SENSIBILITY OF VOLVOX TO SPECTRAL LIGHTS 361 



from stimulation, does away with the polarized condition, by 

 allowing the two layers of ions to mix freely, and the condition 

 of excitation spreads or is propagated. 



We prefer to leave to later development and consideration 

 the exact relations between ionization and permeability ('cause 

 and effect'), and at this time merely call attention to a few mat- 

 ters of rather general nature and import, as they bear upon our 

 subsequent work. 



The study of the relative stimulating effects of lights of dif- 

 ferent wave-lengths on organisms will undoubtedly give informa- 

 tion concerning the nature of the photochemical changes in- 

 volved in light reactions. Any particular chemical reaction is 

 (} produced by a certain group of wave-lengths only, so that the 

 possibility is presented of distinguishing between light of differ- 

 ent wave-lengths, A study of the stimulating effect of various 

 wave-lengths on organisms, combined with radiometric and 

 spectrophotometric examination, showing the degree of 'energy' 

 as well as 'light' absorption of solutions or extracts of the organ- 

 isms, and of the 'visual purple ' of some of the invertebrates will • 

 be of interest and value. 



Although there is a wave-length which is most effective in 

 action on a given photochemical substance, the same wave- 

 length may be maximally effective for more than one substance. 

 Therefore, the fact that the same wave-lengths may be found 

 equally effective for two species of organisms does not per se 

 signify that the photochemical substances or reactions are for 

 that reason the same. This invalidates the value of comparisons 

 which are made, on this basis, between the photochemical sub- 

 stance in a certain organism and the visual purple in the amphib- 

 ian, mammalian, or human eye. The fact that a certain organ- 

 ism shows a maximum sensibility to certain wave-lengths which 

 happen to be the same as those which cause most rapid bleaching 

 of visual purple, and which are maximally absorbed by it may 

 be merely fortuitous, and have no fundamental significance. 

 How otherwise can it be explained that in certain organisms 

 which are nearly related the wave-lengths of light of maximal 

 stimulating efficiency are quite different, while in many that are 



