Permeability 79 
Wodehouse (1917) on the concentration of various substances in the 
cell sap of Valonia (see Chapter xi). The explanation generally 
offered of this phenomenon is that the cell membranes are impermeable 
to the substance in question, so that it can exist in different concen¬ 
trations on the two sides of the membrane. This cannot be the ex¬ 
planation of Pfeffer’s results with dyes, where the accumulation of 
the dye in the cell sap is accounted for by chemical combination of 
the dye with a cell constituent to form a non-diosmosing compound. 
But some of those who have employed the plasmolytic and other 
methods have curiously enough neglected the possibility of such a 
complication in the intake of the substances they have examined. 
We must particularly notice in this connexion the opinion of 
Moore and Roaf (1908) and Moore, Roaf and Webster (1912), who 
rejected the theory of a membrane impermeable to crystalloids as 
the explanation of the permanent difference in concentration on the 
two sides of the membrane, and who held that the observed results 
could be adequately, and more satisfactorily, explained as due to 
adsorption of the penetrating substance by cell constituents or to 
chemical combination between the penetrating substance and some 
cell constituent or constituents in the same way as Pfeffer explained 
the intake of dyes. 
The absorption ratio. Actual determinations of the position of 
equilibrium attained in the penetration of dissolved substances are 
not numerous, and in practically no case is the method of determining 
it free from every objection. Such as they are, however, they do 
provide almost overwhelming evidence to the effect that in no 
observed case is the position of equilibrium in the intake of any 
substance necessarily one of equality of concentration in the external 
solution and the cell sap. 
Nathansohn (1903) made chemical analyses of the external 
solution and the expressed sap of the marine alga Codium after 
immersion for some days in solutions of sodium nitrate. Further 
experiments were carried out by the same author (1904 a , b) on the 
same lines with slices of tubers of Dahlia and Helianthus tuberosus 
and of the root of beet, a number of salts being used. Nathansohn’s 
observations were later extended by Meurer (1909). 
The relation between the final internal and external concen¬ 
trations can best be expressed by the ratio of the first quantity to 
the second. To this ratio, that of final internal to final external 
concentration, the name absorption ratio has been given by Stiles 
and Kidd (1919 a), who calculated the absorption ratios given by 
