4.6 BOTANICAL GAZETTE. 



tenable. While the vertical position assumed by the growing 

 plant's axis is attributed to geotropism and heliotropism, Frank at- 

 tributes the xiosition taken by leaves (with one face to the sky and 

 the other to the ground) to transverse geotropism and transverse 

 heliotropism. "An organ which grows transverse-geotropically, 



places itself horizontally In the same way a transversa helio- 



tropic organ has an inherent tendency to place itself at right-angles 

 to, instead of parallel [as does the stem] to the direction of incident 

 light. It may be said that this is no explanation at all; and this 

 is true in a certain sense. But there is no reason whv it should be 

 more unsatisfactory than the accepted explanation of the vertical 

 growth of stems and roots, namely, that they have an inherent 

 power of growing in these directions." Indeed the one is just as 

 satisfactory or unsatisfactory as the other. The terms "geotrop- 

 ism," " heliotropism," "apheliotropism" and the like, transverse or 

 otherwise, — now a numerous brood, — are useful and not misleading, 

 when held to mean only that, in fact, a leaf " has an inherent ten- 

 dency to be horizontal, ' or "to place itself at right-angles to the di- 

 rection of incident light" and "the hypocotyl of a seedling has an 

 inherent tendency to grow vertically upwards. "The technical terms 

 are unsatisfactory only when they are supposed to carry more expla- 

 nation than the simple and untechnical statement does. But do leaves 

 take their position in virtue of an inherent tendency ? This is sub- 

 stantially denied by DeVries,or at least he maintains that the assump- 

 tion is needless, for that it may be the result of an antagonism be- 

 tween positive and negative heliotropic and geotropic forces, or be- 

 tween these and what he calls epinasty and hyponasty. These new 

 technical terms are simple in meaning. An organ, such as a leaf, 

 is epinastic when the upper half grows in length more than the 

 lower, and the effect is convexity above and concavity below, or 

 downward curvature: it is hyponastic when the reverse takes place. 

 And the suggestion is, that the horizontal position of a leaf under 

 zenith illumination may be due simply to an equilibrium between 

 two or more of these opposing forces. It is unnecessary to enter 

 into particulars of how this obviously might produce the effect: for 

 the result of Mr. Francis Darwin's experiments is a clear disproof of 

 DeVries' suggestion. In the "fundamental experiment:, -if a plant 

 whose leaves have the power of placing themselves at right-angles 

 to incident light is growing normally in the open air and lighted 

 from above, its leaves will be horizontal. Let the plant be now 

 made to rotate on a klinostat [so slowly that no centrifugal effect 

 will be produced, but rapidly enough to destroy all geotropic action] 

 so that the axis of rotation coincides with the axis of the plant. 

 And let the direction of the incident rays of light be parallel to the 

 axis of rotation, so that the morphologically upper side of the 

 leaves is illuminated by rays striking them at right-angles, just as 

 they were when the plant grew on the ground. Then, if the normal 

 horizontal position is the result of a balance between geotropism 



