RESPONSE OF THE ROOT-TIP 367 



that point, in consequence of which the organ would curve 

 away from the stimulus. Thus the mechanical and elec- 

 tric methods of investigation both lead to the identical 

 conclusion that unilateral stimulation of the tip of the root 

 gives rise to a movement such that the organ recoils from 

 the source of stimulation ; since tropic movement towards 

 the stimulus is termed positive, this opposite response must 

 be termed negative. 



Table XXXIV. — Effect of Unilateral Stimulation of the 



Root-Tip, 



A similar electric method was employed for the detection 

 of geotropic excitation in the root, both at the root-tip and 

 at the zone of growth in which geotropic curvature is 

 effected. The two diametrically opposite contacts at the 

 tip will be distinguished as a and h, the corresponding points 

 higher up in the growing region being A and B [cf. figs. 212, 

 213). When the root is vertical the electric conditions at 

 the two diametrically opposite points are practically the 

 same. But when the root is rotated in a vertical plane 

 through 90° a geo-electric response is found to take place ; 

 the responsive current disappears when the root is brought 

 back to the vertical. Rotation through — 90° gives rise to 

 a responsive current in the reverse direction. 



Geo-Electric Response of the Root-Tip 



Experiment 198. — I took the root of a Bean {Vicia 

 Faha) and made two electric contacts at two diametrically 

 opposite points, a and h (see fig. 212), on the root-tip at 

 a distance of about i -5 mm. from the extreme end. Owing 



