112 TEXTBOOK OF PLANT PHYSIOLOGY 



osmotic and gaseous pressures, these two pressures show certain 

 differences, the chief of which is that gases always develop a pres- 

 sure on the walls of the container enclosing them, while the osmotic 

 pressure of solutions 6hows itself only in case they are placed 

 in a container with a semi-permeable membrane separating the 

 solution from water. Osmotic pressure arises only when the solu- 

 tion and the solvent are separated by a semipermeable membrane 

 and, according to the determination of Findley, it is equivalent to 

 the hydrostatic pressure produced. 



The osmotic pressure produced when an osmometer is immersed 

 in water, is explained by the attraction between the molecules of 

 the solvent and those of the solute, determined by the influence of 

 the chemical affinity between them. Water enters the osmometer 

 under the influence of this affinity until the hydrostatic pressure 

 on the walls of the osmometer becomes equal to the osmotic 

 pressure of the solute. If the hydrostatic pressure acts upon a 

 mercury column, as in Pfeffer's osmometer the rise ceases when the 

 pressure of the mercury column balances the tendency of the solu- 

 tion to increase its volume by the absorption of water. The height 

 of the mercury column indicates the magnitude of osmotic pressure 

 which may be determined as the equivalent of the external pressure 

 to which the solution must be subjected to prevent the penetration 

 of the solvent through the membrane, i.e., to stop the increase in 

 the volume of the solution. 



36. Impermeability of Protoplasm and the Phenomenon of 

 Plasmolysis. The Osmotic Properties of Cell Sap and Methods 

 of Its Determination. — The easiest way of convincing oneself that 

 protoplasm is but slightly permeable, or is impermeable, to the sub- 

 stances dissolved in water, is by observing cells with colored cell 

 sap; for instance, the cells of the red beet root or the leaves of red 

 cabbage. While alive, these cells may be kept in water for many 

 days without losing their pigment. But, as soon as the structure 

 of the protoplasmic membrane is destroyed, for instance, by boiling 

 or even by heating at 60 to 70° C, or by the addition of some 

 poison, the colored sap diffuses into the surrounding water 

 through the cell walls. More exact analytical methods have shown 

 that other substances dissolved in water, as for example, the salts 

 and sugars, are not lost to any extent from the living cells. Other- 

 wise, the very existence of submerged plants would be impossible. 

 The phenomenon of impermeability of the protoplasm to sub- 



