118 PRINCIPLES OF SOIL MICROBIOLOGY 



alkali salts and other salts stimulate the maturity of the culture. 

 Prazmowski also observed changes of the dark brown A. chroococcum 

 into a colorless race similar to A. vinelandii and A. agile and a yellow 

 race similar to A. beijerinckii; he concluded, therefore, that the various 

 species described are merely races of one greatly variable species. These 

 results are confirmed by Omeliansky, 69 who found that pigment forma- 

 tion by Azotobacter depends entirely on the composition of the medium. 

 However, this is not sufficient to deny the existence of the various 

 species altogether, especially since the genus itself is very variable, and 

 it is difficult to establish the limits for its systematic position. The 

 organism most commonly studied is A. chroococcum. It goes through a 

 regular life cycle. Drying stimulates spore formation. The membrane 

 surrounding the cell becomes more compact and thinner while the cell 

 itself changes into a spore. 70 Prazmowski observed spore formation 

 also in ordinary media, containing soil extract. Several spores were 

 found to be produced in a cell and these are believed to be responsible 

 for the irregular packet and sarcina forms observed in mature cultures. 

 The latter are formed by simple fission of the cell. 71 The granules 

 arising from the splitting up of the supposed nuclear body may act as 

 gonidia spores. 



Lohnis and Smith claim that the genus Azotobacter is characterized 

 by seven different cell types: 



1. Large non-sporulating, globular, oval, or rod-like cells, with polar or 



peritrichous flagella. 



2. Coccoid cells, the vegetative growth of the regenerative bodies, identical 



with Micrococcus. 



3. Dwarfed cell type, the vegetative growth of the gonidia. 



4. Irregular, fungoid cells, similar to Mycobacterium. 



5. Small non-sporulating rods, identical either with Bad. lactis viscosum or 



Bad. putidum. 



6. Small sporulating rods, identical either with Bac. fusiformus or Bac. 



pumillus A. M. et Gotteil. 



7. Large sporulating cells. 



69 Omeliansky, 1916 (p. 109). 



70 Fischer, H. Ein Beitrag zur Kenntnis der Lebensbedingungen von stick- 

 stoffsammelnden Bakterien. Centrbl. Bakt. II, 14: 33-34. 1905; 15: 235-236. 

 1906. 



71 Jones, D. H. A morphological and cultural study of some Azotobacter. 

 Centrbl. Bakt. II, 38: 14-25. 1913; 42: 68-9. 1914; Trans. Roy. Soc. Canada. 

 Ser. 3, 7: 1913, Sect. IV. Further studies on the growth cycle of Azotobacter. 

 Jour. Bact. 5: 325-342. 1920. 



