106 NOT ALTOGETHER ABOUT PLANTS 



outside, these molecules are surely going to move through 

 the spaces in the walls. They will do it in fulfillment of 

 the law of solution. They will do it unless other mole- 

 cules of their own kind are already as abundant in the sap 

 inside the plant as they are in the water just outside the 

 plant. This is the thing which they actually do, and it is 

 called osmosis. The solutes pass through the spaces of 

 the root-hair wall in accordance with the law of solution, 

 and this is an example of osmosis. Osmosis appears to be 

 simply the process of solution occurring through a mem- 

 brane. (A membrane is a sheet of organic substance; 

 the wall of the root-hair is an example of it.) However, a 

 simple experiment shows that in osmosis we get evidence 

 of a certain force which is not evident in the process of so- 

 lution alone. This force is what is called osmotic pressure. 

 You can show the way osmosis works by a simple 

 experiment. Parchment or the wall of an animal's 

 intestine is a kind of membrane. Tie an unbroken piece 

 of such membrane firmly over one end of an open glass 

 tube. Partly fill the tube with a sugar solution and then 

 place it so that the covered end stands in pure water. 

 After you have let such an arrangement stand for some 

 time, you will find that the liquid in the tube has risen, 

 while the water surrounding the tube has sunk. Water 

 seems to have entered the tube. This appears to indicate 

 a power of osmosis which we have not yet mentioned. It 

 appears that the abundance of sugar molecules inside the 

 tube draws molecules of water from outside the tube 

 into it; it appears that, to satisfy the law of osmosis, 

 the molecules of the solvent will move as well as the 

 molecules of the solute. If the sugar cannot easily get 

 out through the membrane, the water comes in. What- 



