408 COMPARATIVE ELECTRO-PHYSIOLOGY 



but it is complicated by other periodicities, such as that of 

 temperature variation, which do not always coincide in 

 maxima and minima with these variations of light. 



We have seen from mechanical indications that multiple 

 excitations are induced by light. We have, therefore, to 

 determine whether similar multiple excitations can be detected 

 electrically. Fig. 242 is a photographic record of a series of 



such electrical responses ob- 

 tained from the lamina of 

 a leaf of Bryophyllum sub- 

 jected to the continuous 

 action of light We see here 

 an alternation of phase in 

 the response, negative being 

 followed by positive in each 

 case throughout the series. 

 This constitutes a parallel 

 case to that of the mechanical 



FlG. 242. Photographic Record of r 



Multiple Electrical Response in response in fig. 241, inasmuch 



Leaf vi Bryophyllum under Con- as owing to incomplete re- 

 tmuous Action of Light 



covery from each negative 



phase, the base line is gradually tilted upwards. But this 

 need not always occur, for the two phases may be equal, and 

 in that case the base line remains horizontal. 



We have seen, in the chapter on Multiple and Autono- 

 mous Response, that these effects are due to the absorption 

 of an excess of energy. When this absorption is great the 

 energy may find an outward expression, even after the 

 cessation of the stimulus. An example of this was seen in 

 the mechanical response of a Desmodium leaflet (fig. 141)- 

 The plant was at first in a sub-tonic condition, and the auto- 

 nomous pulsation of its lateral leaflets had come to a stand- 

 still. One of these was now exposed to the continuous 

 action of light and its record taken. It will be noticed that 

 under this stimulus of light multiple responses were initiated, 

 which persisted for a time as an after-effect, even on the ces- 

 sation of light. In taking electrical records of the after-effect 



