Page:Popular Science Monthly Volume 2.djvu/315

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length. Then he threw the solar spectrum by a prism on this little field. The plants inclined toward a common axis. Those exposed to the red, orange, yellow, and green rays, leaned toward the deep blue, while the part lighted by violet bent in the opposite direction. Thus the crop took the appearance of a wheat-field bowing under two contrary winds. The turnips placed in the violet-blue region looked toward the prism. Gardner thus determined, as Payer had done, that the most refrangible rays are those which effect the bending of the young stems. He proved also that the plants grow erect again in the dark.

These experiments, repeated and varied in many ways by Dutrochet and Guillemin, uniformly gave like results, but the phenomenon itself still remains almost unexplained. This remark also applies to the very singular facts of the twisting of running plants. The stems of these plants, in twining about their supports, usually curl from the left to the right. Others follow the contrary course, and some twist indifferently in either way. Charles Darwin inferred, from his investigation, that light has an effect on this phenomenon. If twining plants are put in a room near a window, the tip of their stalk takes longer to complete the half circuit during which it turns toward the darker part of the room than that which is described nearer the window. Thus one of them, having gone through a whole turn in five hours and twenty minutes, the half circle toward the window employed a little less than an hour, while the other was not traversed in less than four hours and a half. Duchartre placed some China yams in full vegetation in a garden, and others in a completely dark cellar. The stems of the plants uniformly lost in the dark the power of twisting around their supporting sticks. Those exposed to the sun presented one portion twisting, but when put in the cellar they shot out straight stems. Yet some twining plants are known that seem to be independent of light in twisting.

The sleep of plants, in connection with light particularly, is still less understood. The flowers and leaves of certain vegetables droop and wither at fixed hours. The corolla closes, and after quiet inaction the plant again expands. In others, the corolla drops and dies without closing. In others still, as the convolvulus, the closing of the flower occurs only once, and its sleep marks its death. Linnæus noted the hours of opening and shutting in certain plants, and thus arranged what has been called Flora's clock; but the relations of these closings, with the intensity of light have not yet been scientifically determined.

The green coloring of vegetable leaves and stems is due to a special substance called chlorophyll, which forms microscopic granulations contained in the cells which make up these stems and leaves. These grains are more or less numerous in every cell, and it is their number as well as intensity of color that determines the tint of the plant's tissues. Sometimes they are closely pressed together, covering