38 THE PLANT CELL WALL 



per cent lignin, 14 per cent pentosan, and 4 per cent cellulose. 

 The deposition of a carbohydrate wall generally precedes ligni- 

 fication. 



It has been claimed that lignin and the polyuronides are closely 

 associated and that cellulose is aggregated with xylans or mannans. 

 On the other hand the chemical combination of xylose and xylo- 

 biose with lignin has been demonstrated in beechwood, where 

 there is no evidence for a lignin-uronide linkage. 



The existence of chemical as well as physical associations 

 between lignins and wall carbohydrates seems well established 

 even though the nature of the chemical linkage may vary. The 

 intimacy of the association between lignin and other interstitial 

 substances whether physical or chemical is further shown by the 

 diminished affinity of lignified tissues for various stains. Reduced 

 staining with ruthenium red indicates the masking of or combina- 

 tion with polymeric acid groups and the reduced affinity for congo 

 red has been interpreted as evidence for the infiltration of lignin 

 into intermicrofibrillar spaces. Finally, metal particles deposited 

 in lignified fibers are smaller than those deposited in delignified 

 cell walls. 



From the foregoing considerations we may envisage a frame- 

 work of oriented cellulose fibrils which form in depth a crossed 

 (orthogonal) system of considerable strength. This frame is then 

 embedded in a gelatinous matrix composed of generally lower 

 molecular weight polysaccharides. In maturing tissues, the 

 polysaccharide matrix and lignins interpenetrate, and, in many 

 instances, the latter becomes the dominant interstitial polymer. 



The major constituents to be found in various regions of the 

 cell wall differ primarily in their proportions, rather than in the 

 unique presence or complete absence. Thus, cellulose varies from 

 a dispersed fibrillar condition in the intercellular layer to a dense 

 reticulum in the inner region of the secondary wall. The interstitial 

 substances accordingly show a reciprocal variation. Both the 

 intercellular and primary layers contain open structures which 

 could conceivably be deformed considerably by compression. 

 Although the compact interstitial gel, unlike cellulose, can have 

 little tensile strength it may exhibit more resistance to compres- 



