OSMOTIC PRESSURE OF CELL CONTENTS 263 



Lowering of 

 freezing-point of 



Organism. tissue-fluids. 

 Ccelenterata : 



Alcyonium palmatum 2.196 



Echinodermata : 



Asteropecten aurantiacus 2.312 



Holothuria tubulosa 2.315 



Vermes: 



Sipunculus nudus 2.31 



Crustacea: 



Majasquinado 2.36 



H omarus vulgaris 2.29 



Cephalopoda : 



Octopus macropus 2.24 



With the enhanced specialization, therefore, which characterizes 

 the higher and especially the vertebrate forms of life, independence 

 of the external milieu has been acquired and the cells are bathed 

 in a medium of relatively constant concentration and, as we shall 

 see, of even more constant composition. 



THE OSMOTIC PRESSURE OF CELL-CONTENTS. 



The osmotic pressure of cell-contents can, of course, be determined 

 indirectly by expressing the cell-sap and determining its freezing- 

 point. In many cases, however, the measurement may be made in 

 a very much more convenient manner by employing the method of 

 Plasmolysis devised in 1884 by the Dutch botanist, de Vries. 



In many plants the protoplasm of the cells lies closely adherent to 

 the cellulose cell-wall, and it is found if these cells be immersed in 

 concentrated solutions of salts, sugars, urea or other diffusible sub- 

 stances, that the protoplasm shrinks away from the supporting wall 

 of cellulose, indicating that the protoplasm has diminished in volume. 

 This loss of volume can only be due to the abstraction of water from 

 the protoplasm, and since the agencies which accomplish this abstrac- 

 tion of water are solutions of relatively high osmotic pressures, we 

 infer that the external limiting membranes of the cells, within the 

 cellulose cell-wall but bounding the exterior of the protoplasmic con- 

 tents, is Semipermeable, admitting water but not admitting a variety 

 of diffusible dissolved substances. 



If this interpretation be the correct one, then any solution having 

 a higher osmotic pressure than the cell-fluids will cause plasmolysis, 

 while the solutions which are of just the same osmotic pressure as 

 the cellular fluid will fail to cause plasmolysis. The solutions which 

 just fail to cause plasmolysis, or which are Isotonic with the cell- 

 fluids, should therefore all be of the same molecular or molecular 

 plus ionic concentration, independently of the nature of the dissolved 

 substance, provided, only, that it is not able to penetrate the cell- 

 membrane in measurable proportion within the period of time con- 

 sumed by the shrinkage of the protoplasm. 



