CHAPTER VI.

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.