SUMMARY OF THE CHAPTER 211 



revealed in modifications of metabolism and of movement, and 

 in the production of death. Upon metabolism we can distin- 

 guish an effect of the red rays, which are greatly absorbed by 

 chlorophyll and are chiefly active in assimilation, and an effect 

 of the blue rays, which seem to produce important chemical 

 changes, increasing the production of carbon dioxide in plants, 

 creating an electric current in the retina as it falls thereon, 

 and bleaching visual purple. These chemical changes become 

 more vigorous with increased intensity of light and may lead 

 to death ; while, at the opposite extreme, complete absence of 

 light may prove fatal by withdrawing the necessary thermic 

 and chemical energy. Again we find light sometimes necessary 

 to movement in protoplasm, at other times by its absence or 

 too great intensity inhibiting movement, or, again, by sudden 

 change in intensity, creating abrupt changes in movement. 

 Thus light undeniably has a great effect upon the processes of 

 metabolism and movement. 



Finally, in those complex processes involved in locomotion, 

 light produces very widespread effects ; for the direction 

 (and, though only indirectly, perhaps, the rate) of locomotion 

 is influenced in so important a way that when light is with- 

 drawn the organism wanders aimlessly about. Of the various 

 rays, those with wave length = 40/i to 49/z are the most active 

 in controlling locomotion. Movement towards the light takes 

 place at intensities of light varying greatly with the species 

 and also with the conditions other than light in which the indi- 

 vidual finds itself, two factors upon which depends the degree 

 of attunement. Light having an intensity above that to which 

 the organism is attuned repels the organism. Two kinds of 

 effects are produced by light : one by the direction of its ray 

 phototactic ; the other by the difference in illumination of 

 parts of the organism photopathic. 



We thus see that organisms respond to light, and that this 

 response, exhibited in movements, is not of a widely different 

 order from the disturbances produced in metabolism, which in 

 turn are of the same order as the chemical changes produced by 

 light in our laboratories upon non-living substances. In a word, 

 response to light is the result of chemical changes in the proto- 

 plasm wrought by light. 



