244 PLANT RESPONSE 



external sources. Immediately on this reaching the threshold 

 of response, it will give rise to an excitatory discharge. And 

 it is clear that this excitation will be transmitted preferentially 

 along the line of least resistance, that is to say, in the 

 direction of the greatest conductivity, or outwards. 1 have 

 been able to obtain further experimental verification of this 

 conclusion, by applying a gradually increasing stimulus of 

 condenser discharge to an intermediate point on a petiole of 

 Biophvtum. When this stimulus had reached a certain value, 

 it was found that while excitation, as indicated by the fall of 

 the leaflets, travelled through a great distance forwards, or 

 outwards, its transmission backwards was extremely limited. 

 Could we have adjusted the stimulus, so as to have been 

 slightly above the threshold of response, there would have been 

 no transmission backwards. When stimulus, on the other 

 hand, is excessive, the entire excitatory effect cannot be carried 

 forward ; there is an overflow backwards ; and under these 

 conditions excitatory movements take place in both directions. 



Effect of fatigue on velocity of transmission. - We shall 

 next deal with the modification of the velocity of transmission 

 by fatigue. Specimens of Biophytuiii were used for the 

 purposes of this investigation, and experiments were per- 

 formed by ascertaining the times taken for the transmiission 

 of a repeated uniform stimulus, through the same distance, 

 under shortening periods of rest. 



A stimulus was given to a leaf of Biopliytum, and the 

 record of time taken— the transmission being in a centripetal 

 direction. The plant was now given an interval of rest ot 

 three minutes. Stimulus was again applied, and the time- 

 records obtained in the usual manner. The next stimulus was 

 applied after a resting-interval of two minutes, the following 

 after one, and the last after half a minute, the time of trans- 

 mission and the records of response being taken throughout. 

 From the results given below, it will be seen how regular is the 

 decrease in velocity with the increase of fatigue. The 

 distance to be traversed, 27 mm., was kept the same in all 

 cases. The time taken at the beginning, when the plant was 



