f plants; but they are believed to represent the best which
is now known of the physical chemistry of the plant-cell activities.

HETEROGENEOUS STRUCTURE OF THE CELL

Examination of cell protoplasm under the microscope reveals that it is not
a simple homogeneous mass. In the first place, it has a definite
structure, composed of (_a_) a nucleus; (_b_) numerous granular bodies of
different sizes and kinds; and (_c_) a clear mass of colloidal material,
which (if observed under the ultramicroscope, or photographed by
ultra-violet light) is apparently made up of very minute particles of many
different types of materials; the whole mass, in the case of plant
protoplasm, being generally surrounded by (_d_) a differentiated layer
known as the cell-wall. The actual internal structural arrangement of the
clear colloidal mass is uncertain; but its properties indicate that it may
be considered to be like a mass of foam (resembling a compact mass of
soap-bubbles) the compartments of the foam being, of course, very minute
and the films themselves almost infinitely thin, the contents of each
compartment being probably liquid, and the whole composing a typical
colloidal gel of complex composition.

This conception may not be accurate in every detail, but it seems to fit
very closely the conditions and reactions of cell protoplasm. Furthermore,
it is obvious that the definite structure, or form, of the cell is
essential to its life; since, if the structure be destroyed by any kind of
mechanical injury (freezing of the cell contents, resulting in the
puncturing of the membranes by ice crystals; rupturing of the films, or
cell-walls, by grinding with sharp sand, etc.) so as to bring about an
intermingling of the parts which are segregated from each other in the
organized structure, there results an immediate exhibition of abnormal
chemical actions, accompanied by the liberation of carbon dioxide, and the
death of the cell.

A proper mental picture of the organization of the cell structure and of
the interrelation of all its working parts is suggested by the figure of a
well-organized chemical factory, with the different chemical
transformations which are involved in the whole process being carried on in
different portions, or rooms, of the factory, with the various intermediate
and final products regularly and systematically transported from one room
to another as they are needed to keep each individual step in the