50 



THE PLANT CELL WALL 



methyl carbon of partially esterified pectic acid (pectinic acid) 

 can originate from a variety of sources, including methionine, 

 formaldehyde, glycine, and serine. Of the Q compounds, formate 

 is the best methyl source and serine the poorest. In contrast to the 

 efficiency with which formate becomes ester methyl, it seems to 

 be a relatively poor Ci donor in the O-methylation of lignin 

 and the N-methylation of nicotine when compared with methio- 

 nine. 



Although information about the actual chemical transformations 

 taking place in the cell wall is meager, these studies give some 

 indication of processes at the cellular level in youthful tissues. 

 During maturation and development, the biochemical changes 

 which take place in the wall may show a variety of patterns. The 

 maturation and ripening processes in fruits are, for example, 

 closely associated with pectin chemistry. 



In the peach, the parenchyma cells of the mesocarp increase 

 in diameter to a maximum whereas wall thickness passes though 

 a maximum and declines. The esterification of pectins also ex- 

 hibits this kind of behavior. Thus, in the variety "Nectar", we 

 find: 



Additional evidence also suggests that little esterified pectin 

 is present in the meristematic tissues of very young peaches until 

 the cessation of mitosis. A high and relatively constant level of 

 esterified pectin is present during cell enlargement and is presum- 

 ably associated with increasing methylating capacity. 



During the ripening of pears it has been observed that soluble 

 glucosan, galactan and araban increase in the pectic fraction 



