“A leaf is riddled with an infinite number of excessively minute orifices, each encircled by two lips which give it the appearance of a half-open mouth. They are called stomata. On a single leaf of the linden more than a million can be counted, but so small are they as to be quite invisible without a magnifying-glass. This picture shows you how they look under a microscope. Well, through these orifices the plant breathes, not pure air such as we breathe, but poisoned air, fatal to an animal but wholesome for a plant. It inhales through its myriads of millions of stomata the carbonic acid gas diffused through the atmosphere; it admits this gas into the inner substance of its leaves, and there, under the sun’s rays, a marvelous process follows. Stimulated by the light, the leaves operate upon the deadly gas and take from it all its carbon. They unburn (the word is not in the dictionary, more’s the pity, for it gives the right idea)—they unburn the burnt carbon, undo what combustion had done, separate the carbon from the air with which it is bound up; in a word, they decompose the carbonic acid gas. “And do not think it any easy thing to unburn a burnt substance, to restore to their original condition two substances united by fire. Scientists would need all the ingenious means and powerful drugs they possess to extract carbon from carbonic acid gas. This task, which would tax the utmost resources of the man of science, leaves accomplish noiselessly, without effort, even instantaneously, and with the sole requirement that they shall have the aid of the sun. “But if sunlight fails, the plant can do nothing with the carbonic acid gas, the chief item in its diet. It then pines away with hunger, shoots up as if in quest of the missing sunshine, while its bark and leaves turn pale and lose their green color. Finally it dies. This sickly state induced by the absence of light is called etiolation. It is artificially produced in gardening for the purpose of obtaining tenderer vegetables and of lessening or even entirely removing the too strong and unpleasant taste of some plants. In this way some salad greens are bound with a rush so that the heart, deprived of the sun’s rays, may become tender and white; and thus, too, celery is banked up and left to whiten, since otherwise its taste would be unbearable. If we cover grass with a tile or hide a plant under a pot turned upside down, we shall after a few days of this enforced darkness find the foliage all sickly and yellow.