrates so formed become available to the 

 roots of the green plant. On the other hand, the 

 nitrates of the soil may be seized upon by yet other, 

 denitrifying micro-organisms and, becoming con- 

 verted into ammonia compounds, may be lost to the 

 vital circulation. The constant leakage of nitrogen 

 from combined forms to the free and inert form of 

 nitrogen gas results in a shortage of nitrogen available 

 for the formation of the nitrogenous food of plants. 

 We may thus speak of the problem which besets all 

 living organisms that of obtaining adequate supplies 

 of organic nitrogen compounds as the nitrogen 

 problem, and we may well believe that the sum-total 

 of life supported on our planet is determined ulti- 

 mately by the amount of available nitrogen present 

 in the earth and sea. Occasionally, organisms are 

 met with which have solved the nitrogen problem 

 in a fundamentally satisfactory manner. Among 

 such organisms are nitrogen-fixing bacteria, legu- 

 minous plants and man. Each of these organisms 

 has evolved methods of bringing back into vital 

 circulation the nitrogen which has escaped as nitrogen 

 gas into the air. 



The nitrogen-fixing bacteria which occur in the 



