OSMOSIS AND IMBIBITION 235 



the laws of diffusion. There we saw that substances diffused in- 

 dependently of each other and according to their own partial 

 pressures. We can view an osmometer in the same way. On the 

 inside we have, e. g., a 40% solution of sugar. That means there 

 is only a 60% solution of water on the inside. On the outside, 

 however, there is a 100% solution of water. In other words, the 

 water is more " concentrated" (to use a term which is generally 

 used for the solute only and not for the solvent) on the outside 

 than on the inside, and so it moves from the place where it is the 

 more concentrated (has a higher partial pressure) to the place 

 where it is the least. This movement continues until the differ- 

 ence between the two solution pressures is balanced by the hydro- 

 static pressure exerted by the column of liquid. If the liquid in- 

 side the osmometer is rich in solutes, i. e., poor in water, the column 

 will hence be higher than if the solution inside is relatively poor 

 in solute and rich in water. The salts or other solutes also tend 

 to diffuse out, and do so as much as the membrane will permit. 



The Membrane. — The second question to consider is how the 

 materials pass through the membrane. Some think that the sol- 

 vent dissolves in the membrane (or vice versa) and passes through 

 in this fashion. Others have suggested that reversible compounds 

 are formed which tie up the solvent with the membrane on one 

 side (the outside) and release it on the other. In certain cases such 

 as the passage of alcohol, pyridine, etc., through rubber membranes 

 there may be some truth in the solution and reversible compound 

 theories, but most of the work can be explained more easily on the 

 assumption that the membranes are porous. These pores of a 

 certain size let water pass through but not the larger molecules 

 of sugar, salt, etc. In very dilute solutions electrical phenomena 

 play a part, but in the study of elementary plant physiology this 

 type of osmosis can be neglected. 



A third question is why the water stays in after it gets in. 

 Since it is free to pass out, why doesn't it do so? One answer is 

 that the particles inside attract the water and hold it so that once 

 inside it is not free but is bound up with the particles present. 

 Since they cannot pass out, neither can the water which is bound 

 to them. A second explanation is that the osmotically active 

 substances inside act as a trap and block the way. Molecules of 

 water can enter freely but once inside they are hindered from 

 passing out by the sugars, salts, etc., in the path. Any theory 



