IMPORTANCE OF WATER 



113 



stances dissolved in water may be detected also in another way. If 

 a cell, preferably with colored cell sap, is placed into a sufficiently 

 strong solution of some harmless substance, such as sugar or potas- 

 sium nitrate, the following results may be observed under the 

 microscope. First, the cell decreases somewhat in volume, after 

 which the protoplasm begins to retreat from the cell wall. If the 

 solution is very strong, the protoplasm will shrivel up into a ball- 

 like mass containing the highly concentrated cell sap (Fig. 45). 

 The space between the cell wall and the 

 protoplasmic sac will be filled with the 

 external solution, which has penetrated 

 through the wall. This phenomenon of 

 shrinkage of the protoplasm has been 

 called "plasmolysis." 



The different stages obtained in 

 watching the cell in water or in a 

 solution may be explained in the following 

 way: The protoplasm is readily per- 

 meable to water and impermeable to 

 substances dissolved in the cell sap. 

 When the cell is surrounded by pure 

 water, these substances attract the latter, 

 causing the protoplasmic sac to increase 

 its volume. As a consequence of this FlG 4 5._s U ccessive stages of 

 increase in size, the protoplasm distends piosmoiysis in cells of a moss 



, . ., 11 11 leaf (after Molisch). 



and becomes appressed to the ceil wall, 



transmitting its pressure to the latter (Fig. 42). This condition 

 is obtained when the elastic tube of a bicycle tire transmits the 

 pressure of the air pumped into it to the more solid outer rim. 

 As a result of this pressure, the wall of the cell is in a rigid con- 

 dition known as "turgidity," and the volume of the cell is slightly 

 increased. 



If a cell is put into a concentrated solution of any substance, 

 then, owing to the impermeability of protoplasm, this substance 

 will exert an osmotic pressure on the latter. The direction of this 

 pressure, however, will not be toward the outside of the cell, but 

 inward. The cell wall being permeable, the solution readily 

 passes through it. Under the influence of the pressure, water 

 escapes from the cell sap into the outer solution and the plasma sac 

 decreases in volume. If the total molecular concentration of the 



