CHEMO-TROPISM AND GALVANO-TROPISM 557 



of the current. The current was now interrupted, and as an 

 effect of the kathode-break we obtain an expansion and 

 acceleration of growth above the normal, as seen from the 

 up curve. This acceleration persisted for nearly three 

 minutes, after which the growth-rate became again normal. 

 The current was now sent in a reversed direction, and the 

 result of this anode-make was a sudden expansion and 

 acceleration of growth, which, as in 

 the case of motile response, persisted 

 for a relatively short time, when the 

 growth-rate, as seen from the return 

 to the horizontal, became once more 

 normal. The current was now in- 

 terrupted, and as the result of anode- 

 break we have a contraction and 

 retardation of growth, which per- 

 sisted for nearly a minute and a 

 half, after which the growth-rate 

 became again normal. From this 

 experiment we again see that not 

 only are the anodic and kathodic 

 effects opposite, but that the effects 

 on each of make and break are also 

 opposite. It is also seen that the 

 expansional effect caused by anode- 

 make is relatively smaller, and 

 occurs, and is completed, more 

 rapidly than the kathode-make con- 

 traction, whose effect is more per- 

 sistent. From these data we are 

 able to arrive at a more comprehensive law of polar ex- 

 citation in the case of vegetable tissues than was given 

 in Chapter XVL, including not only ordinary mechanical 

 responses, but also the modifications induced in growth. 

 It will be understood that with regard to growth, expan- 

 sion means acceleration of growth, and contraction means 

 retardation. 



Fig. 231. Effects of Anode 

 and Kathode on Variation 

 of Rate of Growth in Root 

 of Bindweed exhibited by 

 Balanced Growth-record 



Down curve represents con- 

 traction and retardation of 

 growth, here and in fig. 233. 

 Up curve represents expan- 

 sion and acceleration of 

 growth. Km - contraction 

 at kathode-make ; Kb = ex- 

 pansion at kathode-break ; 

 Am = expansion at anode- 

 make, and Ad = contraction 

 at anode-break. It will be 

 noticed in this and other 

 figures that the contraction 

 at kathode-make is stronger 

 and more persistent than 

 the expansion at anode- 

 make. 



