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TEXTBOOK OF BOTANY 



may use the apparatus represented in Fig. 69. Various kinds of mem- 

 branes and solutes may be tested by this means. The thistle tube with 

 attached membrane is first placed in a vessel of water as in Fig. 69A to 

 see if the membrane is permeable to water. The solute may then be added 



Fig. 69A. Diagram to illus- 

 trate the passage of water 

 through a membrane: A, rep- 

 resents a molecule of inside 

 water; B, a molecule of outside 

 water; and C, the membrane. 

 Equal numbers of water mole- 

 cules are in contact with the 

 inside and outside of the 

 membrane, and the movement 

 of water molecules through 

 the membrane in both direc- 

 tions is the same. Hence the 

 level of the water in the tube 

 remains unchanged. 



to the water in the thistle tube. If there is a definite continued rise of 

 water in the thistle tube the membrane is more permeable to water than 

 to the solute. A temporary rise that soon subsides, such as one obtained 

 with a membrane of filter paper, is due merely to the fact that the molec- 

 ular motion of water molecules is more rapid than that of the molecules 

 of the solute. One may also tie the membrane across the mouth of a 



B 



Fig. 69B. Diagram to illus- 

 trate osmosis: A, represents a 

 sugar molecule; B, a water 

 molecule; and C, a differen- 

 tially permeable membrane. 

 The sugar in solution dilutes 

 the water so that fewer water 

 molecules are in contact with 

 the inside than with the out- 

 side of the membrane. Hence 

 water passes in more rapidly 

 than it passes out, and the level 

 of the water in the tube rises. 



