D. Thoday, 
257 
THE “OSMOTIC HYPOTHESIS:” A REJOINDER. 
I N a recent number of this journal 1 exception has been taken to 
certain criticisms in a paper on “ Turgescence and the Absorp¬ 
tion of Water” 2 in which I drew attention to the need for a stricter 
use of terms in dealing with the water relations of cells and tissues 
and suggested the conception of water-absorbing power, which 
depends on the water-deficit of a cell or system and not solely on 
the osmotic strength of the cell-sap. 
Stiles and Jorgensen admit the assumption involved in 
their use of the term isotonic, viz. that the turgor pressure was 
negligible in the potato tissue used in their experiments. When, 
however, they claim that the implicit assumption should have been 
obvious, I must demur. On the contrary, it is surely a contradiction 
in terms to call a tissue turgescent in which the turgor pressure is 
practically nil. 
The word turgescent is, it is true, used loosely. The border¬ 
line between turgidity and flaccidity is not sharply defined. Appear¬ 
ance and rigidity change gradually as turgor is lost; and even when 
there is no turgor pressure deformation will bring the elasticity of the 
wall into play, so that the loss of rigidity is not complete. At the same 
time the term turgescent does imply the existence of turgor pressure, 
and the unqualified use of the term isotonic was calculated to 
mislead. 
Moreover, the assumption that turgor pressure is negligible 
must be justified by experiment for each kind of tissue used. Stiles 
and Jorgensen’s new experiments with beet-root indicate that the 
roots they used were in a condition bordering on flaccidity; but it 
does not follow that their potato tissue was in a comparable 
condition, nor that beet-root would always be in that condition. 
1 must confess that the point of some of their counter-criticism 
escapes me. 
The implication “that the solution which just brings about 
plasmolysis is to be regarded as isotonic with the cell-sap” does 
not involve neglect of the observation of Pfeffer that “ at the 
commencement of plasmolysis the osmotic pressure of the external 
fluid is slightly higher than that of the turgid cell.” It is clear 
from the context that Pfeffer is referring to the original turgid 
condition of a cell and had in mind the contraction of the vacuole 
1 Voi. XVIII, 1919, p. 40. 
* Ibid., Vol. XVII, 1918, p. 108. 
