Relation of PIasmolysis to Shrinkage of Plant Tissue. 47 
remains uninjured by comparatively strong solutions (0-25 N) of 
sodium chloride fora surprisingly long time. 
A second point we wish to emphasize is that the conditions in 
a tissue in regard to tissue tensions and water relations must be 
exceeding complex. To enter into a discussion of these questions is 
outside the scope of this note, but we may refer for example to tbe 
discussion of this question by Pfeffer, 1 in which the complexity of 
the matter is made very clear. While we may agree with Thoday 
that, “ in all applications of physical and chemical principles to the 
analysis of complex physiological phenomena a clear understanding 
of their action in simple cases is an essential preliminary to the 
consideration of observed complications or discrepancies,” 2 it 
seems to us Thoday makes this an excuse for attempting to 
explain phenomena which are actually complex on the hypothesis 
that they are simple. Thus Thoday attempts to explain the water 
relations of the cells of a tissue in terms of osmotic pressure and 
the tension of the cell wall. How little further this takes us is 
obvious when we realise that the two quantities are not indepen¬ 
dent of one another, and that the tension of the cell wall must 
depend on a number of factors including the modulus of elasticity 
of the cell wall, the permeability of the cell wall to water, the 
osmotic pressure of the ceil sap, etc. We do not know how far 
the permeability of the cell wall influences the modulus of elasticity 
and what factors influence the permeability. We know little 
enough about the elasticity of the cell wall. In considering the 
water relations between different cells of the same tissue Thoday 
assumes that the colloidal properties of the cell contents can be 
neglected. This reduces the applicability of Thoday’s remarks to 
those tissues in which the protoplasm forms a negligible part of 
the volume of the cell, and also assumes in this case that the 
colloidal properties of the cell sap are negligible. It seems to us 
that while 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, it is going too far to accept the osmotic theory 
as a fact and to derive from it “important corollaries” not founded 
on fact but on the unproved theory, especially since we know the 
conditions are certainly more complex than the theory assumes. As 
we pointed out in our earlier paper, 3 mathematical treatment of the 
subject is possible and in the future will no doubt be helpful, but 
1 Physiology of Plants, English Edit., Oxford. Vol. 1, pp. 134 147, 1900, 
Vol. II, pp. 50-67, 1903. 
* New Phyt., 17, p. 109. 1918. 
s Ann. of Bot., 31, p. 431, 1917. 
