Grafts et al. 



48 



Water in Plants 



Practically all liquids become solids below the critical temperature when 

 pressure becomes sufficiently high, and so pass out of the liquid state where 

 the laws of diffusion pressure apply. In the liquid state, however, the 

 diiTusion pressures of both solute and solvent are lowered as the solute 

 dissolves, and it is this diffusion pressure lowering that becomes of para- 

 mount importance in the mechanism of osmosis. 



Pi 



N, 



N, ho 



0.1 

 0.9 



02 

 0.8 



0.7 



0.4 

 O.d 



OJ 

 0.S 



0.6 

 0.^ 



07 0.8 



0.3 O.i 



0,9 

 OJ 



1.0 

 



Fig. 13.— Diagram showing the relations of partial and total vapor 

 pressures of solutions according to Raoult's Law. pi° is the vapor pres- 

 sure of component 1 in the pure state; pi is its (partial) vapor pressure 

 in the solution, and Ni its mol fraction. p2°, P2, and N2 are similar 

 quantities for component 2. 



The Mechanics of Osmosis : — The process of osmosis is independent 

 of the pressure state under which the total osmotic system operates; the 

 osmotic pressure and the turgor which the system is capable of developing 

 are the same whether the system is atop a high mountain, at sea level, or 

 within an evacuated chamber. This is true because turgor pressure is not 

 an absolute pressure but an excess or differential pressure, and osmotic 

 pressure is a physical property of the solution nearly independent of pres- 

 sure. For convenience, it is usual to consider the system at atmospheric 

 pressure and measure pressure differences from that base line setting the 

 diffusion pressure of water under one atmosphere at zero. The principal 

 difficulty that the plant physiologist encounters is in the treatment of prob- 

 lems in plants where the water is subject to tension and is not at atmospheric 

 pressure. 



