Ig18| STILES & JORGENSEN—PERMEABILITY 527 
explanation of the facts. Indeed, DE Vries himself realized some- 
thing of the complexity of the system, for he lays emphasis on the 
presence of two membranes which function in permeability phe- 
nomena, the outermost layer of the protoplasm, the plasma mem- 
brane, and the layer separating the rest of the protoplasm from the 
vacuole, the vacuole wall. 
In the simplest case of a plant cell immersed in a solution we 
have four phases: the external solution, the cell wall, the protoplast, 
and the vacuole; and in addition there are the limiting layers 
between these various phases which may have properties differing 
from those of either phase. We may represent such a system by 
the following scheme: 
external solution cell wall | protoplast vacuole 
phase boundary phase boundary phase boundary 
(plasma membrane) (vacuole wall) 
Again, in plant tissue intercellular spaces may also affect the 
results of investigations. Obviously in dealing with such a complex 
system the term permeability used in regard to the cell should only 
be used as a general expression to cover the various phenomena 
concerned in the passage of substances between living tissue and the 
external medium or between cell and cell in the living organism. 
It is in this sense that we have used the term permeability in our 
series of papers on these questions in Annals of Botany; we do not 
mean the capacity of substances to pass through any one particular 
phase of the system. 
The permeability of living cells being then such a complex 
matter, it seems advisable not to use such expressions as “ permeabil- 
ity coefficient,” “measure of permeability,” and “temperature 
coefficient of permeability,” unless it is made clear what part of 
the system it is whose permeability is being considered. In our 
opinion the only legitimate use of such expressions is when they 
refer to the passage of substances into and out of the cell, or between 
one cell and another. Generally it is impossible by the methods of 
‘Cf. Prerrer (rr, p. 90): ‘‘In order to reach the cell sap a particle of water or 
dissolved substance must diosmose first through the cell wall and the plasmatic 
membrane which is closely applied to it, and finally pass through the internal limiting 
plasmatic membrane, which bounds the vacuole.” 
