INQUIRY INTO POSITIVE GEOTROPISM 539 



of the tip can only be seen under considerable stimulation, 

 or when that part of the tissue is highly excitable. 



Localisation of geotropic sensibility at the root-tip. — 

 I next turn to Darwin's very important demonstration of 

 geotropic perception as residing in the root-tip. He showed 

 this by extensive researches on the fact observed by Ciesielski, 

 that on the decapitation of the root-tip the geotropic action 

 is found to disappear. Various objections have been urged 

 against this view, which I shall be able to show to be 

 groundless. 



Sachs, for instance, has argued that if such sensibility 

 resided in the root-tip, there is no reason why the tip of 

 the shoot should not exhibit the same. This argument is 

 not, however, valid, inasmuch as the statolithic or other 

 elements, which by their weight cause stimulation, may 

 be concentrated locally in the root-tip, and more diffusely 

 distributed in the shoot. And that the general sensibility 

 of the root-tip to other forms of stimulation is not different 

 from that of the shoot-tip I have already fully demonstrated. 

 Francis Darwin has shown, again, that localised gravi- 

 perceptive areas may occur in organs other than the root. 

 In the seedling of Sorghum, for example, he finds this sensi- 

 bility to reside in the cotyledons. 



The second objection that has been raised is that the 

 shock of amputation might either abolish the general ex- 

 citability or arrest the growth of the root, on which the 

 responsive growth- curvature depends. With regard to the 

 abolition of excitability, it is true that the effect of a strong 

 stimulus, such as that of cut, would persist for some time, 

 yet there is always recovery, after a longer or shorter 

 interval. I have tested the persistence of the fatigue caused 

 by amputation, by the electrical method, and I find that 

 with certain plants, such as Bryophyllum, the recovery from 

 the effect of amputation-shock is very rapid, taking place 

 in the course of about a minute. The longest persistence 

 of the after-effect which I have been able to observe was 

 in the case of Colocasia, where it lasted for a period of 



