396 COMPARATIVE ELECTRO-PHYSIOLOGY 



response. This he ascribes to their ' low average metabolism.' 

 It will thus be seen that no satisfactory explanation is offered 

 either of the mutual opposition of the electrical effects ob- 

 served or of the way in which the presence of ' chloroplasts ' 

 acts as a determining factor. 



To turn now to the subject of my proper inquiry, it has 

 to be determined whether those electrical responses which 

 may be observed in vegetable tissues under the action of 

 light are or are not another expression of the same excita- 

 tory reactions under light which I have already demonstrated 

 by means of mechanical response. And it may be as 

 well to say at the outset that it is the excitability of the 

 tissue, and not the presence or absence of ' chloroplasts,' that 

 is the critical factor in determining this electrical response. 

 For I have obtained strong responses under light from pul- 

 vini, stems, and other tissues which are relatively deficient in 

 ' chloroplasts ' ; and again, while the lamina of a plant rich 

 in ( chloroplasts ' would give but moderate response to light, 

 the petiole of the same plant, characterised by less ' chloro- 

 plasts,' would often give much stronger response. The etio- 

 lated stem of celery, moreover, gives strong electrical response. 

 And, finally, it is an error to suppose that petals of flowers 

 are irresponsive to light, for I have obtained strong response 

 from petals of Sesbania coccineum and from Eucharis lily. 

 Animal nerve, again, in which there is no chlorophyll, gives 

 response to light. 



As I have already shown the typical heliotropic effects 

 exhibited by plants to be brought about by differential 

 excitatory action on the proximal and distal sides of the 

 same organ, I shall now proceed to exhibit the electrical 

 counterparts of these experimentally. As I wish, moreover, 

 to show that the general electrical response to the unilateral 

 action of light is fundamentally the same as that induced by 

 other forms of stimulus, the first experiment to be described 

 will be one depending on the unilateral application of a non- 

 luminous stimulus, say thermal. 



If on one side of a growing organ we apply a series of 



