196 CONTROL MECHANISMS IN CELLULAR PROCESSES 



redirected to the hemicelluloses, which comprise the bulk of the 

 cell wall matrix. Our own analyses (Ray, 1958) also showed that 

 the major part of both the hot-water-soluble "pectin" and the hot- 

 dilute-mineral-acid-soluble "protopectin" fractions is pentosans and 

 hexosans, that is hemicelluloses, rather than polyuronides, so it no 

 longer seems reasonable to consider that these primary walls have a 

 "pectin matrix." However, our determinations have indicated a total 

 polyuronide content of the order of 5 per cent of the cell wall, in 

 good agreement with the figures of Jansen et al. ( 1960 ) . Neither 

 the carbazole nor the COo-yield methods of determining uronides, 

 which were employed, can be corrected accurately for the effects of 

 simultaneously present sugars, especially when uronides constitute 

 a small percentage of the material, so these data overestimate the 

 amount of uronides. But they certainly appear to be closer to the 

 actual amounts than the figure given by Bishop et al. (1958), be- 

 cause we have been able to recover more than 50 per cent of the 

 amounts indicated in crude extracts, as actual uronic acids after 

 acid hydrolysis and separation of uronic acids from sugars by anion 

 exchange resin chromatography. The conclusion of Bishop et al. 

 (1958) concerning polyuronide content seems to be erroneous, be- 

 cause it was based on a method (removal of sulfuric acid as BaS04) 

 which in our experience causes large losses of uronic acids when 

 these are present in small amount. 



We have recently found that the uronic acid of oat coleoptile cell 

 walls consists of both galacturonic and glucuronic acids, with the 

 amount of glucuronic being equal to or greater than the amount of 

 galacturonic in all the fractions, apparently even the hot water solu- 

 ble. The composition of these polysaccharides is, therefore, that of 

 "polyuronide hemicelluloses" rather than pectin, and we wonder to 

 what extent any of the work discussed has really dealt with pectin.- 



It might finally be noted that no direct evidence establishes that 

 any of the "pectin" effects observed actually occur within the cell 

 wall rather than in the middle lamella, where direct observation of 

 Ruthenium Red stained sections suggests that most of the uronides 



2 Since this was written, we have found that the uronic acid which is consistently 

 found at the same Rf as ghicuronic acid, in hydrolysates of oat coleoptile cell wall 

 polysaccharides, is not glucuronic acid. It appears to give galacturonic acid and xylose 

 on further hydrolysis, and is probably an oligosaccharide derived from incomplete 

 hydrolysis of a polyuronide hemicellulose containing both xylose and uronic acid 

 residues. Its abundance indicates that a considerable proportion of tlie cell wall 

 uronic acid may be in polyuronide hemicelluloses as opposed to pectin. 



