CHAPTER III 



The Chemical Nature of the Constituents of the 



Secondary Wall 



BOTH THE primary and the secondary walls, whose formation has 

 thus been briefly considered, are built up from a wide variety of 

 constituents, into whose chemical nature some further inquiry is essen- 

 tial before proceeding to the physical aspects of their organization with 

 which this book is mostly concerned. The basic constituent of the walls 

 of cells in plants, with the particular exception of the fungi and of 

 some algae, is the polysaccharide cellulose and it is natural therefore 

 that discussion throughout the whole of the following pages will centre 

 largely round this substance. With the exception perhaps of the pro- 

 teins, there is no other substance produced by living things which has 

 received so much attention, and about which so much is known. This 

 is primarily because cellulose occupies such a prominent place in human 

 economy; but naturally details of structure are also of paramount 

 importance to an understanding of cell growth. Nevertheless there are 

 many other substances associated with the skeletal cellulose which are 

 not without considerable interest and which can modify profoundly the 

 properties of the whole wall and therefore the nature of the cells con- 

 cerned. It is difficult, if not impossible, to characterize chemically these 

 "incrusting substances"* but they maybe said with some justification to 

 fall into three main groups. Whereas cellulose on hydrolysis yields 

 glucose only, there are a number of wall constituents which yield either 

 a different sugar or a sugar derivative and these have been collectively 

 given the unfortunate name hemicelluloses — unfortunate since in 

 structure and function they are quite distinct from cellulose proper. 

 Taking the latter group first, there is a large number of substances 

 which yield a sugar acid (a glucuronic acid whose nature will be dis- 

 cussed later); these are much more labile than the cellulose and some- 

 times act as a food store, appearing in the wall and disappearing again 

 during the metabolic processes associated, for instance, with seed 



• This term is used to imply that the structural substance in the wall is cellulose, 

 and that these other substances are deposited within it in such a way that the properties 

 of the wall are modified in degree but not in kind. 



21 



