THE ROOT AND ITS FUNCTIONS 40 



Osmotic Movement of Solvents. — This phciioincnon of osmosis 

 is complicated, however, bj^ the fact that the dissolving liquid 

 or solvent (water in this case) as\well as the dissolved substance 

 will pass through the membrane, and by the remarkable circum- 

 stance that where such movement occurs, it is always more rapid 

 in one direction than in the other. Experiment has shown that 

 if two solutions of different densities (or a solution and pure 

 water) are separated by a membrane, a movement of water takes 

 place from the less concentrated to the more concentrated solution, 

 and tends to continue till both are of the same density; and that 

 the rate of this movement is proportional to the difference in 

 concentration. The more concentrated solution will therefore 

 tend to expand through this access of water, and, if it is confined 

 within a closed space, a pressure, often of considerable magni- 

 tude, will develop.* 



As to why such a movement of water occurs no complete 

 agreement of opinion yet exists, for the process of osmotic inter- 

 change involves some of the less clearly understood phenomena of 

 physical chemistry. We may assume that the dissolved sub- 

 stance has an affinity or attractive power for water, and that this 

 attraction increases with the concentration of the substance; or 

 that the molecules of the dissolved substance interfere with the 

 free molecular movement of water molecules, so that where there 

 is little material in solution the water molecules strike the 

 membrane and pass through it oftener than they can where much 

 material is in solution; or we may regard the whole phenomenon 

 as really a manifestation of the fundamental principle of diffusion, 

 since the tendency is for the solutions on both sides of the mem- 

 brane to become equal in concentration, although this is here 

 brought about by a movement of the dissolving liquid as well as 

 of the dissolved substance itself. None of these explanations is 

 entirely satisfactory, but they may perhaps help to picture the 

 process more clearly to our minds. The essential fact remains 

 that water will pass through a membrane toward the denser 

 solution, explain it as we may; and upon this fact depends the 

 power of the plant to withdraw water from the soil. 



Permeability of the Membrane. — Thus far, we have assumed 

 that both the dissolved substance and water may pass with per- 



* Osmotic phenomena arc shown by other solvents than water, but since 

 water is the all-important solvent in physiology, we shall confine our at- 

 tention only to those osmotic processes in which it is involved. 



