GEOTROPIC AND PHOTIC STIMULATION 373 



only in regard to gravitational stimulation, the seat of 

 which is at the root-tip ; for the decapitated root continues 

 to respond to other forms of stimulation, such as chemical 

 or photic. 



Difference between Effects of Geotropic and 



Photic Stimulation 



In the case of light, the source of stimulation is outside 

 the plant : but in geotropism the force of gravity is by 

 itself inoperative ; it is only, as already explained, through 

 the weight of the starch-grains contained in the geo- 

 perceptive cells that the gravitational stimulus becomes 

 effective. Want of recognition of this fundamental difference 

 has led many observers to attempt to establish an identity 

 of reaction of the root to geotropic and to photic stimula- 

 tion, in spite of facts which plainly contradict it. The 

 impulse under geotropic stimulation is originated at the 

 root-tip, and transmitted to the growing region at a distance. 

 This indirect stimulation makes the root curve away from 

 the incident vertical lines of force of gravity. There is, 

 however, no necessary restriction in regard to the point of 

 application of light, which can be directly applied at the 

 growing region, causing movement of the root towards the 

 incident light. 



The distribution of the cells (statocysts) containing the 

 starch-grains in the shoot and in the root, furnishes material 

 for an explanation of the different geotropic response of 

 the two organs. In this connection the results of the 

 investigations of Haberlandt and of Nemec are highly 

 suggestive. Haberlandt finds statocysts present in the 

 responding region of the stem ; in fact, every section of the 

 growing stem not only possesses the responding tissue, but 

 also the apparatus for initiating excitation — namely, the 

 statocysts. The geotropic stimulation of the stem is 

 therefore direct. Nemec's investigations on the distribu- 

 tion of statocysts in the root show, on the other hand, that 

 it is in the central portion of the root-cap that the cells 



