54 RESEARCHES ON IRRITABILITY OF PLANTS 



a similar delay of *i second being again due, as in the 

 previous case, to the latent period. Thus we see that 

 while the stimulus of the feebler intensity of 75 was effec- 

 tive at ' break,' it took the stronger stimulus of 1 to 

 induce response at ' make.' 



Additive Effect of Stimulus 



In the responsive tissue of the animal a single stimulus, 

 by itself ineffective, is found to become effective on repeti- 

 tion. In order to test whether this holds good in the case 

 of the plant also, I carried out the experiments which I 



Fig. 22. — Stimulus of intensity - 5 became effective 

 on being repeated four times. 



shall now describe. With a given specimen I found that a 

 single make-and-break shock of intensity 75 was ineffective 

 in inducing excitation. I then adjusted the secondary for 

 intensity of "5, and made a reed-interrupter interposed in 

 the primary circuit give a series of make-and-break shocks 

 till the leaf responded by a fall. The interrupting reed was 

 adjusted to vibrate five times per second and the number of 

 interruptions is recorded below in the usual manner. It 

 will be seen in the record given in fig. 22 that the make-and- 

 break stimulus, which singly was ineffective, here became 

 effective on being repeated four times. 



Desiring next to observe the effect of still further 

 reducing the intensity of stimulus with the same specimen, 

 I adjusted the secondary for an intensity of "I. It must be 

 remembered that this is the intensity of tetanisation, which 



