174 LIGHT AND PROTOPLASM [Ch. VII 



pounds, the rapid oxidation of which is incompatible with life. 

 In any case it is clear that the bactericidal effect of light is a 

 chemical one. 



Concerning the range of organisms which are thus affected, 

 it must be said that chiefly pathogenic species, such as the 

 bacteria of anthrax, of typhus fever, and of cholera have been 

 experimented with and have shown themselves most suscep- 

 tible. Other bacteria are, however, likewise affected. Among 

 the other 'fungi, Wettstbin ('85) found that the conidia of 

 Rhodomyces Kochii, a human intestinal parasite, did not de- 

 velop in the light. Klein ('85) found the same thing to be 

 true for the conidia of Botrytis cinerea, and showed that the 

 blue-violet rays were the most effective ones. Elving ('90, p. 

 105) gained similar results with Aspergillus, although several 

 days or weeks of insolation did not kill the fully ripe spores. 

 Ward ('93) determined that insolated spores, cultivated on 

 agar or gelatine plates, of Oidium lactis (5 cases), Saccharomyces 

 pyriformis (4 cases), and " a ' Stysanus ' conidial form " found 

 as a saprophyte on the screw-pine, Pandanus, (2 cases) became 

 injured. These are all hyaline and colorless except Stysanus, 

 which is nearly so. Certain colored spores which Ward experi- 

 mented with gave negative results, and Ward concluded that 

 this is because the blue end of the spectrum is cut off before 

 reaching the deeper protoplasm. However this may be, we 

 actually find that in many, but not all, fungi the metabolic 

 processes of the spores are disturbed and even death is pro- 

 voked by intense light. 



Why the spores should be especially susceptible to the action 

 of light is an important inquiry. Ward believes the answer 

 to be that the spores contain oily substances, which are espe- 

 cially liable to oxidation in light, as we have already seen. 



Finally, we have to consider the experiments which demon- 

 strate that a strong sunlight may be injurious even to green 

 plants. This result follows clearly from the work of Prings- 

 HEiM ('81). When strong sunlight is focussed for a short time 

 upon cells of Spirogyra, Nitella, Mesocarpus, or Tradescantia 

 stamen hairs in atmospheric air (5 to 15 minutes), they are 

 killed. No result occurs, however, when the same light falls 

 upon green cells in which the atmosphere has been replaced by 



