134 RESEARCHES ON IRRITABILITY OF PLANTS 



order to eliminate the errors that might be inherent in 

 personal observation, it is desirable that all the data 

 for this determination should be furnished automatically 

 in records made by the plant itself. Successive records, 

 therefore, should enable us to determine with equal accuracy 

 not only the normal velocity but also its variation under 

 given changed conditions. 



And here the preliminary questions arise : With what 

 degree of accuracy can we determine the normal velocity 

 of transmission ? And how far may we depend on the 

 constancy of this velocity, in successive experiments, under 

 normal conditions ? As regards these points, some mis- 

 givings might naturally arise. For the factors calculated 

 to interfere with this constancy will in all probability 

 prove to be numerous. First, we may have the variation 

 of excitability at the point of application induced by the 

 stimulus itself. We have, therefore, to find out what is 

 the maximum intensity of stimulus that may be employed 

 without causing fatigue or other deleterious changes in 

 the tissue. Another point to be remembered is the question 

 already discussed in previous chapters of our ability to 

 apply stimuli, in successive experiments, of identical intensity 

 and duration. Unless this can be secured we cannot look for 

 consistent results, inasmuch as the velocity of transmission 

 may to some extent be dependent on the intensity of stimulus. 

 Likewise, if the transmission of excitation should prove to 

 be due to the transmission of a protoplasmic change, it is 

 easy to see that we must allow the tissue a definite time for 

 protoplasmic recovery after each application of stimulus, 

 without which interval consistency of results could hardly be 

 expected. 



It was only after a long course of investigations — some 

 of which will be described in the course of the present 

 chapter — that I was able to analyse and provide against 

 these several sources of variation. But even after this, 

 I was by no means prepared for the very great consistency 

 of the results which it has been my good fortune to obtain. 



