THE PLAGIOTROPISM OF THE I VF. 



711 



arise, however, grow upwards obliquely at an acute angle, but behave otherwise like 

 the parent shoot. There thus results a fan-shaped radiating system of climbing 

 shoots, which sooner or later reach the top of the wall. As soon as this happens 

 the shoot-axes curve over at the angle of the wall, and then go on growing 

 closely appressed to its horizontal surface, till they come to the other angle 

 of the top of the wall : here they do not bend sharply downwards, however, to 

 grow down attached to the hinder surface of the wall, but they go on growing 

 free straight onwards from the hinder angle of the wall, often quite horizontal 

 for 50 cm., and then sink down obliquely under their own weight. When the one 

 vertical side of the wall is thickly covered with such appressed shoots, another pheno- 

 menon makes its appearance; there arise numerous shoots which grow out away 

 from the wall freely sweeping in the air, and horizontal, at least at first, until 

 they bend down obliquely under their own weight. The orlhotropic fruit-bearing 

 shoots of the Ivy arise usually at the highest point which the plagiotropic climbing 

 shoots have reached ; the former are distinguished from the latter by their radial 

 structure, their free upright growth and by their differently shaped leaves, which 

 are here spirally arranged with a divergence of -|, whereas in the plagiotropic 

 shoots they stand in two rows on the right and left flanks. 



We need however no longer concern ourselves with the orthotropic shoots, 

 since they behave towards light and gravitation like ordinary seedling stems; 

 only the plagiotropic shoots climbing up the wall or freely sweeping horizontally 

 require further consideration. Such shoots, cut off and placed with their lower 

 ends in soil in flower-pots, where they soon become rooted, and fastened up 

 to near the apex to vertical rods stuck in the soil, form convenient subjects 

 for experiment. If such a plant is now placed at a window so that the aerial 

 roots are turned towards the room, and the upper sides of the leaves towards 

 the light, the apex of the shoot curves away from the window in a few 

 days and then goes on growing horizontally, while the long thin leaf-stalks 

 curve towards the window, because they are orthotropic (cf. Fig. 402 A). The 

 plagiotropic shoot-axes of the Ivy are thus at any rate in some way negatively 

 heliotropic ; only it turns out that they continue to curve away from the source 

 of light only until they become horizontal. If they were negatively heliotropic 

 in the same way as most aerial roots, or the primary root of the White Mustard, 

 they would then curve not merely till they assumed the horizontal, but until 

 they were directed obliquely downwards. That this does not occur is at 

 least in part due to the geotropism of these shoot-axes ; in proportion as they 

 approach the horizontal, they come into a more and more favourable position 

 for the action of the geotropic influence under which they tend to become 

 erect. We may therefore assume that the horizontal position of plagiotropic 

 shoots results from the co-operation of light and gravitation ; how this occurs, 

 however, and particularly in what degree each of the influences is concerned 

 in the realization of the direction of growth cannot be stated in detail even in this 

 case. 



It is easily intelligible that the power of the climbing shoots of the Ivy to 

 cling closely to a vertical wall or to a horizontal surface is due to the behaviour just 

 described — i. c. to their negative heliotropism — while the striving of the primary shoot 



