COMPARATIVE ELECTRO-PHYSIOLOGY 



during a period that depends on the intensity of stimulus and 

 the condition of the tissue, after which it declines slowly. 

 After recovery, however, the rate is generally somewhat 

 higher than at firs'". This is due to the persistent after- 

 effect of stimulus absorbed. If 

 the enhanced suctional rate be 

 now again balanced, and 

 stimulus applied once more, 

 there will be a still further 

 enhancement of the rate. In 

 this way, owing to the succes- 

 sive increase of latent energy, 

 the suctional activity is enhanced 

 till it reaches a limit, after which 

 there is but little additional effect 

 to be induced by stimulus. 



There is another and in- 

 teresting effect which is often 

 observed, in consequence of 

 energy absorbed from previous 

 stimulation. This is the diminu- 

 tion of the latent period, after 

 which response takes place. 

 This will be seen in the following 

 record (fig. 228). The initial 

 rate of suction was in this case balanced, as usual, and 

 stimulus of thirty seconds' duration was applied. The 

 responsive acceleration is seen to take place forty-five 

 seconds after the cessation of stimulus. The enhanced rate 

 was balanced, and a stimulus of thirty seconds' duration 



however, does not decide the question ; for in making it, is involved the 

 strong mechanical stimulus of a cut. The outgrowths may, no doubt, obstruct 

 the passage of water, and yet the total abolition of suction not be due to this 

 cause alone. The more effective cause is, in fact, the run-down of energy, as 

 proved by the experiment described above. In another experiment I took a cut 

 stem in which suction had come to a standstill, and, without disturbing the 

 mucilaginous end, I supplied it with water somewhat above the ordinary tem- 

 perature. This thermal stimulation at once initiated renewed suctionaL activity 

 with great vigour. 



FIG. 228. Variation of Latent 

 Period as After-effect of Stimulus 



The record was taken under balanced 

 conditions. Half- shaded portion 

 represents application of stimulus 

 for 30 seconds. Lower record 

 shows latent period to be 45 

 seconds. After re-balance, 

 stimulus of 30 seconds was once 

 more applied. Upper record 

 now shows reduction of latent 

 period to 30 seconds. 



