198 CONTROL MECHANISMS IN CELLUlAff PROCESSES' 



example Clelancf (1959) simply regards plasticity as the "biological 

 process of wall deformation." Such a usage tends to obscure the 

 interesting and fundamental question as to the immediate mecha- 

 nism of plant cell growth, and the possibility that growth may in 

 fact be a passive stretching, but involving a property of the cell wall 

 (plasticitv) which is maintained by metabolism. The language of 

 most authors who discuss plasticity still suggests that they are in- 

 deed thinking in terms of such a passive process as the immediate 

 m.echanism of growth. Recently, for example, Jansen et al. (1960) 

 stated, "it is established that auxin . . . increases the rate of cell 

 elongation in Avena coleoptile sections by causing the cell walls to 

 become more stretchable, more plastic." 



Three principal methods have been employed to measure the 

 "plasticity" of cell walls of growing tissues. The first, introduced by 

 Heyn (1931), is to hold sections by one end in a horizontal position 

 and to applv a weight at the other end. The angle of bending which 

 remains after the weight is removed is considered to result from 

 plastic stretching and to measure plasticity. Heyn and subsequent 

 workers found that auxin increases the angle of bending. Unfor- 

 tunately the method has never been made quantitative by analyzing 

 the stresses to which the tissue is subjected in the experiment. Al- 

 though it is generally felt that the measurement is made in the 

 absence of any real growth, Heyn ( 1931 ) did record that isolated 

 coleoptiles deprived of water grew appreciably, and we have often 

 observed such coleoptiles to show geotropic curvature, a growth 

 response. It cannot be assured that the "plastic" bending might 

 not simply be a differential growth response to the different stresses 

 on the two sides of the tissue, even if growth were occurring by a 

 mechanism other than plastic stretching. 



The second method is to study the "irreversible enlargement" of 

 the tissue under its own turgor pressure, by comparing the lengths 

 of totally plasmolyzed sections before and after a period of expan- 

 sion in some medium in which the cells possess turgor. At best this 

 is, of course, simply a measure of growth itself and cannot arbitrarily 

 be called plastic stretching unless plastic stretching is defined as 

 growth, as by Cleland (1958, 1959). Cleland and Bonner (1956) 

 and Cleland ( 1958 ) found that auxin pretreatment in 0.25 M manni- 

 tol, in which coleoptile sections did not grow, would cause an in- 

 crease in irreversible enlargement when the sections were subse- 

 quently transferred to water in the absence of O2. Since in this last 



