*258 The Osmotic Hypothesis: a Rejoinder. 
and consequent increase of concentration that takes place. This 
is not inconsistent with the statement that when equilibrium is 
reached, involving plasmolysis, whether incipient or pronounced, the 
solution bathing the protoplast and the sap within the protoplast 
are isotonic (that is, of course, ignoring inward pressure due to 
viscosity and surface tension). But, as Pfeffer points out in the 
passage referred to, when cells with extensible walls are turgid the 
osmotic pressure of the sap can be determined by de Vries’ method 
only if the contraction in volume can he measured with sufficient 
accuracy and a correction applied for it. 
It should hardly be necessary to emphasise that in giving a 
definite value for the osmotic pressure of the sap of a cell it is 
necessary to define exactly the condition of the cell. In the treat¬ 
ment of the conditions of equilibrium in my paper it is made 
perfectly clear that reference is to the actual condition of the cell- 
sap at the moment, not to any previous or “ normal ” condition. 
As for “ attempting to explain phenomena which are actually 
complex on the hypothesis that they are simple,” I have merely, 
like Stiles and Jorgensen themselves, taken the osmotic hypothesis 
of Pfeffer and de Vries as a working hypothesis which as a first 
approximation, fits the facts. I do not think they would wish their 
own views to be judged on the principle that to adopt a simple 
working hypothesis is to declare the facts to he as simple as the 
hypothesis. 
We are admittedly ignorant of many things that it would be 
desirable to know, in this field as in others. Stiles and Jorgensen 
seem to suggest that because we “ know little enough about the 
elasticity of the cell-wall ” we are hardly justified in using the fact 
that it is elastic. The difference between us seems to be that 
whereas I frankly find the old working model still useful, they are 
looking beyond it to the complexities. I do not ignore the complex¬ 
ities ; but surely, if “ the osmotic theory of the cell may be used 
as a working hypothesis to explain observed facts while 
our knowledge is in its present incomplete state ” one should 
attempt to the best of one’s ability to understand the theory and its 
corollaries. 
And in what sense is the theory still hypothetical ? Do Stiles 
and Jorgensen doubt that the cell-wall is elastic and in varying degree 
extensible, or that the protoplasm is hardly permeable to the solutes 
of the cell-sap ? The osmotic hypothesis, so-called, is merely a 
working model framed on the basis of of these facts. It applies in 
