246 



PLANT RESPONSE 



FiCr. 105. Curve showing Decline in 

 Heights of Responses, with Diminish- 

 ing Periods of Rest 



Abscissa represents resting-periods, and 

 ordinate heights of response 



motile excitability under shortened periods of rest. This 

 curve when produced will cut the abscissa. Such a point 

 would mark the time-interval between two successive stimuli, 



at which the response would 

 be zero, that is to say, the 

 motile excitability would be 

 abolished. In other words, 

 the leaf would, when the 

 resting-interval was short- 

 ened to this period, prove 

 refractory to stimulus. In 

 Chapter XXI I the existence 

 of this theoretical refractory 

 period will be demonstrated 

 by experiment. 



It is thus seen that owing 

 to imperfect protoplasmic 

 recovery, or, in other words, 

 to residual molecular strain, 

 not only is the conductivity of the tissue gradually diminished, 

 but the excitability also. Thus we obtain some idea of the 

 processes by which fatigue is brought about. 



Effect of intensity of stimulus on velocity. — We have 

 next to study the variation in the velocity of transmission of 

 the excitatory condition, with increasing strength of stimulus. 

 In the case of animal nerve, it has been ascertained by 

 different observers (Helmholtz, Vintschgau, and Fick) that 

 the velocity of transmission of the nervous impulse is not 

 independent of the strength of stimulus, but increases with 

 increasing intensity. The experimental verification of thi.s 

 with conducting animal tissue is, however, extremely difficult, 

 owing principally to the shortness of time involved. 



1 have carried out an investigation on this subject with 

 vegetable tissues, which shows in an unmistakable manner 

 that velocity does undergo an increase, with increasing 

 stimulus. These experiments were carried out with leaves of 

 BiopJiytuvL I first demonstrated this in a qualitative manner. 



