690 



LECTURE XXXIX. 



been cut off", is a matter of some difficulty. But if Darwin's view were correct it is 

 probable that in the case of geotropic shoots also the growing-point at the end of 

 the shoot-axis plays a similar part ; this is, however, by no means the case, as I 

 showed long ago, since pieces of growing shoot-axes from which not merely 

 the growing-point but even the whole apical portion has been removed, are able to 

 make vigorous geotropic curvatures, and even thin lamellae prepared by means 

 of two longitudinal sections from such decapitated shoots are still geotropically 

 irritable. 



For roots also the statement holds good, that only the parts which are growing in 



length react geotropically, and are therefore capable of curving, and that the 



curvature is due to alterations in growth. Since now the whole of the growing 



region, as has already been repeatedly shown, is usually 



^ ^ but 8-10 mm. long, even in the case of thick strong 



primary roots, and the curvature makes itself evident only 



in the middle transverse zones of this portion — i. e. the 



zones which are growing most actively — the curvature of 



roots in general appears to be much more pronounced than 



in the case of long geotropically curved shoot-axes ; or, 



in other words, the radius of curvature is much smaller. 



The accompanying figure (Fig. 389) aff'ords a 

 sufficiently clear idea of the manner in which a vigor- 

 ous primary root, previously growing vertically, when 

 laid horizontally in loose damp soil, curves down- 

 wards behind a thin plate of talc. The six marks in- 

 dicated were drawn on the root with Indian ink, so 

 that the mark o denoted the growing-point enclosed 

 in its root-cap, while the others were about 2 mm. 

 apart from one another. On the transparent plate of 

 talc, behind which the root was situated, a triangular 

 index of paper was gummed, as shown in the figure, 

 and by the aid of this the movements of the root-apex 

 could be better determined ; the figure shows how, in 

 consequence of the growth in length, the marks o, i, 

 2, &c. gradually pass beyond the point of the index, 

 because they are driven forward by the elongation of the parts lying behind. 

 The relative displacement of the marks shows at the same time the distribution of 

 growth within this region; at first it is the portion 2-3 which elongates most, 

 then the piece 1-2 grows more strongly, and finally the principal elongation takes 

 place in the youngest portion o-i. The oldest portion 4-5 has grown but little 

 during the observation, because it was already towards the end of its growing 

 phase at the beginning of the experiment, and, during the course of the experiment, 

 soon ceased to grow altogether ; similarly with the portion 3-4. In the figure, A 

 denotes the condition of the root at the commencement of the experiment ; B 

 after i hour; C after 2 hours ; D after 7 and Rafter 23 hours. 



The curvature is, as observed, after two hours (C) distributed along the whole 

 of the growing region, though still feeble everywhere ; after seven hours [D) the 



Fig. 389.— The growing end of the 

 radicle of a Bean, curving under the 

 influence of geotropisni. 



