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Walter Stiles 
From these experiments Lloyd thus comes to the conclusion that 
imbibition by the protoplasm rather than osmotic pressure is the 
dominant factor in growth. 
While in general the water relations of the non-vacuolated cell 
can be explained by assuming the presence of a semi-permeable 
layer limiting the protoplast, there is yet a considerable body of 
evidence and opinion indicating the importance of imbibition by the 
colloidal protoplasm in determining the uptake and excretion of 
water by the living cell. Yet it has been shown that change in volume 
of cells of the kidney (Siebeck, 1912) and muscle cells (Beutner, 1912 a) 
placed in solutions of a substance which does not enter the cell are 
those which are to be expected if the protoplast is surrounded by a 
Semi-permeable membrane. If the loss or gain in water by the cells 
were related solely to the imbibition properties of the protoplasm, 
a quite different relation should hold. 
The evidence in regard to the presence of a semi-permeable 
membrane limiting the protoplasm of the non-vacuolated cell is thus 
rather conflicting. The observations at present available are much 
too scanty for us to draw any definite conclusions. There seems, 
however, to be definite evidence that in some cases at any rate, 
imbibition by the protoplasm plays a very definite part in determining 
the intake of water into the cell. How far the presence of a semi- 
permeable membrane is necessary for an understanding of water 
intake into the cell has still to be made clear. 
Whatever may be the case with the non-vacuolated cell, there 
appears to be good reason for supposing that the intake of water by 
the vacuolated cell can be largely related to osmotic phenomena. 
Whether or not the outer layers of the protoplast are differentiated 
from the bulk of the protoplast so as to constitute semi-permeable 
membranes differing in permeability from the rest of the protoplast, 
it is reasonable to suppose that the essentially colloidal protoplast 
must itself act as a semi-permeable membrane between a liquid 
external to the cell and that contained in the vacuole. 
4. Evidence from the Facts of Selective Permeability 
and Allied Phenomena 
A great number of observations have been made which indicate 
that different substances penetrate into cells at very different rates and 
to very different extents. This behaviour of the cell is directly com¬ 
parable to the behaviour of the membranes considered in Chapter V, 
