22 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



development and germination. Strictly speaking, one of the most 

 widely occurring non-cellulosic wall materials— pec/m— should be 

 included in this group since this substance also yields a sugar acid on 

 hydrolysis; and it seems safe to say that, with the possible exception of 

 the fungi, there is no plant known which does not contain pectic sub- 

 stances. In the former group, on the other hand, there are a number of 

 substances which resemble in general molecular morphology the 

 ubiquitous cellulose, are closely associated with it, and are therefore 

 extracted from cellulosic bodies only with difficulty. These are referred to 

 as cellulosans. All these substances occur in both primary and secondary 

 walls and, in particular, are undoubtedly present in the former during 

 growth (see Table I). Lignin, however, a substance whose composition 

 is as yet incompletely known, becomes prominent only as the cell 

 ceases to be metabolically active and in many cases its development 

 precedes the death of the protoplast. It is therefore associated in 

 tissues largely with mature or dead elements, notably the cells of xylem 

 or wood (Table I) and the fibres of the phloem. It is prominently 

 present in wood parenchyma cells, which remain alive for long periods 

 after its deposition. All these substances are naturally of very con- 

 siderable importance both academically and in technology, so that 

 there is a very extensive literature, much of which is readily available. 

 It is proposed here, therefore, to give only the briefest sketch of this 

 aspect of wall chemistry; for further information reference may be 

 made to any one of a range of excellent text-books (3). Since cellulose 

 itself occupies the prime place in any discussion of wall investigation, 

 its chemical nature is the first to come under review. 



Cellulose 



It has been recognized now for more than a hundred years that 

 cellulose on acid hydrolysis yields large quantities of glucose, and it is, 

 in fact, now nearly thirty years since it was first established that the 

 yield of glucose is almost quantitative. This completely insoluble and 

 comparatively inactive substance must therefore be composed almost 

 exclusively of a complex of the soluble monosaccharide. The con- 

 ception of the synthesis of small molecular species into large molecular 

 combinations with completely different properties is, of course, now 

 a commonplace in this age of plastics; and it is perhaps redundant 

 to point out the implication that the glucose units in cellulose must 

 be linked in such a way that many of the —OH groups are to a large 

 extent protected against the approach of water molecules, in order 

 to confer insolubility. This is achieved by a now familiar trick of 



