CHAPTER VIII 



OSMOSIS AND OSMOTIC PRESSURE 



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Although the freedom of movement of the molecules of liquids is to some 

 extent restrained by internal cohesive forces, they also possess kinetic activity. 

 Like gases and solutes, therefore, liquids exhibit diffusion phenomena. If, for 

 example, water is brought into contact with another liquid such as ether with 

 which it is only slightly miscible, a slow diffusion of water molecules into 

 the ether will occur. Simultaneously a slow diffusion of the molecules of 

 the ether will take place into the water. Such diffusion will continue until 

 the two liquids are mutually saturated. 



Osmosis. — This is by far the most familiar process involving the dif- 

 fusion of liquids. An understanding of the dynamics of this process, and 

 of the significance of the physical quantity termed osmotic pressure is essential 

 to an interpretation of the water relations of plant cells and tissues. 



Let us first consider an experiment ar- 

 ranged as in Fig. 19. A sac-like membrane 

 of collodion is nearly filled with a strong 

 sucrose solution and immersed in a beaker 

 of water. The top of the sac is tightly 

 plugged with a rubber stopper. The col- 

 lodion membrane is so prepared that it is 

 permeable to water, but impermeable or 

 practically so to sucrose; in other words it 

 is differentially permeable (Chap. X). It 

 is also slightly elastic. 



After a short time the originally limp 

 sac becomes rigidly distended. This is due 

 to the movement of water through the col- 

 lodion membrane into the interior of the sac 

 example of osmosis 



SUCROSE 

 'SOLUTION 



DISTILLED 

 - WATER 



Fig. 19. Apparatus for the 

 demonstration of osmosis through 

 a collodion membrane. 



Such a diffusion of water is an 

 The pressure developed as a result of the entrance of 

 water is exerted against the inside wall of the membrane. This pressure is 

 counterbalanced by an equal and oppositely directed pressure exerted upon the 

 solution by the distended wall. If the membrane is virtually impermeable to 



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