320 LIGHT AND THE BEHAVIOR OF ORGANISMS 



mentioned above agree in that they indicate that the 

 region of maximum effect for all the plants tested is lo- 

 cated somewhere toward the violet end of the spectrum. 

 It may therefore be definitely concluded that the distri- 

 bution in the spectrum of stimulating efficiency for plants 

 is not primarily dependent upon energy or brightness as 

 judged by the human eye, since the maximum for these is 

 located in the yellow. 



(3) The reactions to light in plants are in all proba- 

 bility associated with photochemical changes induced by 

 the light. We have seen that photochemical changes are 

 specific. If, e.g., one chemical reaction takes place only in 

 blue and another only in green, other conditions being the 

 same, it may be concluded that the chemical constituents 

 taking part in the two reactions are not the same. The 

 experimental results presented above show that the effect 

 of the different rays on reactions is not the same for all 

 plants. It is therefore probable that the photochemical 

 changes associated with the reactions to light are not the 

 same in all plants. I do not consider this point definitely 

 established, owing to the possible effect of difference in 

 selective absorption of light in the different plants. It is 

 however more strongly supported by the observed reac- 

 tions in unicellular forms than by those in sessile plants. 



