196 Report of the Department of Bacteriology. 



likewise unsupported by any direct evidence. It is a particularly 

 hard theory either to prove or to disprove because of the difficulty 

 in determining whether protozoa live in the soil in their active state. 

 If Russell's theory is correct, the increase in germ content which 

 takes place in frozen soil is closely related to that which has been 

 shown to occur in partially sterilized soil. Until recently the best 

 supported explanation of the latter phenomenon was that the treat- 

 ment necessary to effect partial sterilization disturbed the equilibrium 

 of the soil bacteria and as a result allowed certain kinds to multiply 

 abnormally. Now that Russell has proposed his protozoan theory, 

 opinion is divided. Whichever explanation is the more probable, it 

 is possible that the rapid increase in numbers of bacteria in partially 

 sterilized soil and their multiplication in frozen soil may be due to 

 similar causes. The improved crop-yields in the former case raise 

 the question as to whether the increased germ content in the latter 

 case has any practical importance. If the bacteria that multiply 

 during the winter are favorable to plant growth, a cold winter may 

 have a more beneficial effect on following crops than a warm one. 

 This question leads into the unsolved problem of seasonal variation 

 among soil bacteria. It shows the necessity of knowing what kinds 

 of bacteria predominate at different seasons, and what influence 

 each kind has upon plants. 



