Permeability 177 
In the case of a cell without a wall in which it is assumed that the 
resistance to stretching offered by the protoplast is negligible, a cell 
immersed in a solution isotonic with the cell sap will undergo no 
change in volume, while in a hypertonic solution contraction, and in 
a hypotonic solution increase in volume of the cell will take place 
until the osmotic pressure of the liquid on the two sides of the proto¬ 
plasmic membrane is the same. 
In the case of turgid cells we have already noted that the suction 
pressure is less than the difference in the osmotic pressures by the 
turgor pressure, so that when, after immersion in a solution, no change 
in volume occurs in a turgid cell, the external solution is hypotonic, 
the tendency for water to enter the cell on account of the difference 
in osmotic pressure, being just balanced by the turgor pressure tend¬ 
ing to force water out. 
If now, a turgid cell is immersed in a solution of such a strength 
that a contraction of the protoplast away from the cell wall is just, 
but only just, brought about, the osmotic pressure of the solution 
external to the cell will be equal to that of the cell sap at the moment 
when the turgidity of the cell just disappears. It will be observed 
that the osmotic pressure will be somewhat greater than that of the 
turgid cell, as the latter has a somewhat greater volume than the 
cell at the commencement of plasmolysis, and consequently, as the 
total amount of solute in the cell remains unaltered, the osmotic 
pressure of the liquid in the vacuole must increase as the vacuole 
contracts. 
If the concentrations of a number of different substances which 
just bring about plasmolysis of similar cells are determined, these 
different solutions should have the same osmotic pressure, and are 
therefore described as isosmotic. De Vries (1884 a) regarded such 
solutions as having the same attraction for water, so that the attrac¬ 
tion of a molecule of any one substance for water would be inversely 
proportional to the concentration of its isosmotic solution. By the 
term isotonic coefficient of a substance de Vries (1884 a, 1888, 1889) 
signified the magnitude of the attraction of a molecule of the body 
in dilute aqueous solution for water. As unit he took one-third the 
attraction of a molecule of potassium nitrate. 
De Vries concluded that the isotonic coefficient in strongly diluted 
solutions is a constant for every compound, the constant depending 
in a certain w r ay on the composition of the compound and being 
expressible approximately by a simple number. The isotonic co¬ 
efficients found by de Vries are given in the following table. 
