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Page:Popular Science Monthly Volume 18.djvu/527

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DARWIN ON THE MOVEMENTS OF PLANTS.

chanical resistance, because it occurred when there was not pressure enough to produce it; and, besides, Sachs has shown that the growing part is more rigid than the part just above it, which should have yielded first to resistance. Moreover, objects that yield with the greatest ease will deflect a radicle. After various attempts to explain the phenomenon, Darwin was led to suspect that the tip was sensitive to contact, and that it transmitted an effect to the upper part of the radicle, so exciting it to bend away. Such a thing had never been suspected, although Sachs discovered that the radicle is sensitive a little above the apex, and bends, like a tendril, toward the touching object.

Full details are given of the experiments by which this suspicion was verified. We can only say, briefly, that "germinating beans were pinned hilum downward inside the well-moistened cork lids of glass vessels which were half filled with water, and the light excluded. When the protruding radicles were the tenth of an inch or more long, bits of card about one twentieth of an inch square, or bits of sand-paper, were affixed to the sloping sides of their tips by means of thick gum-water, which by itself had no effect. To avoid confusion from the bending known as Sachs's curvature, the bits were never put in front." That the reader may have a clear idea of the kind of movement excited by the bits of card, we give, Fig. 14, sketches of three beans thus treated, which show the gradations in the degree of curvature. Out of fifty-five beans experimented upon, fifty-two were considerably bent away from the object attached, and the remaining three seemed to become sickly. As the radicle of the pea

PSM V18 D527 Pisum sativum.jpg
Fig. 15.—Pisum Sativum: Deflection produced within twenty-four hours in the growth of vertically dependent radicles, by little squares of card affixed with shellac to one side of the apex: A, bent at right angles; B, hooked.

was found to be rather more sensitive at a point above the apex than that of the bean, he experimented with twenty-eight peas which had been soaked for twenty-four hours, and then left to germinate in damp sand. He tried them first with bits of card above the apex for Sachs's curvature, and thirteen of them bent toward the card, the greatest curvature being 62°. Bits of card were then fastened to one side of