TRANSMISSION OF EXCITATORY WAVES IN PLANTS 239 



Difficulties in exact determination of velocity of trans- 

 mission of excitation.— In the course of the investigation 

 carried out on this subject, I have found that the discrepancies 

 between the velocities, determined in the way described, are 

 largely to be accounted for, first, by indefinite changes of 

 excitability at the point of application, due to the injury 

 caused by excessive stimulation ; and, second, to the changes 

 of conductivity, caused by fatigue, in the rest of the tissue. 

 I have also found that the velocity of transmission is only 

 a determinate quantity when the intensity of stimulus is 

 constant. It undergoes variation, with changes in the stimu- 

 lation-intensity. 



These difficulties are met by using a stimulus which does 

 not cause injury, and which can be repeated at uniform 

 intensity. Such a stimulus is given by means of the con- 

 denser discharge. As regards the changes of conductivity 

 due to fatigue, I have found that fatigue is removed, and 

 conductivity fully restored, after a definite period of rest, 

 which, in the case of Biophytum, is about four to five minutes. 



The next difficulty to be overcome is concerned with the 

 question of recording the exact moment of application of 

 stimulus, and that of the initiation of response at a distant 

 leaflet. A further source of uncertainty in the last respect, 

 lies in the existence of an unknown latent period of the 

 leaflet, which may delay the visible response, even after the 

 effect of stimulus has reached the point at the base of the 

 leaflet. 



It is evident that the times of application of such rude 

 modes of stimulation as cut, or contact of hot wire, cannot be 

 accurately determined, and the exactmoment of the beginning 

 of the responsive movement of the motile leaflet is equally 

 difficult to ascertain by the unaided eye. These difficulties 

 are, however, removed, if we use the discharge from a con- 

 denser as our mode of stimulation, and the magnified move- 

 ment of the spot of light from the Optic Lever, as the 

 indicator of the commencement of response. The observer, 

 following the spot of light from the Optic Lever, makes two 



