OSMOTIC PRESSURE IN ANIMALS AND PLANTS 563 



or molecules, that entrance itself being regulated by the 

 alterations in permeability of the protoplasmic membrane. 

 Changes in osmotic pressure are therefore an expression of 

 the net result of cell activities in altering the total number of 

 molecules and ions at large in the water it contains. 



Measurements of osmotic pressure may be made directly by 

 measuring the hydrostatic pressure developed inside a semi- 

 permeable membrane in contact with the solvent on the other 

 side, or they may be made indirectly by measurements of freez- 

 ing-point, boiling-point, or vapour pressure of the solution. 

 The connection is evident when one considers that all these 

 constants depend upon the molecular concentration of the 

 solution, and that the same amount of work must be done to 

 change the molecular concentration — and consequently the 

 vapour pressure — of a solution by a definite quantity, quite 

 irrespective of the means by which the alteration is effected. 

 It is therefore immaterial whether the concentration is carried 

 out by forcing a semi-permeable piston through the solution or 

 by separation of the solvent by freezing it out, or by boiling it 

 away. 



In physiological investigations two methods have been 

 employed in the main, that of plasmolysis — the balancing of the 

 internal osmotic pressure of the cell against the external osmotic 

 pressure of a non-poisonous solution of electrolytes or non- 

 electrolytes — and that of cryoscopy — the determination of the 

 freezing-point of the expressed contents of the cell. 



In the plasmolytic method of determining the osmotic pres- 

 sure of a plant cell a series of solutions of different concentrations 

 are brought into contact with the cell, until one is found which 

 just forces back the protoplasm from the cellulose wall against 

 which it had been pressed by the osmotic pressure, a hydrostatic 

 pressure, of the cell sap in the vacuole. The osmotic pressure 

 of the sap is then taken to be equal to that of this solution. 

 In reality this gives a slightly high value, as the external solu- 

 tion at first causes a shrinkage of the cellulose walls to their 

 normal dimensions, for they are somewhat distended by the 

 internal pressure. 



There are also a number of other sources of error due to the 

 fact that the protoplasmic membrane is but rarely completely 

 impermeable to the solute in the external medium, that at 

 different times this permeability may vary, and that the external 



