OSMOSIS AND IMBIBITION 239 



ber of particles in solution, the harder it is to drive off the water by 

 heating. Consequently the vapor pressure will be less and the 

 boiling point will be higher. The osmotic pressure can then be told 

 by the elevation of the boiling point. 



Similarly in freezing it is necessary to separate the water from 

 the solute, and the more solute present the harder it is to freeze 

 the solution, i. e., the lower must be the temperature. Conse- 

 quently the depression of the freezing point is an index to the 

 osmotic pressure, and this method is the one commonly employed 

 with plant and animal liquids. 



In conclusion, it may be briefly added that the osmotic proper- 

 ties of cells are among the most important since, along with the 

 condition of the membranes through which the materials pass, 

 they largely determine the movements of all substances from cell 

 to cell within the plant. 



Imbibition. — Osmosis has explained only how the water and 

 solutes enter the living protoplasm. While this is of the highest 

 importance, there yet remains something to be said of the method 

 by which water passes through the dead parts of the cell such as 

 the cell wall, through seed coats, etc. This is by imbibition. If we 

 place water in a glass tube, the water which wets the glass will rise 

 slightly in the tube to a height which varies with the diameter of 

 the tube. The elements or molecules of the water cohere to each 

 other and adhere to the glass. While the forces of adhesion are 

 stronger than gravity, they are not strong enough to overcome the 

 cohesive forces within the water itself, with the result that a 

 small column of water is pulled up the glass. If, on the other 

 hand, water and sugar are put together instead of water and 

 glass, we find that the adhesive forces between the molecules 

 of water and sugar are stronger than the cohesive forces of the 

 sugar, with the result that the sugar is pulled apart and broken 

 down into its constituent molecules. In other words it goes into 

 solution. Here we have the two extremes. In capillarity, cohesive 

 forces are much the stronger of the two; in solution, the adhesive 

 forces get the upper hand. In imbibition, the forces are much 

 more evenly balanced. The water is attracted very strongly by 

 the particles which make up the cellulose cell wall, and the par- 

 ticles of the cellulose are also attracted by each other; but they 

 are nevertheless forced apart by the water which is able to come 

 in between them. When the water is imbibed, the walls are 



