DrFFERENT REACTIONS' OF BORSI-VENTRAL AND RADIAL ORGANS. 705 



radial organ — lateral roots, side-shoots, leaves, &c. — are as a rule plagiotropic, and 

 in many cases, particularly leaves, are also distinctly dorsi-ventral in structure ; and 

 conversely, orthotropic petioles with radial structure, and, similarly orthotropic 

 radial side-shoots, very commonly arise from plagiotropic shoot-axes. In the case 

 of plagiotropic lateral roots this makes itself evident in that they produce lateral 

 rootlets of the second and thirtl order which are apparently not geotropic at all, 

 as a rule (see Fig. 397). 



The relations between anisotropy and the correlations of growth come out, if 

 possible, still more clearly, when plants are allowed to grow on the axis of a klinostat, 

 and at the same time the effect of heliotropism is eliminated. Even in this case, where 

 gravitation and light can have no effect on the direction of growth of the organs, 

 they nevertheless grow so that they form certain angles with one another ; but the 

 questions here opened up still require investigation. 



I now proceed to describe the relations between radial or dorsi-ventral struc- 

 ture on the one hand, and orthotropic or plagiotropic growth on the other, relations 

 which have hitherto been studied chiefly by myself. 



When an orthotropic shoot-axis or primary root is laid horizontally, there 

 results in the case of the former the upward, and in that of the latter the downward 

 curvature described previously, and it is wholly immaterial which side of the organ 

 lies upwards or downwards ; it is just in this that the radial structure of such organs, 

 even with reference to their irritability, makes itself evident — i.e. they react in the 

 same way on all sides. Radial orthotropic organs also behave in exactly the same 

 way towards the light : wlicn illuminated laterally they curve until the free movable 

 end lies in the direction of the ray of light itself, or, what is the same thing, is 

 illuminated equally on all sides; this part then grows straight forward and in the 

 direction of the ray of light, towards or away from the source of light — I am here, for 

 the time being, excluding certain cases of so-called negative heliotropism. 



Dorsi-ventral plagiotropic organs behave quite differently. If a still growing leaf 

 of a Gourd, for instance, is laid horizontally with the normally upper side downwards, 

 the whole lamina becomes curved concave upwards; if such a leaf is illuminated 

 from beloAV it likewise becomes concave. In this way it is shown that the venation of 

 the leaf possesses the same kind of geotropism and heliotropism as ordinary ortho- 

 tropic shoot-axes; but a great difference makes itself evident in that this only 

 happens when the lower side of the lamina is turned upwards, or towards the source 

 of light. If such a growing leaf is laid horizontally with its upper surface upwards, 

 or if the light is allowed to fall vertically on its upper surface, the latter does not 

 become concave ; on the contrary, the lamina extends itself in one plane. It is thus 

 seen that the lamina of a growing leaf reacts differently according as gravitation or 

 a ray of light affects it from below or from above, and it is just in this the dorsi- 

 ventral structure of the lamina with reference to stimulation is made evident. 



The dorsi-ventral flat shoot of a Marchantia (pp. 69 and 526) and other similar 

 flat shoots, and, generally, all the organs with strictly dorsi-\entral structure known to 

 me, also behave like a leaf. 



If now we suppose a jilane thin organ with dorsi-ventral structure, such as 

 the lamina of an ordinary leaf or the flat shocjl of a Liverwort, rolled togclliL-r 

 parallel to the direction of its longitudinal axis, so that either the ventral or the 



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