CHAPTER XXI 

 THE NON-SYMBIOTIC NITROGEN-FIXING BACTERIA 



THE non-symbiotic nitrogen-fixing bacteria thus 

 far known may be divided into two classes: (1) Anaerobic 

 ferments, first described by Winogradsky in 1893; 

 (2) aerobic bacteria described by Beyerinck in 1901. 



Anaerobic bacteria. The anaerobic ferment described 

 by Winogradsky and named by him Clostridium Pastori- 

 anum (Fig. 29), is a rod-shaped organism, developing 

 in the absence of air (anaerobic), and producing spores 

 and boat-shaped masses (clostridia). Pure cultures of 

 this organism, when grown in solutions of sugar and the 

 necessary mineral salts, but containing no nitrogen 

 compounds, develop rapidly and assimilate the free 

 nitrogen of the air. This may be readily demonstrated by 

 analyzing the cultures at the end of ten days or two 

 weeks, when considerable quantities of combined 

 nitrogen will be found in the liquid. While Clostridium 

 Pastorianum is a distinctly anaerobic organism, it is 

 yet capable of developing under conditions allowing a 

 more or less ready access of air, provided other bacteria 

 are present. Under these circumstances, the accom- 

 panying bacteria use up the oxygen in the solution, and 

 thus make it possible for the anaerobic Clostridium 

 Pastorianum to develop properly. 



(197) 



