Permeability 205 
of substances of low molecular weight which can diffuse freely out 
of the cell. It is obvious that in such a case the change observed is 
not necessarily connected in any way with changes in permeability. 
Nevertheless, it seems clear that substances may pass with 
different degrees of difficulty into or out from the cell under different 
conditions, even when the differences of concentration within and 
without the cell are the same, and it may be convenient to speak of 
any such difference which can be brought about by change in con¬ 
ditions as due to a change in permeability, although it must be 
emphasized that the term permeability is then not used in a strict 
sense (cf. Chapter 1). Even then it is very questionable whether it 
is legitimate to speak of the permeability of the cell or of any part 
of it without reference to the substances concerned, for it is by no 
means certain that a change in permeability to one substance in¬ 
volves the same relative change in permeability to another substance 
(cf. Chapter v). In the following discussion the term “change of 
permeability” is used to indicate a change in the ease with which 
a substance will pass into or out from the cell unconnected with a 
change in the difference in osmotic pressure or of concentration 
within and without the cell. It should, however, be clearly recognised 
that in so doing the term permeability is used loosely and with no 
intention of accepting the point of view that the changes observed 
are entirely due to alterations in the permeability of a cell membrane. 
Such changes may be reversible or irreversible. The latter appear 
always to be in the direction of increased permeability, with exosmosis 
of dissolved substances and, in the case of the turgid cell, of water, 
ending in complete loss of turgidity and in death. Such irreversible 
changes are produced by high temperatures, freezing and subsequent 
thawing, and toxic substances. It is possible they may be also brought 
about by rapid withdrawal of water resulting from immersion of a 
cell in a strongly hypertonic solution. The course of water loss during 
such an irreversible change in permeability is illustrated by the curves 
in Fig. 11 in Chapter x, showing the change in water-content of 
potato tuber immersed in solutions of ethyl alcohol of various con¬ 
centrations. The exosmosis of electrolytes from the same tissue 
immersed in solutions of the same alcohol of various concentrations 
at a temperature of 20° C. is indicated by the curves in Fig. 16 which 
exhibit the increase in electrical conductivity (corrected for the 
presence of non-electrolytes) of 50 c.c. of external solution containing 
20 disks of potato tissue, 17 mm. in diameter and 175 mm. thick. 
A comparison of the two sets of curves shows that the same concen- 
