200 



Nitrogen Fixation— Nonsymbiotic 



sources of energy; this symbiotic action leads to a niaxiinuni economy 

 in the utihzation of energy. In the presence of RJiizo])ium legumino- 

 sartim, Azotohactcr was found to fix luore nitrogen tlian alone. Sym- 



FiG. 84. Clostridium pasteurianiim, showing spore formation (from Wino- 



gradsky ) . 



biosis between pure cultures of cellulose-decomposing and nitrogen- 

 fixing bacteria has also been reported. 



Blue-Green Algae 



The ability of blue-green algae to fix atmospheric nitrogen is now 

 definitely established. Drewes first observed this phenomenon in 

 1928 for species of Anahaena and Nostoc. His discovery was con- 

 firmed by Allison and Morris in 1930. Certain strains of Myxophy- 

 ceae, isolated from nitrogen-deficient warm springs, were also found 

 to be capable of utilizing atmospheric nitrogen. A species of Nostoc 

 punctifonne was isolated from different host plants and found ca- 

 pable of fixing up to 1.95 mg of nitrogen per 50 ml of culture solution, 

 in presence of a suitable sugar source. A culture of Nostoc mus- 

 corum isolated from the soil was found capable of obtaining both 

 its carbon and its nitrogen from the air; it fixed as much as 10 mg 

 of nitrogen in 45 days and 18 mg in 85 days per 100 ml of medium 

 from carbohydrate; in the dark, it fixed 10-12 mg of nitrogen per 

 1 gm of glucose consumed. 



