200 CONTROL MECHANISMS IN CELLULAR PROCESSES 



Our feeling is, therefore, that existing approaches to "plasticity" 

 have not positively distinguished it from other possible mechanisms 

 of cell growth, and we have attempted to think of means by which 

 a plastic stretching and a molecular mosaic mechanism could be 

 distinguished. Some experiments in this direction are described in 

 the next section. 



Experiments on the Immediate 

 Mechanism of Growth 



The essential distinction between what we have called a molec- 

 ular mosaic growth mechanism and a plastic stretching can be 

 thought of in the following terms. Plastic stretching is a forcible 

 phvsical deformation depending upon a viscosity-like property (plas- 

 ticity) which may be maintained by metabolism but can be said to 

 have a particular ( passive ) value at any moment, whereas in a molec- 

 ular mosaic it is some activity of the cell (most easily visualized as 

 a chemical reaction), rather than physical stress, which breaks the 

 critical or controlling bonding forces within the cell wall and leads 

 to expansion. Either mechanism could require turgor pressure for 

 actual displacement, hence visible growth, to occur. There is, of 

 course, a possible middle ground involving processes of both types. 

 But if one confines one's thinking to the simple possibility that the 

 controlling process in growth has one or the other character, it would 

 seem that one operational distinction between them would be a 

 separation in time between the action of metabolism and the occur- 

 rence of growth in the case of plastic stretching, but not in the case 

 of a molecular mosaic. We could speak of the plasticity decaying 

 with some characteristic "half-time" if synthetic metabolism were 

 suddenly suppressed, while growth by molecular mosaic would have 

 to cease immediately. One presumes that the half-time for plas- 

 ticity would be short in comparison with the normally used inter- 

 vals of one-half hour or longer in growth measurements. 



Albert W. Ruesink and the author have experimented with a 

 method of following accurately the growth of oat coleoptile sections 

 over periods of minutes ( usual interval between measurements, one 

 minute). Our original experiments were intended to determine 

 how rapidly the rate of growth changes when one suddenly changes 

 the temperature. We mount a section, about 7 mm long and con- 

 veniently hollow, horizontally on a small thermocouple junction with 



