402 



COMPARATIVE ELECTRO-PHYSIOLOGY 



also obtainable by longitudinal transmission. In order to 

 do this an organ must be selected which is a fairly good 

 conductor. I have thus been able to observe a series of 

 responses to transmitted stimulus of light, using such speci- 

 mens as the petiole of Bryophyllum. Light was here applied 

 at a distance of 5 mm. from the proximal contact, and this 

 gave rise to a series of true excitatory responses of galvano- 

 metric negativity. 



Having thus established unmistakably the negative sign 

 of the excitatory electrical variation induced and transmitted 



under stimulus of light, I shall 

 next proceed to give records of 

 experiments in which light was 

 applied directly. The effect ob- 

 served in these cases is naturally 

 much larger, as there is no 

 enfeeblement by transmission. 

 Fig. 238 shows a series of such 

 responses, obtained at intervals of 

 two minutes, by the application 

 of sunlight previously passed 

 through a stratum of water 

 during five seconds only in each 

 case, on the petiole of a vigorous 

 leaf of Bryophyllum. It will be 

 noticed that after each response 

 of galvanometric negativity there 

 is an after-effect of positivity, in 

 consequence of which the base 

 line of the series, instead of re- 

 maining horizontal, trends down- 

 wards. This power of holding 



stimulus latent, for the increase of internal energy, we shall 

 later see to be important, as heralding the initiation of 

 multiple response. The exhibition of these after-effects, due 

 to increase of latent energy, is also to be observed in the 

 record given already of the mechanical response of Mimosa 



FIG. 238. Photographic Re- 

 cord of Series of Negative 

 Responses of Petiole of 

 Bryophyllum to Stimuli of 

 Sunlight of Five Seconds' 

 Duration applied at Inter- 

 vals of Two Minutes 



Observe the positive after-effect, 

 due to increase of internal 

 energy, which causes down- 

 ward trend of base-line. 



