226 



B. A berg 



MOLES PER LITER 



Fig. 3. Growth effects on Avena coleoptile of: A. Some pheiioxyacetic acids with 

 different para-substituents (X), i.e. bromo (Br), methyl (Me), iodo (I), and ter- 

 tiary butyl (tB). B. Some (+)phenoxyalkylcarboxylic acids, i.e. a-substituted n- 

 butyric acid (B), n-valeric acid (V), and n-caproic acid (C). Data presented as in 

 Figure 1. 



ated by the assumption of the cooperation of several similar, but not 

 identical, auxin receptors. 



An alternative way would be to assume independent systems for 

 auxin and antiauxin actions, but it seems that such an hypothesis 

 generates more problems than it solves. If the initial inhibition of 

 oat coleoptile cylinders is not related to the auxin system, it is diffi- 

 cult to understand why it should be annihilated by the presence of 

 [AA. The final stimulation, on the other hand, corresponds to an 

 inhibition of flax root growth which is relieved by antiauxin applica- 

 tion, and nothing is as yet known which makes it possible to differen- 

 tiate this phase of the action from an ordinary auxin cllect. 



THE EFFECTS OF SUBSTITUENTS IN DIFFERENT 



POSITIONS 



The effect of pora-substitution has been discussed earlier (-1,6). 

 4-F- and 4-ClPOA show a considerably augmented auxin activity 

 compared to ihc luisubstituted PGA. With increasing size of the 

 jbara-substituent a change in the antiauxin direction takes place and 

 is complete with 4-iP-POA and 4-NiPOA. In the mc'ia-substituted 

 POA series a conspicuous antiauxin component of the activity is ap- 

 parent already for .S-ClPO.\, but, on the othei- hand, a strong auxin 



