THE LIVING CKLL 



35 



greater osmotic pressure, water will pass by osmosis from the first 

 into the second solution, the effective osmotic pressure in this case 

 being the difference between the values for the two solutions. 



The parchment membrane used in the experiment described above 

 allows the sugar molecules to diffuse slowly into the outer water ; 

 the membrane is to a certain extent permeable to the sugar. The 

 concentration and therefore the osmotic pres- 

 sure of the contents of the thimble thus fall, 

 and some of the water initially taken into the 

 thimble leaks out again. A permanent osmotic 

 retention of water is only possible with a 

 membrane which, while giving free passage 

 to water molecules, denies passage to the mole- 

 cules of the osmotically active substance. 

 Such a membrane is said to be semi-permeable 

 to the solution, and it is only with such mem- 

 branes that the osmotic pressure of a solution 

 can be determined directly. Copper ferro- 

 cyanide forms a well-known artificial mem- 

 brane which is semi-permeable to solutions 

 of sugar and of various other substances. 

 For purposes of experiment it is usually de- 

 posited within the minute pores of a porous 

 pot, which thus gives it rigid support. 



The plant cell forms an osmotic system. 

 The cell-sap of the vacuole consists of a solu- 

 tion of osmotically active substances, such as 

 sugars, organic acids and salts, and normally 

 exerts a total osmotic pressure of 5 to 20 

 atmospheres. This solution is enclosed in the 

 membrane formed by the thin lining of cyto- 

 plasm. The cell-wall gives support to the 

 cytoplasmic lining : its presence makes possible the development 

 of a high degree of turgor within the cell. Under normal conditions 

 the plant cell is in a state of inflation or of turgor (p. 34), set up by 

 the osmotic absorption of water. This condition can be removed 

 without harm to the cell by immersion in a solution of potassium 

 nitrate or of cane sugar which is sufficiently concentrated to be of 

 higher osmotic pressure than the cell-sap. Such a solution is said 

 to be hypertonic to the sap ; one of lower osmotic pressure than the 

 sap would be termed hypotonic . For many plant cells a 5 per cent. 



Fig. I7a. 



Apparatus for the demon- 

 stration of Osmosis. The 

 parchment thimble contains a 

 sugar solution, while the outer 

 vessel is initiaDy filled with 

 water. 



