Page:Popular Science Monthly Volume 2.djvu/313

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botanist, Prillieux, published the result of a course of experiments made with an entirely different purpose, and taking up the study of the action of light from a new point of view. Resting on the twofold consideration that the distinctly-colored rays are not equally luminous, and that those of the greatest illuminating power are also those which act with most energy on plants, Prillieux undertook to examine what influence will be exercised on plants by rays different in color, but known to be equal in intensity, and whether this influence differs in the case of different colors, or is the same, provided they do not vary in illuminating power. The long and conscientious researches of this experimenter led him to the conclusion, that rays of different colors act with equal force on the green parts of plants, and produce an equal release of gas, when they have the like luminous intensity. He holds that all luminous rays effect the reduction of carbonic acid by vegetables in proportion to their illuminating power, whatever their refrangibility may be. If the yellow and orange rays are more active in this respect, it is because their luminous glare is much greater than that of the extreme rays.

The luminous rays also promote the production of green tissue, the green matter of all vegetables. Gardeners blanch certain plants by raising them in the dark. They thus obtain plants of a pale yellow, spindling, without strength or crispness. They are attacked by a true chlorosis, and waste away, as if sprung from barren sand. The sun also aids the transpiration of plants, and the constant renewal of healthy moisture in their tissues. On failure of the evaporation of moisture, the plant tends to grow dropsical, and its leaves fall, from weakness of the stem.

This love of plants for light, which is one of the most imperious needs of their existence, displays itself also in other interesting phenomena, which show that solar rays are, in very truth, the fertilizer that produces color. The corolla of vegetable species growing at great heights on mountains has livelier colors than that of species that spring in low spots. The sun's rays, in fact, pass more easily through the clear atmosphere that bathes high summits. The hue of certain flowers even varies according to the altitude. Thus the corolla of the Antliyllis vulneraria shades down from white to pale red and vivid purple. In general, the vegetation of open, well-lighted places is richer in color and development than that of regions not accessible to the sun. Some flowers originally white afterward deepen in color by the direct action of light. Thus Cheiranthus cameleo has a flower at first whitish, afterward yellow, and, at last, a violet-red. The Hibiscus mutabilis bears a flower which opens at morning with a white hue, and grows red during the day. The flower-buds of the Agapanthus umbellatus are white when they begin to unclose, and afterward take on a blue tint. If, at the moment of leaving its spathe, the flower is wrapped in black paper, intercepting the light, it remains white, but regains its