The microscope shows us in the plant, which was able to give to the naked eye only obscure signs of its inner life, a highly organized state life of restless development and renewal.
We have represented the citizen of this state, the plant-cell, as an exceedingly simply formed being: it consists of a round body of soft, slimy substance, like a sack, the interior of which is filled with a watery juice. The soft substance, forming the wall of the body, is called protoplasm; it is the most important matter in all nature, for it alone is the bearer of life. With slight changes it forms not only the bodies of all plant-cells, but also the white and yolk of the egg, flesh and blood, the substance of the brain and nerves, milk and cheese, the skin and hair of animals. While in lifeless nature nearly every kind of stone has a different chemical constitution, in the world of life one and the same fundamental substance forms the basis of the bodies of plants, animals, and men.
But if the plant-cell consisted simply of soft protoplasm, it would not be able to resist the presence and assault of strange bodies; therefore it is surrounded with a hard shell, which it prepares as a dwelling and for its protection, in a similar manner as the snail forms its shell, by secreting over its surface a matter which soon hardens into a firm, transparent envelope. This shell, which is called the cell-wall, does not show the most minute opening, but incloses the protoplasm perfectly tight. We might, therefore, liken the cell to an egg in which the soft, living contents are concealed in the hard shell.
Plant-cells vary greatly in size. Those of elder-pith and of the begonia-leaf may be perceived, with the naked eye, as resembling an extremely delicate lattice-work; the pollen of rye and of melons separates in water into little dust-particles—single cells, just at the limit of visibility. A drop of malt-yeast, on the other hand, is resolved under the microscope into millions of oval fungus-cells, one or two thousand of which would hardly fill the space of a centimetre. Plant-cells average about the size of a hair's-breadth, many only about a third or a fourth as much; others are larger, and particularly longer; the single fibers from which cotton and linen threads are spun are plant-cells which, although very slender, are from two to six centimetres long.
But in nature nothing is great and nothing little, and there is room, even in the smallest cell, for the greatest diversity and development of the powers of life. A continuous formation and transformation, origin and decay, a constant change of matter, is going on in every cell; reception and assimilation of food, inspiration and expiration; certain atoms which have become of no use for purposes of life are cast aside, others are taken up from without in their places; on this food and change of substance depend the renovation of the cell and the maintenance of its life. Evidently not solid substances are appropriated, for we know that the cell is incased in a perfectly closed
