Page:Popular Science Monthly Volume 22.djvu/194

182 itself up more closely and thickly, a few doors and windows are still left open in it, through which communication may still take place with the adjoining cells; this occurs by the cell-wall not becoming strengthened at particular points; and when, in the course of time, the shell has become still thicker, these places appear as pores or canals, which lead outwardly from the interior of the cell. And it is worthy of remark that at each point where such a canal penetrates the thickened cell-wall a corresponding passage is also left open in the next cell, so that the two canals meet each other, and are only separated by a thin partition. Communication continues uninterrupted by these pore canals.

The plant-cell is, nevertheless, subject to the fate of all life—it grows old and dies at last. It seldom survives a summer; toward the end of the fall its activity becomes weaker. Dissolution gradually overcomes the dead protoplasmic bodies, and only the empty cell wall is left, which may continue to exist as a vacant chamber for years and centuries after the living nucleus has perished. As a rule, the cell propagates itself before it dies; as an earth-worm may be divided into two parts, each of which will become an independent individual, so the parent-cell divides itself into two daughter-cells, which supply the place of the mother, and continue their life-activity with renewed vigor.

Such in its principal features is the economy of the plant-cell. It is fed by the absorption of fluid and gaseous foods; it elaborates those foods into the most diversified products; it respires; it strengthens and thickens its shell, yet in such a manner that it can continue in living intercourse with its neighbors; it propagates itself by splitting into daughter-cells; it grows old and dies. Let us now glance at the arrangements and laws according to which the cells act in organic connection as citizens of a single state. As there are wild bees that do not live together in a hive, as there are human tribes that wander around in the woods without organic connection, so there are plant cells that remain isolated during all their lives; they all perform in the same manner the business of their whole existence, which is highly primitive, and unadapted to perfection; their progeny does not continue in social connection, but separates into wholly free individuals. Such plants, which always consist of single cells, are called one-celled; they are found among the lowest forms of the microscopic world, among the algae and the fungoids. The green coating that covers the rocks, the tree-trunks, and the shingles of the roof, is resolved by the microscope into innumerable green round cells; the brown scum that floats upon ponds and ditches exposed to the sun, the yeast-plant, the bacteria that produce putrefaction, are one-celled plants of this kind.

Generally, however, the plant-cell is, like man, a social being, which finds its true calling only in state-life. In most growths, from that of