142 RESEARCHES ON IRRITABILITY OF PLANTS 



Having now obtained means for the accurate determina- 

 tion of the velocity of transmission, we next proceed to 

 study the effects of various agencies in inducing changes 

 in the normal rate. And first we must consider the im- 

 portant question of whether or not the velocity is in any 

 degree dependent on the intensity of stimulus. In the 

 corresponding case of conducting animal-nerve there is 

 considerable diversity in the results which have been 

 arrived at. It has been found by some investigators that 

 velocity is independent of the intensity of stimulus. Others 

 have found, on the contrary, that the velocity of trans- 

 mission increases with the intensity of the stimulus, till 

 with a very high intensity it becomes immeasurable. The 

 results which I shall here describe will probably throw 

 light on this debatable question. It may be said, in anti- 

 cipation, that the effects are to some extent modifiable in 

 a definite way by the condition of the conducting-tissue. 



If the specimen happens to be in a sluggish condition, 

 then increasing intensity of stimulus will be found to be 

 attended by increasing velocity of transmission. Again, a 

 moderately strong intensity of stimulus is often found to 

 leave, as an after-effect, increased conducting power. That 

 is to say, to a tissue which has been sluggish, stimulation 

 itself imparts a higher conductivity. In these facts there 

 is a remarkable parallelism to what has already been pointed 

 out in the matter of the amplitude of response of the sub- 

 tonic tissue. In that case we saw that increasing intensity 

 of stimulus gave rise to increased amplitude of response. 

 We saw, further, that following a given stimulus, increased 

 excitability appeared as an after-effect, so that the repeti- 

 tion of an identical stimulus evoked response of enhanced 

 amplitude. 



Effect of Increasing Intensity of Stimulus 



Returning now to the experimental inquiry into the 

 influence of intensity of stimulus on the velocity of trans- 

 mission, I reproduce a set of records (fig. 85) obtained with 



