58 TEXTBOOK OF PLANT PHYSIOLOGY 



cling closely to its colonies and consume oxygen with great 

 avidity. 



The fixation of nitrogen is an endothermic process requiring 

 an external supply of energy. Clostridium bacteria use as a 

 source of energy the process of butyric fermentation. This 

 mechanism will be considered in greater detail farther on (see Art. 

 82). Winogradsky has succeeded in establishing a definite quan- 

 titative relationship between the two processes. He found that 

 2 to 3 mg. of nitrogen are assimilated for each gram of carbohy- 

 drates fermented. 



Somewhat later, the Dutch bacteriologist Beijerinck (1901) dis- 



G° n O 



n%oOoo°o 



Fig. 23. — Clostridium Pasteurianum. Fig. 24. — Azotobacter chroococcum. 



covered another widely distributed bacterium, which he calls 

 Azotobacter (Fig. 24). This organism has the capacity of very 

 intensive respiration, which supplies it with sufficient amount of 

 energy for the fixation of free nitrogen. Since a much greater 

 amount of energy may be obtained from the same quantity of 

 carbohydrates by the process of respiration than by that of fer- 

 mentation, it is but natural that Azotobacter works with greater 

 efficiency than Clostridium, being able to fix as much as 15 mg. of 

 nitrogen per gram of sugar consumed. Besides these two most 

 important nitrogen-fixing bacteria, there are large numbers of 

 others found in soil which assimilate atmospheric nitrogen. Cer- 

 tain authors atttribute the same capacity even to some of the 

 molds and blue-green algae, though this has to be confirmed. 



