Groiath Regulating Effects of Phenoxy Compoimds 231 



Dr. Hansch: I'd like to comment on this paper with respect to 

 2-point attachment in a somewhat opposite fashion from what Dr. 

 Wain has done. It seems to us that the data can be explained rather 

 nicely in terms of the 2-point contact idea. Take for example the 

 4-methylphenoxyacetic acid which is very weakly active as an auxin, 

 and compare this end with the 4-bromo compound. Dr. Wain sug- 

 gested that the first thing was attachment of the ring and then this 

 prevents a further more active ring from attaching. If you look at 

 it the other way around - first, attachment of the carboxyl by rapid 

 reactions of amidization or salt formation, then the inability of the 

 ring to react as fast, say, in the 4-methyl as it would in the 4-bromo 

 — this can be explained very nicely for the whole series of compounds 

 on the basis that the ring is less electron-rich. Electron-poor rings 

 with halogens would latch down more tightly so that groups like 

 methyl and iodo would give antiauxins. These compounds would 

 be attached first by the carboxyl, but you would get the second-point 

 attachment to the ring only very slowly as, with phenoxyacetic acid. 

 Now, then, if you put lAA in the system, the carboxyls attach at 

 about the same rate. But the phenoxyacetic acid would not permit 

 lAA to come in as fast as it normally would for 2-point attachment. 

 Dr. Wain: Just one word on that. I was not doing Dr. Wightman 

 justice in those very few remarks. He did dissect out all these fac- 

 tors and studied a wide range of compounds which could produce 

 what we thought were the specific groupings. The significant thing 

 is that these acids which do not have this highly active ring system 

 are not competitive antagonists at all. He thinks that palmitic acid 

 or any of the aliphatic acids which can provide the carboxyl group 

 are not attached. The point is, that in relation to Dr. Aberg's paper 

 I think it is possibly a little dangerous to use, as antagonists, com- 

 pounds which themselves possess auxin activity. The bromophenoxy- 

 acetic acid has positive activity and it can at best be only in the inter- 

 mediate range. 



Dr. Nitsch: I have been very much interested by the S-shaped 

 curves Dr. Aberg showed us. They reminded me of some work done 

 by McRae and collaborators with 2,4-D (D. H. McRae, R. J. Foster, 

 and J. Bonner. Kinetics of auxin interaction. Plant Physiol. 28: 

 343-355, 1953) and by Barlow et al. (H. W. B. Barlow, C. R. Han- 

 cock, and H. J. Lacey. Studies on extension growth in coleoptile 

 sections. I. The influence of age of coleoptile upon the response of 

 sections to lAA. Ann. Bot., N. S. 21: 257-271, 1957). McRae et al. 

 observed that the addition of very low concentrations of 2,4-D in the 

 presence of low concentrations of lAA reduced growth below that ob- 

 tained with lAA alone. Barlow et al. showed that low concentrations 

 of lAA have a depressive effect upon the elongation of oat coleoptile 



