OSMOTIC PEESSUEE. 227 



In all cases of diffusion it is obvious that any dissolved 

 substances move from the stronger solution to the weaker 

 one, so that equality in concentration is approached. In 

 many cases the attainment of this equality is facilitated by the 

 movement of a greater quantity of the solvent from the weak 

 to the strong solution. Certain substances when arranged as 

 a partition between a solution and the solvent will permit of the 

 passage of the solvent only, but not the dissolved substance. 

 Perfect semi-permeable membranes, as they are termed, are not 

 known, but a near approach to them can be made artificially by 

 producing a precipitate of copper ferrocyanide within a porous 

 earthenware cell. If such a cell be filled with a solution, 

 closed, and its interior be connected with a manometer, it will, 

 when immersed in the solvent, show the production of con- 

 siderable pressure, in some cases amounting to several 

 atmospheres. The pressure attained in any experiment will, 

 when it has become constant, be found to vary directly with 

 the concentration and to increase with a rise of temperature. 

 This osmotic pressure, as it is termed, has been shown to be 

 analogous to gaseous pressure and to be amenable to the laws 

 of Boyle and Charles. 



The cell walls of plants, or rather their protoplasmic linings, 

 are probably composed of an approximately semi-permeable 

 membrane ; hence if they are surrounded with a solution of 

 less concentration than their contents, they will receive more 

 liquid than they will lose, a..nd the pressure within the cell will 

 be increased ; on the other hand, if the solution outside be 

 more concentrated than that within, more liquid will leave 

 than will enter, and the cell will shrink. The outer part of the 

 cell walls (cellulose) is not merely semi-permeable, but allows 

 of free diffusion. It, however, has not much power of exten- 

 sion and so acts as a nearly rigid and strongly elastic envelope 

 for the protoplasm. It is the strain set up by turgescence of 

 the cells which is the chief cause of the rigidity and firmness 

 of vegetable tissues in spite of their high water content. (A 

 turnip, though so firm and hard, for example, contains far 

 more water and less solid matter than milk or beer.) 



The stems and leaves of plants owe their stiffness and erect- 

 ness to the same cause the strain produced by the elasticity 



Q2 



