128 PLANT GROWTH SUBSTANCES 



injured living tissues exhibiting normal growth and bending responses 

 (27). Coleoptiles, which were immersed in boiling water for 15 seconds, 

 did not show any electrical changes when they were placed in the hori- 

 zontal position. Further control experiments also disclosed that the 

 electrical responses were not due to electrode phenomena, which might 

 have been the case in earlier work. 



The curvature induced by gravity, represented by movement of the 

 coleoptile tip measured in ocular scale divisions (18 per millimeter), is 

 shown in Figure 2B. This curve indicates that upward bending starts 

 only after the plant has been in the horizontal position for 22 minutes. 

 When the relationship between the known velocity of transport (19, 

 36,37) and the minimum distance that the auxin has to be displaced in 

 lateral redistribution is evaluated, it is at once apparent that the trans- 

 verse electrical polarity in Avena is established before an unequal distri- 

 bution of auxin is considered possible (23). This sequence of events also 

 indicates that the electrical polarity in the coleoptile is not dependent 

 on the metabolic process for which the auxin is directly responsible. 

 These results and inferences permit the arrangement of coleoptile re- 

 sponses to gravity in the following order: establishment of a transverse 

 electrical polarity in which the underside becomes electropositive; un- 

 equal distribution of auxins (in this instance the auxin is again trans- 

 ported toward the electropositive portion of the plant); and upward 

 curvature. 



Stimulation by light. — Only a limited number of investigators have 

 been concerned with the various phases of the effects of incandescent 

 light on the electrical potentials of etiolated seedlings (11,26,35). When 

 the field is narrowed to the effect of unilateral illumination on the trans- 

 verse electrical polarity of the Avena coleoptile the only information 

 that is available comes from the preliminary experiments of Schrank 

 (25) and Oppenoorth (20). Figure 3 A shows the typical electrical 

 response of the most apical cells of an isolated Avena coleoptile (contacts 

 0.5 millimeter below the apex), which was illuminated continuously by 

 a light intensity of 16 foot-candles at the coleoptile position. As indicated 

 by the curve in Figure 3A, after ten minutes of illumination the shaded 

 side becomes electronegative to the lighted side. This first negative 

 variation was observed in about 65 per cent of the experiments. Later 

 the shaded side always becomes electropositive to the lighted side, 

 reaching an average maximum of 8.2 millivolts. The corresponding bend- 



