Archive image from page 426 of Darwin and modern science; essays. Darwin and modern science; essays in commemoration of the centenary of the birth of Charles Darwin and of the fiftieth anniversary of the publication of the Origin of species darwinmodernscie00sewa Year: 1909 Root-tip 395 less striking result. Although these researches confirmed Darwin's work on roots, much stress cannot be laid on them as there are several objections to them, and they are not easily repeated. The method which—as far as we can judge at present—seems likely to solve the problem of the root-tip is most ingenious
Archive image from page 426 of Darwin and modern science; essays. Darwin and modern science; essays in commemoration of the centenary of the birth of Charles Darwin and of the fiftieth anniversary of the publication of the Origin of species darwinmodernscie00sewa Year: 1909 Root-tip 395 less striking result. Although these researches confirmed Darwin's work on roots, much stress cannot be laid on them as there are several objections to them, and they are not easily repeated. The method which—as far as we can judge at present—seems likely to solve the problem of the root-tip is most ingenious and is due to Piccard1. Andrew Knight's celebrated experiment showed that roots react to centrifugal force precisely as they do to gravity. So that if a bean root is fixed to a wheel revolving rapidly on a horizontal axis, it tends to curve away from the centre in the line of a radius of the wheel. In ordinary demonstrations of Knight's experiment the seed is generally fixed so that the root is at right angles to a radius, and as far as convenient from the centre of rotation. Piccard's experiment is arranged differently. The root is oblique to the axis of rotation, and the extreme tip projects beyond that axis as shown in the sketch. The dotted line A A represents the axis of rotation, T is the tip of the root, B is the region in which curvature takes place. If the motile region B is directly sensitive to gravitation (and is the only part which is sensitive) the root will curve away from the axis of rotation, as shown by the arrow 6, just as in Knight's experiment. But if the tip T is alone sensitive to gravitation the result will be exactly reversed, the stimulus originating in T and conveyed to B will produce the curvature in the direction t. We may think of the line A A as a plane dividing two worlds. In the lower one gravity is of the earthly type and is shown by bodies falling and roots curving downwards: in the upper world bodies fall upwards 1 Pringsheim's Jahrb.
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