CHAPTER X 



THE ELECTRIC RESPONSE TO LIGHT 



Electric response to the stimulus of light — Dual reactions A and D under 

 light — Excitatory D reaction detected by response of galvanometric 

 negativity — Persistence of electric response of motile organs even 

 under physical restraint — The photo-electric cell — The predominant 

 D effect in the electric response of Musa to light — The positive 

 after-effect and the phenomenon of ' overshooting ' — Positive electric 

 response of Hydrilla — Effect of temperature on electric response — 

 Effect of narcotics — Effect of excess of carbonic acid. 



In the two preceding chapters we have discussed the 

 effects of certain physiological variations on photosynthetic 

 activity, and found that they run parallel to those observed 

 in other modes of response. We shall in the present chapter 

 make a special study of the effect of light as determined by 

 the independent method of electric response. 



Light, as previously stated, gives rise to dual reactions 

 A and D, which can be detected by electric means. For 

 example, we make suitable electric connections on a Mimosa 

 plant, one contact being made with the responding pulvinus 

 and the other with a distant indifferent point. Under any 

 mode of stimulation, the D effect of excitatory fall of the 

 leaf is manifested by an electrical variation of galvano- 

 metric negativity. This electric concomitant of the D effect 

 is unfailing and is independent of the mechanical response. 

 For, even after the motile response becomes arrested by 

 physical restraint, external or internal, the electrical response 

 persists. Thus, when water is applied on the pulvinus 

 of Mimosa, it absorbs it in excess, and in this water-logged 

 condition the response by mechanical movement is inhibited. 

 But the excitatory protoplasmic reaction of the tissue may 

 still be detected by the electric response of galvanometric 

 negativity. 



