164 PHYSICAL INFLUENCES. 



phototaxis or heliotaxis. The pigment, bacteriopurpurin, does not take 

 the place of chlorophyl, however, since the bacteria do not produce 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. 54. 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. 



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. 53). It has been 

 found that molds produce mycelium mostly in the dark, while in daylight 

 sporangia are produced mainly. This difference in the development 



