Salt accumulation and mode of action of auxin 



the near future, strongly suggest interference with the pectins of cell walls. It 

 will probably be agreed that the plastic and elastic extensibility of a poly- 

 galacturonic acid or in general of an oxidized hemicellulose will be markedly 

 controlled by the condition of the carboxyl groups. If these long chain 

 molecules are associated with multivalent cations, minimum extensibility 

 will be found owing to electrovalent binding together of adjacent molecules. 

 If the carboxyls are free, hydrogen bonding will provide considerable 

 tensile strength but much less than that found in presence of cations and 

 finally, when, or if, they are converted to methyl esters, there will be minimal 

 tensile strength as hydrogen bonding will be replaced by van der Waals' 

 forces and so extensibility will be maximal. 



10-^\MAA 



W~^V\EDTA 



m''*V\ EDTA 



W'^WEDTA 

 



Figure 3. Extension growth o/Avena coleoptile plotted against time in the presence of lAA 10~^ M 

 and in water as controls and with 10~^. 10~*, and I0~^ M EDTA. 



(iii) The percentage of methoxyl in the walls of coleoptiles before or 

 during extension growth is readily determined. Fuller details will be published 

 later. The critical data consist of ratios of uronic carboxyl to uronic methyl 

 carboxylate. The fraction of total uronic carboxyl occurring as methyl ester 

 in young extensible walls does not appear to exceed 50 per cent. The meth- 

 oxyl content of such wall material is about 3 to 4 per cent of the dry weight, 

 from which one can calculate a probable polyuronide content of 40 to 50 per 

 cent of the total wall weight, as a minimum, in 70-hour-old coleoptiles. 



(iv) It had been found by Brian and Rideal (1952) that strong interaction 

 occurred between 2-methyl-4-chlorophenoxyacetic acid (MCPA) and 

 surface films of phospholipids and lipoproteins. Similar interaction of lAA 

 with such films was also found. This, coupled with results of some chromato- 

 graphy carried out in our laboratory, suggested complex formation of the 

 choline moiety of the phospholipid with lAA. 



The effect of I AA on acetylcholine-esterase was therefore examined and I 

 am indebted to Dr. G. Brownlee for carrying out this study with a sphenic 

 nerve-diaphragm preparation treated with tubocurarine. Reversal of the 

 tubocurarine effect with physostigmine is well known and the explanation 

 given and accepted by pharmacologists is that physostigmine is a specific 

 inhibitor of acetylcholine-esterase and acts in this case by causing accumula- 

 tion of acetylcholine. 



287 



