OSMOSIS 107 



ever it is that has caused the result, one thing is evident: 

 the law of solution is more nearly satisfied than it was 

 before, for the molecules of the solute are farther apart 

 than they were before, and this has resulted from a move- 

 ment of the solvent rather than from a movement of the 

 solute. Some sugar, however, has found its way through 

 the membrane and is in the water outside. Evidently it 

 is easier for water molecules to pass through a membrane 

 than it is for sugar molecules. 



But to go back to the root-hair. It may be that some 

 molecules in the soil water are too big to enter it, but many 

 there are which do enter it, and which are sure to con- 

 tinue to enter it as long as the abundance of them is less 

 within than it is without. The molecules of a solute 

 appear to move from where that solute is more dense to 

 where it is less dense as easily and as surely as a ball rolls 

 down a sloping roof. But suppose that these molecules 

 become equally abundant on both sides of the root-hair 

 wall. That will establish a sort of equilibrium ; the osmotic 

 pressure will be the same on both sides of the wall so far 

 as that particular kind of solute is concerned, and the move- 

 ment will stop. How, then, are we going to account for the 

 fact that the movement does not stop ? Of the molecules 

 which the plant needs there is a steady procession, con- 

 stantly entering as long as the plant is at work. How 

 are we going to explain this fact? 



Not only the solutes keep entering the plant. The 

 water in which they move also keeps entering. Both must 

 keep entering, or the plant could not grow ; it could not 

 live. How is it that the procession of water is kept up 

 as well as the procession of solutes? Here are two things 

 to be explained. 



