INFLUENCE OF LIGHT AND OTHER RAYS. 



163 



therefore, as easily as with the action of heat in the thermal death-point. 

 The different colors of the spectrum do not act alike; the part of the spec- 

 trum from red to green is practically without influence upon micro- 

 organisms, while the blue light acts strongest and the intensity decreases 

 in the violet and ultra-violet. In carrying on experiments with the in- 

 fluence of light, it must be remembered that glass absorbs ultra-violet 

 rays, and further that the heating of the medium by direct radiation must 

 be avoided. 



Yeasts, molds, and bacteria and probably protozoa are equally sensi- 

 tive to light. Even the spores of most bacteria do not show a greater 

 resistance to light, while the mold spores are an exception. The colored 



FIG. 53. Phototropism of Rhizopus nigricans. The mold was grown on gelatin with 

 diffused light coming from right side. (Original.) 



spores of the Penicillium, Aspergillus and Mucor species can be exposed to 

 light for a long time without being killed, but the colorless spores of 

 Oidium and Chalara show no increased resistance. It is supposed that 

 the pigment in mold spores is a protection against light. This is not true 

 with the pigment of bacteria. The colored and colorless strains of pig- 

 mented bacteria show no difference in their resistance to light. Only one 

 group, the so-called purple bacteria, is an exception. These peculiar 

 organisms, many of which feed on hydrogen sulphide, seem to 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 



