13^ PLANT GROWTH SUBSTANCES 



data shown in Table i, it appears even more likely that the hormone 

 mechanism is involved in the bending induced by current. Finally, 

 it has been demonstrated that phototropic bending can be inhibited by 

 transversely applied current, and that curvature responses to electrical 

 and light stimulation are apparently limited by a common factor (29). 

 The premise that the responses to both types of stimulation are con- 

 trolled by auxin would explain this observation. 



Perhaps the most striking fact is that the initial curvature is toward 

 the electropositive side of the plant and not toward the negative side 

 as it was in all of the previous instances. Several implications must be 

 considered in the evaluation of this fact. What is needed most are 

 additional experiments which would expose the details of the pathways 

 of current flow through the tissue. This might help to visualize how 

 bending is induced apically to the contact region, as well as to help 

 account for the direction of the initial bending. Another point to be 

 kept in mind is that the electrical polarity measured in these experiments, 

 in contrast to the previous ones, very likely is due to polarization phe- 

 nomena rather than changes in the inherent electrical pattern. This 

 inference is made only on an indirect basis from the work of Berry et al. 

 (i) on the onion root. 



Current applied longitudinally. — DuBuy and Olson (14) were the first 

 to report that the Avena coleoptile could be made to bend by applying 

 direct current longitudinally to one side of the apical 5 millimeters. 

 The curvature, which they observed, was always toward the side on 

 which the contacts were placed and, according to them, not dependent 

 on the direction of the current flow. When these experiments were 

 duplicated and extended, several additional facts were disclosed (30). 

 Figure 7 presents some of these observations. Ten microamperes applied 

 longitudinally to one side of the apical 5 millimeters (polarity indicated 

 in the insets) cause marked changes in the longitudinal polarity of the 

 coleoptile. Current flowing from the apex toward the base reverses the 

 inherent electrical polarity and gives the response shown by the average 

 curve I in Figure 7A. Effects of the same current flowing in the opposite 

 direction are shown by curve II, which is quite obviously difl^erent from 

 curve I. Current flowing from the apex toward the base enhances rather 

 than reverses the inherent electrical polarity. The magnitude of the 

 initial response is not as large as in curve I, and the sequence of events 



