RESPONSE IN THE CxROWING REGION 369 



specimens gave concordant results. The following table 

 gives the absolute values of electromotive force induced at 

 the tip under geotropic stimulus in three different specimens. 



Table XXXV — Geo-Electric Response of the Root-Tip 



{Vicia Fab a). 



Specimen 



I 

 2 



3 



Induced E.M.F. 



O-OOII volt 

 O-OOIO ,, 

 0-0015 ,, 



Electric Response in the Growing Region 



Experiment 199. — Investigation was next undertaken 

 on the electric variation induced in the growing region under 

 the stimulus of gravity. The experimental difficulties are 

 here less serious, since the available area of contact for 

 galvanometric connection is not so restricted as in the case 

 of the root-tip. The specimen was securely mounted so that 

 the root was vertical. It was then rotated through + 90° 

 into the horizontal plane, so that the point A {see fig. 213) 

 in the growing region occupied the upper position. The 

 electric response in the growing region was very definite 

 and took place in a short time. The induced electric 

 change at A was one of galvanometric positivity indicative 

 of increase of turgor and expansion. 



The next series of experiments was carried out in the 



following order. The root was first rotated through + 90° 



so that A was above. The responsive electric variation, as 



already stated, was galvanometrically positive. The root was 



then rotated back to the vertical position when the current 



disappeared. The root was next rotated through —90° 



and the responsive current became reversed, the now 



upper point B having become electro-positive. Alternate 



rotations through + 90° and — 90° were carried out six 



times in succession with consistent results. The interval 



allowed bet\veen one stimulation and the next was determined 



2 3 



