138 RESEARCHES ON IRRITABILITY OF PLANTS 



is *I2 second. Thus the velocity of transmission as given 

 by both these experiments is identical — namely, 



t 1-5 



V = - = A_ = 20 mm. per second. 



The Differential Method 



In order to put the constancy of these results to a still 

 more rigorous test, I next modified the experiment in the 

 following way, employing the Differential Method. The 

 stimulus was first applied at a distance d from the respond- 

 ing pulvinus, and the total time T was found from this 

 record. In the next experiment the distance of the point 

 of stimulation was reduced to d^, and its corresponding 

 total time, T^, found in the usual manner. And lastly, a 

 record was taken under direct stimulation. This furnished 

 the value of the latent period L. 



It will be seen that we have here three different sets 

 of data for the determination of the absolute value of the 

 velocity of transmission. In two of these we derive the 

 velocity in the usual manner from the distance, the total 

 time, and the latent period. In the third, knowledge of L is 

 not required, as it will be seen that the difference in the 

 times T — T^ of transmission observed in the first two cases 

 represents the time taken by the excitation to travel the 

 difference between the two distances d — d^. Hence three 

 separate determinations, Vi, Vo, and V3, obtained with the 

 same specimen, are 



y ^ . Y — <^i y __d—di 



T-L' ' Ti-L' ' T-Ti 



The rigour of this test of constancy will be gauged by 

 the extent to which the different determinations of Vj, Vg, 

 and V3 are consistent with one another. 



In an experiment carried out in this way the intensity of 

 stimulus appHed was 3 units, and the vibrating recorder 

 had a vibration-frequency of 10 per second. In the first 



