RESPONSE TO STIMULUS OF LIGHT 397 



thermal shocks, by means of the electro-thermic stimulator, 

 the proximal side will undergo contraction, while the ex- 

 pelled water, by its hydro-positive effect, will induce expan- 

 sion on the distal side. By means of these two conspiring 

 actions the organ will be bent towards the source of 

 stimulus. The electrical variation on the distal side will 

 therefore be positive ; but if the stimulus applied be suffi- 

 ciently strong and long-continued, true excitation will be 

 transmitted across the tissue to the distal side. This will 

 neutralise the first mechanical movement, and the corre- 

 sponding electrical effect will be a reversal of the previous 



FIG. 233. Experimental Arrangement for Detection of Electrical Change 

 induced at the Point transversely Distal to Point stimulated 



Upper point stimulated by thermal shocks from electro-thermic stimulator, 

 the lower being the transversely distal point. 



positive into the excitatory negative. Similar electrical 

 effects will also be observed if the organ be restrained from 

 movement, or if it be so old as to have lost its power of 

 motility. 



Taking now a young stem of Bryophyllum^ I applied a 

 series of thermal stimuli at the proximal point (fig. 233). 

 Periodic closure of the electrical circuit by means of a metro- 

 nome caused rapidly succeeding thermal shocks to act on the 

 upper or proximal side. By adjusting the heating current 

 the stimulus was at first made moderate. It will be seen 



