HISTORICAL 251 



with "alinit," however, have not confirmed to any great extent the 

 claims of its exploiters. 



In 1901 Beijerinck's investigations led to an extremely important 

 addition to the history of non-symbiotic nitrogen-fixation. He 

 described a new group of large aerobic bacilli to which he gave the 

 generic name Azotobacter. 



In an early paper published by Beijerinck and van Delden, they 

 maintain that Azotobacter are incapable of fixing appreciable 

 quantities of nitrogen in pure culture, but are dependent to a large 

 extent on Gramilobacter, Radiobacter, Aerobactcr. They considered 

 that in mixed cultures the Granulobader, Radiobacter, and Aerobacter 

 possess the power of fixing nitrogen in the presence* of Azotobacter, 

 which grows at the expense of the combined nitrogen escaping from 

 them into the solution. 



A little later Gerlach and Yogel succeeded in isolating from soil 

 the Azotobacter of Beijerinck and in showing that in pure cultures 

 and in the presence of salts of organic acids, Azotobacter are capable 

 of active nitrogen-fixation. They obtained a fixation of 9 mgm. 

 of nitrogen in a 1 per cent, solution of grape sugar. But Beijerinck 

 challenged this assertion, claiming that their cultures were not pure 

 but were mixed with other forms difficult to separate. The claims 

 of Gerlach and Yogel were substantiated by the work of Freuden- 

 reich, Koch and Lipman. The latter not only show r ed that the 

 Azotobacter possess the power of fixing nitrogen in pure cultures, 

 but he explained the failures recorded by others. 



Although not necessary, the presence of other organisms often 

 proves advantageous. Lipman found that in the presence of such 

 forms as B. radiobacter and B. levaniformus the nitrogen-fixation is 

 faster and goes on at a more regular rate. 



To the two species of Azotobacter A. chroococcum and A. agilis 

 described by Beijerinck and van Delden, Lipman, added A. mne- 

 landii, A. beijerinckii, and A. woodstoicnii. Later Lohnis and 

 Westermann described A. vitreum, and after a study of 21 cultures 

 of various Azotobacter concluded that they represented only four 

 types. A. chroococcum is most widely distributed in the soils so far 

 studied. 



The discussion of the subject thus far has been more or less con- 

 fined to the Azotobacter, but investigations of Beijerinck and van 

 Delden, Lohnis, Moore, Chester, Bredemann and others have 

 brought to light other microorganisms having the power to fix 

 nitrogen. Among these are B. mesentericus (which fixes appreciable 

 quantities of nitrogen), B. pneumonia, B. lactis viscosus, B. radio- 

 bacter, B. prodiyiosus, B. asterosporus and B. amylobacter. 



Bredemann, after a careful study of the morphological and physio- 

 logical characteristics of eleven "original species" of other investi- 

 gators and of sixteen cultures prepared by himself from various soils, 

 concluded that all belong to the single species B. amylobacter of 



