RESPONSE OF TirS OF SHOOT y\ND ROOT 513 



and it will still show an apogeotropic curvature. But the 

 case is quite different with the root. Here the perceptive 

 organ and the responding organ are, as will be seen in the 

 next chapter, distinct and separate from one another. 



Difference in character of response between tip and 

 growing region of root. — We shall next turn our attention 

 to the peculiar characteristics of the tip of the root as dis- 

 tinguished from the responding region of growth. Darwin, on 

 applying moderate artificial stimulus unilaterally to the tip 

 of the root, found that it moved away from the source of 

 stimulus ; whereas Sachs, on applying similar unilateral stimu- 

 lus to the responding growing region directly, found that it 

 moved towards it. Thus we see that two different effects are 

 induced, according as stimulation is applied on the responsive 

 zone itself, or transmitted to it from the distant tip ; and in 

 this fact we may perhaps find a clue to the explanation of 

 the opposite effects produced by stimulus of gravity on 

 positive and negative geotropic organs. 



No explanation has yet been offered of the opposite 

 characters of these responsive effects induced by similar 

 stimuli, according as they are applied on the responsive zone 

 or on the tip of the root. It was suggested by Darwin that 

 *the tip of the radicle is endowed with diverse kinds of 

 sensitiveness ; and that the tip directs the adjoining growing 

 parts to bend to or from the exciting cause, according to the 

 needs of the plant.' ^ These diverse kinds of sensitiveness 

 have in his view been acquired by the tip of the root, for the 

 final advantage of the plant. 



The question, then, which we must investigate is, as to 

 whether the peculiar sensitiveness of the tip has been specially 

 evolved by the burrowing root, or is characteristic of the tips 

 of growing organs in general. Should the latter prove to be 

 the case, we have next to account for this characteristic itself. 

 I have already suggested, as a possible explanation of the 

 difference between the responsive actions in apogeotropic 

 and geotropic organs, that in the one case stimulus acts 



' MoTcnients of r/aii/s, pp. 552 and 573. 



L L 



