36 PLANT RESPONSE 



tion — that is to say, be locally disturbed, say by torsional 

 vibration — two effects will be produced : first, the negative 

 turgidity-variation, which is the true excitatory effect, with 

 its attendant negative electrical variation ; and, second, the 

 electrical effect due to hydrostatic disturbance or water- 

 movement, which is positive. Of these two opposed electrical 

 effects, the first, or true excitatory variation, is generally 

 speaking much the stronger. It therefore completely masks 

 the second, or effect of water-movement, and the resultant 

 response is the normal negative variation. The water-move- 

 ment effect may, however, be unmasked by killing the tissue, 

 and then applying the same torsional vibration as before. 

 The result is now a positive electrical response. 



Or the positive effect may be made to exhibit itself 

 separately, under favourable conditions in a living tissue, by 

 the method of indirect stimulation, that is to say, by the 

 application of stimulus at a distance. When such a distant 

 point is stimulated, there is a sudden expulsion of water 

 from that point, due to stimulation. This gives rise to a 

 wave of increased hydrostatic pressure (with its attendant 

 positive turgidity-variation), which travels with a relatively 

 great velocity. The true excitatory variation, travelling at 

 its slower rate, reaches any given distant point much later. 

 The two effects ought thus to be divided from each other by 

 some interval of time. 



We should therefore expect on stimulation of a sensitive 

 plant to find the hydrostatic disturbance, with its attendant 

 positive turgidity-variation— reaching the distant motile 

 organ the earlier of the two. And since the negative 

 turgidity-variation due to excitation causes a fall of the leaf, 

 the positive turgidity-variation due to hydrostatic disturbance 

 should be expected to produce an abnormal positive or 

 erectile movement, and the same positive turgidity-varia- 

 tion should also find a simultaneous electrical expression 

 in the abnormal positive response. The true excitatory 

 response — with its attendant negative turgidity-variation — 

 should cause, la er the normal negative mechanical response, 



