Permeability 217 
it does not follow that such exosmosis is due to increased permeability 
of the cell membranes to the solutes of the cell. It may, for instance, 
result from a breaking down of complex and indiffusible substances 
into simpler and diffusible ones. Similarly, the so-called reversible 
changes in permeability may be attributed to reversible changes in 
molecular associations rather than to changes in the permeability of 
the cell membranes. Which is the correct view cannot be decided in 
the present state of our analysis; it can only be determined by further 
work. 
CHAPTER XIV 
THEORIES OF CELL PERMEABILITY 
I T will be clear from the review in the preceding chapters of the data 
at present available with regard to the permeability of plant cells, 
that our information is far from complete, and, as far as quantitative 
data are concerned, fragmentary. Nevertheless, in spite of the in¬ 
sufficiency of information, there is no shortage of theories of cell 
permeability. It is one of the weaknesses of the theories of cell per¬ 
meability that they are for the most part not even based on the whole 
of the scanty information available, but only on one particular set 
of observations which fit the particular theory. Under these circum¬ 
stances perhaps none of the theories are of any great value, and no 
attempt will be made here to summarise in any detail all the theories 
that have been put forward to account for certain facts of cell intake 
or excretion. Those theories will chiefly be considered which have come 
into prominence during recent years, and which, in the opinion of 
the writer, may be perhaps of some use, in that they may stimulate 
to further work. 
Two questions which have been discussed in earlier chapters are 
obviously of first importance in regard to the mechanism of perme¬ 
ability in living cells. These are (1) the purely physico-chemical 
question of the permeability of membranes discussed in Chapter v, 
and (2) the plasma-membrane discussed in Chapter vm. From the 
discussion on the plasma-membrane it must be admitted that the 
evidence for the presence of a membrane which acts as a semi- 
permeable membrane separating the bulk of the protoplasm from 
the external solution (supposed to be present as such in the cell wall) 
is not by any means convincing, although to regard the whole body 
of the protoplasm in the vacuolated cell as a more or less semi- 
