VELOCITY OF TRANSMISSION OF EXCITATION 447 



tissues whose responsive peculiarities are indistinguishable 

 from those of animal nerves. These must therefore be 

 regarded as vegetable nerves (Chapter XXX 1 1.). 



The determination of the velocity of transmission of 

 excitation in sensitive plants may be made by applying a 

 stimulus at any point and observing the interval which 

 elapses before motile effects are visible at a given distance. 

 It is, however, impossible by such means to obtain accurate 

 and consistent results until certain factors of variation are 

 successfully eliminated. These are (i) indefinite changes of 

 excitability owing to in- 

 jury caused by stimulus at 

 the point of application ; 



(2) changes of conductivity 

 caused by fatigue ; and 



(3) the unknown effects 

 of varying intensities of 

 stimulus on velocity of 

 transmission. 



As a result of investi- 

 gations on this subject I 

 found that the velocity of 

 transmission can only be 

 regarded as a determinate 

 quantity when the intensity 

 of stimulus has a definite 



value. Excessive stimulation, again, is found, by causing 

 injury, to modify the excitability and conductivity of the 

 tissue. These difficulties, however, are overcome by the 

 employment of a stimulus which does not cause injury, and 

 which is capable of repetition at uniform intensity. One 

 such form of stimulation is obtained by the use of discharge 

 from a condenser previously charged to a known .voltage 

 (fig. 272). As regards those changes of conductivity which 

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

 conductivity fully restored, after the lapse of a definite period 

 of rest, varying in duration in different plants from four to 



FIG. 272. Diagrammatic Representation 

 of Electrical Connections for Deter- 

 mination of Velocities of Centrifugal 

 and Centripetal Transmissions 



A and B are exciting' electrodes, and L the 

 indicating leaflet. 



