Page:Popular Science Monthly Volume 22.djvu/196

184 vascular tissue, which are engaged, by means of well-trodden routes of communication, in supplying the most remote parts of the domain quickly and abundantly with food and raw material and in exporting the finished fabric. But a defenseless kingdom would be an easy prey to its enemies; therefore the cell-state maintains, but peaceably, and with no view to aggression, in the cells of its bark-tissues, a standing army, on which depends the defense of the whole realm at its borders. As Sparta believed it was most securely defended by the living walls of its citizens, so does the cell-state. The cells of the bark-tissue form a close cordon, through which no rain-drops, no hurtful gas-puff, no hostile animal, no disease-generating spore can penetrate. They wear a hard, siliceous armor, or are protected by an impermeable coating of wax. They have no other purpose, they do no other work, than in compact array to ward off hostile attacks. Single cells advance before the line and oppose attacks with sharp-cutting weapons, finely pointed briers or thorns, or weave themselves into intricate abatis, in which hostile insects become entangled by their feet. The points of many of these thorns are poisonous, as in the nettle, which, when touched by the hand, breaks off and remains in the skin, and fills the invisible wound with one of the strongest poisons known to nature and science.

The cells of the bark-tissues are locked so closely to each other that, like the members of a brave phalanx, they would be torn apart before they would separate from each other; and they can be separated from the other tissues only as a connected layer, a thin membrane that may be drawn off from all plants, and is known as the epidermis. This living cell-fortification is interrupted in many places by round openings like gates, which may be closed by a couple of cells as if with double doors, by the opening of which the access and egress of gases and vapors to and from the interior are permitted.

Thus the plant is protected from external enemies; but its most dangerous adversaries are the hungry members of its own kingdom. Not all plants are supported by peaceful labors; there is among them also a predatory horde, whose members, the parasitical plants, unfit for honorable occupation, and bearing the marks of their baseness in their pale color and offensive smell, lurk in the darkness and in concealment till they can find some victim to attack and overcome. Now is the strength of the living wall of the plant tested: as long as it is unbroken, the assault is repelled; but the persistent enemy presses into the smallest opening. Woe to the tree from which the wind has broken a limb, or in which the careless gardener has made a bad cut! The microbes, whose spores are floating through the air in unwholesome clouds, and fall with the dust, settle upon the wounded surface, and soon its whole cell-structure is pervaded by their destructive webs.

In peaceful times the other citizens of the cell-state attend to their