ADDITIVE ACTION-CURRENT 37I 



stimulated point A, in the growing region higher up, 

 becomes galvanometrically positive. If now two galvano- 

 metric connections be made at the points a and A, the in- 

 duced electric difference is increased and the galvanometric 

 response becomes enhanced. 



Experiment 200. — The root was at first held vertical, 

 and two electric contacts made at a and A. In this neutral 

 position there is little or no current. But as soon as the 

 root was laid horizontal, an electromotive response was 

 exhibited which showed that a was galvanometrically 

 negative and A galvanometrically positive (see fig. 213, d). 

 The induced electric response disappeared on restoration 

 of the root to the vertical position. I give below^ the 

 results of typical experiments with a vigorous specimen 

 which gave strong electric response. It was possible to 

 repeat the geotropic stimulation six times in succession, 

 the results being invariably consistent. The responses 

 taken in succession exhibited slight fatigue, the first 

 deflection being 140 divisions, and the sixth 115 divisions, 

 of the galvanometer scale. 



Table XXXVII. — Induced Electromotive Variation between the 

 Tip and the Growing Region (a Negative and A Positive). 



These results lead to the conclusion that under geotropic 

 stimulation : 



(i) the induced galvanometric negativity at the upper 

 half of the root-tip is due to direct stimulation ; and 



(2) the induced galvanometric posit ivity of the growing 

 region on the same side is due to indirect stimulation 

 by transmitted impulse. 



