84 Fundamentals of Auxin Action 



organ may abscise. The vertical orientation effects of this sort are called 

 "correlation effects." The term is especially common in the early litera- 

 ture. Auxin has been found to be the primary agent responsible for 

 the correlation effects, and the polarity of its movement is the basis 

 for its correlative action. 



Lateral movement of auxins can be induced by the lateral applica- 

 tion of such stimuli as light, gravity or physical contact. The physio- 

 logical significance of lateral transport in the tropic movements of 

 plants is discussed in the section on tropisms (chapter IV). 



The physiological basis for the polar movement of auxins is not 

 understood. One promising theory has been advanced to explain it, 

 and that is that movement is controlled by bioelectric potentials in the 

 plant. The concept was first suggested by Brauner (1927), and has 

 since found much support in the work of Lund (1947) and Schrank 

 (1951). Some of the best evidence in support of the theory has been 

 obtained from studies with the Avena coleoptile. A natural bioelectric 

 gradient exists in the coleoptile, the base of which is electropositive to 

 the apex. Upon exposure to light, the shaded side becomes electro- 

 positive to the lighted side. Upon lateral exposure to gravity, the lower 

 side becomes electropositive to the upper. In each instance the auxin 

 moves toward the more positive charge. The bioelectric change pre- 

 cedes the curvature response as one would expect if the charge bears 

 a causal relationship to the curvature. Furthermore, the charge fades 

 at the same rate as the curvatvne action (Webster and Schrank, 

 1953). There are some unsettled questions, however. One would ex- 

 pect that the artificial application of an electric gradient to a 

 coleoptile would simulate the tropic stimuli. Instead, however, the 

 application of a potential results in a bending of the coleoptile to- 

 ward the positive pole of the applied current — quite the opposite to 

 the response which is associated with natural tropisms where the bend- 

 ing is toward the negatively charged side (Schrank, 1951). Also the 

 bioelectric gradient down the coleoptile shows a peak in electro- 

 negative charge some 5 mm. below the apex, and no comparable re- 

 versal of auxin polarity from that point to the apex has been found. 

 However, the theory is still the best one at present. 



A further suggestion to explain the movement of auxin has been 

 advanced by Showacre and du Buy (1947), who note that many of the 

 same factors influence both tropisms and protoplasmic streaming. 

 Light retards streaming in the cytoplasm of cells of Avena coleoptiles, 

 and this effect would be expected to be most pronounced on the lighted 

 side when unilateral light is used. It is suggested that the retarded 

 streaming may result in retarded auxin transport, and as a conse- 



