RESPONSE TO STIMULUS OF LIGHT 



395 



Case 



III. 



Description 

 of tissue 



Tissues which 

 exhibit mul- 

 tiple or auto- 

 nomous re- 

 sponse. 



Action 



Considerable absorp- 

 tion of energy, im-- 

 mediate or prior. 



Effect observed 



Initiation of multiple 

 response in Desmo- 

 diuni gyrans pre- 

 viously at standstill ; 

 multiple response 

 under continuous 

 action of light in 

 Biophyttim ; photo - 

 tactic movements 

 of swarming spores ; 

 cf. multiple visual im- 

 pulses in retina. 



Turning now to the electrical responses induced by light, 

 our investigation is resolved into the inquiry whether these 

 are not the electrical concomitants of those excjtatory effects 

 which we have already been able to analyse through 

 mechanical response. But, before proceeding to this question, 

 I shall first briefly refer to certain ' electrical effects of light 

 upon green leaves ' which have been observed by Dr. Waller. 1 



His experiments were performed by making galvanometer 

 connections with two halves of the same leaf, one being 

 strongly illuminated and the other unilluminated. With leaves 

 of different plants he obtained opposite electrical effects under 

 the action of light. From the leaves of Iris, for example, during 

 illumination, he obtained response of galvanometric negativity, 

 with reversal, or positivity, on the cessation of light as its after- 

 effect. With leaves of Tropceolum and Mathiola, on the con- 

 trary, he obtained positive response during illumination and 

 subsequently negative. Beyond the suggestion that negativity 

 may be associated with dissimilation and positivity with 

 assimilation, Dr. Waller offers no explanation of this opposi- 

 tion of effects observed by him. He states, however, that he 

 regards the presence of ' chloroplasts ' as essential to these 

 electrical reactions under light, inasmuch as petals, he found, 

 gave no response. Even in the case of the green leaves 'of 

 ordinary garden shrubs and trees,' moreover; he found no 



1 Waller, Proc. Roy. Soc. vol. Ixvii. pp. 129-137. 



