522 -LECTURE XXXI. 



and to what extent the influence of gravitation succeeds in affecting the position in 

 relation to it of the new organs. 



Since, then, as Voechting had in part found, and as results definitely from my 

 experiments on Thladiantha and Opuntia, gravitation acts in such a way as to move 

 the root-forming substances downwards and the shoot-forming substances upwards, 

 we must assume that this is the case not merely with cut-off pieces, but also with entire 

 plants living and rooted in the soil ; in this case, however, the internal disposition and 

 the influence of gravitation work to the same end, at least in so far as plants with 

 erect stems and down-turned roots are concerned. In accordance with this it is 

 also the rule that in horizontal creeping or climbing shoots the formation of roots 

 takes place on the axis progressively behind the terminal bud, on the underside of 

 the shoot, and in such cases the illumination of the dorsal side of the shoot also 

 evidently has for its effect that the roots make their appearance exclusively on the 

 ventral or shaded side. 



Instructive examples to illustrate the fact that the light in many cases prevents 

 the formation of the growing-points of roots upon the directly illuminated side of the 

 shoot-axis, so that they only appear on the shaded side, may be afforded in the first 

 place by the leaf-shoots of the Ivy *. When these climb up on a wall, or on the stem 

 of a tree, the clinging roots necessary for climbing arise exclusively on the side turned 

 towards the solid substratum : in this case, however, the active cause lies, not 

 simply in the contact or pressure suffered by this side of the shoot-axis, but in the 

 fact that it is turned from the light. The horizontal pendent shoots of the Ivy teach 

 this directly : the anchoring roots here are alway* formed only on the side turned 

 away from the light. But still more : it is possible experimentally to cause shoots of 

 Ivy to form aerial roots on what was hitherto the lighted or dorsal side, by illuminating 

 them for some time on the side which was previously the ventral or shaded side. Fig. 

 339 illustrates this in the case of an Ivy-shoot, the lower end of which was rooted in a 

 flower-pot, and fastened upright to a rod. The arrows give the direction of the rays of 

 light. At A we have the upper part of the shoot with what was hitherto its dorsal side 

 turned towards the light, and bearing roots only on the side turned from the light ; at 

 the same time the shoot-axis exhibits a negatively heliotropic curvature towards the 

 shaded side. A shoot, together with its pot, was now placed at the window so that .the 

 side which bore roots hitherto was turned towards the light. The shoot now made in 

 the first place the heliotropic curvature represented at C, opposed to the previous one, 

 and then grew on horizontally : meanwhile new groups of aerial roots arose on the now 

 horizontal and at the same time shaded side — the previously illuminated dorsal side. 

 With many other dorsi-ventral shoots of course this reversal of ventral and dorsal side is 

 not so easy as in the case of the Ivy, since all the influences of external forces here 

 treated of are dependent upon the specific capacity for reaction of the particular plant. 



Another object resembhng the Ivy in this connection was found by Voechting 

 in the broad, two-winged, leafless shoot-axes of a cactus-like plant, Lepismium 

 radicans, in which likewise each of the two flat sides is able to produce roots, 

 but only when it is turned away from the source of light ^- 



^ The properties of the Ivy shoot here referred to are described in detail in my treatise 'Uber 

 orthotrope und flagiotrope Pflanzentheik' (Arb. des bot. Inst. Wzbg. B. II, p. 257). 



2 That the formation of roots is promoted by the absence of light I have already insisted upon 



