OSMOSIS 



71 



capillarity does not account for soil water getting inside the cell. 

 It has to go through the cellulose wall and the delicate membrane 

 within. The walls of cells, like wood, absorb soil water readily by 

 a process known as imbibition. This brings the soil water in con- 

 tact with the cell membrane. Inside the cell membrane is a liquid 

 which would diffuse freely with the soil water if the membrane 

 were removed. But a membrane acts peculiarly toward diffusing 

 substances. An experiment will help us to understand this. 



Osmosis. — If we carefully break away part of the shell of an 

 egg so as to expose the delicate skin or membrane underneath, we 

 have a picture of the relation of the cell membrane (like the egg 

 skin) to the cell wall (like the egg shell) . If this egg is placed in a 

 glass of cold water, within a short time the membrane will bulge 

 out, showing that water has passed 

 into the egg through the membrane. 

 If, however, we test the water in the 

 glass for protein, the organic sub- 

 stance of which white of egg is 

 composed, we shall find none. Evi- 

 dently the egg membrane will per- 

 mit the passage of water but not of 

 protein. Such a membrane is said 

 to be semi-yer'meable. It is this kind of membrane that surrounds 

 plant and animal cells. It will permit certain substances such as 

 water to pass through it readily in either direction, and it will per- 

 mit certain substances in solution to pass less readily, while still 

 other substances will not be permitted to pass through at all. 



Another experiment will help us. If we take a thistle tube, 

 fill the lower end with a solution of grape sugar and water, then 

 tie tightly over it an animal membrane (such as pig's bladder), 

 and place the apparatus in water, as shown in the figure on the 

 following page, we notice that after a very little time the fluid in the 

 thistle tube begins to rise. Evidently water passes into the tube 

 more rapidly than the substance inside can pass out. If we could 

 see the separate particles, or molecules, of the water and of the 

 solution of water and sugar, they would be found to arrange them- 

 selves on each side of the membrane so as to cover it completely. 

 But since the water molecules diffuse easily through the membrane 



Osmosis through the membrane of 

 an egg. 



