FIXATION OF ATMOSPHERIC NITROGEN. 271 



of nitrogen by Ps. pyocyanea and Lohnis secured similar results with 

 Bad. pneumonia, B. lactis viscosum, B. radiobacter and B. prodigiosus. 

 Gottheil has detected fixation by B. ruminatus and B. simplex; Pill ai has 

 described a nitrogen fixing aerobic bacillus, B. malabarensis; Westermann 

 studied a similar organism that he named B. danicus; while Beyerinck 

 and van Delden observed, some years earlier, that certain strains of 

 B. mesentericus could fix relatively large amounts of nitrogen. Similarly 

 Ps. radicicola has been found to possess a slight, but nevertheless an 

 appreciable power to fix elementary nitrogen in culture solutions or in 

 the soil. 



FIG. 67. Azutobacter vinelandi, a non-symbiotic nitrogen-fixing organism. (After 



Lipman.) 



But while nitrogen fixation among aerobic soil bacteria is not as un- 

 common as was at one time supposed, this function is so feeble and so 

 variable in most instances, as to be of negative, or, at best, of doubtful 

 economic significance. On the other hand, the aerobic Azotobacter, first 

 described by Beyerinck in 1901, may be regarded not only as possessing 

 a very pronounced ability to fix atmospheric nitrogen, but as playing a 

 role of some moment in maintaining the supply of combined nitrogen in 

 the soil. 



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

 by Beyerinck and van Delden, Lipman added A. vinelandii, (Fig. 67) A. 

 beyerincki and A. woodsfownii, and Lohnis and Westermann, A. vitreum. 



