120 PHYTOHORMONES 



actual formation of new protoplasm, in the stage of growth 

 we are concerned with, may or may not take place. 



The tendency of the cell to take up water, which may be 

 defined as the Suction Force, is given by the difference 

 between the osmotic pressure of the cell contents and the 

 pressure exerted on them by the walls (Ursprung and Blum, 

 1924). The tendency to take up water can be increased 

 either by increasing the osmotic pressure of the cell contents 

 or by decreasing the counter-pressure exerted by the wall. 

 (The rate at which water is taken up can be increased tem- 

 porarily by increasing the permeability for water, but this 

 does not affect the ultimate dynamic equilibrium.) That 

 growth itself is probably not due to an increase in osmotic 

 pressure is shown by the observation of Ursprung and Blum 

 (1924) that there is no increase in osmotic pressure in 

 rapidly growing tissue. Nevertheless Czaja (1935a) has 

 suggested that auxin acts by changing the osmotic value 

 of the cell. Auxin, being an acid, is supposed to change the 

 charge on the cell membrane from negative to positive; 

 this would cause negative osmosis, and thus increase the 

 turgor. As sole support for this theory, Czaja cites some 

 experiments on plasmoptysis (bursting) of root-hairs in 

 4 per cent sucrose solution, in which indole-acetic acid 

 gave a somewhat greater effect than HCl at the same pH. 

 In the absence of further experimental support and in view 

 of the observations of Ursprung and Blum, the theory does 

 not seem to justify further comment. 



While there is thus no direct relation between growth and 

 osmotic pressure, investigations directed at the cell wall 

 have shown that there is a close relation between growth 

 and certain changes in its properties. During the last 

 50 years a considerable literature has grown up around the 

 cell wall, which we cannot attempt to review here. The 

 reader may be referred to Frey-Wyssling (1935, section I), 

 or Anderson (1935). Briefly, it may be said that irreversible 

 extension of the wall could be brought about by plastic 

 stretching alone, by active intussusception, that is by the 



