NUTRITION IN THE LIVING BEING AND IN THE CRYSTAL.
Assimilation and growth in the crystal.—Methods of growth
in the crystal and in the living being; intussusception;
apposition.—Secondary and unimportant character of the process of
intussusception.
I have already stated (Chap. VI. p. 209) that nutrition may be
considered as the most characteristic and essential property of living
beings. Such beings are in a state of continual exchange with the
surrounding medium. They assimilate and dissimilate. By assimilation
the substance of their being increases at the expense of the
surrounding alimentary material, which is rendered similar to that of
the being itself.
_Assimilation and Growth in the Crystal._—There exists in the crystal
a property analogous to nutrition, a kind of nutrility, which is
the rudiment of this fundamental property of living beings. The
development of a crystal starts from a primitive nucleus, the germ
of the crystalline individual that we will presently compare to
the ovum or embryo of a plant or an animal. Placed in a suitable
culture-medium—_i.e._, in a solution of the substance—this germ
develops. It assimilates the matter in solution, incorporates the
particles of it, and increases, preserving at the same time its form,
reproducing its specific type or a variety of it. Its growth proceeds
without interruption. The crystalline individual may attain quite a
large size if we know how to nourish it properly—we might say, to
fatten it. Very frequently, at a given time, a new particle of the
crystal serves in its turn as a primitive nucleus, and becomes the
point of departure for a new crystal engrafted upon the first.
Taken from its mother liquor, placed where it cannot be nourished,
the crystal, arrested in its growth, falls into a condition of rest
not without analogy to that of a seed or of a reviviscent animal. Its
evolution is resumed with the return of favourable conditions—the bath
of soluble matter.
The crystal is in a relation of continual exchange with the surrounding
medium which feeds it. These exchanges are regulated by the state of
this medium, or, more exactly, by the state of the liquid stratum
which is in immediate contact with the crystals. It loses or it gains
in substance if, for example, this layer becomes heated or cooled
more rapidly than the crystal. In a general way, it assimilates or
dissimilates according as its immediate environment is saturated or
diluted. Here, then, we have a kind of mobile equilibrium, comparable,
in some measure, to that of the living being.
_Methods of Growth of the Crystal and of the Living Being.
Intussusception. Apposition._—In truth, there seems to be a complete
opposition between the crystal and the living being as regards
their manner of nutrition and growth. In the one case the method
is intussusception; in the other it is apposition. The crystalline
individual is all surface. Its mass is impenetrable to the nutritive
materials. Since only the surface is accessible, the incorporation of
similar particles is possible only by external juxtaposition, and the
edifice increases only because a new layer of stones has been added to
those which were there before. On the contrary, the body of an animal
is a mass essentially penetrable. The cellular elements that compose
it have more or less rounded and flexible forms. Their contact is by
no means perfect. They have neither the stiffness nor the precision
of adjustment that the crystalline particles have. Liquids and gases
can insinuate themselves from without and circulate within the meshes
of this loose construction. Assimilation can therefore take place
throughout its whole depth, and the edifice increases because each
stone is itself increasing.
_The Secondary and Commonplace Character of the Process of
Intussusception._—The apparent opposition of these two processes is
doubtless diminished if we compare the simple mineral individual
with the elementary living unit, the crystalline particle with the
protoplasmic mass of a cell. Without carrying analysis so far as
this, it is yet easy to see that apposition and intussusception are
mechanical means that living beings employ at one and the same time and
combine according to their necessities. The hard parts of the internal
and external skeleton increase both by interposition and superposition,
at once. It is by the last method that bones increase in diameter,
and the shells of molluscs, the scales of reptiles and fishes, and
the testae of many radiate animals are formed. In these organs, as in
crystals, life and nutrition occur at the surface.
Apposition and intussusception are then secondary, mechanical
arrangements having relation to the physical characters of the
body—solidity in the crystal, semi-fluidity in the cellular protoplasm.
If we compare the inorganic liquid matter with the semi-fluid organized
matter, we recognize that the addition of substance takes place in
the same manner in each—_i.e._, by interposition. If we add a soluble
salt to a fluid, the molecules of the salt separate themselves and
interpose themselves between those of the fluid. There is, therefore,
nothing especially mysterious or particularly vital about the process
of intussusception. Applied to fluid protoplasm, it is merely the
diffusion that ordinarily occurs in mixed liquids.