lids into or out of
the cell; i.e., if the protoplasm acted as an ideally semi-permeable
membrane, only water could pass into or out of it. But we know that mineral
salts from the soil must pass into any cell before the synthesis of
proteins, etc., can proceed, and that the fats, carbohydrates, proteins,
etc., which are synthetized in vegetative cells pass from these to other
organs of the plant for use or storage. The obvious explanation for this
condition of things in the plant is that protoplasm (and, indeed, this is
equally true for practically all known membranes) is not absolutely
impermeable to dissolved crystalloids; or, in other words,
semi-permeability generally means only that the solvent passes through the
membrane more readily and more rapidly than do the dissolved materials in
it. Even colloidal materials will diffuse through most common membranes,
although at so slow a rate that the process is scarcely observable by
ordinary methods of study. Hence, the actual permeability of the protoplasm
permits the movement of both water and dissolved solids from one part of
the organism to another; but its approximation of semi-permeability
produces osmotic pressure and induces freer movement of water than of
dissolved substances, and so provides for turgidity of the cells and for
equalization of the water content of different portions of the protoplasmic
mass.

It is clear, therefore, that osmotic pressure plays an important part in
the physical mechanism of cell activities and in the regulation of the
proportion of water contained in the protoplasm, with its consequent
effects upon the chemical reactions which may go on in the cell.

Actual measurements of the osmotic pressure of plant cell have been made.
The results are more or less uncertain, because, as has been pointed out, a
plant cell is not a definite quantity of uniform protoplasm surrounded by
an ideal semi-permeable membrane, but is instead a mass of living matter
which is approximately semi-permeable throughout its entire volume and is
in a constantly changing condition because of the anabolic and catabolic
activities which are going on in it; but values have been obtained which
show a normal osmotic pressure as high as fourteen atmospheres in the cells
of very turgid plants, such as those of some of the green algae. Animal
cells probably have an osmotic pressure similar to that of the blood which
circulates around them, which is approximate that of sev