160 TROPIC MOVEMENTS 



A plagiotropic position may, however, also be attained without the aid 

 of any epinasty, as when a leaf, owing to the position of the stem, has to 

 curve beyond the epinastic position of equilibrium. In such cases the 

 epinasty is no longer essential, and may modify the position assumed 

 little or not at all if the leaf orients itself definitely in regard to gravity, 

 whether it has to overcome epinasty, photonasty, and the like, or not. The 

 fact that the angle the leaf makes with the stem may vary indefinitely 

 suffices to show that the orienting action of the stem is either absent or is 

 so weak as to be ineffective. 



De Vries considered that tropic stimuli always produced a parallelo- 

 tropic reaction, so that a plagiotropic position could only result from 

 the combination of a tropic action with some other attempted curvature. 

 This view is, however, not supported by the facts, nor is it easy to see any 

 reason why a responding organ should not be able to directly set itself 

 at right angles to an orienting agency. 



When the expansive tissues are symmetrically arranged, an autogenic 

 epinastic curvature may be prevented, but may take place when the organ 

 is split longitudinally, and may then cause the parallelo-geotropic halves 

 to assume plagiotropic positions. In the same way two leaves bound 

 together with their upper surfaces together form a symmetric arrangement, 

 and may in certain circumstances react parallelo-geotropically because the 

 opposed plagiotropic tendencies only equilibrate in a vertical plane. 



Dorsiventral organs are much more liable to nastic curvatures than 

 radial ones, and any dissimilarity in the sensitivity or power of reaction 

 of the upper and under surfaces is bound to affect the tropic responses. 

 Thus the physiological dorsiventrality of certain tendrils results in the fact 

 that a curvature is only produced when contact is applied to the sensitive 

 concave side. In addition, a stem cannot place itself parallel to the 

 incident rays of light when one side has a feebler heliotropic irritability 

 than the other, or when one side is smeared with indian ink. Hence 

 a plagio-phototropic orientation is to be expected when the structure is 

 such that light penetrates more readily on one side than on the other. 

 Under such circumstances a photonastic curvature might result in diffuse 

 daylight, although this is actually due to unequal phototropic stimulation. 

 Care is needed in the interpretation of such phenomena, as is well shown 

 in the case of dorsiventral tendrils ; for although contact on the convex side 

 does not excite a curvature, it is able to suppress one when the concave side 

 is also stimulated, so that both sides are irritable, though in unlike degree. 



It is difficult to determine from the tropic reactions in what degree the 

 irritabilities of the upper and under sides differ in intensity or in quality. 

 By altering the incidence of the light a plagio-phototropic leaf may be 

 caused to assume its proper position of equilibrium either by a positively 

 or negatively directed movement, whereas illumination of the under side 



