2 i 8 Walter Stiles 
permeable membrane separating the external solution has more in 
favour of it. 
Most of the propounders of theories of cell permeability assume 
a semi-permeable plasma-membrane forming the external layer of 
the protoplasm in all cells, and if this were the case, it might perhaps 
be possible to find an explanation of cell permeability that would 
hold in all cases. But if this supposition is erroneous it is clear that 
we cannot expect a theory which explains the passage of substances 
from the outside of the cell through the protoplasm into the vacuole, 
necessarily to explain also the intake of the substance by the proto¬ 
plasm, that is, the membrane. 
To those who believe in a more or less semi-permeable plasma- 
membrane limiting the protoplasm externally, the three theories of 
membrane permeability described in Chapter v are obviously ap¬ 
plicable to the penetration of substances into the cell, and we do 
actually find that all three theories, the sieve theory, the solution 
theory and the chemical combination theory, have been put forward 
in some form or another as theories of cell permeability. But to those 
who do not accept the presence of such a plasma-membrane, these 
theories in their simple form obviously cannot account for the intake 
of substances by meristematic cells or excretion from them, while 
in the case of vacuolated cells they can at most only account for the 
passage of substances into the vacuole and not for their uptake by the 
protoplasm. This essential dependence of theories of cell permeability 
on belief or disbelief in the existence of a plasma-membrane of 
restricted permeability has not always been sufficiently emphasized 
in discussions on the mechanism of absorption by cells, even if it 
has been fully realised. 
Since some theories of cell permeability rely for some considerable 
measure of support on observations on the intake of dyes by living 
cells (“vital staining ,, ) it will be as well to preface a consideration 
of the individual theories with a brief summary of the main facts of 
dye intake. 
The work of Pfeffer and subsequent investigators suggested that 
while plant cells rapidly absorb basic dyes, acid dyes are in general 
absorbed not at all or only to a slight extent. Overton (1899, 1900), 
whose theory of permeability will be discussed later, came to the 
conclusion that only dyes soluble in lipoid substances enter the cell. 
Basic dyes are, however, on the whole, soluble in lipoid substances, 
while acid dyes are not. It was later found by Hober and Kempner 
(1908) and Hober and Chassin (1908) that many acid dyes are taken 
