CHAPTER XXIV 



THE AFTER-EFFECT OF LIGHT 



Two types of diurnal movement have been considered : one 

 in response to the predominant effect of variation of hght, 

 and the other to that of changing temperature. There are, 

 however, certain other organs which are sensitive to varia- 

 tions both of hght and of temperature. The effect of hght is, 

 generally speaking, antagonistic to that of rise of tempera- 

 ture ; hence the movement which is the resultant of the 

 two effects requires careful analysis. 



Still greater complexity is introduced by the conflict- 

 ing factors of the immediate and the after-effect of light. 

 Great obscurity surrounds this after-effect phenomenon, 

 and I endeavoured to determine its characteristics by 

 the electric method of investigation. A fuller account of 

 the after-effect of light on the response of various plant- 

 organs and of the animal retina will be found elsewhere.^ 

 I here refer only to one or two characteristic results which 

 have immediate bearing on the present subject. 



Direct stimulation by light induces an excitatory reac- 

 tion, which is exhibited mechanically by contraction and 

 electrically by induced galvanometric negativity. Under 

 continuous stimulation the excitatory effect, whether of 

 positive curvature or of induced galvanometric negativity, 

 is found to attain a maximum. The positive tropic curva- 

 ture and the induced galvanometric negativity exhibit, 

 on account of fatigue at the directly stimulated proximal 

 side, a decline and neutralisation. This neutralisation is 



^ Comparative Electro-Physiology , p. 392. 



