20 SCIENCE PROGRESS. 



most important make use of the phenomena of plas- 

 molysis. It appears that the living protoplasm of a 

 vegetable cell behaves as a semi-permeable membrane. 

 It seems to allow water to freely pass through it, 

 whereas it retains the sugar and saline constituents of 

 the cell sap. If such a cell be immersed in water it 

 becomes turgid, and the cell wall may eventually be 

 ruptured, owing to the osmotic pressure of the cell sap 

 being set up within it. If the cell, on the other hand, 

 be immersed in a strong aqueous solution, the cell sap 

 loses water. The protoplasmic envelope which in the 

 normal condition is closely applied to the cell wall 

 contracts, and thus within the cell wall vacant spaces 

 are formed which become filled with transparent solution. 

 On using cells with coloured contents it is easy to 

 observe under the microscope when this entrance of 

 solution takes place, and, by having a series of solutions 

 of gradually diminishing concentrations, and immersing 

 in each preparations of living cells from the same plant, 

 it is possible to ascertain the strength of solution at 

 which entrance of solution within the cell wall or plas- 

 molysis just ceases to occur. This particular solution 

 will be isotonic with the cell sap, as neither gains or loses 

 water when separated by the semi-permeable protoplasm 

 of the living cell. Solutions of different substances which 

 are isotonic with the sap of cells from the same plant may 

 in this way be obtained, and which may, therefore, be 

 regarded as isotonic with one another. 



The normal concentration of the cell sap may be 

 altered by plasmolysing the cells in a standard solution 

 of known concentration. Solutions of other substances 

 may then be found in which the state of plasmolysis 

 remains unchanged, and these will evidently be isotonic 

 with the standard. Several series of solutions of differ- 

 ent substances, isotonic with different standards, may 

 also be obtained by using cells from plants of different 

 species. 



Tamman obtains isotonic solutions by allowing a drop, 

 say of copper sulphate solution, to fall into a solution of 



