198 Bacteria in Relation to Country Life 



Winogradsky showed also that the amount of nitro- 

 gen fixed bears a certain relation to the amount of 

 sugar supplied. In other words, the bacteria employed 

 used the sugar not only as food, but also as fuel, the energy 

 of which was utilized partly for making the free nitrogen 

 of the air to combine with other elements. Winogradsky 

 demonstrated, likewise, that the fixation of nitrogen 

 is discouraged when nitrogenous substances are present 

 in the culture medium. Thus, when salts of ammonia 



Fig. 29. Non-symbiotic nitrogen-fixing bacteria. Rods, clostridia, and spores 

 of Clostridium Pastorianum. (Winogradski.) 



were purposely added, the fixation of free nitrogen de- 

 creased in proportion to the combined nitrogen sup- 

 plied. A point was finally reached when there was 

 practically no fixation of nitrogen by the bacteria. This 

 fact is of great interest in teaching us that the nitrogen- 

 gathering bacteria, like leguminous plants, prefer to 

 employ the nitrogen compounds already at hand, and 

 turn to the atmosphere only when combined nitrogen 

 is not to be had. 



Clostridium Pastorianum has been isolated also by 

 other investigators. It seems to be widely distributed in 

 cultivated soils, although we are still in ignorance of the 



