713 



LECTURE XL. 



to climb vertically upwards, and that of its lateral shoots to climb obliquely upwards 

 are to be ascribed to their differing geotropism, in so far as this makes itself evident 

 chiefly at the flanks where the leaves are situated. 



It has already been shown in a previous lecture that in the case of Marchantia 

 and similar structures the dorsi-ventral organisation is itself induced by the direction 

 of the incident "light; this once accomplished, however, the side which was hitherto 

 ventral and shaded cannot again be transformed, by being subsequently illumi- 

 nated, into an organically upper side — the dorsi-ventral structure cannot be reversed. 



The Ivy behaves very differently in this 

 respect— i. e. so far as the shoot-axes of 

 the plagiotropic shoots are concerned; 

 if one of these rooted shoots is fastened 

 to a vertical rod up to close beneath the 

 apex, and is placed at a window so that 

 the side which had hitherto borne roots, 

 or, what is the same thing, had hitherto 

 been the shaded side of the shoot-axis 

 is turned towards the light, as in Fig. 

 402 B, it is noticed in the first place," 

 that the backward curving of the apex of 

 the shoot takes place much more slowly 

 than in the reverse position, evidently 

 because the sensitive organisation which 

 had been induced by the previous rela- 

 tions is only slowly altered; but it is 

 actually altered, for the growing apex 

 of the shoot becomes not only horizon- 

 tal, as in Fig. 402 C, but even deve- 

 lopes roots on the side which was 

 previously illuminated, and which is now 

 converted into the shaded side. 



There is still to be mentioned here 

 an interesting peculiarity of the plumule 

 of young seedlings of the Ivy. When 

 seedlings growing at a window have 

 emerged from the soil, the first seg- 

 ment of the shoot (the hypocotyl) is 

 curved concavely towards the window; it is therefore positively heliotropic. On 

 further growth, however, the new internodes of the axis curve away from the 

 window, and the same then takes place with the first one also; its heliotropism, 

 at first positive, thus becoming negative. 



A similar behaviour, but in some respects different, is exhibited also by the 

 Indian Cress {Tropaolum majus). I mentioned this then unknown case so long 

 ago as 1865. The seedling-stem (here the epicotyledonary segment) is at first 

 decidedly positively heliotropic ; but if the plants remain undisturbed at a bright 

 window, in summer, the seedling-stem together with the new developing internodes 



FIG. 402.— Plagiotropic curvatures of the shoots of Ivy 

 (sec text). 



