Turgesce?ice and the Absorption of Water. 11 i 
no turgor pressure .' This is the condition which was used by 
de Vries in his plasmolytic method of determining isotonic 
coefficients; for he determined the strength of solution which just 
began to plasmolyse the cells, e.g., of the coloured epidermis of 
Tradescnntia discolor. 
The relations between the various factors are represented in 
graphic form in the accompanying diagram, in which the osmotic 
pressure of the sap, the water-absorbing power of the cell and the 
turgor pressure are plotted as ordinates against the volume of the 
protoplast, which diminishes from left to right, or the “ saturation 
deficit ” which increases from zero in the same direction. 
The assumption has been made that, as for most elastic 
materials, the extension of the cell wall is proportional to the dis¬ 
tending force, so that the increase of volume is also approximately 
proportional to the increment of turgor pressure from the point at 
which the protoplast begins to distend the wall. As the volume 
increases (from right to left in the diagram) the concentration of 
the sap will diminish, and consequently also its osmotic pressure: 
these changes will be more or less considerable according to the 
extensibility of the wall. 
Applications of the conception of water-absorbing power. 
Some important corollaries which are by no means generally 
realised may now be considered. 
(1) Condition of equilibrium between adjacent cells. It is evident 
that the power of one cell to withdraw water from another cell in 
contact with it does not depend directly or solely upon the 
osmotic pressure of its sap in relation to that of the other, but 
upon their relative water-absorbing power. As the condition of 
• Cf. Renner, Flora, 103, 1911, pp. 239-241. 
