CHAPTER II. 



ABSORPTION, DIFFUSION, OSMOSE. 



29. We may next endeavor to learn how plants absorb 

 water or nutrient substances in solution. There are several 

 very instructive experiments, which can be easily performed, 

 and here again some of the lower plants will be found useful. 



30. Osmose in spirogyra. — Let us mount a few threads of 

 this plant in water for microscopic examination, and then draw 

 under the cover glass a five per cent solution of ordinary table 

 salt (NaCl) with the aid of filter paper. We shall soon see 

 that the result is similar to that which was obtained when glycer- 

 ine was used to extract the water from the cell-sap, and to con- 

 tract the protoplasmic membrane from the cell wall. But the 

 process goes on evenly and the plant is not injured. The proto- 

 plasmic layer contracts slowly from the cell wall, and the move- 

 ment of the membrane can be watched by looking through the 

 microscope. The membrane contracts in such a way that all 

 the contents of the cell are finally collected into a rounded or 

 oval mass which occupies the center of the cell. 



If we now add fresh water and draw off the salt solution, 

 we can see the protoplasmic membrane expand again, or move 

 out in all directions, and occupy its former position against the 

 inner surface of the cell wall. This would indicate that there is 

 some pressure from within while this process of absorption is 

 going on, which causes the membrane to move out against the 

 cell wall. 



The salt solution draws water from the cell-sap. There 

 is thus a tendency to form a vacuum in the cell, and the 

 pressure on the outside of the protoplasmic membrane causes it 



13 



