112 



RELATIONS OF PLANTS TO LIGHT 



ketones, and sulphides. Actual disintegrations may be induced 

 among which may be named the breaking down of chlorophyl. 



The waves of shortest length and greatest frequency are gen- 

 erally supposed to be most active in producing these effects, 

 although it has been proven that rays from the entire range of 

 the spectrum participate in the disintegration of chlorophyl. 



172. Critical Points in the Chemical Action of Light. No 

 minimum intensity of light is to be found for non-chlorophylla- 

 ceous forms since they may exist in total darkness during the entire 

 period of development of several generations of individuals, or 



FIG. 51. Plate of anthrax spores exposed for five hours to the solar spectrum in 

 August, then incubated for 48 hours. The horizontal line shows the length of the 

 spectrum, the vertical lines the limit of the principal regions of the spectrum. The 

 letters R, G, V and B denote the regions of principal colors, of which they are the 

 initials. The clear area is where fewest spores of bacteria have developed after ex- 

 posure to light. After Ward. 



perhaps forever. An optimum may be . distinguished only for 

 certain special forms of this class which make use of radiant en- 

 ergy in the synthesis of foods of which Bacterium photometricum 

 is an example, since none of the direct chemical effects of light 



