ASSIMILATION OF NITROGEN BY PLANTS 229 



is burned. Likewise, in the process of denitrification of nitrates 

 in the soil, the formation of free nitrogen in the soil usually occurs 

 as a result of a superabundant accumulation of nitrogenous 

 matter. If the processes resulting in the fixation of molecular 

 nitrogen were not present, the nitrogen balance of the earth 

 naturally would show an ever increasing deficit, and the organic 

 world would be in danger of perishing from nitrogen starvation. 

 This does not occur, because among the living organisms 

 populating the earth's surface there are many that possess the 

 capacity of carrying out the extremely difficult synthesis of 



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no 



Fig. 66. — Clostridium pastorianum. Fig. 67. — Azotobacter chroococcum. 



organic nitrogen compounds from nonnitrogenous substances 

 and free molecular nitrogen. These are the so-called ''nitrogen- 

 fixing" bacteria. 



The majority of these nitrogen-fixing bacteria live in the soil. 

 It has been known for a long time that fallow land becomes 

 richer in nitrogen and that the accumulation of fixed nitrogen 

 occurring in it is a biological process, since it is interrupted by 

 the addition of antiseptics to the soil. Vinogradsky (1893) was 

 the first to succeed in isolating from the soil one group of these 

 bacteria and in studying their physiology. It proved to be a 

 sporogenous bacillus, causing butyric acid fermentation m 

 sugar-containing mediums and simultaneously fixing molecular 

 nitrogen (Fig. 66). The discoverer has named it Clostridium 

 pastorianum. This organism belongs to the type of anaerobic 

 bacteria; i.e., it is able to live and develop only in the absence 



