Theories of Auxin Action 183 



of water could be due to changes in the cytoplasm itself, especially 

 changes in osmotic value; or it could be due to changes in the proper- 

 ties of the wall and the cell membranes, particularly with respect to 

 extensibility and permeability. Each of these two factors in water 

 uptake has been defended as a possible mechanism through which 

 auxin may bring about growth. 



Changes in the osmotic value of the cytoplasm were first proposed 

 by Czaja (1935) to account for water uptake. Essentially his idea was 

 that auxin may increase the osmotic value of the sap which would 

 result directly in water uptake and growth. A basic limitation to this 

 concept lies in the fact that growth is not necessarily associated with 

 an increase in osmotic value. In fact, van Overbeek (1944) and 

 Hackett (1951) have been able to obtain growth with an associated 

 decrease in osmotic value. 



Changes in the cytoplasm do occur, however, following auxin 

 treatment. Among these changes there appear to be decreases in cyto- 

 plasmic viscosity (Northen, 1942). There is some quantitative relation- 

 ship between viscosity changes and auxin treatment, but the effect is 

 not specific for auxins, and some active auxins do not produce these 

 viscosity changes. 



A very interesting suggestion has been made by Commoner et al 

 (1942-3) who pointed out that since water uptake in growth may be 

 linked with a respiratory mechanism, one might explain growth as 

 being due to the osmotic uptake of water activated by a respiration- 

 induced uptake of salts. By this theory auxin may stimulate the respir- 

 atory uptake of salts, and the resultant increase in the osmotic value 

 of the cell sap would cause water uptake and hence growth. The 

 major objection to this theory is van Overbeek's observation, already 

 mentioned, that under some circumstances growth, associated with a 

 drop in osmotic value, can occur in potato slices. Clearly such growth 

 cannot be a function of salt accumulation bringing about an in- 

 creased osmotic value. Another fact which contradicts the theory is 

 that auxin-induced growth can occur in pure water in the absence of 

 salt uptake (Reinders, 1942). On the other hand the presence of salts 

 in the water can greatly facilitate growth (Steward et al, 1940). 



In an effort to establish whether auxin-induced growth was a 

 function of water uptake, Reinders (1942) has measured the effects of 

 auxin on the uptake of water (measured by weight) of potato slices. 

 Auxin does induce growth of these tissues, so of course she found a 

 similar increase in water uptake. She further established that the 

 water uptake response was strictly dependent upon the aerobic respira- 

 tion of the tissues, for in anaerobic conditions the uptake essentially 



