2 
Walter Stiles 
even be difficult to discover whether the substance were permeable 
at all to living cells. For many substances which are harmless to 
cells when presented to them in low concentration have a toxic effect 
in stronger concentrations, and as a result of toxic action the per¬ 
meability of the cell may be greatly increased. It may therefore 
often be necessary to use substances in dilute solutions if permeability 
of living cells is to be examined, and if no marked accumulation of the 
substance in the cell sap takes place its appearance there maybe very 
difficult to recognise on account of the small quantity of it present. 
The necessity of distinguishing between intake or absorption of a 
substance by the cell and the permeability of the cell membranes 
to the substance has been rightly emphasized by recent writers 
(Hober and Nast, 1913; Brooks, 1917 a; Osterhout, 1917 d). 
It may again be emphasized in this place that the cell is a very 
complex system with regard to its permeability. A dissolved sub¬ 
stance in diffusing into the cell sap from the solution outside the cell 
has to pass through the cell wall and the protoplasm and possibly 
through plasmatic membranes on the inner and outer surfaces of the 
protoplasm which differ in permeability properties from the bulk of 
the protoplasm (cf. Stiles and Jorgensen, 1918). It is very generally 
impossible to distinguish between these different phases in attempt¬ 
ing an analysis of permeability phenomena, so that in the following 
where the permeability of the cell or of the cell membranes is men¬ 
tioned it means the permeability of the whole series of cell membranes 
which separate a liquid external to the cell from the cell sap in the 
vacuole. 
It should be perfectly clear from what has been said of the per¬ 
meability of membranes in Chapter V, that we may expect the per¬ 
meability of the same cell to differ exceedingly towards different 
solutes. We may also expect that the order of permeability to a 
series of substances may differ in the case of different cells. Also as 
cells differ so much in their structure and properties we may expect 
that methods applicable for the determination of the permeability 
of some kinds of cells may not be applicable in the case of others. 
This is indeed the case, as will be very evident from a consideration 
of the various methods that have been evolved for the determination 
of permeability. These methods are described below. 
