132 PLANT GROWTH SUBSTANCES 



Without arrogating the space to review the specific data, two other 

 impHcations of these experiments should be contemplated. It was pre- 

 viously noted that when the coleoptile is placed in the horizontal 

 position the under side becomes positive to the upper. Simultaneous 

 mechanical stimulation of the apical 10 millimeters of the under side 

 establishes a reversed electrical polarity (opposite to the polarity induced 

 by gravity alone), inhibits upward curvature for 25 minutes, and de- 

 creases the subsequent rate of bending. Geotropic bending can be 

 entirely prevented by repeated appHcation of mechanical stimuli to the 

 under side at five-minute intervals. This inhibition apparently is not 

 due to an injury phenomenon because similar stimulation of the upper 

 side does not inhibit upward curvature. Stimulation of the upper side 

 results in an electrical polarity larger than that obtained from gravity 

 alone. 



Indications are that the curvature observed in these experiments 

 is due to growth. Coleoptiles that have had the apical 3 millimeters 

 removed 2 hours and 10 minutes before stimulation cannot be induced 

 to bend by a combination of mechanical and gravitational stimuli. This 

 is taken to verify the necessity of the presence of auxin for the curvature 

 responses that were previously observed. The fact that the electrical 

 polarity established by these plants is similar to that in nondecapitated 

 coleoptiles has been interpreted to indicate further that the auxin- 

 controlled mechanism is not required for the generation of electrical 

 polarities of this nature. 



In the series of experiments that has just been reviewed it was demon- 

 strated that the Avena coleoptile responds to stimulation by gravity, 

 light, or mechanical means by establishing a transverse electrical polarity. 

 These stimuli also induce bending. In all of these instances the polarity 

 is established before the bending starts, and the direction of bending is 

 such that the electropositive side always becomes the convex side. If it 

 is assumed that the lateral transport of auxin is the fundamental inter- 

 mediate link in these curvature responses, then it follows that the auxin 

 is invariably transported toward the electropositive side of the plant. 

 This implies that the auxin is transported as a negatively charged particle. 

 These data are considered to be compatible with the Went-Kogl 

 elect rophoretic transport theory. However, extreme caution should be 

 exercised in drawing categorical and far-reaching conclusions about the 

 causal relationship between the electrical polarity and the auxin transport. 



