RESPONSIVE f.ROWTH-CURVATURES IN PLANTS 729 



effect of stimulus. These have been shown to be classified 

 as follows : 



I. Direct unilateral stimulus on the responding 

 organ : (a) Positive response under moderate stimulation. — - 

 The proximal, by the direct action of stimulus, contracted ; 

 and the distal, by the indirect action of stimulus, expanded. 

 The result was a concavity of the proximal, and convexity 

 of the distal, conspiring to bring about movement towards 

 stimulus. 



(J)) Intermediate or neutral response. — Though the trans- 

 verse conductivity of a tissue may be feeble, yet under 

 somewhat 'strong .stimulation the true excitatory effect 

 is transversely conducted from proximal to distal. The 

 result is that when the two opposite sides are equally excited, 

 there is a neutralisation, or disappearance of responsive 

 curvature ; or, by alternate fatigue of the two sides again, 

 the organ may be made to. oscillate to and fro about a more 

 or less mean position. 



{c) Negative response. — When stimulus is \e.\-y strong and 

 long continued, we obtain not only the transverse conduction 

 of effect, but also temporary induction of anisotropy of the 

 organ. The proximal side is now, owing to fatigue brought 

 about by the direct impact of excessive stimulus, the less 

 excitable ; and the internally diffused stimulus, causing 

 greater contraction of the more excitable, induces concavity 

 of the distal, or a negative responsive movement. 



Besides this we have organs which are characterised b}- 

 a permanent anisotropy or dorsi-ventrality, and we then 

 obtain two classes of effects, according as the transverse 

 conductivity is very feeble or moderately strong. Owing to 

 the dorsi-ventral structure, the responsive movement can 

 only take place at right angles to the plane which .separates 

 the anisotropic halves of the organ. These effects are the 

 same in growing organs, such as plagiotropic shoots and 

 dorsi-ventral petioles, and in mature dorsi-ventral organs, 

 such as pulvini. 



{d') Dorsi-ventral positive response. — When the transverse 



