382 



P. M. Ray 



I 



© 



I 



Polyuronide choin 



# Carboxyl group 



O Methyl esterified carboxyl 



© Calcium ion 



Me Methyl group 



©. 



I 



I 

 ©« 



Me © 



Me ©■ 



^1 



Fig. 1. Illusliation of the plastic and molecular mosaic mechanisms of growth 

 based on the pectate theory. A shows cross link in unstrained wall. B shows 

 elastic expansion under turgor. In C the cross link is being broken by forcible 

 separation; the polyuronide chains now assume relaxed configurations (D), and the 

 carboxyl groups enter into salt linkage with other adjacent carboxvls (E). B' sliows 

 again a bond under turgor stress; in C, bond is broken by methylation of the 

 carlioxyls. After the released chains reach relaxed positions (D'), methyl ester 

 groups arc remo\ed and carboxyls enter into salt linkage with other adjacent 

 carboxyls (E'). 



Under the force of turgor these critical bonds are put under stress, 

 as shown in Figure IB. This will involve an increase in the bond 

 length from the rest length, as well as elastic strain elsewhere in the 

 cell wall structure. One way in which the structure could expand 

 irreversibly would be that at some point the strain on the electro- 

 static bond becomes great enough to break the bond — i.e., further 

 increase in bond length does not increase the restoring force between 

 the bonded groups. The two polyinonide chains will then spring 

 apart, taking up more relaxed configinations, and the carboxyls will 

 enter into salt linkages with other carboxyls to which they now find 

 themselves adjacent. This process will be repeated after similar bond 

 breaking elsewhere in the structure has reimposed strain on the re- 

 gions in which the two original carboxyl groups are located. 



This process amounts to a passive distortion of the cell wall struc- 



