Permeability 
171 
Quantitative Relations of the Cell in regard 
to Water on the Simple “Osmotic” View 
Omitting from our consideration the complications introduced if 
the protoplasm is in fact separated from cell wall and vacuole by 
semi-permeable membranes, which, in the absence of definite evi¬ 
dence, we are at least partly justified in doing, it is clear from what 
has been said in preceding chapters that in a full consideration of 
the permeability of the vacuolated plant cell to water we have to 
take into account the water relations of three distinct phases: the 
cell wall, the protoplasm and the vacuole. 
It is generally assumed that the cell wall is completely permeable 
both to water and substances dissolved in the water, while it appears 
to be accepted that the protoplasm is readily permeable to water, 
but is impermeable or permeable in varying degrees to dissolved 
substances. The only part the cell wall is supposed to play in the 
water relations of the cell is to act as an elastic covering as described 
above, while the protoplast acts as a semi-permeable membrane en¬ 
closing the solution of salts and other crystalloids in the vacuole. 
This view of the cell I would speak of as the simple osmotic view, 
for it regards the living cell in its water relations as a simple osmotic 
cell. From what has been said in earlier chapters of the constitution 
of the protoplasm and cell wall it is obviously a considerable simpli¬ 
fication of the actual state of affairs, neglecting as it does absorption 
of water and salts by the cell wall and protoplasm, with consequent 
changes in properties of these essential parts of the system, together 
with all electrical effects such as negative osmosis. Nevertheless the 
simple osmotic view is the one generally taken in discussions on the 
water relations of the plant cell, the reason for this being no doubt 
partly a natural preference for the simple rather than for the com¬ 
plex, partly to the beautiful work of de Vries (1884 a, 1885) which 
did so much to render the view popular, but no doubt also because 
in many cases the assumptions involved do not introduce errors of 
any considerable magnitude. This will most probably be the case 
where the cell wall is thin and the volume of the protoplast very 
small in comparison with that of the vacuole. 
The simplest case we can consider is that of the single isolated 
cell. This is to be regarded as a solution of crystalloids (the cell sap) 
surrounded by a semi-permeable membrane (the protoplast), itself 
surrounded by the elastic cell wall. In the discussion immediately 
following it is assumed that the protoplast is absolutely semi- 
12—2 
