520 PLANT RESPONSE 



effect as between A and B can be inferred by the algebraical 

 summation of these separate effects. Or it can be directly 

 obtained by making electrical connections with the dia- 

 metrically opposite points A and B, as in fig. 212, c, and 

 applying gradually increasing stimulus at C near A. 



It will be remembered that these opposite effects of 

 direct and indirect stimulation have received independent 

 demonstration, in both mechanical and growth responses. 

 In Biophytum and in Artocarpus the positive turgidity- 

 variation was proved to be the indirect effect of stimulation, 

 by the erectile responses of the motile organs (pp. 24, 420). 

 In the case of growth- response, again, the indirect effect of 

 stimulation, with the concomitant positive turgidity-variation, 

 was shown in the increased rate of growth (p. 430). The 

 effect of direct stimulation in inducing a negative turgidity- 

 variation, again, was exhibited in Biophytum and Artocarpus, 

 by the responsive depression of the motile organs. In the 

 case of growth-response, it was exhibited by contraction and 

 concomitant retardation of growth. 



Mechanical response inferred from observed electrical 

 response. — From the results of the electrical investigation on 

 these turgidity-variations, induced by direct and indirect 

 effects of stimulus, as just described, we are led to conclude 

 that : 



1. {a) Moderate unilateral stimulation of the tip gives 

 rise by longitudinal transmission to the indirect effect of 

 stimulus, namely a positive turgidity-variation, on the same 

 side of the distant responding-organ. This will give rise to 

 acceleration of growth and convexity of that side, and by 

 the consequent responsive movement the tip will be carried 

 away from the source of stimulation. 



(d) When the unilateral stimulation of the tip is a little 

 stronger, the indirect effect of stimulus is neutralised by the 

 subsequently transmitted true excitatory effect, and there is 

 no resultant action. 



Very strong unilateral stimulation of the tip gives rise by 

 longitudinal transmission to the direct effect of stimulus, 

 namely a negative turgidity-variation, on the same side of 



