PHYSICOCHEMICAL ORGANIZATION OF THE PLANT 13 



crystalloids. The cellulose of the cell wall is a membrane of 

 this character. It is permeable to almost all substances dissolved 

 in water. 



If a sac made of a membrane, such as animal bladder, parch- 

 ment paper, or a hardened film of collodion, is filled with a solu- 

 tion of a substance that does not diffuse very fast, e.g., cane 

 sugar, and if it is then tightly closed and placed in water, the 

 sac will swell rapidly, its walls will become 

 turgid, and finally it may rupture when it 

 is unable to withstand the pressure from 

 within. If instead of the opening being 

 tied up, a glass tube is inserted in it, under 

 the influence of internal pressure the level 

 of the liquid in the tube soon begins to rise, 

 first rapidly, then slower and slower, until 

 it stops at a certain definite level, after 

 which it begins to fall again. 



By analyzing the changes in the contents 

 of the sac, it is found that at first a rapid 

 entrance of the water takes place, which 

 leads to a considerable increase in volume. 

 Afterward, the diffusion of the dissolved 

 substance from the sac into the surround- 

 ing water becomes slower, and finally the 

 concentration of the outer and inner solu- 

 tions is almost equal, and the walls begin 

 to collapse. The entrance of the water into 

 the sac is called ''endosmosis," the out- 

 ward escape of the solute, ''exosmosis." 

 The whole apparatus, the sac with the 

 inserted tube (Fig. 4), has been called an ''osmometer" by 

 Dutrochet, the French botanist, who was the first (1826) to study 

 this phenomenon. The diffusion of liquid and dissolved sub- 

 stances through a membrane has been given the general name 

 ''osmosis." The hydrostatic pressure, developing as a result of 

 an increase of endosmosis over exosmosis and swelling of the sac, 

 is called "osmotic pressure." 



Further investigations have shown that besides minutely 

 porous membranes that only slightly check the diffusion of 

 water and its solutes, there exist membranes that allow water or 



Fig. 4. — Osmometer (after 

 Dutrochet) . 



