RESPONSIVE CURVATURE OF ANISOTROPIC ORGAN 93 



It has thus been clearly established that there is no 

 specific sensibility of the dorsi-ventral organ which is in 

 any way distinct from that of radial organs — the responsive 

 lateral movements of leaves being merely a special or dif- 

 ferential form of the longitudinal contraction which has thus 

 been found to be widely prevalent. 



Summary 



When the two sides of an organ become unequally 

 excitable by reason of molecular differentiation, a resultant 

 lateral response, due to differential longitudinal contraction, 

 is obtained, the more excitable side becoming concave. This 

 molecular differentiation may be induced artificially by uni- 

 lateral application of cold, or of strong stimulation. 



This molecular differentiation occurs under natural con- 

 ditions in plagiotropic stems, the upper surface being acted 

 on by stimulus of vertical sun-light. 



In a hollow tubular organ, such as the petiole of Cucurbita, 

 or the peduncle of Allium, the outer surface, which is 

 constantly acted on by light, is found to be less excitable 

 than the inner surface. 



The spiral formed, by unilateral stimulus of contact, in 

 such tendrils as that of Passiflora is less excitable on the 

 already stimulated, or concave, than on the outer, or convex, 

 side ; diffuse stimulation, causing greater contraction of the 

 more excitable convex side, gives rise here by differential 

 contraction to the responsive movement of uncurling. 



Molecular anisotropy culminates in dorsi-ventral in- 

 equality, as seen in the petioles or in the pulvini of leaves. 

 Here, too, diffuse stimulation causes lateral response, by 

 inducing concavity of the more excitable half. 



Dorsi-ventral organs do not possess any specific sensi- 

 bility different from that of radial organs, the lateral 

 responsive movement, being the result of the differential 

 longitudinal contraction of two unequally excitable halves. 



The universal law of responsive movement is : Mechanical 

 response takes place by the concavity of the more excited side. 



