. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). 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
. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). 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 0 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 o
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