Permeability 141 
For the sake of clearness therefore, the term plasma-membrane 
will be used to denote a surface layer of protoplasm which behaves 
as a membrane surrounding the bulk of the protoplasm, and which 
may exhibit different degrees of permeability to different substances, 
readily permitting the passage of some, less readily permeable to 
others, and to yet other substances quite impermeable. The mem¬ 
brane bounding the outside of the protoplast, and so in contact with 
the cell wall, will be called the external plasma-membrane, and that 
bounding the vacuole, the internal plasma-membrane. For the visibly 
distinct outer layer of the protoplasm the term hyaloplasm will be 
used. Of the existence of this in certain cells there is no doubt. 
What we are immediately concerned with here is the existence of the 
plasma-membrane. Should the evidence for the existence of the 
plasma-membrane be regarded as convincing, the question then 
arises of the identity of the plasma-membrane with the hyaloplasm. 
There are five main lines of evidence for the existence of plasma- 
membranes. These will now be considered. 
1. Evidence from the Physical Laws of Surfaces 
Surface phenomena have been discussed in some detail in 
Chapter III, where it has been pointed out that the surface of 
separation between two phases, such as we have at the boundary 
between the protoplast and the cell wall, and between the protoplast 
and the vacuole, has physical properties different from those of the 
bulk of either phase. It has been noted that one result of these 
different physical properties is that any substance present in solution 
which lowers the surface tension of the solvent will tend to ac¬ 
cumulate at the surface. Hence substances present in the protoplasm 
which lower the surface tension of water will be present in the surface 
layer in higher concentration than in the bulk of the protoplasm. 
That the protoplasm contains many such substances there can be 
no doubt, and the presence of these substances in greater concentra¬ 
tion in the surface layer than in the internal mass of the cytoplasm, 
might in itself account for a difference in permeability properties 
between the surface and the interior of the cytoplasm. But the cell 
wall and the vacuole no doubt also contain substances which lower 
the surface tension of water, and these will also tend to accumulate 
at the surface of the cell wall and vacuole respectively. Thus at the 
interfaces, cell wall-protoplast and protoplast-vacuole, there must be 
present thin layers of material which differ both physically and 
