Collected Physical Papers/On a Vegetable Photo-Electric Cell
XXIII
ON A VEGETABLE PHOTO-ELECTRIC CELL
When light is incident on the sensitive pulvinus of Mimosa pudica, the response is by contraction and the resulting fall of the leaf. A diminution of electric resistance of tissues of plants under stimulation has also been demonstrated in the previous paper. The electromotive response of galvanometric negativity, under the excitatory action of light, has previously been demonstrated in my work on Comparative Electro-physiology. I here describe a new and interesting method for this demonstration.
Normal Response to Light
For obtaining the normal response, we take a vigorous leaf and pin it on a paraffined block of wood. Two pieces of thin muslin in connection with non-polarisable electrodes are spread over two areas of the leaf A and B; when these pieces of muslin are moistened with normal saline, they become practically transparent. When light from an arc lamp is thrown on A, that area becomes galvanometrically negative and the responsive current flows in {he direction GAB. Light thrown on B (A being shaded) causes a response in the opposite direction, (left illustration fig. 96).
The fact, that the electromotive response under light is the same as that under a different mode of stimulation such as mechanical, is demonstrated as follows. The moist piece of cloth on A is rubbed against the surface of the leaf by means of a glass rod; or the surface is struck with a glass hammer. In both these cases, A end of the leaf becomes galvanometrically negative, the direction of the current of response being the same as when A is stimulated by light.
Having given a simple demonstration of the fundamental reaction, I describe the photo-electric cell made of two pieces of leaf. In the experiment just described, the resistance of the circuit is very great, on account of the high resistance of the two non-polarisable electrodes, and the resistance offered by the leaf. The non-polarisable electrode, moreover, is a source of trouble; an attempt was therefore made to discard it, and employ other means for diminishing the resistance of the circuit. For the following experiments we employ the leaf of Musa sapientum which is divided into two longitudinal halves by a slit along the thick midrib. Two pieces of leaves are thus obtained about 10 × 10 cm. which are hung parallel and separated from each other in a rectangular glass vessel filled with normal saline; the distance between the two leaves is 3 cm. Two gold wires are thrust through the length of the two divided midribs; they serve as external electrodes of the photo-voltaic cell, which lead to the galvanometer G. The glass trough is placed inside a rectangular wooden chamber with two hinged doors on opposite sides, by which the leaf A or B could be alternately exposed to light (fig. 96). When the doors are closed, A and B are in darkness; they are practically iso-electric, there being no current in the galvanometer. But exposure of A to light gives rise to a, difference of potential between A and B, A becoming galvanometrically negative, the resulting deflection being in one direction. Exposure of B gives rise to a responsive deflection in the opposite direction. The two leaves serve as the two plates in a voltaic cell; but unlike ordinary voltaic cell with elements of different metals,
the two plates of the vegetable cell are made of two halves of the same leaf, the electromotive force being generated by the excitatory action of light on one of the two half leaves. The advantages of this method of obtaining electromotive response are: (1) that the troublesome employment of the non-polarisable electrodes with their high resistance is dispensed with; (2) that the area of the surface of the leaf exposed to light is considerably increased; (3) that the electric resistance of the circuit is greatly decreased, since the interposed resistance is that of normal saline about 3 cm. thick with a broad section of 100 square cm.; and (4) that alternate and opposite responses may be obtained by successive exposures of the two leaf-plates to the parallel beam of an arc lamp, this being easily secured by turning the rectangular plant chamber round a revolving base.
Response of the Leaf to Light
The photo-voltaic cell thus constructed is stimulated by light from an arc lamp which passes through a trough of alum solution for absorption of the heat rays. Successive exposures are made for 10 seconds and records obtained on a moving photographic plate. The normal responses are uniform, exhibiting induced galvanometric negativity as seen in the up-curves. On the cessation of light there is a complete recovery; the recovery shows in fact, an overshooting towards galvanometric positivity from which it returns almost to the original zero position before stimulation (fig. 97). The records indicate the existence of dual reactions, a negative variation or D-effect followed by a positive variation or the A-effect.
Positive Response to Light
Some observers have obtained with green leaves a response of galvanometric positivity. The results of following investigation offer an explanation of the apparent anomaly. I have shown elsewhere that a positive response occurs under a stimulus below the critical intensity, and that this critical point is low in highly excitable tissues, whereas it is relatively high in others which are less excitable. The excitability, I find, is modified by the age and vigour of the specimen. It is very considerable at moderately young age, being feeble when the tissue is either very young or very old. The same stimulus which evokes a negative response in a vigorous middle-aged leaf may, therefore, be expected to give rise to a positive response in a very young leaf or in a leaf which is becoming yellow with age.
The above anticipations have been found verified in the following experiments. In figure 98 is seen the positive response of a very young leaf of Musa; the next figure shows similar positive response given by a
Fig. 97. Normal electromotive response in a vigorous specimen.
Note the transient positive after-effect.
Figs. 98 and 99. Abnormal positive response in a very young and in a very old specimen.
very old leaf. There is an additional factor, to be presently described, which also tends to induce a positive response.
Effect of Increasing Duration of Exposure
The reaction under light, is within limits, proportional to the quantity of light, that is to say, on intensity of light multiplied by duration. The effect of increasing intensity of light has already been shown (cƒ. fig. 95). 
Fig. 100. Effect of increasing durations
of exposure of 5, 10 and 15 seconds.When the intensity is maintained constant, the amplitude of response undergoes an increase with the increased duration of exposure (fig. 100). But this increase does not go on indefinitely, for the continuous action of light causes a maximum negative response beyond which a decline sets in. It is probable that this is due to an opposing element which tends to neutralise the normal excitatory D-effect. The existence of this opposing A-reaction has already been seen in the transient after-effect in figure 97. Other results which I obtained, show that stimulus induces, in general, both the D- and A-effects; in excitable specimens the D-effect is predominant and therefore, masks the A-effect; the positive A-effect is, however, exhibited under feeble or even as an after-effect of strong stimulation.
Certain conditions, moreover, are specially favourable for the exhibition of the A-effect. When the green leaf has an abundant supply of chlorophyll, the photo-synthetic process of building up becomes specially marked. I have thus obtained under the action of light, a positive response with the green leaf of Luctuca sativa in which chlorophyll is present in great abundance.
The fact that positive response is associated with assimilation is proved by my experiments on photo-electric response of water plants. This building up process by photosynthesis is here independently demonstrated by profuse evolution of oxygen. These aquatic plants exhibit marked positive electric response during strong illumination, recovery taking place on the cessation of light (fig. 101).
Fig. 101. The positive electric response of Hydrilla.
Effect of Continued Action of Light
The positive element in the response may be indirectly demonstrated even in the normal Musa leaf. In figure 102 is seen the effect of continuous action of light which at first exhibits the predominant negative attaining a maximum; the positive element now begins to increase with the duration of light and causes a reversal; at a certain stage, the two elements, D and A, balance each other, the resulting response becoming horizontal. On the stoppage of light, the antagonistic A element ceases to be active, while D appears to be persistent.
Fig. 102. After-effect of light. The
maximum negative is opposed by
growing positive and the balance
attained is seen in the horizontal portion
of the curve. Stoppage of light causes the
unmasking of the negative followed by recovery.The result is a sudden unmasking of the negative, hitherto held in balance during exposure to light. A negative response with subsequent recovery thus occurs on the cessation of light.
The responses of Musa and of actively assimilating Hydrilla are seen to exhibit characteristic differences on account of the relative predominance of the D- or A-effect. In Musa, D is predominant, A being exhibited either as a positive after-effect or by the overshooting of response in the positive direction. In an actively assimilating Hydrilla plant, on the other hand, A is predominant and the response is positive. In less vigorous Hydrilla the positive becomes masked by the negative, when the resultant response appears to be similar to that of Musa.