132 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE 



Positive tropic curvature is brought about by the con- 

 joint expansion of the distal and contraction of the proximal 

 side. 



Summary 



The response of an organ is modified by the point of 

 application of the stimulus. 



The closest parallelism has been established between the 

 response to stimulation given by pulvinated and by growing 

 organs respectively. Conditions which give rise to negative 

 mechanical or electric response also give rise to negative 

 variation or retardation of growth. This is also true of 

 positive mechanical and electric response and positive 

 variation or enhancement of growth. 



Effective stimulation is shown to give rise to two distinct 

 impulses : one of these, the positive, is of a hydraulic nature ; 

 the negative, on the other hand, is of an excitatory charac- 

 ter. The positive is transmitted quickly ; the latter, being 

 a phenomenon of conduction of protoplasmic change, is 

 propagated slowly. The positive impulse gives rise to 

 expansion, the excitatory negative to contraction. 



Feeble stimulus, especially when acting on a subtonic 

 organ, gives rise only to positive response. 



The results of investigation of the effect induced by all 

 forms of stimulation lead to the establishment of the follow- 

 ing law : direct stimulation induces contraction ; indirect 

 stimulation gives rise to expansion. 



Direct stimulation of the responding region causes a 

 contractile fall of the motile leaf, and a retardation of growth 

 in a growing organ. The transmitted or indirect effect of 

 stimulus applied at a distance is to induce an erection of 

 the leaf and an acceleration of the rate of growth. 



Tropic movements illustrate the laws of direct and 

 indirect stimulation. The directly excited proximal side 

 undergoes contraction, the opposite distal side undergoes 

 expansion ; these two factors conspire to produce a positive 

 curvature. 



