130 PLANT GROWTH SUBSTANCES 



experiments was much greater than the amount required to give the 

 first maximum positive curvature. In this instance the imphcation is 

 that photoinactivation of auxin may have been an unusually prominent 

 factor in the induction of the curvature. Finally, the data in Figure 3A 

 are not in apparent agreement with the results obtained by Oppenoorth 

 (20), who found that unilateral illumination of the coleoptile by 500 

 meter-candle-seconds of unfiltered mercury light caused the lighted side 

 to become electropositive to the shaded side. He obtained this polarity 

 by subtracting the longitudinal electrical changes of the shaded side 

 from the simultaneous changes of the lighted side. A common basal 

 contact was used in measuring these electrical responses. 



Mechanical stimulation. — Growth curvature responses to mechanical 

 stimulation, which were first studied in tendrils, are also prevalent in 

 etiolated seedlings. Years ago Stark (33) demonstrated that he could 

 induce bending of the coleoptile by stroking it on one side with a cork 

 rod. These observations, along with the frequently reported fact that 

 mechanical stimulation of a segment of living tissue causes it to become 

 electronegative to the unstimulated portion (16,18), led Schrank (24) 

 to investigate the relationship between the electrical and curvature 

 responses of the Avena coleoptile to mechanical stimulation. Preliminary 

 experiments confirmed Stark's observation that the coleoptile would 

 bend toward the side that was lightly tapped and demonstrated that a 

 transverse electrical polarity was established with the stimulated side 

 becoming electronegative to the opposite side. In subsequent experi- 

 ments mechanical stimuli were applied by the use of an electrically 

 operated vibrator, which was mounted on a micromanipulator in order 

 that the position of the stimulating device could be accurately duplicated. 



Curves in Figure 4A show the average magnitude of the transverse 

 electrical polarity that is established in the apical region oi Avena when 

 the apical 10 millimeters of one side are mechanically stimulated by the 

 vibrator for the duration indicated. The magnitude of the polarity and 

 the rate of its decrease are dependent on the duration of the stimulation. 

 In Figure 4B the corresponding curvatures are shown. The magnitude 

 and rate of bending are also dependent on the duration of the stimulation 

 with the direction of bending being toward the negative side of the plant. 

 Since mechanical stimuli cannot be applied to the coleoptile without 

 bending or displacing it from its original position (the reason for starting 

 the curves below the zero line), there is no way to determine exactly 



