Page:Collected Physical Papers.djvu/337

 XXII

THE QUADRANT METHOD OF RESPONSE TO STIMULUS OF LIGHT

In addition to the method of electromotive variation, I succeeded in perfecting an independent means for recording the response of plants by the variation of its electric resistance. The living tissue excited by any mode of stimulation—mechanical, electrical or photic—responds by a diminution of resistance. A new method of resistivity variation of extreme delicacy will now be described by which the response of the leaf to the stimulus of light can be recorded.

The principle of the method will be understood from the diagram given at the lower end of figure 92, which represents a leaf blade of Tropaeolum in which its four quadrants P, Q, R, S, serve as the four arms of a Wheatstone Bridge. The diagonal connections are made with the battery and the galvanometer respectively. The three contacts with the leaf may be fixed, and the fourth moved slightly to the right or to the left till an exact balance is obtained in darkness, when PQ = RS. One of the pairs of opposite quadrants P and Q is shaded by a double V-shaped screen. Exposure of the leaf to light produces a variation of resistance not of one, but of two opposite arms of the bridge R and S; the upsetting of the balance is thus due to the product of the variations of resistance in the two opposite quadrants. The responsive galvanometer deflection is found to be very large indicating a diminution of resistance in the quadrants stimulated by light. The double V-shaped