CHAPTER X 



BACTERIA AND THE CIRCULATION OF NITROGEN 



IN NATURE 



1. Introduction : The Assimilation of Free Nitrogen by the Bacteria 

 of the Root Nodules of Leguminous Plants, and by Bacteria in 

 the Soil. 



APART from the activity of organisms like the pigment and phos- 

 phorescent bacteria, and the remarkable metabolism of the sulphur- and 

 iron-bacteria, the work of bacteria in nature embraces three great processes : 



1. The circulation of nitrogen: effected by putrefaction, the formation of nitrates, 

 and the assimilation of atmospheric nitrogen. 



2. The circulation of carbon by the, 'fermentation ' of carbohydrates and other non- 

 nitrogenous products of animals and plants. 



3. The causation of disease in other organisms, particularly in man and the higher 

 animals. 



There are in nature five sources of nitrogen open to plants and animals : 



1. The atmosphere (79 per cent, by volume of free nitrogen). 



2. The nitrates of the soil and the traces of nitrous acid formed in the air during 

 thunderstorms. 



3. Ammonia, which occurs in minute quantities in the air, and is set free abundantly 

 by the putrefaction and decay of dead organisms. 



4. Animal excreta, which contain nitrogen compounds of many kinds, even down to 

 ammonia ; and 



5. The tissues of plants and animals. 



The first three of the above-named sources are useless to animals, for 

 these obtain their nitrogen from plants only, either directly (herbivora), or 

 indirectly through other animals (carnivora). To plants, on the other hand, 

 nitrogen seemed until comparatively recently to be available only in one 

 of these three forms. Vegetable physiologists had come to the conclusion 

 that in nature the nitrates of the soil were the one and only form in which 

 nitrogen was taken up by plants. For, although it was known that in 

 experiments plants could be got to take up ammonia salts and even gaseous 

 ammonia, it was evident that ammonia was not a common source of nitrogen 

 in nature. And, finally, the atmosphere, the greatest storehouse of nitrogen in 

 nature, seemed closed to plants. These views, however, were overturned by 



