RESPONSE TO STIMULUS 'OF LIGHT 



413 



It has thus been seen that the electrical response of 

 plants to light is not essentially different from their response 

 to other forms of stimulus. The various effects observed 

 are the electrical concomitants of 

 those excitatory effects which we 

 had already seen to be exhibited 

 in mechanical response. In a 



FiG. 246. Photographic Record of Phasic" 

 Alternation, showing Direct and After 

 Effects in Type III., represented by 

 Petiole of Cauliflower 



Continuous line represents effect during appli- 

 cation of light. Stimulus was withdrawn 

 slightly before the attainment of the second 

 maximum negativity, resulting in over- 

 shooting of curve in negative direction. 

 Dotted portion of record represents after- 

 effect on cessation of light. 



PiO. 247, Photographic Record 



- of Pair of Responses obtained 



with a Second Specimen of 



Cauliflower, representative of 



Type III. 



The stimulus of light was in 

 these cases withdrawn exactly 

 on the attainment of the 

 negative maximum. Dotted 

 portions of the record ex- 

 hibit the after-effect. 



highly excitable tissue the direct effect gives rise to galvano- 

 metric negativity. The indirect effect of stimulus, however, 

 is one of galvanometric positivity. Positive response may 

 also be obtained from a tissue which is not highly ex- 



