42 THE PLANT CELL WALL 



increase in diameter is accompanied by marked increase in neutral 

 polysaccharide and lignin, and slight increase in cellulose, and 

 a marked decline in pectic substance. The 25 mm hypocotyl 

 contains about 42 per cent cellulose, 14 per cent pectin, 24 per 

 cent neutral polysaccharide, and 20 per cent lignin. Analysis of 

 woody species shows similar changes. For example, in ashwood, 

 the cambium contains about 22 per cent cellulose, 31 per cent 

 pectin, and 6 per cent lignin. In new wood, these components run 

 55 per cent, 4 per cent and 25 per cent, respectively, and in sapwood, 

 59 per cent, 2 per cent, and 21 per cent. These examples show 

 that the proportions of cell wall components change markedly 

 during growth and maturation, but they can be assigned no other 

 significance. The apparent decrease in pectic substances which occurs 

 as lignin and other constituents increase demonstrates only that the 

 synthesis of lignin has become a dominant process and that pectin 

 synthesis has become slower, or has perhaps ceased. Although 

 such data show no biogenetic relations, they are nevertheless 

 useful. A change in proportions among wall components is import- 

 ant in the study of developmental chemistry and reflects the basic 

 character of growth and maturation processes in plant cells. 



Remarkably little is known about the biochemistry of wall 

 synthesis in vascular plants. General biochemical information 

 about polysaccharide synthesis can be drawn upon for analogies; 

 however, such excursions do not provide specific information 

 about the relation of wall synthesis to general cell economy. In 

 contrast to polymers of a-glucose such as amylose whose biochem- 

 istry has been moderately well known for some years, information 

 pertaining to the enzymology of cellulose synthesis has been ac- 

 quired only recently. Indeed, the only definitive information has 

 been derived from studies of the micro-organism Acetobacter 

 xylinum which is notable as a cellulose producer. When C 14 -labeled 

 carbohydrate intermediates are supplied to Acetobacter the label 

 appears in cellulose when provided by D-glucose-1-C 14 , D-glucose- 

 2-C 14 , D-glucose-6-C 14 , D-mannitol-1-C 14 , or glycerol- 1-3-C 4 

 the sole carbon source. Although ethanol enhances incorporation 

 of labeled glucose in cellulose, labeled ethanol and acetate are 

 not themselves incorporated when provided either as sole carbon 



