A. R. SCHRANK 



129 



ing toward the light expressed as horizontal tip movement is shown in 

 Figure 3B. As in the previous experiments the final transverse electrical 

 polarity is again established before the curvature starts. The orientation 

 of the electrical polarity with respect to the subsequent bending is 

 the same as it was when the plants were stimulated by gravity. 



These observations are beset by several limitations. The indicated 

 electrical changes have been observed only for the most apical cells of 

 the isolated sheath. Whether or not unilateral illumination induces a 

 transverse electrical polarity along the entire longitudinal axis is not 

 known. Furthermore, the quantity of light that was used in these 



MILLIVOLTS 



+ 10 

 + 5 



-5 



1.0 

 0.5 



LIGHT ON 



0.5- 



100 WATT 



1 



■ ' ' ' 



: MILLIMETERS 



- Curvature 



I I I 



10 20 MINUTES 60 70 



Figure 3. A. Effect of continuous unilateral illumination of an isolated 

 Avena sheath on its transverse electrical polarity 0.5 millimeter below the 

 apex. B. Phototropic bending of the same coleoptile. 



