THE CELL 



35 



within the cyHnder increases and rises above its original level. 

 Presently this rising column of liquid develops a distinct hydro- 

 static PRESSURE that finally restrains the inward passage of water 

 completely. The pressure developed varies with the concentration 

 of the sugar; the greater the concentration of the sugar in solution 



iMT£ff 



MEMBRANE 



SUGAR 

 MOLECULE 



SUGAR 

 SOLUTION 



WATER 

 MOLECULE 



Fig. 6. — Diagram to illustrate the nature of osmotic pressure. The content of 

 the suspended tube is a solution of sugar in water. The mouth of the tube is 

 closed by a semi-permeable membrane and the beaker contains pure water. While 

 the sugar molecules constantly bombard the membrane, they do not pass through; 

 the water molecules readily pass into the tube containing the sugar solution. The 

 result of the passage of water into the tube is a rise in the level of the tube 

 contents above that of the surrounding water and a consequent development of a 

 hydrostatic pressure that exactly equals the osmotic pressure of the sugar molecules. 



the greater the pressure that is necessary to arrest further passage 

 of water through the membrane. Now the passage of a substance 

 through a semi-permeable membrane by diffusion is termed osmosis 

 and this hydrostatic pressure that is developed by the invasion of 

 water into the sugar solution is exactly equal to the osmotic pres- 

 sure of the sugar solution. Osmotic pressure is derived from the 

 fact that all the dissolved molecules of the sugar are in motion 

 and collide with the membrane; obviously the greater the number 



