188 CONTROL MECHANISMS IN CELLULAR PROCESSES 



Other than potato. Measurement of diffusion of labeled water (Thi- 

 mann and Samuel, 1955; Ordin and Bonner, 1956) suggesed too that 

 osmotic equilibrium should practically prevail, although this conse- 

 quence was not shown explicitly, and it should be noted that diffu- 

 sion of water is not identical with the process of osmosis by which 

 it is assumed the cells are absorbing water. 



In our own experiments of the type to be described below, we 

 find that the rate of expansion of oat coleoptile sections in auxin is 

 of the same magnitude as the rate of osmotic swelling or shrinking 

 immediately after the section is transferred between two growth- 

 inhibitory (but non-plasmolyzing) concentrations of mannitol 0.1 M 

 apart ( differing by 2.4 atm of osmotic potential ) . This would sug- 

 gest that the growing cells could not be close to osmotic equilibrium. 

 There is still considerable uncertainty about the actual value of P 

 in rapidly growing cells, despite the importance of this quantity in 

 the consideration of possible mechanisms of growth. 



Regardless of whether effects on rate of water uptake are in- 

 volved in auxin effects on growth rate, some explanation is still 

 needed of the fact that water uptake into growing cells is accom- 

 panied by irreversible enlargement of their cell walls, whereas in 

 non-growing cells the walls merely stretch elastically until an equilib- 

 rium value of P has built up, and absorption ceases. This feature 

 of growth is of primary interest at the present time, and it would be 

 expected to be the controlling process if, as many assume, the P of 

 growing cells never departs far from the osmotic equilibrium value 

 as they take up water. 



Growth of the Cell Wall 



The idea that expansion of the cell wall is the controlling process 

 in growth and in the action of auxin on it goes back to the early 

 auxin literature and was made prominent especially by Heyn ( 1931, 

 1933). He obtained evidence that growth is a passive "plastic ex- 

 tension" of the cell wall under the force which turgor exerts on it, 

 and that auxin causes increase in the "plasticity," or rate of irre- 

 versible stretching under unit force. That growth seems to depend 

 upon turgor has long been known, for a wilted plant does not grow. 

 The inhibitory effect of mannitol on growth of stem and coleoptile 

 sections, at osmotic concentrations approaching that of the cells, is 

 widely accepted as affording a measure of the dependence of growth 



