280 



PHYSICAL INFLUENCES 



thrive better in light than without it. Direct sunlight does not kill 

 them, it rather attracts them and they move toward the light. This is 

 called phototaxis or heliotaxis. The pigment, bacteriopurpurin, does 

 not take the place of chlorophyl, however, since the bacteria do not pro- 

 duce oxygen in light and always need organic food. 



The effect of light upon microorganisms is mainly brought about by 

 a chemical change in the protoplasm, and also, to some extent, by a 

 chemical change in the medium, namely the formation of a peroxide or a 

 similar oxidizing agent. 



The germicidal action of light is of importance in the purification of 

 rivers. It is applied also in curing diseases of the skin, as lupus and 



FIG. 119. Two cultures of an Aspergillus^one grown in the dark the other in 

 diffused light, showing rings. (Original.) 



leprosy, by exposing the diseased parts to a very concentrated light of 

 the electric arc. This light contains plenty of blue and violet rays and 

 is preferable to sunlight because it is always ready for use and its com- 

 position and intensity can be controlled easily. Ultra-violet light is 

 used in the sterilization of water and of milk. 



Diffuse light is not nearly as harmful to microorganisms as direct 

 sunlight. Long exposures to diffuse light will kill most bacteria, while 

 molds are not at all sensitive. They rather like a very dim light, and 

 many molds grown in a dark room with light only from one side will 

 grow toward the light. This property, which is characteristic for all 

 green plants, is called heliotropism or phototropism (Fig. 118). It has 



