Life Movements in Plants Vol 1/Chapter 20

In the preceding Paper I have shown the essential similarity of effect of stimulus on autonomous activity of the Desmodium leaflet, and of the growing organ. It was shown how stimulus revived the pulsatory activity of Desmodium leaflet in a state of standstill, in the same way as it renewed the arrested growth-activity.

The investigation of this subject was rendered possible by the successful device of my Oscillating Recorder. A very light glass fibre was used for the construction of the lever, which was supported on jewel bearings. The short arm of the lever was 2 cm. in length, and the long arm 8 cm. This gave a magnification of 4 times. But it is quite easy to increase the magnification to 10 times or more.

The pull exerted by the pulsating leaflet is extremely slight, and the relatively heavy lever made of steel wire used in the Resonant Recorder is not well-suited for our purpose. The pulsation of the leaflet is relatively slow, being once in two minutes or so. The intermittent contact of ten times in a second, given by the Resonant Recorder, is therefore too quick. In the Oscillating Recorder tie intermittence was, therefore, reduced to once in a second, or once in five seconds, the recording plate itself being made to move to-and-fro at this rate. The carrier of the plate-holder slides backwards and forwards on ball bearings; a ​wheel in the clockwork connected with an eccentric is released periodically, at intervals which may be varied between one and five seconds. By the action of the eccentric, the plate carrier approaches the writing lever with diminishing speed till the movement is zero at the contact. This contrivance is essential, since any sudden shock of the plate against the lever is apt to give rise to after-vibrations of the writer. The plate carrier is quickly withdrawn after the production of a dot on the smoked glass plate by contact with the writing lever.

The clockwork is governed by a revolving fan which can be gradually opened out by a regulating screw. The speed can thus be adjusted within wide limits, and maintained constant and at any desired speed. A second set of wheels connected with the clockwork moves the plate-holder in a lateral direction. A series of records may thus be taken for fifteen minutes, half an hour, or an hour.

The record obtained in this way is very perfect. Not only is the effect of an external agent shown by variation in the amplitude and frequency of pulsations, but the change of speed in any phase of the pulse becomes automatically recorded.

The whole plant can not be conveniently manipulated for different investigations. It is, however, possible using the precautions described below to use the detached petiole carrying the pulsating leaflets. The terminal large leaf may also be removed. The necessary amputation is often followed by an arrest of pulsation. But as in the case of isolated heart in a state of standstill, the movement of the leaflet may be revived by the application of internal hydrostatic pressure. Under these conditions, the rhythmic pulsations may easily be maintained uniform for many hours.

​The petiole carrying the leaflet is mounted water-tight in the short arm of an U-tube filled with water; for producing internal hydrostatic pressure in the plant the height of water in the longer arm is suitably raised. The U-tube holding the specimen may be adjusted up and down, and laterally. A hinged support also allows the specimen to be placed at any inclination. The movement of the leaflet, it is to be remembered, does not always take place in a vertical direction. The object of the mechanical adjustments is to place the specimen at such an angle that its up and down movements when in a straight line should be vertical, or have its long axis vertical when the movement is elliptical. It is important that the specimen should be illuminated equally from all sides; for one-sided illumination causes a bending over of the leaflet towards light.

The pulvinule of the leaflet acts like the pulvinus of Mimosa, that is to say, the leaflet undergoes a sudden fall to down position by the contraction of the more effective lower half of the pulvinule; the 'up' position denotes recovery and expansion of the more effective half. The up-and-down movements of the leaflet correspond to the diastolic and systolic movements of the animal heart. There is, indeed, as I have shown elsewhere^[1] a very close resemblance between the activities of rhythmic tissue in the plant and in the animal.

Experiment 92.—For the study of effect of light on Desmodium, I first obtained record in darkness. A horizontal beam of divergent light from an arc lamp placed at a distance of 200 cm. was made to act diffusely on the leaf from all sides. This was done by means of three inclined mirrors, the first throwing the light vertically downwards, the ​second vertically upwards, and the third horizontally forward from the side opposite the lantern. The effect of light is seen demonstrated in Fig. 86.

Light was applied at the second pulsation. It will be seen that light retards or arrests the autonomous activity. On the cessation of light the normal activity was found to be gradually restored. It is of much interest to note here the similarity of action of light on autonomous activity of the leaflet of Desmodium and of a growing organ. In both, we find that while in the sub-tonic condition of the tissue the effect of light is to enhance or renew the autonomous activity of growth and pulsation, in normal condition the effect is to retard it.

Inspection of the record exhibits another very interesting characteristic. We saw that light retarded growth by inducing an incipient contraction. In the Desmodium leaflet the contractile reaction of light is exhibited by the characteristic modification of its pulsations. The duration ​of application of light is represented by the horizontal line. In Fig. 86 the up-curve represents up-movement of diastolic expansion, and the down-curve of systolic contraction. The contractile reaction of light is seen to counteract the normal expansion, with diminution of diastolic limit of pulsation.

It has been shown that while rise of temperature up to an optimum enhanced the rate of growth, the effect of light was to retard it. Hence the effects of light and warmth are antagonistic.

Effect of rise of temperature on pulse-record: Experiment 93.—In studying the effect of rise of temperature on the pulsation of leaflets of Desmodium, we discover similar
Fig. 87.—Effect of rise of temperature on pulsation of leaflet of Desmodium gyrans. Horizontal line represents the duration of gradual rise of temperature from 30°C. to 35°C. Note the expansive effect of rise of temperature in reduction of systolic limit of pulsation. antagonistic reactions of light and warmth. The leaflet was placed in a plant-chamber with an electric arrangement for gradual rise of temperature. The first two ​records were taken in the normal temperature of the room, which was 30°C. The temperature was gradually raised to 35°C, the record being taken all the time. It will be seen (Fig. 87) that the effect of warmth is diametrically opposite to that of light. The record in Fig. 86 exhibited the contractile effect of light by reducing the diastolic limit of expansion. In the present case the expansive reaction of warmth is exhibited by the reduction of systolic limit of contraction. The temperature of the plant chamber was now allowed to return to 30°C., and we observe the gradual restoration of normal systolic limit of contraction.

Two different effects are found in the action of the stimulus of light alike on the autonomous activity of leaflet of Desmodium gyrans and of growing organs. In condition of sub-tonicity light renews pulsation of Desmodium and enhances the activity of growth. In normal tonic condition the effect induced is the very opposite, light causing an arrest of pulsation and retardation or arrest of growth.

The contractile effect of light is seen not only in the retardation of growth, but also by the characteristic modification of pulsation of Desmodium in the diminution of diastolic limit of expansion.

The antagonistic reactions of light and warmth are found not only in growth but also in the rhythmic activity of Desmodium gyrans. In the pulsation of Desmodium the contractile effect of light induces a rapid diminution of the diastolic limit of expansion, while the expansive reaction of warmth brings about a marked reduction of the systolic limit in successive pulsations.