Page:Popular Science Monthly Volume 18.djvu/526

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through radiation. The movements of various organs to the light, which are so general throughout the vegetable kingdom, and occasionally from the light, or transversely with respect to it, are all modified forms of circumnutation, as again are the equally prevalent movements of stems, etc., toward the zenith, and of roots toward the center of the earth. In accordance with these conclusions, a considerable difficulty in the way of evolution is in part removed, for it might be asked. How did all their diversified movements for the most different purposes first arise? As the case stands, we know that there is always movement in progress, and its amplitude or direction, or both, have only to be modified for the good of the plant, in relation with internal or external stimuli."

The discovery of the sensitiveness of the apex of the radicle was made by Darwin when he was looking for something else. Wishing to know how the radicles of seedlings passed over obstacles in the ground, he placed germinating beans in such a way that the tips of

PSM V18 D526 Vicia paba.jpg
Fig. 14.—Vicia faba: A, radicle beginning to bend from the attached little square of card; B, bent at a rectangle; C, bent into a circle or loop, with the tip beginning to bend downward through the action of geotropism.

the radicles came into contact with opposing surfaces at a high angle. When the root-cap touched such an obstacle it was at first a little flattened, but this flattening soon disappeared, and the apex took a direction at right angles to its former course. Straight lines had been painted along the growing terminal part of some of these radicles before they met the opposing objects, and the lines became sensibly curved in two hours after the apex had come into contact with them. The explanation of this curvature of the growing part could not be me-