Permeability 207 
The Effect of Permeability of the Protoplasm to Solutes 
on the Water Relations of the Cell 
In considering the water relations of the cell on the simple osmotic 
view it has been assumed that the protoplasm is impermeable to the 
solutes contained in the cell sap, and also to the solutes present in 
the external liquid. It is further assumed that the external medium 
exerts no influence on the permeability of the protoplasm. Where 
these assumptions do not hold our considerations have to be modified. 
In the first case let it be supposed that a proportion of the solutes 
in the cell sap are capable of passing through the protoplasm. These 
solutes will then diffuse through the protoplasm until the concentra¬ 
tions of each particular solute within and without the protoplasmic 
membrane are in equilibrium with one another. This will mean that 
the osmotic concentration of the cell sap is probably lessened so that 
in consequence the cell would take up less water, or lose more, than 
it would if the protoplasm were completely impermeable to all the 
solutes in the vacuole. On the other hand, if there are substances in 
the liquid external to the cell which can penetrate the protoplasm, 
the cell, when equilibrium is reached, will contain more water than 
it otherwise would, since the osmotic concentration is raised. Prob¬ 
ably in most cases both phenomena occur, although the former may 
take place to a very limited extent. Some substances, such as ethyl 
alcohol, ether, acetone and chloral hydrate, may penetrate the proto¬ 
plasm so rapidly that plasmolysis is not produced by hypertonic 
solutions (Overton, 1895). Should the substance external to the cell 
influence the permeability of the protoplasm, as, for instance, by 
adsorption or chemical action, so that less or more solute passes 
through the protoplasmic membrane than otherwise would, the water 
relations of the cell will be correspondingly altered. 
The Effect of Imperfect Permeability of the Cell Wall 
to Solutes on the Water Relations of the Cell 
It is generally assumed on the simple osmotic view of the cell 
that the cellulose cell wall is completely permeable to most dissolved 
substances, at any rate, such crystalloidal substances as acids, 
alkalies, salts and sugars. Apart from the well-known cases of semi- 
permeable cell walls which have been dealt with in Chapter VII, it 
is by no means certain that cellulose walls are always as permeable 
to dissolved substances as to water. Indications of an imperfect 
permeability of the cell walls for sugar molecules were observed 
by Wachter (1905) in beet root and onion bulb scales and by 
