Page:Life Movements in Plants Vols 3 and 4.djvu/25

 the second volume of the Transactions of the Institute (1919), a very sensitive electric method is described for the determination of the exact position of the sensitive layer in the interior by which the plant is enabled to perceive the vertical direction, so that the shoot or the petiole places its length parallel to the. direction of the lines of force of gravity, with the apex upwards. This directive movement is accomplished by a responsive curvature, the upper side becoming concave and the lower convex. The concavity of the upper side is brought about by an excitatory contraction, which I have shown .takes place under all forms of stimulation, mechanical, electrical, chemical and photic. It has also been shown that the characteristic signs of excitation — diminution of turgor, contraction, and diminution of the rate of growth — may be detected electrically by an induced change of galvanometric negativity If suitable electric connections are made so that one contact is on one side of the stem, and the other on a distant indifferent point, then on laying the plant horizontal, the upper side of the stem is found to exhibit an electric change of galvanometric negativity indicative of excitation. The geotropic irritation and the electric sign of excitation disappears as soon as the plant is restored to the normal vertical position.