CHAPTER XXXIX 



INQUIRY INTO POSITIVE GEOTROPISM 



?<o specific difference as regards their responses between shoot and root — 

 Darwinian curvature — Localisation of geotropic sensibility at the root-tip— 

 Experiments as to whether amputation of root-tip abolishes excitability — The 

 tip of the root the organ of gravi-perception — The perceptive 7'crsiis the 

 responding organ — True perceptive region. 



We have in the course of the last chapter studied in detail 

 the expression in growth-curvatures of the direct and indirect 

 effects of stimulus, and have found them to be, under known 

 conditions, very definite. We have seen that the responsive 

 effects produced at the tips of root and shoot are in no way 

 different from those which have been observed in other parts 

 of the tissue. The specific sensitiveness hitherto so generally 

 attributed to the root-tip is thus found not to be justified. 

 The differences of effect which have been noticed, according 

 as tip or responding region is the point of stimulation, we have 

 seen to be due merely to the transmission of the indirect instead 

 of the direct effect. I shall next proceed to show that the 

 opposite curvatures induced in shoot and root by stimulus of 

 gravitation are really due, not to two distinct sensibilities 

 positive and negative, but to the action of a single stimulus 

 which acts in one case directly, and in the other indirectly. 



Darwinian curvature. — It has thus been shown that the 

 responsive curvature manifested by the radicle on stimulation 

 of its tip is not characteristic of roots alone, but, under 

 similar circumstances, of the shoot also. It has not, then, 

 been specially evolved for the advantage of the plant. As 

 proving this, the abrupt conclusion of certain experiments 

 of my own has a peculiar interest. The tip of the root 



