PHYSICOCHEMICAL ORGANIZATION OF THE PLANT 17 



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 increase in size, the protoplasm distends and becomes 

 appressed to the latter (Fig. 3). This condition is obtained when 

 the elastic tube of an automobile tire transmits the pressure of 

 the air pumped into it to the more solid outer casing. As a result 

 of this pressure, the wall of the cell is in 

 a rigid condition known as 'Hurgidity," 

 and the volume of the cell is slightly 

 increased. 



If a cell is put into a concentrated 

 solution of a substance, then owing to 

 the impermeability of protoplasm, this 

 substance will not penetrate into the cell 

 sap but will remove water from it. The 

 cell wall being permeable, the solution 

 readily passes through it. Under the 

 influence of the attraction, water escapes 

 from the cell sap into the outer solution, 

 and the plasma sac decreases in volume. 

 If the total molecular concentration of 

 the external solution is higher than the 

 total molecular concentration of the cell 

 sap, the pressure of the external solution 

 directed inward exceeds the pressure of the cell sap directed out- 

 ward. Consequently, turgor pressure disappears, and the cell 

 decreases in volume. Moreover, the protoplasmic sac withdraws 

 somewhat from the cell wall, which, with lessened internal 

 pressure, contracts slightly. 



The phenomenon of plasmolysis is of importance in many 

 ways. In the first place, it shows a difference in the osmotic 

 properties of the protoplasm and the cell wall. Second, it may 

 frequently be used to indicate whether the cell under investi- 

 gation is alive or not, because impermeability is peculiar only to 

 living cells. Finally, plasmolysis enables one to determine with 

 great accuracy the magnitude of the osmotic pressure within 

 every living cell. 



A method of determining osmotic pressure in cells was devised 

 by De Vries as early as 1884. It is based on the following con- 



FiG. 6.- — Successive stages 

 of plasmolysis in cells of a 

 moss leaf {after Molisch). 



