LAWS OF RESPONSIVE GROWTH-CURVATURES 535 



Laws of responsive growth-curvature.— It must be 

 remembered here that the effect of indirect stimulation is to 

 cause an increase in the rate of growth, and that of direct 

 stimulation a retardation of the rate. By a positive effect is 

 meant a responsive mechanical movement towards, and by 

 negative, away from, the source of stimulation. 



1 . Unilateral stimulus of moderate intensity, applied at the tip 

 of root or shoot, gives rise to a negative effect, the tip being moved 

 away. 



2. Stronger or long-continued stimulus, applied unilaterally 

 at the tip of root or shoot — being conducted gradually to the 

 growing region — results in a neutralisation of the first negative 

 by a subsequent positive effect. Or there may be a resultant 

 positive, due to the predominance of the transmitted effect. 



3. Direct unilateral stimulation of moderate intensity on the 

 growing region causes a positive response or movement towards 

 stimulus. 



4. Strong or long-continued unilateral stimulation of the 

 growing region, on account of the transmission of effect to the 

 distal side, may give rise either to neutralisation of the normal, 

 or to a reversed or negative effect, that is to say, to movement 

 away from stimulus. 



Summary 



All the mechanical effects induced at the responsive 

 growing region of either root or shoot, by unilateral stimula- 

 tion, may be summarised as follows, it being understood that 

 positive response means movement towards, and negative, 

 movement away from, stimulus : 



(i) Positive response is induced, first, by direct unilateral 

 application of stimulus on the growing region ; second, by the 

 long-continued unilateral application of moderately strong 

 stimulus at the tip. 



(2) Negative response is induced, first, by the unilateral 

 application of feeble stimulus at the tip ; and second, by long- 

 continued unilateral application of strong stimulus at the 

 growing region, causing fatigue of the proximal, and trans- 

 mission of true excitation to the distal, side. 



