184 THE POPULAR SCIENCE MONTHLY. 



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 open- 

 ings 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 con- 

 cealment 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 unwhole- 

 some 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 



