270 MICROBIOLOGY OF SOIL. 



described them as anaerobic bacilli allied to those of the butyric 

 group. In 1901 our knowledge of Azo-bacteria was enriched by Bey- 

 erinck's discovery of a group of large, obligate aerobic bacteria that he 

 designated as Azotobacter. Since that date it has been found that the abil- 

 ity to fix atmospheric nitrogen is possessed also by certain molds and by 

 various species of bacteria. However, this ability is not only extremely vari- 

 able, but is also very feeble as compared with that of the members of the 

 two groups described by Winogradski and Beyerinck. These two groups 

 may, therefore, be designated as including the nitrogen fixing bacteria 

 par excellence. 



ANAEROBIC SPECIES. The species isolated by Winogradski was named 

 by him B. (Clostridium) pasteurianus (Fig. 66). It was found to grow 



FIG. 66. B. (Clostridium) pasteurianus, a non-symbiotic nitrogen-fixing organism. 



(After Winogradski from Lipman.) 



readily under anaerobic conditions in culture solutions containing dextrose 

 and the necessary mineral salts, but no combined nitrogen. The products 

 of growth included protein, butyric and acetic acids, carbon dioxide and 

 hydrogen. In the presence of other bacteria B. (Clostridiwii) -pasteur- 

 ianus was found to develop also under aerobic conditions. Subsequently 

 studies by Winogradski and other investigators showed that.5. (Clostridium) 

 pasteurianus, and varieties of this species are very widely distributed in 

 cultivated soils. More recently Bredeman made a thorough and extended 

 study of anaerobic Azo-bacteria and demonstrated their almost invariable 

 presence in a large number of soil samples from Europe, Asia and America. 

 In his opinion they correspond more or less closely to B. amylobacter 

 described many years before by van Tieghem. 



AEROBIC SPECIES. A more or less pronounced power to fix atmos- 

 pheric nitrogen is apparently possessed by a considerable number of 

 aerobic species. Lipman has demonstrated the fixation of small amounts 



