CELL WALL DYNAMICS 49 



cell division all three groups of carbohydrates are low, but increase 

 proportionally to the increase in wall surface area. These carbo- 

 hydrates increase markedly with the onset of cell elongation, 

 their rate of synthesis exceeding in magnitude the increase in cell 

 surface area. 



The apical initials of the Allium root contain small amounts 

 of pectin, protopectin and cellulose whether calculated on a per 

 cell or wall area basis. Protopectin is relatively more abundant 

 in the walls of apical initials than in radially enlarging cells. The 

 proportion cellulose :pectic substance is the same in apical initials 

 and cells in radial enlargement. In the transition between radial 

 enlargement and elongation where both processes coincide, the 

 open-work wall of the radial phase becomes more continuous, 

 the gaps are filled in, and the primary pit fields are reduced in 

 size. During this transition, all carbohydrates increase, and pectin, 

 soluble non-cellulose polysaccharide, and cellulose all increase 

 more than the surface area of the wall. 



With the cessation of radial enlargement and onset of rapid 

 elongation the discontinuities of the wall are filled in with cellulose, 

 pectin, hemicellulose and non-cellulosic polysaccharide. Thus, 

 these components increase per unit area immediately following 

 the end of the radial phase and then become proportional to wall 

 area at a new high level, although the wall as a whole is not thicker. 

 It has been suggested that elongation may occur only when there 

 is an approximately continuous wall, hence is limited during the 

 radial phase by its fragmentary character. 



In proliferating tobacco callus tissue, some 25 per cent of total 

 cell nitrogen is associated with the insoluble cell wall fraction, 

 whereas non-proliferating pith walls contain but 2 per cent of 

 the cellular nitrogen. Wall protein is rich in hydroxyproline 

 relatively abundant in histidine, and poor in thio-amino acids. 

 Tyrosine-C 14 , is rapidly incorporated into the wall protein, but 

 its radioactivity remains fixed and is not redistributed throughout 

 the various amino acids. It is evident that the tyrosine metabolism 

 of the wall is of a terminal rather than intermediary character. 



The introduction of methyl ester groups is one of the important 

 changes in the pectic substances during wall development. The 



