INQUIRY INTO POSITIVE GEOTROPISM 545 



that the stimulation of the shoot is direct, and that of the 

 root indirect — that a single identical stimulus should in 

 the two cases induce opposite responsive effects. 



Thus, though for the sake of convenience it is necessary 

 to distinguish the upward curvature of the stem from the 

 downward curvature of the root, by such terms as apogeo- 

 tropic and geotropic, yet these words must not be allowed to 

 connote two different sensibilities to the action of gravity ; 

 for the sensibility of irritable tissues to stimulus is always of 

 one kind, and of one kind only. 



Summary 



The responses induced by stimulation of the tips of shoot 

 and root are similar under similar circumstances. 



The amputation of the tip of the root does not, normally 

 speaking, abolish the excitability of the growing region of 

 the root. 



The abolition of geotropic action in the root, after 

 amputation of the tip, shows therefore, as pointed out by 

 Ciesielski and Darwin, that with regard to gravity it is the 

 tip that is the perceptive organ. This conclusion is also 

 supported by the experiments of Pfeffer and Czapek, in 

 which amputation was not included. The statolithic particles, 

 again, through whose weight stimulation by gravity is pro- 

 bably brought about, are, generally speaking, found con- 

 centrated at the root-tip. 



Direct stimulation has been shown to induce a responsive 

 movement in one direction, and indirect stimulation in the 

 opposite. The opposite geotropic curvatures in apogeotropic 

 and geotropic organs are therefore due, not to the possession 

 of different sensibilities, but to the fact that in the former 

 stimulation is direct, and in the latter indirect. 



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