Page:Popular Science Monthly Volume 45.djvu/545

Rh the word; for it is connected by continuity with all the heads of wheat that preceded it and with all those that will follow it. The important part is the seed, or the germ which it includes, continuing itself by a stem and a flower into another seed like it. The root, the straw, the glumes are accessories—all to be abandoned every year by the seed incessantly reviving of itself, which veritably incarnates the species wheat.

The molecular movement being at the very basis of life, to what extent does it regulate its manifestations? Does it make its influence felt only to maintain the external form or to exert a certain amount of command upon it? It does command it in effect, and all the external characteristics of the species and the individual appear to us definitely as subordinated to the conditions of their inner chemistry. Chevreul was the first who formulated the principle of the absolute dependence of life on the physicochemical laws of inert matter. The demonstration of it is furnished in the manure and fertilizers by means of which we succeed in prodigiously modifying the external appearance of the plant, to the point of rendering it almost unrecognizable. This sprout, in a dry, arid soil, is stunted, coriaceous, and hairy; that other one, from the same kind of seed, growing in the shade, on a soil constantly moist, is large, plump with water, soft and smooth. Without more knowledge, we should see in them two distinct species, if all the intermediate terms did not meet here and there on grounds half dry or half shaded, to show that we are simply dealing with two individuals of the same species, the molecular constitution of which is not absolutely identical because of the different conditions in which each one has lived.

It was long thought that the plant could choose by its roots the substances in the earth useful in its support and growth. This is not correct. The root, in contact with the extremely complex bodies which are continually formed and unformed in the soil around it, takes all those which the spongy terminal tissue of each radicle can dissolve. The plant is in this case only a reagent like any other; it is passive, and suffers itself to be penetrated by every substance, useful or injurious, in the quantity in which that substance is susceptible of mingling and combining with its superficial tissues. By virtue of the molecular constitution of the walls of the root, and especially of the extreme cells of their fibers, plants absorb particular mineral principles, and these principles in their turn, drawn into the vital molecular movement, favor it, impede it, or modify it in some way, and at last provoke a perceptible change in the aspect of the plant. This direct, immediate influence of molecular constitution on the forms of living beings appears to be more sharply marked in plants, but that is perhaps