Walter Stiles 
170 
a number of gels and other colloids, the evidence that this will com¬ 
pletely account for the water relations of the non-vacuolated cell 
lacks definiteness. A considerable amount of quantitative work is 
necessary both on non-vacuolated plant cells and on colloidal systems 
before the question can be regarded as in any way settled. 
Similarly in the vacuolated cell, where the evidence that we have 
to do with a system constituting an osmotic cell is more conclusive, 
it is not at all clear that it is correct to regard the limiting layers of 
the protoplast as semi-permeable membranes separating the proto¬ 
plasm on the one side from an external solution contained in the 
cell wall, and on the inner side from the liquid in the vacuole. On 
the other hand, in the present state of our knowledge it does seem 
reasonable to suppose that the whole layer of protoplasm may act 
as a membrane separating vacuole and cell wall, and to some extent 
semi-permeable inasmuch as it may allow the ready passage of water, 
while varying greatly in its permeability to different substances dis¬ 
solved in the water, and even completely preventing the passage of 
some. 
Turgidity 
Whether on account of the swelling of the colloidal constituents 
of the cell, or because the cell behaves as a solution of crystalloids sur¬ 
rounded by a semi-permeable membrane, or on account of both these 
reasons, under most circumstances the cell will absorb or has absorbed 
water to such an extent, that it exerts a pressure on the cell wall, 
which being capable of extension, is stretched. But as the cell wall 
is elastic, that is, tends to return to its original form on being stretched, 
it exerts a pressure against its extension, and consequently against 
the increase in volume of the cell. The cell is therefore only capable 
of absorbing water until the inwardly directed pressure resulting 
from the stretched condition of the cell wall is equal to the 
pressure sending water into the cells. The rigidity of the cell 
resulting from this state of affairs has been not inaptly compared 
with that of an inflated bicycle tyre or football ( e.g . by Livingston, 
1903). It is spoken of as turgidity 1 , while the cell is said to be 
turgid. 
1 The term “ turgescence " is sometimes used to express the same condition 
of the cell (e.g. de Vries, 1874, and many later writers). The term “turgor" 
is sometimes used in the same sense (e.g. by Gager, 1916). This term is some¬ 
times used to express a definite quantity, but it is perhaps best to limit it to 
its qualitative sense. 
