CHEMICAL NATURE OF THE CONSTITUENTS 27 



discussed later on suggests that there is a close association between the 

 cellulosans and cellulose (7), though there are some aspects of more 

 modern work which make it clear that the kind of association involved 

 is by no means so certain as was at one time thought. This, however, 

 will be a subject for inquiry later on when the general picture of wall 

 organization has been painted in. 



Among these substances, two are very widespread. Xylan, a poly- 

 merized xylose, is universal in the Angiosperms, and Mannan, giving 

 mannose on hydrolysis, is characteristic of the Gymnosperms. This is 

 a most remarkable distribution of a chemical substance and must, 

 presumably, be of some significance, though no particular attention has 

 hitherto been devoted to its implications. Both are extractable with 

 difficulty from cellulose, either acid hydrolysis — which is liable also to 

 attack the cellulose itself — or treatment with 5% caustic soda being 

 necessary for their removal. 



The cell walls of plants thus constitute a most complicated mixture 

 of a number of sugars joined together to form molecular chains, whereby 

 soluble and reactive units become linked into insoluble and less reactive 

 bodies, and it is therefore a somewhat difficult task to reveal in its 

 entirety the detailed organization of the wall. The problems involved 

 are rendered even more difficult when, as so often happens in dead 

 tissue and more particularly in those tissues most easily handled, the 

 wall becomes impregnated also with other substances of a non-sugar 

 type. Of these undoubtedly the most common, and the only one with 

 which it is necessary to deal here, is lignin. 



Lignin 



The chemical nature of lignin is as yet unknown and the many 

 structural formulae which have been proposed as representing its con- 

 stitution (8) alone emphasize the dearth of concrete facts. It was, in fact, 

 at one time considered that lignin might be a decomposition product of 

 the saccharides in the wall consequent upon attempts to achieve its 

 isolation! The difficulties involved in its isolation are certainly very 

 considerable, and it is in this that the obstacles in the way of a final 

 elucidation of its structure are to be found; but there can be no doubt 

 now but that lignin does represent a complex of substances which are 

 actually present in the wall. Of the various attempts which have been 

 made to define the substance, perhaps the most convincing and certainly 

 the one in most complete harmony with the chemical data is that 

 proposed by Freudenberg (9). According to his view, lignin is built up 

 from aromatic nuclei such as I, II and III (Fig. 9) to which may be 



