190 RESEARCHES ON IRRITABILITY OF PLANTS 



application of the stimulus. When the distance of application 

 is reduced to half, we find that the negative or excitatory 

 response makes its appearance in succession to the positive. 

 The positive is seen to take place "8 second and the negative 

 10 seconds after the application of the stimulus. It will be 

 noticed that the reduction of the distance to half causes 

 only a slight diminution in the transmission-period of 

 the positive impulse — from i second to -8 second. The 

 transmission-period of the negative impulse, on the other 

 hand, undergoes as we have seen a rapid diminution with 

 diminishing distance. 



It has thus been shown that when a feeble or moderate 

 stimulus is applied at a relatively great distance there occurs 

 only the positive response. As the distance is reduced the 

 antagonistic negative response makes its appearance, there 

 being at first a considerable time-interval between the posi- 

 tive and the negative. On reducing the distance still further 

 the interval becomes reduced. The positive will then, as 

 we shall see later, become masked by the predominant 

 negative. 



Characteristics of the Positive Impulse 



I shall now pass under review the various characteristics 

 which distinguish the positive from the negative impulse. 



(i) The positive impulse travels much faster than the 

 negative. 



(2) The conditions for the exhibition of the positive 

 impulse are that the stimulus should be moderate or 

 feeble and applied at a distance. A semi-conducting tissue 

 again is more suitable for its exhibition than a highly 

 conducting tissue. 



(3) While an increase of intensity of stimulus gives rise 

 to enhanced velocity of negative impulse, it produces 

 little or no change in the velocity of positive impulse. 

 Thus Table IV shows that while raising the intensity of 

 stimulus from 5 to 8 units shortened the transmission- 



