THE POSITIVE RESPONSE 191 



period of the negative from 9*4 seconds to 4*6 seconds, 

 it induced no variation in the transmission-period of 

 the positive impulse which remained unchanged at 

 •6 second. 



(4) When the distance to be traversed is reduced say 

 to half, the period of transmission of the negative impulse 

 is also reduced to half. But with the positive impulse 

 the diminution is very slight. Thus in fig. 103 we find 

 that with a distance of 20 mm. the period for the propa- 

 gation of the positive impulse was i second ; when the 

 distance was reduced to half, the transmission-period 

 was reduced not to half a second but to eight -tenths of a 

 second. 



It is difficult from these data to arrive at any definite 

 conclusion as regards the nature of the positive impulse. 

 Is this impulse physical or physiological ? If the former, 

 could it be hydro-mechanical ? In consequence of a 

 feeble stimulus applied at a distance, we have at the 

 responding pulvinus, a positive turgidity variation, expan- 

 sion and erection of the leaf, evidently brought about by 

 the forcing in of water. This presupposes a forcing out of 

 water somewhere else, probably at the point of application 

 of stimulus. It may be supposed that an active contraction 

 occurred in the plant-cells under excitation, in consequence 

 of which the sap was forced out giving rise to a hydraulic 

 wave. On this supposition the positive impulse is to be 

 regarded as hydro-mechanical. There is, however, no 

 definite experimental proof in support of this theory. I 

 shall presently describe the experiment I devised to settle 

 the question. Unfortunately the results obtained were 

 not as decisive as I hoped they would be. 



It should be clearly understood that even if the positive 

 impulse were found to be hydro-mechanical, it has nothing 

 whatever to do with the propagation of the excitatory or 

 negative impulse. For if the negative response took place 

 in consequence of the impact of the positive impulse, 

 then the negative should take place at a definite interval 



