174 



Science of Plant Life 



diffuse inward. We may say that the water is more con- 

 centrated outside the tube than inside, so water passes from 



Fig. ioo. Diagrams to illustrate 

 the passage of water through a 

 membrane : A , molecule of inside 

 water; B, molecule of outside 

 water; C, membrane. 



Fig. ioi. Diagram to illustrate 

 osmosis: A, sugar molecule; 

 B, water molecule; C, selectively 

 permeable membrane. 



the place of greater concentration to the place of less con- 

 centration. 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, like the water, tends to pass from 

 the place of greatest concentration but is restrained by the 

 membrane from moving outward. 



If we close the upper end of the thistle tube, the water will 

 continue to rise and compress the inclosed air. The pres- 

 sure developed under these conditions is called osmotic pres- 

 sure. If a large amount of sugar is put inside the tube, the 

 water will rise rapidly and exert great pressure. If only a 



