ary within wide limits
and whose reactions are responsive to the usual phenomena incident to the
colloidal condition of matter.

According to the latter conception, vital activities of cell protoplasm may
be due to changes in water content, to electrical disturbances, to the
phenomena resulting from the conditions brought about by surface boundaries
between the different phases of the gel, to varying osmotic pressure, to
changes in chemical reaction, etc., and may be controlled by various
stimuli of chemical, physical, or mechanical nature. This conception seems,
therefore, to fit most closely the actual conditions under which the
protoplasm exists and carries on its vital functions.

With this conception in mind, we may now proceed to a consideration of how
the various components of the complex organic colloidal system, and their
specific properties, can affect its chemical activities.

The components of the system are, of course, water, salts, and the various
organic compounds (fats, proteins, carbohydrates, and enzymes in all cells;
and other groups, such as essential oils, tannins, pigments, etc., in cells
which have certain special functions to perform) which constitute the solid
phase of the colloidal mixture. In addition to the definite chemical
properties of each of these component groups, which have been studied in
detail in preceding chapters, there are many physical, or
physical-chemical, properties of the system as a whole, and of its
component parts, which are of the utmost importance in the physiological
activities of the protoplasm. These we may now proceed to consider in some
detail.

WATER

Water constitutes the largest proportion of the weight of active
protoplasm. In living cell contents (except those of such bodies as resting
seeds, etc.), water comprises from 70 to 95 per cent of the total weight of
the substance; the average proportion being usually between 85 and 90 per
cent. The fact that protoplasmic material can exist in turgid form with
such high percentages of water as these is due, as has been pointed out, to
its existence as a colloidal gel. It is because of this condition that
increases in the proportion of water generally increase the turgidity, or
turgor, of the protoplasm; instead of, as in all other cases, rendering the
mixture less solid and more labile. Losses of water from the protoplasmic
gel decrease its "swollen" condition and so render th