its work away from the indigo-plant by artificially producing indigo. But a raw material which has at some time been brought forth out of the laboratory of a living plant-cell always lies at the foundation of all these manipulations of the chemist, wonderful as they are. And, notwithstanding the immense progress that modern chemistry has made within the last ten years, its art is still limited at this point: no prospect yet exists that it will be able, artificially, to produce the most important of all the substances that go to build up the bodies of animals and plants, and to form their living cell-tissues—protoplasm, or the envelope of the plant-cells, the matter of the muscles and nerves. Chemistry shares this limitation of its means with animals. No animal can live on air, water, and earth alone, like the plants; no animal can combine the simple chemical combinations, as they occur in lifeless nature, into the life-substance protoplasm. The animal must draw the substance of his flesh and blood from the plant, for his own vital forces are not competent to produce it. The plant-cells alone possess the faculty of ennobling the simple combinations of lifeless nature into matter fitted for life. Every cell, furthermore, possesses another art, that of forming different fabrics out of the same raw material. Hence arises that infinite diversity of substances of different properties which are drawn from the vegetable kingdom. Close together, in the shadow of the same wood, grow crow-foot and wood-ruff, centaury and nightshade; the same soil gives food to their roots, the same air plays around their foliage; and yet the cells of one secrete a pungent, those of another a narcotic poison, those of a third a bitter medicinal juice, those of a fourth an aromatic flavor. The cell utilizes a part of its food for its own growth; but, sooner or later, the growth ceases, and the cell, keeping the form and size it has acquired, becomes a permanent cell. It is round or oval, or resembles a many-sided crystal. Some cells become flat and square, like a tile; some put out rays, like a star, or form a zigzag, like the wall of a fortress; many lengthen themselves out. The inner structure, also, of the cell changes with age; the envelope, delicate and thin in youth, afterward receives accretions and ornaments. Some cells have within a hollow screw-way, like a winding stair; in others, the inside is covered with beautiful nettings, rings, flutings, or lattices. Most cells thicken their casings, as the oyster does, by adding new layers over the older ones; and, when their hollows are quite filled up, they may rival stones and bones in hardness, as, for example, the cells of the iron-wood and the ivory nut.
As the cell-wall grows thicker, fluids and gases penetrate its invisible pores with more difficulty; and with continuous increase of thickness the living protoplasmic bodies inhabiting its interior must finally die for want of food. They in effect build their own coffin, immure themselves living in their own cell-prison. But a wonderful provision prevents the food being entirely cut off. While the cell-wall is arch-
