366 CHAP. XXXIII. THE RESPONSE OF THE ROOT 



by indirect stimulation, direct unilateral stimulation of the 

 growing region gives rise to a positive curvature (p. 143). 



I now go on to describe the electric response resulting 

 from (i) indirect stimulation of the root-tip, and (2) direct 

 stimulation of the growing region. 



Electric Response to Indirect Stimulation 



Experiment 197. — One of the two electric connections 

 with the galvanometer is made at a point A {see fig. 211) on 



one side of the growing region 



^ Y\ of the root of the Bean (Vicia 



I I Faha), the other connection 



being made at the diametrically 

 opposite point B. Unilateral 

 stimulation was applied at the 

 root-tip a, and on the same side 

 as A. The tip was subjected to 

 various modes of unilateral 

 stimulation ; mechanical stimu- 

 lation was effected by friction 

 with emery-paper, or by pin- 

 prick ; chemical stimulation was 

 produced by application of 

 dilute hydrochloric acid ; ther- 

 mal stimulation was effected by 

 the proximity of an electrically 

 heated platinum wire. In 

 every case the response was that 

 of induced galvanometric posi- 

 tivity at A. The electric varia- 

 tion took place within about 10 seconds of the application 

 of stimulus; the time-interval obviously depends on the 

 length of path to be traversed by the transmitted impulse 

 causing indirect stimulation. 



The galvanometric positivity at A gave indication that 

 there was an increase of turgor and expansion induced at 



Fig. 211. Diagrammatic repre- 

 sentation of mechanical and 

 electric response of root to 

 indirect stimulation, the 

 stimulus having been applied 

 at the tip a. Figure to the left 

 shows responsive mechanical 

 movement away from the 

 stimulus. The electric response 

 to indirect stimulation is in- 

 dicated in the figure to the 

 right, the point on the same 

 side exhibiting galvanometric 

 positivity at a. The shaded 

 part indicates the responsive 

 region of growth at some 

 distance from the tip. (Vicia 

 Faba.) 



