HORMONAL REGULATION OF PLANT CELL GROWTH 199 



step auxin would not promote if added directly, they regarded this 

 as a "residual effect of auxin on the cell wall," of a plastic nature. 

 This effect has the interesting peculiarity of disappearing if 3 X 10"^ 

 M KCN is applied in the last ( plastic stretching ) step, all the odder 

 because this step is conducted under anaerobic conditions, so the 

 KCN effect presumabh' could not be simph' on cell respiration. It 

 makes one wonder whether the residual effect can in fact be a per- 

 sistent physical alteration of the cell wall. 



The third and most direct approach to plasticity is to apply known 

 forces to plasmohzed or otherwise inactivated tissues and directly 

 measure their reversible and irreversible deformations. Heyn ( 1931, 

 1933) used it but did not explore it extensively except to measure 

 elastic properties. The kind of difficulty encountered in trying to 

 relate such measurements to growth can be illustrated with the re- 

 sults of a careful study by Brauner and Hasman ( 1952 ) . They cut 

 strips of potato tissue 3X3 mm in cross-section, followed their 

 growth, then plasmolvzed them and subjected them to a tension, and 

 determined the amount bv which the tissue remained stretched after 

 release of the tension (plastic stretching). Tissue which had been 

 growing in auxin, was stretched "plastically" 1.7-1.9 per cent by a 

 tension of 20 g acting for 5 minutes. As previously mentioned, 

 Brauner and Hasman established also that growing tissue was prac- 

 tically in osmotic equilibrium with its dilute medium, so the turgor 

 pressure of the growing cells must have been practicallv equal to 

 their osmotic potential, which was determined to be about 6 atm. 

 It can readily be computed that in the growing tissue a force of more 

 than 500 g must have been acting on the cell walls across a 3 X 3 mm 

 cross-section; this tissue actuallv grew 26-27 per cent of its initial 

 weight in 24 hours, or only 0.09 per cent in 5 minutes (which in- 

 cludes all three dimensions, not just the direction of the cross-section 

 considered). Thus, the apparent rate of extension in the stretching 

 experiment was at least 20 times the rate of wall extension during 

 growth, despite the fact that the stretching force was less than one- 

 twentieth of the force acting during growth. It would seem that the 

 property being measured in the stretching experiment could not 

 have been the property which was controlling the growth rate, even 

 had this involved plastic stretching. The stretching treatment may 

 possibly measure the ability of a force to distort the shapes of the 

 cells on which it acts rather than to expand the cell wall "plas- 

 tically." 



