Problems in Biophysics of Cell Growth 383 



ture under force and can be called a plastic extension. The rate of 

 extension will be greater, the greater the turgor pressure, since more 

 bonds will be under a strain great enough to break them. To make 

 the structure more plastic and to stimulate growth at constant turgor 

 pressure (as auxin does), it is necessary to reduce the total number of 

 bonds present. In terms of the pectate theory this requires an in- 

 crease in the methyl ester content, so that fewer carboxyl groups are 

 left unesterified and capable of forming salt linkages. 



Another way in which the cell wall structure under strain might 

 grow may be recognized by supposing that, in fact, turgor force is not 

 great enough to break critical bonds in the structure — salt linkages in 

 the pectate theory. Through metabolic machinery the cell, however, 

 may be capable of breaking these bonds by insertion of a methyl ester 

 group on one or both carboxyl groups (Figure IC). When this hap- 

 pens, the no-longer linked polyuronide chains will move apart to 

 relaxed configurations, just as in the previous example, and subse- 

 quently they may enter into bonding with adjacent carboxyl groups 

 after metabolic removal of the methyl groups (Figure IE'). The cell 

 wall will extend as these events take place successively in all parts of 

 the structure. 



This kind of growth is not a passive process as every unit of en- 

 largement depends upon an act of metabolism; it cannot be consid- 

 ered a passive plastic stretching. Nevertheless, it is dependent upon 

 turgor, because in the absence of strain in the structure, the bonded 

 uronide chains will not separate when the bond between them is 

 broken by methylation, but instead they will merely resume bonding 

 subsequently. Also, note that the amount of growth achieved per 

 active growth event will be greater, the greater the strain on the units 

 acted upon, so the growth rate will increase with increasing turgor 

 pressure, as was the case with pure plastic stretching. So an increase 

 in the growth rate at constant turgor pressure is not to be looked for 

 in a change in the number of bonds present but in an increased rate 

 of bond breaking - of methylation in the pectate theory. One can see 

 that an increased growth rate could be obtained in the presence of 

 auxin, without any change in methyl ester content, provided there is 

 an increased rate of metabolic turnover of uronide methyl ester groups. 

 This second picture of how the cell wall might grow is really an 

 extension, to molecular dimensions, of the concept of mosaic growth 

 advanced by Frey-Wyssling and co-workers (3,4). This conceives 

 of extension as resulting from localized transformations of elastic 

 strain into fixed deformation through some loosening process carried 

 on by the protoplasm. The areas of loosening (as interpreted from 

 electron micrographs) were much larger than the bond distances 



