7o8 LECTURE XL. 



curving away from the centre of rotation, which removes all doubt as to their 



geotropism. 



When I reversed the box, in which the seedling just referred to was grow- 

 ing, so that the apex of the primary root was turned upwards, the tip of the 

 root curved vertically downwards; the direction of growth of the lateral rootlets 

 changed also, their apices likewise curving downwards not vertically, but only 

 obliquely, and this so that they grew on at about the same angle with respect to the 

 horizon as before the reversal of the box. Those portions of the lateral rootlets 

 which are quite black in the figure are the portions which were growing during the 

 inverted position. The box was then placed upright again, and the lateral roots 

 once more curved obliquely downwards, again to grow straight out at the same angle 



with the horizon as before the 

 first inversion of the box. It has 

 not yet been explained hitherto 

 what relations of organisation cause 

 these geotropic organs to grow 

 obliquely to the direction of the 

 earth's radius. Elfving found sub- 

 sequently, it is true, that runners 

 growing horizontally in the soil — 

 e. g. shoot-axes of Heleocharis, Spar- 

 ganium, and Scirpus marUimus — 

 owe this horizontal direction of 

 growth to geotropism; if they are 

 placed obliquely or vertically up- 

 wards they curve under the influence 

 of geotropism, until the bud lies 

 horizontally, and it then goes on 

 growing in this direction. 



When sub-aerial shoots with 

 dorsiventral structure are at the 

 same time heliotropic and geotropic, 

 two cases may occur; either the 

 heliotropic influence coincides with 

 the geotropic one, or each strives to give the organ different directions, so that, as 

 a matter of fact, a middle direction results. In order to be correctly understood 

 in what follows, I may premise that in the case of plants growing in the open the 

 heliotropic effect is to be supposed as if a vertical ray of light fell on the plant from 

 a luminous point in' the zenith; since just as we must suppose all the influence of the 

 gravity of the earth to proceed in a straight line only from the centre of gravity 

 of the earth to the plant, so we may also imagine the effects of the light reflected 

 from the whole sky as produced by a single resultant ray. We shall now make use 

 of this supposition in what follows. 



The well-known Liverwort, Marchantia polymorpha, forms broad shoots which 

 are green on the upper side, and on the lower side colorless and provided with 

 numerous root-hairs : these shoots are also seen to be strictly dorsiventral structures 



FIG. 400.— Roots of Vicia Faha growings in soil behind a pane of 

 ]7laS5, at first in the normal, then in the inverted, and then again in 

 the normal position. The arrows show the direction in which 

 gravitation acted with respect to tite lateral roots in the different 

 positions. 



