MECHANICAL AND ELECTRICAL RESPONSE OP ROOT 465 



electrical variation took place within about ten seconds 

 of the application of stimulus ; the interval would ob- 

 viously depend on the length of path to be traversed 

 by tho transmitted effect of indirect stimulation. 



The galvanometric positivity at A indicated that there 

 was induced at that point an increase of turgor and ex- 

 pansion, in consequence of which the organ would move 

 away from stimulus. Thus both by the mechanical and 

 electrical methods of investigation we arrive at an identical 

 conclusion that the efiPects of unilateral stimulus at the 

 tip of the root gives rise to a movement, by which the 

 organ is moved away from the source of stimulus ; since 

 tropic movement towards stimulus is termed positive, this 

 opposite response must be regarded as negative. 



TABLE KXXVI. — EFFECT OF INDlREl.'T STIMULJS UNILATERALLV APPLIED 

 AT THE ROOT-TIP. 



Effect at tlie proxitnal side A in the Effect at the distal 



growing i-f^gioii. side B. 



Galvanometric positivity, indicative of 

 increase of turgor and expansion. 



Negligible. 



Tlie corresponding tropic ciirv;itiire is negative, /.e., a movement 

 away from stinmhjs. 



The root-tip when burrowing its way underground comes 

 in contact with hard substances and moves away from the 

 source of irritation. The irritability of the root-tip is 

 generally regarded as being specially evolved for the ad- 

 vantage of the plant. But reference to experiments that, 

 have been described shows that this reaction is not unique 

 but exhibited by all plant organs, growing and non-grow- 

 ing. Indirect stimulus has been shown to give rise, in 

 both shoot and root, to a negative tropic curvature in 



