envelope; but liquid and gaseous foods can be easily absorbed. Although the most perfect microscopes have never made any holes visible in the cell-envelope, there is not the slightest doubt that this envelope is porous, like a fungus, but that the pores are infinitely finer. Therefore, we may understand that, when a cell is placed in a fluid, the envelop absorbs it to fullness, and conveys to the inner protoplasmic body as much as it requires; and, inversely, certain parts of the cell-juices, which the living protoplasmic body does not need for itself, are transpired through the pores of the envelope and become applicable to the use of other cells; and the same may take place with air and gases.
The old naturalists believed that all bodies were composed of four elements—fire, water, air, and earth. Modern physics and chemistry have divested these elements of their high importance; but they are still full of meaning to the life of plants. Earth, air, and water are the food of plants; fire, or rather light and heat, are the forces that set agoing the play of life in the cells. The most important food of plants is contained in the mineral solutions which the water, penetrating the soil, extracts from it, and in the oxygen and carbonic acid which they derive from the air.
Water, earth salts, and the gases—the raw materials which the plants suck up—are changed within the cells into starch and sugar, gum and woody fiber., albumen and wax, oil and resin, into powerful medicines and deadly poisons. The simplest plant possesses an art which the most skillful chemist has not been able to learn from it. It is true that the chemist can artificially prepare in his laboratory many of the substances which the plant-cell likewise produces; he can convert the starch of the potato into the sugar that gives the wine-grape its sweetness; this, again, he can transform into the fruit-acids which, in connection with the sugar, give the berries their fresh and agreeable taste; he can even produce the flavor of the fruits from the fusel-oil which he obtains by the fermentation of the sugar. He can make the oil of bitter almonds from benzoic and formic acids; he can, with as good art, imitate the pungent taste of the pepper, and the biting one of the mustard-seed, and the narcotic poison which only the nightshade has hitherto prepared for the healing of sore eyes. He can produce from the sap of firs the crystal-needles of the vanilla, for which a Mexican orchid has heretofore been obliged to give up its pods; from the distillation of wood he obtains a smoky fluid, from which he procures salicylic acid, for the production of which the flowers of the meadow-sweet or the bark-tissues of the willow were formerly required; and from this he makes also the ink-coloring gallic acid, which formerly only a little wasp knew how to draw out by its sting from the cells of the oak, and the aroma of the wood-ruff. He has made the work of the cells in the madder-root superfluous, for he has fabricated its costly dyes, along with a hundred other splendid pigments, out of tar-oil and stone-coal; and is now on the point of taking
