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Collected Physical Papers/The Quadrant Method of Response to Stimulus of Light

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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 screen is next turned through 90°; the quadrants P and Q are now exposed to light, and R and S shaded

Collected Physical Papers Fig. 92.tif

Fig. 92. The Quadrant Method of response by variation of electric resistance. Two opposite quadrants of the leaf are shaded. Electric connections are made at the junctions of the quadrants and not at the middle as shown in figure.

from it. The resulting upset of the balance and the galvanometer deflection is now in an opposite direction.

The reliability and the sensitiveness of the Quadrant Method may thus be tested by obtaining equal and opposite responses under alternate illumination of the two pairs of quadrants; the test in confirmation of the above will be found in records given in figure 93.

After securing exact balance, the double V-shaped screen is kept fixed, and the leaf mounted in a rectangular

Collected Physical Papers Fig. 93.jpg
Fig. 93. Equal responses in opposite directions by alternate illumination of the two pairs of quadrants.

dark chamber closed except at the front, which carries a photographic shutter by which one pair of quadrants is exposed to light for a definite duration. The electric connections with the leaf are led to four binding screws; the petiole protruding from the box is dipped in an U-tube filled with water (cƒ. right-hand illustration of fig. 92).

The source of light is an arc or an incandescent lamp, placed inside a lantern, the condenser of which sends a parallel beam of light. A rectangular glass trough filled with alum solution is interposed in the path of light to absorb the heat rays. The duration of exposure is varied according to the sensitiveness of the specimen; the usual period of exposure is about 20 seconds.

response to light from a single spark

The extreme sensitiveness of the quadrant method is demonstrated by the record given in figure 94.

Collected Physical Papers Fig. 94 and 95.jpg

Fig. 94.
Fig. 95.

Fig. 94. Response to light from a single spark.
Fig. 95. Effect of stimulus of light increasing in the ratio of 1:3:5:7.

The duration of a spark-discharge of a Leyden Jar may be regarded as of the order of hundred thousandth part of a second. The discharge took place at a distance of 15 cm. from the leaf and the response is seen to consist of a preliminary positive twitch followed by a large negative response, indicative of normal diminution of resistance. The leaf exhibited a complete recovery.

Effect of increasing Intensity of Light

The arc lamp is taken out of the lantern, and the diverging beam employed for the following experiment. As the intensity of light varies as the square of the distance, suitable marks were made on the scale fixed on the table, so that the intensity of light incident on the leaf was increased in the proportion of 1:3:5:7 by bringing the arc nearer the leaf at the particular distances marked on the scale. The duration of exposure was kept the same.

The responses under increasing intensities of light are seen in figure 95. The resistance is seen to undergo a diminution with the increasing intensity of stimulus.

Effects of Stimulants and Depressants

The responses are appropriately modified by stimulating or depressing agents. Dilute vapour of ether, for example, increases the excitability and enhances the amplitude of response. Strong dose of chloroform, on the other hand, causes depression and death as indicated by gradual diminution and final abolition of response.

The results of other investigations, fully described elsewhere, show that external stimulation gives rise to two definite protoplasmic reactions, which may be described as the A- and the D-effects. The A-effect, usually induced by sub-minimal stimulus, finds outward expression, by induced expansion, increase of turgor, enhancement of the rate of growth, galvanometric positivity and increase of electrical resistance. The D-effect (predominantly induced under stimulus of moderately strong intensity) is outwardly manifested, on the other hand, by contraction, diminution of turgor, diminished rate of growth, galvanometric negativity and diminution of electric resistance. Under very strong stimulation the response tends to become multiple. The following table shows the parallel effects exhibited by diverse modes of response.

Table showing parallelism in different modes of response

External change Mechanical response Variation of growth Electro-motive response Resistivity variation
Sub-minimal stimulus Expansion; erectile response Acceleration of growth Galvanometric positivity Increase of resistance
Moderate stimulus Contraction and fall of leaf Retardation of growth Galvanometric negativity Diminution of resistance
Strong stimulus Multiple response Multiple response Multiple response Multiple response
Depressants Diminished response Retardation of growth Diminished response Diminished response
Stimulating agents Enhanced response Acceleration of growth Enhanced response Enhanced response

(Life Movements in Plants, Vol. IV, 1923.)