SPORE-FORMING BACTERIA 195 



2. When soil-infusion was heated before plating at a tempera- 

 ture (75-85°C.) high enough to kill the vegetative forms of 

 bacteria, nearly if not quite as many colonies of these spore- 

 forming bacteria developed as when it was plated unheated. 

 In about one-third of the cases, indeed, their numbers were 

 actually shghtly higher on the plates made after heating; al- 

 though all such differences undoubtedly lay within the Umits of 

 the experimental error. This suggests that these bacteria occur 

 in normal soil as spores rather than in a vegetative state. 



3. No increase in the total number of these organisms nor 

 decrease in the number of their spores could be detected in a 

 pot of soil to which fresh manure had been added, 



4. These results throw considerable doubt on the common 

 assumption that these organisms are important ammonifiers 

 in the soil. They raise the question as to what possible soil 

 conditions favor their growth and multiphcation. 



