230 PLANT PHYSIOLOGY 



making dry cells and to fill it with a solution of copper sulphate 

 (CuS0 4 ). The whole is then placed in a solution of potassium 

 ferrocyanide [K 4 (FeCN) 6 ]. Since the cylinder is porous, both 

 liquids penetrate the wall and meet in the middle of it where a 

 membrane of copper ferrocyanide is precipitated : 



K 4 Fe(CN) 6 H-2CuS04 = Cu2Fe(CN) 6 +2K 2 S04. 



Copper ferrocyanide is a differentially permeable membrane. It 

 will permit water to pass through, but not salts in solution. If 

 such a cell is filled with a sugar solution, closed at the top, and 

 supplied with a manometer, an increase in the height of the mer- 

 cury column will be observed as soon as the cell is immersed in 

 water, thus indicating an increase in the internal pressure. Such 

 an instrument is called an osmometer, and with it Pfeffer and his 

 students obtained the following results using cane sugar: 



1. Atl5°C. 



Cone, of sugar Pressure (cm. of mercury) 



1% 53.2 



2 101.6 



4 208.2 



6 287.7 



That is, the osmotic pressure developed within a solution varies 

 directly with the concentration. 



2. A 1% solution. 



Temperature Pressure 

 6.8° 50.5 



13.7 52.5 



22.0 56.7 



That is, the osmotic pressure within a solution increases with the 

 temperature. 



3. A 6% solution at 15° C. 



Membrane Pressure 



Animal bladder 14.5 



Parchment paper 29.0 



Copper ferrocyanide 287.7 



That is, the pressure derived varies with the membrane used 

 because some membranes are more permeable than others. 



Osmotic Pressure Similar to Gas Pressure— Van't Hoff (1887) 

 studied these results carefully and, after comparing them with 

 the results previously obtained in studies made on the pressures 

 in gases, he concluded that the dissolved substances were giving 



