Permeability 209 
It is generally supposed that all living cells in their normal 
condition undergo changes in regard to the ease with which sub¬ 
stances pass into and out from them. These are sometimes spoken of as 
functional (Bayliss, 1915) changes in permeability. As these changes 
are reversible they can obviously take place in either direction, that 
is, so as to increase or diminish the capacity of substances to pass into 
or out from the cell. In the former case, if the action is prolonged, 
we may expect the change to become ultimately irreversible. 
Factors which may bring about changes in permeability are 
those of the environment of the cell, namely, temperature, light and 
the composition of the external medium. The changes produced by 
various factors will now be considered. 
Effect of Temperature on Permeability 
The influence of temperature on the rate of intake and exosmosis 
of water by plant cells, and on the permeability of cells to water, has 
already been considered in Chapter x, and the influence of tempera¬ 
ture on the rate of intake of dissolved substances has been discussed 
in Chapter xn. The effect of temperature over the range o° C. to 
30° C. is to increase the rate of intake of both water and dissolved 
substances, although the magnitude of the temperature effect varies 
considerably. A good case can be made out for regarding the change 
in the rate of intake of water into the vacuolated cell with change 
in temperature as due to a change in permeability of the protoplasm. 
That the same is the case with dissolved substances is not in any way 
evident. Within the range of temperatures indicated, changes related 
to changes of temperature are certainly reversible, even if the con¬ 
ditions are continued over comparatively long periods of time, but 
if the temperature rises too high, or if the tissues are allowed to 
freeze, the tissues or cells quickly disorganise owing to other changes 
which as far as we know may have nothing to do with permeability. 
Effect of Light on Permeability 
The influence of light on the intake and excretion of salts has 
already been dealt with in the preceding chapter. Such changes in 
relation to light appear undoubtedly to be reversible. Here again 
they can only be spoken of as changes in permeability if the latter 
term is used in a loose sense. 
Haupt (1902) concluded that light influences the permeability of 
the cells of extra-floral nectaries in Euphorbia and Vida, He found 
excretion of sugar only took place from the cells when illuminated, 
Phyt. XXII. 4. 
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