86 



General Botany 



molecules. Consequently there are fewer water particles on the 

 inside tending to diffuse outward. Sugar dilutes the water in the 



Fig. 47. Diagram to illustrate 

 the passage of water through a 

 membrane : A represents a mole- 

 cule of inside water, B a molecule 

 of outside water, and C the mem- 

 brane. Equal numbers of water 

 molecules are in contact with the 

 inside and outside of the mem- 

 brane, and the rate of movement 

 through the membrane in both 

 directions is the same. Hence 

 the level of the water in the tube 

 remains unchanged. 



Fig. 48. Diagram to illustrate 

 osmosis: A represents a sugar 

 molecule, B a water molecule, 

 and C a differentially permeable 

 membrane. The sugar in solu- 

 tion dilutes the water so that 

 fewer water molecules are in 

 contact with the inside than with 

 the outside of the membrane. 

 Hence water passes in more 

 rapidly than it passes out, and 

 the level of the water in the tube 

 rises. 



tube. Consequently the water is more concentrated outside the 

 tube than inside, and in keeping with a general law of diffusion 

 the water passes from the place of greater concentration to the 

 place of less concentration. 



Moreover, sugar is a highly soluble substance ; that is, it has a 

 great affinity for water, and the sugar particles tend to hold the 

 water particles in contact with them inside the thistle tube. The 

 sugar, hke the water, tends to pass from the place of greatest 



