Life Movements in Plants Vol 1/Action of Stimulus on Vegetable Tissues
III.—ACTION OF STIMULUS ON VEGETABLE TISSUES
By
Sir J. C. Bose,
Assisted by
Narendra Nath Sen Gupta.
The leaf of Mimosa pudica undergoes a rapid fall when subjected to any kind of shock. This plant has, therefore, been regarded as "sensitive," in contradistinction to ordinary plants which remain apparently immobile under external stimulus. I shall, however, show in course of this Paper that there is no justification in regarding ordinary plants as insensitive.
Let us first take any radial organ of a plant and subject it to an electric shock. It will be found that the organ undergoes a contraction in length in response to the stimulus. On the cessation of excitation the specimen gradually recovers its original length. Different organs of plant may be employed for the experiment, for example, the tendril of Cucurbita, the pistil of Datura, or the flower bud of Crinum. The shortening may be observed by means of a low power microscope. Greater importance is, however, attached to the detailed study of response and its time relations. The pull exerted by a delicate organ during its excitatory contraction is slight; hence arises the necessity of devising a very sensitive apparatus, which would give records magnified from ten to a hundred times.[1]
RESPONSE RECORDERS.
The magnification of movement is produced by a light lever, the short arm of which is attached to the plant organ, the long arm tracing the record on a moving smoked plate of glass. The axis of the lever is supported by jewel bearings. The principal difficulty in obtaining accurate record of response of plant lies in the friction of contact of the recording point against the glass surface. This difficulty I have been able to overcome by providing a device of intermittent instead of continuous contact. For this, either the writer is made to vibrate to and fro, or the recording plate is made to oscillate backwards and forwards.
1. The Resonant Recorder.—In this the writing lever is made of a fine steel wire. One end of this wire is supported at the centre of a circular electromagnet; this latter is periodically magnetised by a coercing vibrator, which completes an electric circuit ten hundred, or two hundred times in a second. The writing lever is exactly tuned to the vibrating interrupter and is thus thrown into sympathetic vibration. Successive dots in the record thus measure time from 0·1 to 0·05 second. The employment of the Resonant Recorder enables us to measure extremely short periods of time for the determination of the latent period or the velocity of transmission of excitation.
2. The Magnetic Tapper.—Measurement of very short intervals is not necessary in ordinary records of response. In this type of recorders, the circular magnet is therefore excited at longer intervals, from several seconds to several minutes; this is done by completion of the electric circuit at the required intervals, by means of a key operated by a clock.
3. The Mechanical Tapper.—In this, magnetic tapping is discarded in favour of mechanical tapping. The hinged writing lever is periodically pressed against the recording plate by a long arm, actuated by clock-work.
4. The Oscillating Recorder.—Here the plate itself is made to oscillate to-and-fro by eccentric worked by a clock. The frame carrying the plate moves on ball-bearings. The advantage of the Oscillating Recorder lies in the fact that a long lever, made of line glass libre, or of aluminium wire, may be employed for giving high magnification. A magnification of a hundred times may be easily obtained by making the short arm 2·5 mm. and the long arm 25 cm. in length.[2]
RESPONSE OF A RADIAL ORGAN.
Experiment 10.—As a typical example I shall describe the response of a straight tendril of Passiflora. A cut specimen was mounted with its lower end in water. Suitable electric connections were made for sending a feeble induction shock of short duration through the specimen. In this and all other records, unless contrary be stated, up-curve represents contractile movement. On application of stimulus of electric shock, an excitatory movement of contraction occurred which shortly reached its maximum; the apex-time was one minute and forty seconds, and recovery was completed after a further period of five minutes (Fig. 10). Stronger shocks induce greater contraction
with prolongation of the period of recovery. The specimen was afterwards killed by application of poisonous solution of potassium cyanide; this brought about a permanent abolition of response. The experiment just described may be taken as typical of response of radial organs.
In a radial organ contraction takes place equally in all directions; it therefore shortens in length, there being no movement in a lateral plane. But, if any agency renders one side less excitable than its opposite, diffuse stimulation will then induce greater contraction on the more excitable side which will therefore become concave.
RESPONSE OF AN ANISOTROPIC ORGAN.
Excessive stimulation is found to reduce the excitability of an organ. Under unilateral mechanical stimulation a tendril of Passiflora becomus hooked or coiled, the concave being the excited side. From what has been said, the unexcited convex side will relatively be the more excitable.
Experiment 11.—I took a specimen of hooked tendril, and excited it by an electric shock. The response was by the greater contraction of the more excitable convex side, on account of which the curved specimen tended to open out. The record of this response is seen in Fig. 11;
the apex-time was nearly two minutes, and the recovery was completed in the further course of 15 minutes.
From the responses of organs rendered anisotropic by the differential action of the environment we pass to others which show certain amount of anatomical and physiological differentiation between their upper and lower sides. I find that many petioles of leaves show movement in response to stimulus. Many pulvini, generally regarded as insensitive, are also found to exhibit responsive movements.
RESPONSE OF THE PULVINUS OF MIMOSA PUDICA.
The most striking and familiar example of response is afforded by the main pulvinus of Mimosa pudica of which a record is given in Fig. 12. It is generally assumed
that sensibility is confined to the lower half of the organ. It will be shown in a subsequent Paper that this is not the case. The upper half of the pulvinus is also sensitive though in a feeble degree, its excitability being about 80 times less than that of the lower half. On diffuse stimulation the predominant contraction of the lower half causes the fall of the leaf, the antagonistic reaction of the upper half being, in practice, negligible. In order to avoid unnecessary repetition, I shall ignore the feeble antagonistic reaction of the less excitable half of the organ, and shall use the word 'contraction' for 'relatively greater contraction.'
It is interesting in this connection to refer tn the response of the leaf of Water Mimosa (Neptunia oleracea). Here the reaction is very sluggish in comparison with that of Mimosa pudica. A tabular statement of contractile response of various radial, anisotropic and pulvinated organs will show a continuity in the contractile reaction: the difference exhibited is a question of degree and not of kind.
TABLE 1.—PERIODS OF MAXIMUM CONTRACTION AND OF RECOVERY OF DIFFERENT PLANTS.
| Specimen | Period of maximum contraction. | Period of recovery. | |||
| Radial organ: Tendril of Passiflora… |
… … … … |
100 120 180 3 |
seconds „ „ „ |
4 13 57 16 |
minutes „ „ „ |
As regards the excitatory fall of the leaf of Mimosa pudica, Pfeffer and Haberlandt are of opinion that this is due to the sudden diminution of turgor in the excited lower half of the pulvinus. The weight of the leaf, no longer supported by the distended lower cells, causes it to fall. This is accentuated by the expansion of the upper half of the pulvinus which is normally in a state of compression. According to this view the excitatory fall of the leaf is a passive, rather than an active, movement. I have, however, found that in determining the rapidity of the fall of Mimosa leaf the factors of expansive force of the upper half of the pulvinus and the weight of the leaf are negligible compared to the active force of contraction exerted by the lower half of the pulvinus (p. 87).
With regard to the fall of turgor, it is not definitely known whether excitation causes a sadden diminution in the osmotic strength of the cell-sap or an increase in the permeability of the ectoplast to the osmotic constituents of the cell. Pfeffer favours the former view, while others support the theory of variation of permeability.[3]
RESPONSE OF PULVINUS OF MIMOSA TO VARIATION OF TURGOR.
Whatever difference of opinion there may be in regard to the theories of osmotic and permeability variations, we have the indubitable fact of diminution of turgor and contractile fall of the pulvinus of Mimosa under excitation. The restoration of the original turgor brings about recovery and erection of the leaf. In connection with this the following experiments on responsive movements of the leaf under artificial variation of turgor will be found of interest:—
Effect of Increased Turgor: Experiment 12.—A young Mimosa plant was carefully transplanted and the root embedded in soil placed in a linen bag. This was held securely by a clamp, and one of the leaves of the plant attached to the recorder. Withholding of water for a day caused a general loss of turgor of the plant. A vessel full of water was now raised from below so that the linen bag containing the roots was now in water. The effect of increased turgor by suction of water by the roots became apparent by the upward movement of the leaf. The distance between the immersed portion of the plant and the leaf was 2 cm. and the up-movement of the leaf was indicated within 10 seconds of application of water (Fig. 13). The velocity with which the effect of
increased turgor travelled was thus 2 mm. per second. The leaf exhibited increasing erection with absorption of water.
Effect of Diminution of Turgor: Experiment 13.— While the leaf in the above experiment was in process of erection, a quick change was made by substituting KNO3 solution for the water of the vessel in which the roots were immersed. The plasmolytic withdrawal of water at the roots gave rise to a wave of diminished turgor, the effect of which became perceptible within 40 seconds by the movement of fall of the leaf. (Fig. 13.)
DIFFERENT MODES OF STIMULATION.
In Mimosa excitation is manifested by the contraction of the pulvinus and the consequent movement of the leaf. But in most plants, excitatory movement cannot be realized on account of the rigidity of the plant structure, the thickness of the cell-wall and the want of facility for escape of water from the excited cells. I shall show later how excitation may be detected in the absence of mechanical movement.
As regards stimulation of vegetable tissues, there are various agencies besides electric shock, which induce excitatory contraction; these agencies I shall designate as stimuli. Excitation is detected in Mimosa by the downward movement of the leaf. It will be found that such excitatory movement is caused by a mechanical blow, by a prick or a cut, by the application of certain chemical agents, by the action of electric current and by the action of strong light. The study of the action of these stimuli will be given in greater detail in subsequent Papers.
I shall give below a general classification of different stimuli which cause excitation in vegetable tissues.
Electric Stimulus.—Induction shock, condenser discharge, the make of kathode and the break of anode.
Mechanical Stimulus.—Mechanical blow, friction, prick or cut.
Chemical Stimulus.—Effect of certain acids, and of other chemical substances.
Thermal Stimulus.—Sudden variation of temperature; application of heated wire.
Radiation Stimulus.—Luminous radiation of the more refrangible portion of the spectrum; ultra-violet rays; thermal radiation in the infra-red region.
All these different forms of stimulus induce an excitatory contraction, a diminution of turgor, and a negative mechanical response or fall of a motile leaf.
SUMMARY.
A radial organ responds to stimulus by contraction in length; as all its flanks are equally excitable there is no lateral movement under diffuse stimulus.
Physiological anisotrophy is induced in an organ, originally radial and isotropic, by the unequal action of the environment on its different sides. Diffuse stimulus induces a greater contraction of the more excitable side.
In a curved tendril the concave side is less excitable than the convex. Diffuse stimulus tends to straighten the curved tendril.
In the pulvinus of Mimosa pudica, the lower half is eighty times more excitable than the upper, and the fall of the leaf is due to the predominant contraction of the more excitable lower half.
A diminution of turgor takes place in the excited cells. Restoration of turgor brings about recovery of the leaf to its normal erect position. Independent experiments show that the fall of the leaf may be brought about by an artificial diminution of turgor, and the erection of the leaf by an increase of turgor.
- ↑ For detailed description cf. Bose.—"An Automatic Method for Investigation of Velocity of Transmission of Excitation in Mimosa."— Phil. Trans., B. vol. 204, (1913).
- ↑ Bose—"Researches on Irritability of Plants," p. 279— Longmans, Green & Co.
- ↑ With reference to the fall of Mimosa leaf Jost says: "When the pressure of the cell decreases we naturally assume this to be due to a decreasing osmotic pressure due to alterations in the permeability of the plasma, and an excretion of materials from the cell. It is a remarkable fact that plasmolytic research (Hilburg 1881) affords no evidence of any decrease in osmotic pressure. No complete insight into the mechanism of the stimulus movement in Mimosa has yet been obtained, although one thing is certain, that there is a decrease in the expansive power on the under side of the articulation."—Jost, "Plant Physiology"—English Translation, p. 515. Clarendon Press (1907). Blackman and Paine think that the loss of turgor on excitation "is probably due to the disappearance or inactivation of a considerable portion of the osmotic substances of the cells."—Annals of Botany, Vol. XXXII, No. CXXXV, Jan. 1918.