XI. THE PLANT 225 







In this analogy, the small shot represent the particles of the 

 solvent (e.g., water), the clay balls, those of the dissolved substance and 

 the clay complexes, the molecular aggregates resulting from the 

 association of the dissolved substance with some of the solvent, while 

 the net represents the semi-permeable membrane. 



In reality, the mechanism, by which the membrane effects the 

 stoppage of the motion of the dissolved substance and permits the 

 passage of the solvent, may be very different to a net-work anpl the 

 difference between the particles of dissolved matter and of the solvent 

 may not be one of size, but the analogy will still serve to explain the 

 phenomenon. 



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, and 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 extension 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, for example, though so firm and hard, 

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



The stems and leaves of plants owe their stiffness and erectness to 

 the same cause the strain produced by the elasticity of the cellulose 

 envelopes on the one hand and the water-distended state of their 

 protoplasmic contents on the other. When this turgescence relaxes, 

 e.g., by evaporation, the plant wilts and droops, becoming quite flaccid. 



A solution within a cell composed of a semi-permeable membrane 

 and closed by a manometer, if immersed in the pure solvent, takes in 

 the latter until the osmotic pressure is in equilibrium with the pressure 

 produced by the mercury column of the manometer. If more mercury 

 be added to the manometer some of the solvent will be forced out ; 

 on the other hand, if the mercury column be shortened more, solvent 

 will enter and the volume of liquid within the cell will increase. It is 

 found that the pressure produced by a given weight of dissolved sub- 

 stance per litre of solution, at a given temperature, is exactly equal to 

 the pressure which would be exerted by the substance if it could exist 

 as a gas under the same conditions as to volume and temperature. 

 This latter, in the case of different substances, varies inversely with the 

 molecular weight of the substance. Consequently it is found that two 

 solutions exert an equal osmotic pressure when there are present in a 

 given volume of the solution the same number of moleculesof the dissolved 

 substances. In the case of most metallic salts and the stronger acids in 

 aqueous solution, this law is subject to a correction, because of the dissocia- 

 tion of these compounds into ions, each of which acts as a molecule. 1 



1 Vide p. 76. 



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