6<S6 



LECTURE XXXIX. 



final effect is the same, i.e. the whole growing region finally erects itself vertically, 

 and then goes on growing in that direction. 



In order to avoid unnecessary details, and still to give an idea of how geotropic 

 erection of a shoot laid horizontally is finally accomplished, we may suppose that 

 abed, in Fig. 386, is a shoot which grows at the apex, the bud of which is 

 indicated by the arrow-head. A few hours after it was laid horizontally, it had 

 assumed the form efcd\ it is noticed that the fully developed portion cd now, as 

 also subsequently, undergoes no alteration, whereas the still growing portion efc has 

 undergone a curvature which is nearly the arc of a circle. Some hours later, again, 

 this portion of the shoot-axis has assumed the form gh c\ in consequence of the 

 geotropic after-effect, the region h, which is here that of strongest growth, has 

 curved so much that the part hg has been carried passively back beyond the vertical. 

 This phenomenon, however, comes forth clearly only in the case of very vigorously 

 growing, long, and especially thin shoots ; in those of the Corn-flower {Agro- 



FIG 38S.— Diagram of g-eotropic curvatures gradually made by the shoot 

 abed which has been laid horizontally (see the text). 



siemma Gilhagn), for instance, the part^/z may after 6-8 hours be directed not only 

 horizontally but obliquely downwards and backwards. Fig. 386, however, represents 

 the curvatures of a horizontally laid flower-scape of a bulbous-plant. Allium atro- 

 purpureum. When the apical portion ^,^, then, has assumed the position indicated, 

 it is once more submitted to geotropic influence, which, however, must induce a 

 curvature in a direction opposed to the preceding one, while at the same time the 

 part h c goes on further erecting itself in the same way as before ; hence the whole 

 growing portion now obtains the form ike, and this subsequently passes over into 

 the still more erect and elongated form m n c, which finally extends itself perfectly 

 straight. It is to be noticed that at last the persistent curvature of the geotropically 

 erected shoot lies in the region n c, i. e. in that region where, during the whole pro- 

 cess, the growth in length was at first slow and then ceased altogether. This region 

 is, by its connection with the fully developed portion e d, in the most favourable position 

 to remain nearly at right-angles to the direction of gravity for a long time, and 

 thus to be submitted to strong geotropic influence during the whole process, although 

 all other factors in the process are not favourable to the curvature of just this part. 



