Walter Stiles 
246 
in the slightest the soundness of the method or of their conclusions. 
Examples of this kind can be multiplied without difficulty. 
It is hoped that the correlation of work along many different lines 
attempted in this account of permeability will make it easier for the 
student to grasp the present position of our knowledge, and also 
make it easier for the investigator to avoid pursuing a line of attack 
which acquaintance with other work in the same field would show him 
could not further the advance of knowledge satisfactorily. 
From the review attempted in the preceding chapters it should 
be very evident how imperfect at present is our knowledge of the 
permeability of plant cells, and, moreover, how very doubtful is the 
significance of much of the experimental data on which our knowledge 
rests. It cannot then be surprising if a review of the current theories 
of cell permeability should lead to the conclusion that overwhelming 
evidence in favour of any one of them is not forthcoming. To this 
conclusion we are certainly led, and, indeed, it would seem rather 
unlikely in any case that all substances should enter or be excluded 
from the cell on account of the same mechanism, and ultrafiltration, 
solubility in lipoid substances and other constituents of the proto¬ 
plasm, adsorption and other surface effects, and chemical combination 
may all play a part in determining whether any particular substance 
is absorbed or not by the living cell. One point to be noticed in 
particular is that with all these theories of cell permeability the 
influence of the cell wall is usually neglected. Yet it seems to me, 
especially in view of the work on semi-permeable cell walls dealt with 
in an earlier chapter, and that of Hansteen-Cranner on the consti¬ 
tution of the cell wall, that this neglect is not wholly justifiable, and 
that to assume that the cell wall acts in no other way than as a dead 
porous envelope is scarcely in accord with the facts. 
In conclusion it should be emphasized how overloaded the whole 
subject of permeability is with theories, and with observations based 
on the assumption of the correctness of unproved theories. For this 
reason many of the conclusions drawn from their observations by 
various investigators are valueless. Not even the membrane theory 
of the cell, and the simple view of the plant cell as an osmotic cell 
surrounded by an elastic envelope, are really proved, although the 
latter, within limits, has formed a good working hypothesis in regard 
to the water relations of the cell. It clearly breaks down when used 
to interpret the relation of living cells to dissolved substances. The 
correctness of the theory of a semi-permeable membrane surrounding 
the protoplasm is also dubious (cf. Davidson, 1916). 
