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DISCUSSION 



Dr. Bonner: I think that everybody agrees that for a molecule 

 to be active as an auxin, it has to have some appropriate geometry, 

 presumably because it has to fit into particular kinds of holes, and 

 it has to have two particular reactive groups. It seems clear to me 

 that the reactivity of the aromatic nucleus has an important function 

 in determining the activity of an auxin. Dr. Hansch referred to the 

 strange case of the 2,6-dichlorophenoxyacetic acid and 2,6-dichloro- 

 propionic acid. At low concentrations these compounds behave as 

 auxin antagonists, and at higher concentrations they are active and 

 behave as auxins, but their activity has some very strange character- 

 istics. 



Dr. Osborne: If 2,6-dichlorophenoxyacetic acid is tested in the 

 Avena straight growth test, one finds that even with relatively low 

 concentrations there is an initial but small stimulation of growth 

 lasting for a few hours followed by a retardation in growth. I am 

 not sure how one would explain this on a 2-point attachment theory. 



Dr. Bonner: The 2,6-dichlorophenoxyacetic and propionic acids 

 are active, but their activity has the interesting feature that this ac- 

 tivity is manifested for a very short time in contrast to 2,4-D or other 

 auxins. It is a "short-term" auxin; it goes in and does something for 

 only a short time. 



Dr. Osborne: This appears true for 2,6-dichlorophenoxyacetic acid, 

 but if one does similar experiments with the propionic derivative, the 

 growth stimulation can be continuous for a period up to 24 hrs. One 

 does not find the subsequent retardation of growth. The stimulation 

 with this 2,6-substituted phenoxy compound is apparently there all 

 the time. 



Dr. Wain: In relation to the 2-point contact theory which we have 

 been hearing so much about, the carboxyl group probably does react 

 chemically in the manner you have suggested. The question is, what 

 is happening in this second contact which you have postulated. It 

 might be reacting with some essential or unessential thiol group in 



