56 BACTERIOLOGY 



dence is observed, for instance, in complete nitrification, 

 as already noted; in the course of putrefaction, where, 

 through exhaustion of free oxygen by the actively germinat- 

 ing aerobic varieties, the conditions are supplied that enable 

 the anaerobic species to develop and exercise their biological 

 activities. Again, through the proteolytic activity of 

 enzymes produced by certain species of bacteria, other 

 speces are supplied with nutrition that would otherwise be 

 unassimilable or only imperfectly so. Similar cooperative 

 or symbiotic relations between bacteria and higher plants 

 are also noticed, notably that between certain bacteria of 

 the soil and the group of leguminous plants, whereby the 

 latter are enabled, through the assistance of the former, to 

 make up their nitrogen deficit in large part from the free 

 nitrogen of the atmosphere. This latter relationship is 

 probably an example of true symbiosis.^ 



Influence of Light. — Light is not only unnecessary to the 

 performance of functions by bacteria but appears to be 

 in varying degrees inhibitory. 



Direct sunlight is destructive to many species. It is a 

 matter of common experience that cultures of particularly 

 important species retain their type characteristics better 

 and longer if cultivated in the dark than in diffuse daylight. 



Electric light has likewise a depressing influence upon 

 the viability of bacteria. Beyond the fact that bacteria in 

 vacuo are unaffected by light we have no knowledge of the 

 mechanism of its action. Presumably it has something to 

 do with oxidation processes. 



The germicidal action of the direct rays of the sun may 

 be easily demonstrated by preparing a plate of colon bacillus, 

 shading a portion and allowing the sun to shine upon it for 



1 See Nitrogen fixing bacteria. 



