OSMOTIC PRESSURE AS A FACTOR 



175 



rate of intake, the environment of the cell being supposed to 

 remain the same. 



From the foregoing considerations it appears probable that all 

 plant cells excepting those to which the environmental complex 

 presents on all sides identical conditions of water-supplying or of 

 water-extracting power, are to be thought of as practicalh' never 

 attaining even to the dynamic equilibrium of balanced rates of 

 water intake and of water exit. Much less do they attain the 

 static equilibrium of balanced forces presupposed in the earlier 

 part of this discussion. The problem of the water relations be- 

 tween cells and their environment is thus seen to be an exceedingly 

 complex affair, but it is nevertheless a problem in the solution of 

 which we may hope to make rapid progress, for many of the forces 

 and rates of water movement, with which it is concerned, are 

 already susceptible of quantitative study. 



In conclusion, I may emphasize, in a somewhat different way, 

 some of the points which have come up in my earlier paragraphs, 

 beginning now with the emdronment instead of with the cell. 

 The envirormiental complex, as far as the water relation is con- 

 cerned, may be looked at as exerting upon the entire peripheryof 

 the organism an influence which tends toward water extraction. 

 The only exception to this generalization is to be found in the 

 purely ideal case where the surface of the organism may be sup- 

 posed to be bathed with pure water. In such an ideal case, the 

 cell wall (or the outer layer of the protoplasm) would soon beconie 

 imbibitionally saturated with a pure solvent in which no osmotic 

 pressure would be manifest, and the saturation of the wall would 

 preclude any outward imbibitional attraction for water. 



If the environment presents an aqueous solution to the exposed 

 surface, then there must be developed in the cell wall (which may 

 be assumed to be imbibitionally saturated with this solution) an 

 osmotic pressure which tends directly to oppose the outward force 

 arising from osmotic pressure in vacuole and protoplasm. As 

 this environmental solution might become more and more concen- 

 trated, the inwardly directed pressure should consequently in- 

 crease, and the stretching strain upon the wall from within should 

 decrease and finally disappear, after which plasmolysis should 

 occur. 



