VI.—ON CONDUCTION OF EXCITATION IN PLANTS
By
Sir J. C. Bose.
The plant Mimosa offers the best material for investigation on conduction of excitation. With regard to this question the prevailing opinion had been that in plants like Mimosa, there is merely a transmission of hydro-mechanical disturbance and no transmission of true excitation comparable with the animal nerve. I have, however, been able to show that the transmission in the plant is not a mechanical phenomenon, but a propagation of excitatory protoplasmic change. This has been proved by the arrest of conduction by the application of various physiological blocks. Thus local application of increasing cold retards, and finally abolishes the conducting power. The conducting tissue becomes paralysed for a time as an after-effect of application of cold; the lost conducting power may, however, be quickly restored by tetanising electric shocks. The conducting power of an animal nerve is arrested by an electrotonic block, the conductivity being restored on the cessation of the current. I have succeeded in inducing similar electrotonic block of conduction in Mimosa. Conductivity of a selective portion of petiole may also be permanently abolished by local action of poisonous solution of potassium cyanide.[1]
Having thus established the physiological character of the transmitted impulse in plants I shall now proceed to give some of the principal results of my earlier and recent investigations on the effects of various agencies on conduction of excitation in plants.
Apart from any question of hydro-mechanical transmission, it is important to distinguish two different modes of transmission of excitation. In a motile tissue contraction of a cell causes a physical deformation and stimulation of the neighbouring cell. Examples of this are furnished by the cardiac muscle of the animal, the pulvinus of Mimosa, and the stamen of Berberis. This mode of propagation may better be described as a convection of excitation.
The conduction of excitation, as in a nerve, is a different process of transmission of protoplasmic change. The conducting tissue in this case does not itself exhibit any visible change of form. In the plant the necessary condition for transmission of excitation to a distance is that the conducting tissue should be possessed of protoplasmic continuity in a greater or less degree. This condition is fulfilled by vascular bundles. There being greater facility of transmission along the bundles than across them, the velocity in the longitudinal direction is very much greater than in the transverse.
For accurate determination of velocity of transmission the testing stimulus should be quantitative and capable of repetition. Abnormal high velocity has been observed in Mimosa by applying crude and drastic methods of stimulation, by a transverse cut or a burn. This is apt to give rise to a very strong hydro-dynamic disturbance, which travelling with great speed, delivers a mechanical blow on the responding pulvinus. Such hydro-dynamic transmission is not the same as physiological conduction.
In the primary petiole of Mimosa the highest velocity under electric stimulation I find to be about 30 mm. per second. This velocity is considerably lower than the velocity in the nerve of higher animals, but higher than in the lower animals. As an example of the latter, mention may be made of the velocity of 10 mm. per second in the nerve of Anodon and 1 mm. per second in the nerve of Eledone.
PREFERENTIAL DIRECTION OF CONDUCTION.
Experiment 33.—The conduction of excitatory impulse takes place in both directions. This can be demonstrated by taking a petiole of Biophytum sensitivum or of Averrhoa carambola. These petioles are provided with a series of motile leaflets. Stimulation at the middle point of the petiole gives rise to two waves of excitation, one of which travels towards the central axis of the plant, and the other away from it. The centrifugal velocity is greater than the centripetal as will be seen from the following results:
| Biophytum… | Velocity in centrifugal direction… | 2.9 mm. per second. |
| Velocity„ in centripetal direction„… | 2.92 mm. per„ second.„ | |
| BiophytumAverrhoa… | Velocity„ in centrifugal direction„… | 0.5 mm. per„ second.„ |
| Velocity„ in centripetal direction„… | 0.26 mm. per„ second.„ |
EFFECT OF TEMPERATURE.
Variation of temperature has a marked effect on the velocity of transmission of excitation. Lowering of temperature diminishes the velocity, culminating in an arrest. Rise of temperature, on the other hand, enhances the velocity. This enhancement is considerable in specimens in which the normal velocity is low, but in plants in optimum condition, the velocity being already high, cannot be further enhanced. The following tabular statement gives results of effects of temperature on velocity of transmission in Mimosa and Biophytum:—
TABLE IV.—EFFECT OF TEMPERATURE ON VELOCITY OF TRANSMISSION.
| Specimen. | Temperature. | Velocity. |
| Mimosa (winter specimen)… | 22°C | 3.6 mm. per second. |
| 28°C | 6.3 mm. per„ second.„ | |
| 31°C | 9.0 mm. per„ second.„ | |
| Biophytum……… | 30°C | 3.7 mm. per„ second.„ |
| 35°C | 7.4 mm. per„ second.„ | |
| 37°C | 9.1 mm. per„ second.„ |
EFFECT OF SEASON.
The velocity of transmission is very much lower in winter than in summer. In the petiole of Mimosa, the velocity in summer is as high as 30 mm. per second; in winter it is reduced to about 4 mm. The lowering of velocity in winter is partly due to the prevailing low temperature and also to the depressed state of physiological activity.
EFFECT OF AGE.
In a Mimosa plant, different leaves will be found of different age. Of these the youngest will be at the top. Lower down, we obtain a fully grown young leaf, and near the base, leaves which are very old. The investigation deals with the effect of age on the conducting power of the petiole.
Comparison of conducting power in different leaves: Experiment 34.—Selecting three leaves from the same plant we apply an identical electric stimulus at points 2 cm. from the three responding pulvini. The electric connections are so made that the same tetanising shock is applied on the three petioles, very young, fully grown, and very old. The secondary coil is gradually pushed in till the leaves exhibit responsive fall. The fully grown leaf was the first to respond, the velocity of transmission being 23 mm. per second. The secondary coil had to be pushed nearer the primary through 6 cm. before excitation could be effectively transmitted through the young petiole; for the oldest leaf still stronger stimulus was necessary, since in this case the secondary had to be pushed through an additional distance of 4 cm. for effective transmission of excitation. I also determined the relative values of the minimal intensity of stimulus, effective in causing transmission of excitation in the three cases. Adopting as before the intensity of electric stimulus which causes bare perception in a human being as the unit, I find that the effective stimulus for a fully grown young petiole is 0.3 unit, while the very young required 2.5 units, and the very old 5 units. Hence it may be said that the conducting power of a very young is an eighth, and of the very old one-sixteenth of the conductivity of the fully grown young specimen.
It will thus be seen that the conducting power of a very young petiole is feebler than in a fully grown specimen. The conducting tissue, it is true, is present, but the power of conduction has not become fully developed. This power is, as we shall see later, conferred by the stimulus of the environment. In a very old specimen the diminution of conducting power is due to the general physiological decline.
EFFECT OF DESICCATION ON CONDUCTING TISSUES.
I have already shown that transmission in the plant is a process fundamentally similar to that taking place in the animal nerve; it has also been shown that the effects of various physical and chemical agents are the same in the conducting tissues of plant and of animal.
Effect of application of glycerine: Experiment 35.—It is known that desiccation, generally speaking, enhances the excitability of the animal nerve. As glycerine, by absorption of water, causes partial desiccation, I tried its effect on conduction of excitation in the petiole of Mimosa. Enhancement of conducting power may be exhibited in two ways: first, by an increase of velocity of transmission; and, secondly, by an enhancement of the intensity of the transmitted excitation, which would give rise to a greater amplitude of response of the motile indicator. In Fig. 41 are given two records, N, before, and the other after
Fig. 41.—Action of glycerine in enhancing the speed and intensity of transmitted excitation. Stimulus applied at the vertical line. Successive dots in record are at intervals of 0.1 sec.
the application of glycerine on a length of petiole through which excitation was being transmitted. The time-records demonstrate conclusively the enhanced rate of transmission after the application of glycerine. The increased intensity of transmitted excitation is also seen in the enhanced amplitude of response seen in the more erect curve in the upper record.
INFLUENCE OF TOXIC CONDITION ON CONDUCTIVITY.
Different specimens of Mimosa are found to exhibit differences in physiological vigour. Some are in an optimum condition, others in an unfavourable or sub-tonic condition. I shall now describe certain characteristic differences of conductivity exhibited by tissues in different conditions.
Effect of intensity of stimulus on velocity of transmission.—In a specimen at optimum condition, the velocity remains constant under varying intensities of stimulus. Thus the velocity of transmission in a specimen was determined under a stimulus intensity of 0.5 unit; the next determination was made with a stimulus of four times the previous intensity, i.e., 2 units. In both these cases the velocity remained constant. But when the specimen is in a sub-tonic condition, the velocity is found to increase with the intensity of the stimulus. Thus the velocity of conduction of a specimen of Mimosa in a sub-tonic condition was found to be 5.9 mm. per second under a stimulus of 0.5 unit; with the intensity raised to 2.5 units, the velocity was enhanced to 8.3 mm. per second.
After-effect of stimulus.—In experimenting with a particular specimen of Mimosa I found that on account of its sub-tonic condition, the conducting power of the petiole was practically absent. Previous stimulation was, however, found to confer the power of conduction as an after-effect. It is thus seen that stimulus canalises a path for conduction.
The effect of excessive stimulus in a specimen in an optimum condition is to induce a temporary depression of conductivity; the effect of strong stimulus on a sub-tonic specimen is precisely the opposite, namely, an enhancement of conductivity. I give below accounts of two typical experiments carried out with petiole-pulvinus preparation of Mimosa. Excessive stimulation in these cases was caused by injury.
Action of Injury on Normal Specimens: Experiment 36.—A cut stem with entire leaf was taken, and stimulus applied at a distance of 15 mm. from the pulvinus. From the normal record (1) in Fig. 42 the velocity of transmission was
Fig. 42.—Effect of injury, depressing rate of conduction in normal specimen: (1) record before, and (2) after injury. (Dot-intervals, 0.1 sec).
found to be 18.7 mm. per sec. The end of the petiole beyond the point of application of the testing stimulus was now cut off, and record of velocity of transmission taken once more. It will be seen from record (2) that the excessive stimulus caused by injury had induced a depression in the conducting power, the velocity being reduced to 10.7 mm. per sec. Excessive stimulation of normal specimens is thus seen to depress temporarily the conducting power.
Action of Injury on Sub-tonic Specimens: Experiment 37.—I will now describe a very interesting experiment which shows how an identical agent may, on account of difference in the tonic condition of the tissue, give rise to diametrically opposite effects. In demonstrating this, I took a specimen in a sub-tonic condition, in which the conducting power of the tissue was so far below par, that the test-stimulus applied at a distance of 15 mm. failed to be transmitted (Fig. 43). The end of the petiole at a distance
Fig. 43.—Effect of injury in enhancing the conducting power of a sub-normal specimen; (1) Ineffective transmission becoming effective at (2) after section; (3) decline after half an hour, and (4) increased conductivity after a fresh cut.
of 1 cm. beyond the point of application of test-stimulus was now cut off. The after-effect of this injury was found to enhance the conducting power so that the stimulus previously arrested was now effectively transmitted, the velocity being 25 mm. per sec. This enhanced conducting power began slowly to decline, and after half an hour the velocity had declined to 4.1 mm. per sec. The end of the petiole was cut once more, and the effect of injury was again found to enhance the conducting power, the velocity of transmission being restored to 25 mm. per sec.
SUMMARY.
There are two different types of propagation of excitation: by convection, and by conduction. In the former the excited cell undergoes deformation and causes mechanical stimulation of the next; example of this type is seen in the stamen of Berberis. The conduction of excitation consists, on the other hand, of propagation of excitatory protoplasmic change. The tranmission in the petiole of Mimosa is a phenomenon of conduction.
This conduction takes place along vascular elements. The conductivity is very much greater in the longitudinal than in the transverse direction.
Rise of temperature enhances, and fall of temperature lowers, the rate of conduction. Excitation is transmitted in both directions; the centrifugal velocity is greater than the centripetal.
Dessication of conducting tissue by glycerine enhances the conducting power. Local application of cold depresses or arrests the conduction. Application of poison permanently abolishes the power of conduction.
Conductivity is modified by the effect of season, being higher in summer than in winter.
The power of conduction is also modified by age. In young specimens the conducting power is low, the conductivity is at its maximum in fully grown organs; but a decline of conductivity sets in with age.
The tonic condition of a tissue has an influence on conductivity. In an optimum condition, the velocity is the same for feeble or strong stimulus. Excessive stimulation induces a temporary depression of the conducting power.
The effects are different in a sub-tonic tissue: velocity of transmission increases with intensity of stimulus; after-effect of stimulus is to initiate or enhance the conducting power. The conducting path is canalised by stimulus.
- ↑ Bose—"An Automatic Method for the Investigation of Velocity of Transmission of Excitation in Mimosa." 'Phil. Trans.' 'B, Vol. 304 (1913) and also "Irritability of Plants." Longman's Green & Co. (1913), p. 132.