SENSIBILITY OF VOLVOX TO SPECTRAL LIGHTS 363 



though we do not know whether the products of its change them- 

 selves stimulate the receptors or whether they act catalytically. 



It seems to us justifiable to conclude that the influence of dif- 

 ferent wave-lengths, as far as the chemical reactions which are 

 associated with the action of light are concerned, are funda- 

 mentally the same for animals and plants. This is all that we 

 understand that Loeb claims when he speaks of the ' 'identity of 

 heliotropism in plants and animals," viz., that the reactions of 

 plants and animals are both due to the action of light on the 

 photosenstitive substances, resulting in transformation. For dif- 

 ferent organisms (plants and animals) there are different wave- 

 lengths which have a maximum of stimulating effect. This sig- 

 nifies that there is a difference in the photosensitive substances, 

 photochemistry telling us that the most efficient wave-length 

 varies with the nature of che photochemical substance. 



We find it difficult to understand just what Mast ('17, p. 522) 

 means when he says, "the reactions are not wholly dependent upon 

 wave-lengths, for while there is clearly a region of maximum 

 stimulating efficiency in the spectrum, stimulation is not confined 

 to this region and the stimulating effect of the wave-lengths on 

 either side of it can be made greater by simply increasing their 

 intensity." 



It is found in plants and animals that certain wave-lengths 

 have a maximal stimulating effect as compared with the effect 

 of other wave-lengths. It stands to reason, of course, that this 

 effect is due to the energy of radiation (i.e., light and heat effects, 

 which together make up the stimulating value of the wave- 

 lengths in question). For different wave-lengths to produce the 

 same effect would require that the amount of energy absorbed 

 by the photochemical substance would be the same in all cases 

 (that is, to have the same relative penetrating power). There- 

 fore it follows that by increasing the intensity of any wave- 

 length we would increase its stimulating effect. It is thus pos- 

 sible that if the entire series of wave-lengths be made increasingly 

 intense or greater in radiant power beyond a certain limit, a 

 considerable portion of the spectrum in the neighborhood of the 

 wave-lengths of maximum stimulating efficiency at a lower in- 



