MOVEMENTS OF IRRITATION. 515 



shoots of Hedera Helix. The shoots of the ivy are markedly 

 dorsiventral and plagiotropic, with the exception of the flower 

 shoots, which are not developed until the plants have attained a 

 considerable age, and which are orthotropic. If free-swinging 

 ivy shoots, about 30 cm. in length, are cut off and planted in 

 flower-pots, and after six to eight weeks, when they are well 

 rooted, are bound to a vertical stick, we can, under suitable con- 

 ditions, observe interesting phenomena at the summit of the shoot 

 projecting beyond the end of the rod. 



In August I placed Ivy plants, obtained as above, in front of 

 a window with a north aspect, and found that the shoot ends 

 speedily turned away from the window. They grew horizontally 

 inwards into the room, and after four weeks presented the appear- 

 ance depicted in Fig. 172. The shoots performed negatively helio- 

 tropic and photoepinastic curvatures (as to the modus operandis 

 however, further investigations are necessary). The illuminated 

 side of the shoot was consequently forced to become convex and 

 bring the shoot ends into a horizontal position. This having been 

 effected, they did not curve still further downwards, since now 

 negative geotropism (the geotropic properties of ivy shoots have 

 been specially investigated by Sachs) was able to assert itself very 

 energetically. It appears, therefore, that the horizontal position 

 of ivy shoots is the resultant of the directive influence of light on 

 the one hand, and gravity on the other (?). The same factors 

 also determine the direction assumed by the shoots of ivy plants 

 growing in nature. 



In the preceding pages we merely discussed in the first place 

 certain properties of leaf structures and many shoots. We have 

 now sought to explain -the normal position of leaves and shoots by 

 reference to these properties. We conceived the normal position 

 of the organs as brought about by the co-operation of various 

 forces. Thus the usual orientation of leaf-blades, e.g., will be 

 essentially due to the co-operation of their photoepinasty on the 

 one hand and their negative geotropism on the other. 



It is, in fact, certain that the normal position of leaf-stalks, and 

 also certainly of many shoots, can be explained in the manner 

 indicated. But it is precisely as regards the leaf-blades that the 

 method is unsatisfactory. Vochting, Schwendener, and others 

 have shown that neither geotropic movements of the lamina?, nor 

 conditions of loading, need directly and of necessity be concerned 

 in bringing about their fixed light position. On the contrary, 



