300 PLANT RESPONSE 



time. This was because the energy stored up in the tissue 

 had become exhausted. If, then, a single stimulus were 

 given, say, by condenser discharge, a single response would 

 be found to ensue. This showed that in such a case the 

 arrest of the automatic movements was not due to the 

 abolition of motility in the leaflet, but only to exhaustion of 

 the energy stored up, which would have given rise to oscilla- 

 tion. It was also shown that if Desuiodiuni in a state of 

 standstill were subjected to a single strong thermal stimulus, 

 it exhibited a multiple series of rhythmic responses. 



Multiple response to constant stimulus. — Having 

 observed the production of multiple responses as an after- 

 effect of a single strong stimulus, I shall now proceed to 

 demonstrate the production of similar multiple responses by 

 the action of a constant excitatory condition. 



1. Chemical. — I have shown that in Biophytiiiii, and in 

 Desmodium at standstill, rhythmic pulsations are produced 

 by the constant action of a chemical stimulant. Analogous 

 phenomena are also known in animal tissues ; in the isolated 

 heart in a state of standstill, rhythmic pulsation can be renewed 

 by chemical stimulation. 



2. Electrical. — I have often observed that when a strong 

 current is sent through Averrhoa carainbola, a number of 

 rhythmic pulsations are found to take place. In animal 

 tissues, again, similar rhythmic excitations have been ob- 

 served, not only in cardiac muscle, whose rhythmicity is 

 so marked a characteristic, but also in skeletal and other 

 muscles. 



3. Stimulus of light : {a) On retina. — And now before I 

 describe the experimental demonstration of periodic excita- 

 tion in plants, as caused by constant stimulus of light, I shall 

 refer in detail to certain remarkable periodic effects which I 

 have observed in the retina, under the action of the same 

 stimulus. I showed in the last chapter that strong stimulus 

 of light gives rise in the retina to multiple responses, in the 

 form of recurrent after-images. I shall now prove that during 

 the continuance of constant light, pulsatory visual effects are 



