Growth Regulating Effects of Phenoxy Compounds 



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Fig. 2. Growth effects upon Avena coleoptile cylinders (A), wheat roots (W), and 

 flax roots (F) of (+)a-(4-methylphenoxy)propionic acid (4-MePOP) and (4-)a- (3, 

 5-dichlorophenoxy)propionic acid. Data presented as in Figure \. 



pionic and butyric acids are auxins of similar strength, the valeric 

 acid shows intermediate character (at least in the oat cylinder test), 

 while only antiauxin properties are revealed for the caproic acid. 



The presence of an auxin component in the activity of intermedi- 

 ate substances like 2-naphthoxyacetic acid (4) or 4-BrPOA can hardly 

 be disputed. In view of the gradual transition from such substances 

 to those for which the restoration of oat coleoptile growth at higher 

 concentrations remains well below the control level (e.g., 4-MePOA, 

 Figure 3), the author does not hesitate to accept even a slight tend- 

 ency to such a restoration as a sign of weak auxin activity. Such 

 signs are often paralleled by indications of auxin activity in the flax 

 root test. It shoidcl also be noted that the tendency to growth restor- 

 ation at high concentrations of 4-MePOA is changed to a real stimu- 

 lation for the nearly related substance (-|-)4-MePOP (Figures 2 and 3). 



The quantitative elucidation of the action of intermediate sub- 

 stances along the lines used for the pure auxins and antiauxins meets 

 with difficulties. It seems probable, however, that these can be obvi- 



