56 THE VEGETATIVE FUNCTIONS OF PLANTS 



sweet, showing that some of the dissolved sugar had 

 passed, by its own motion, up through the mass of the 

 water to the top. This simple experiment illustrates diffu- 

 sion of liquids. The dissolved sugar behaves in a manner 

 quite similar to that of the gaseous "odor" of the musk. 



53. Osmosis. If, now, the denser sugar solution in the 

 bottom of the tumbler were separated from the less 

 dense water above by a porous membrane, such as a piece 

 of bladder or parchment, the diffusion would take place 

 through the porous membrane, and the water above would 

 soon become sweet, as in the previous case. In other 

 words, it is possible to have diffusion through a membrane. 

 Diffusion through a membrane is osmosis. 



The conditions realized in the experiment described 

 above are a denser liquid (in the bottom of the tumbler), 

 separated from a less dense liquid (at the top of the 

 tumbler) by a porous membrane. Moreover, not only 

 would the sugar solution pass up through the membrane, 

 but the water above would pass in the opposite direction, 

 and. more rapidly than the sugar solution. This would 

 continue until the solution was of the same density 

 (equal amounts of sugar in equal amounts of water) 

 on both sides of the membrane. Thus the action of 

 osmosis may be stated as follows: 



When two fluids (liquids or gases) of different densities 

 are separated by a porous membrane, diffusion through the 

 membrane will take place until equilibrium results. The 

 diffusion will be more rapid from the less dense to the more 

 dense fluid. 



Or, again, osmosis may be denned as the interchange oj 

 two fluids of different densities when separated by a porous 

 membrane. 



