26 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



galacturonic acid residues connected through one of galactose and one 

 of arabinose. Such attempts to formulate a structural formula are, 

 however, of historical importance only and need not be dwelt upon. 

 It seems now clear, through close examination of chemical as well as 

 physical properties, that the structure based on polygalacturonic acid 

 (Fig. 8) proposed by Meyer and Mark in 1930(6) is in essence correct, 

 though chemical evidence for the type of linkage (whether, for instance, 

 it is the 1 :4 link as in cellulose and as illustrated in Fig. 8) is lacking. 

 It seems safe, therefore, to regard pectic acid as a long chain polymer, 

 and pectin itself would then be the almost completely methylated 



o- 



COOH H OH COOH 



Fig. 8. Polygalacturonic acid. Three glucuronic acid residues are shown linked by 

 1 : 4 primary valences. The chain could obviously be continued indefinitely at either 



end. 



product. Pectin occurs in the walls of almost all plants and forms the 

 middle lamella in growing tissues. A jelly-like substance such as this 

 is obviously very suitable for maintaining cells in close proximity to 

 each other, while allowing mutual displacements, and is therefore very 

 suited to conditions in the growing points. In adult tissues, however, 

 the cells must be cemented together very firmly, and it is of interest to 

 note that in these adult tissues the middle lamella is often converted to 

 calcium pectate, forming a hard cement soluble in 0-5% ammonium 

 oxalate only after treatment with acid {e.g. by a mixture of equal parts 

 of alcohol and hydrochloric acid in the cold for twelve hours or so). This 

 would appear to involve the conversion of calcium pectate into pectic 

 acid (which, incidentally, occurs in nature only as the calcium salt) and 

 its subsequent solution in the oxalate. A third type of pectic substance, 

 or probably class of substances, is given the name of protopectin. This, 

 too, forms a hard cement, but its relative insolubiUty is probably due 

 rather to a close association with other wall substances than to any 

 special feature of its molecular composition. 



Cellulosans 



The cellulosans, on the other hand, form a distinct group of sub- 

 stances distinguishable from cellulose in that they yield pentose sugars 

 on hydrolysis, and from the polyuronides in that they do not yield 

 uronic acids. The chemical and physical evidence which will be 



