INQUIRY INTO POSITIVE GEOTROPISM 543 



in this respect as a diffuse pulvinoid. It should be re- 

 membered, however, that the excitability of a tissue does 

 not disappear on the cessation of growth, and responsive 

 curvatures are thus sometimes seen to extend even beyond 

 that zone. 



Contraction takes place, as we have seen, in all tissues, but it 

 is relatively much less in the very young and the very old. The 

 responsive contraction at the tip is therefore slight and com- 

 paratively inconspicuous. Indeed, it can only be made visible 

 under very strong stimulation. That contraction is induced, 

 however, is shown in the fact that a wave of positive turgidity- 

 variation reaches the zone of growth by excitation of the tip. 

 This can only be brought about by the expulsion of water, 

 which is the result of excitatory contraction at that point. 

 Under direct stimulation, then, every tissue, whether at the tip 

 or in the growing region, undergoes a similar contraction. 

 The sensitiveness, or sign of response, of all tissues is therefore 

 the same ; but about these signs we are liable to be deceived 

 if we pay attention to the conspicuous effects alone, for the 

 local application of a unilateral stimulus at the tip gives rise 

 to little perceptible concavity, while its transmitted indirect 

 effect, lower down in the growing region, induces a relatively 

 great convexity. Moreover, the whole length of the organ 

 above the point of greatest convexity acts as a kind of lever- 

 index, causing high magnification of the responsive movement. 

 The tip, on the other hand, possesses no such magnifying 

 index. These considerations will clearly show that there is 

 no difference in kind between the sensitiveness of the tip 

 and that of the growing region, both alike having the power 

 of perceiving and responding, in different degrees, to external 

 stimulation in general. Just as the tip itself merges physically 

 into the growing region, so these respective responses pass 

 imperceptibly into one another. Though moderate unilateral 

 stimulus of the root-tip causes the characteristic movement 

 away from stimulus, yet strong stimulation induces movement 

 towards it. Stimulus applied on other parts of the plant 

 also will induce movements similar to that caused by mode- 



