712 COMPARATIVE ELECTRO-PHYSIOLOGY 



was thought that the effect of this stimulus was different 

 in different cases, the specific reaction in each organ being 

 determined by the ultimate advantage of the plant But I 

 have been able to show that the excitatory effect of light is 

 normal and like that of any other form of stimulus. The 

 various results induced by it depend, first, on the question 

 whether stimulus has remained localised at the point of 

 application, or been transmitted to distant areas. The effect 

 is thus modified by the intensity of the stimulus and the 

 conductivity of the tissue. These results, however, may be 

 further modified by the differential excitability of the organ. 

 Here, as in other cases of stimulation, the general rule holds 

 good that response is by greater contraction and galvano- 

 metric negativity of the more excited. As a concrete example 

 may be mentioned the case of the pulvinus of Mimosa, 

 when the upper surface alone is subjected to the stimulus 

 of light. Here, owing to local excitatory contraction of 

 the upper, the expelled water reaches the lower half of the 

 pulvinus and induces there the hydro-positive effect of 

 expansion, both of these effects conspiring, in this first stage 

 of response, to erect the leaf. The electrical variation at 

 the lower half is here, then, found to be positive. But as 

 the excitatory effect itself is gradually conducted to the 

 lower half, it induces there an increasing contraction. The 

 mechanical response is now therefore reversed, from one of 

 erection to one of depression, the electrical variation of the 

 lower half of the pulvinus undergoing at the same time a 

 corresponding change from positivity to negativity (fig. 237). 

 From this experiment it is clear that the electrical response 

 under light exhibits the same stimulatory changes which are 

 also visibly demonstrated by mechanical response. We see, 

 moreover, from this experiment that light in general acts 

 as a moderate stimulus. For while mechanical or thermal 

 stimulus induces a sudden collapse of the leaf of Mimosa, the 

 application of light brings about only a gradual fall. 



Owing to this moderateness of the stimulus of light, and 

 to the fact that its application is strictly local, it is easy to 



