The Motor Mechanism of Plants

In dealing with* the movements of plants I will discuss them in the following order : The Mot or*M eche^i sm of Adult Members such as Leaves of Sensitive ahd other Plants. 9 In addition to the above there still remains The Motor Mechanism of Growing Organs and of Tropic Movements, which will be treated in detail in another volume. The most striking and familiar plant-movement is that shown bv the sensitive plants, of which M i. nosa ftudica is the one that has been most extensively investigated. The study of the general features of the motor mechanism may well begin with a detailed account of the leaf of this plant.

The motile organ consists of a large mass of highly excitable and contractile cortical cells in the cushioned ido. i. Leaves of M imosa pudica in expanded condition ^left) and contracted condition after stimulation (right). leaf-joint, the pulvinus. A representation of a pair of Mimosa leaves is given in fig. i ; the right Leaf has fallen in consequence of a shock. A contraction of the cortical cells of the pulvinus occurs on stimulation. Since the lewer half of the pulvinus is, however, much larger, more excitable and more actively motile than the upper half, on diffuse

stimulation the greater contraction of the lower half causes the leaf to fall. The actual shortening of the lower half of the pul vims is very small, but as the long petiole acts as a magnifying index, the resulting movement is made conspicuous. The expulsion of sap from the contracting cells simul¬ taneously with the contractile movement can be successfully demonstrated in a vigorous specimen, as follows : Experiment I. Expulsion of sap from the excited cells. — A leaf is detached by a..eut made at the junction of the pulvinus with the "stem. The cut end is immediately placed in a very dilute solution of sodium chloride for several hours so that it may absorb a small quantity of the solution. Although a water-logged condition of the pul/inus, due to excessive absorption of water, causes a diminution or abohtion of motility, nevertheless a vigorous pulvinus partial^ regains its excitability under favourable conditions. The cut end of the pulvinus is then carefully washed with distilled water, and the, leaf, held in a clamp, is so arranged that the cut end is just immersed in a dilute solution of silver nitrate. On application of strong thermal shock to the pulvinus there is an expulsion ol sap from the cortex, which had previously absorbed traces of sodium chloride. The result is a stream of expelled sap projected into the silver nitrate solution, visually manifested as a string o£ white precipitate of silver chloride.

It may be asked what happens to the sap expelled from the excited pulvinus in the intact plant. Since the cortex is continuous in the pulvinus and the stem of the plant, the sap expelled from the contracting cells will pass into adjacent unstimulated cells. It will, in fach be shown in a succeeding chapter that the movement of sap takes place normally along the cortex from cell to cell, from the more stimulated to a less stimulated region. Under strong stimulation the sap may be expelled into the intercellular spaces, and also injected into the xylem, which serves as a reservoir.

Movement being an expression of the contractile reaction, the extent of the movement is a measure of the amount of contraction. If a quantitative stimulus of uniform intensity were applied to the leaf at regular and suitable intervals, and if the physiological condition of the responding tissue remained constant, there would be obtained a series of mechanical responses which would be practically similar to each other. But if the condition were to undergo any change, due to some variation in the environment, then the record would give indications of the invisible internal change. When the vital condition of the plant undergoes depression, the fact is indicated by a diminished amplitude of the mechanical response. If, on the other hand, the moto- excitability of the plant should in any way be enhanced, the amolitude of the response would undergo an increase. The varying effects of freshness and fatigue, of stimulating and depressing drugs, of heat and cold, of light and darkness are in this way exhibited by characteristic modifications of the response. By means of testing-shocks the plant itself can thus be made to reveal those obscure internal changes which are otherwise entirely beyond our scrutiny.

In the investigation of the response of plants two special difficulties have to be overcome. The first of these is to securn quantitative stimulation of uniform intensity, which may be repeated time after time ; the second is the accurate record of the responsive movement. lhe plant can be stimulated by employing any agents that excite the animal tissues. The following are the different modes of stimulation : Specific reactions have been assumed for different stimu¬ lations. In reality there is no such difference, for results wiii be described which show that all kinds of direct stimula¬ tion of effective intensity give rise to contraction, actual or incipient. It must be borne in mind that there is little vitality in the epidermis or outside skin of the plant-body upon which an external stimulus impinges ; hence arises the necessity for special contrivances by which stimulation at the surface may reach the deeper and more active cortex .-in an intensified form. There are in the plant various adaptations by which frictional stimulation or the stimulus of light may induce excitation in the living cells, lactile •hairs or bristles accentuate the external stimulus of contact. In Mimosa, such hairs occur on the under side of the pulvinus, and by their leyet -action cause stimulation of the motor tissue. In many of the tendrils, again, there are tactile pits by which the stimulus of contact is accentuated. As regards the perception of light, Haberlandt has shown that ir: many leaves the epidermal cells are lens-shaped, so that the incident light becomes focussed on the sensitive proto¬ plasmic iayer, and thus a reaction to the stimulus oi ligh* is facilitated. It is obvious that in cases where f he epidermis is perfectly Opaque there can be no protoplasmic excitation under the action of light. As regards electric stimulation there is no such drawback, since electric shocks can penetrate to the internal tissue.

The ideal form of stimulation for quantitative purposes is one of which the intensity can be kept constant in successive experiments, or be varied in a graduated manner. The two most .effective methods for stimulating the plant which fulfil most of v the experimental requirements are fi) the electro-thermic stimulus, and (2) the stimulus of an induction-shock. t Stimulation by direct application of a heated wire causes injury to the tissue. Thermal stimulation, without injury, is rendered practicable by the electric generation of a moderate amount of heat. A loop of line platinum wire surrounds the petiole which is to be stimulated, the current from a battery being led to the loop by means of a fine flexible silver wire (fig. 2). The electric heating of the platinum loop causes a thermal shock, which may be applied once or several times in succession by means of a metronome -inter¬ rupter. The ■ intensity of the stimulus can be increased or diminished by adjusting the heating current. Care must be taken that the heat pro¬ duced in the platinum loop is not so great as to injure the tissue. An additional precaution is to put a drop of water between the platinum loop and the enclosed tissue, the xcess being removed* by blotting-paper. The thin film of water protects the tissue from a burn.

A single shock or a succession of shocks from an induction- coil may be employed. In my earlier experiments this method appeared to be unsuitable for long-continued experi¬ mentation, for the plant became insensitive on account of injury caused by successive shocks. Being misled by the prevailing belief that the excitability of the plant was considerably lower than that of the animal, I had been led to employ an intensity of current wliicn was unnecessarily

uniform stimulation ; metronome em¬ ployed in place of key k. for closing circuit for a definite lenglh of time. high. 1 discovered afterwards that Mimosa, in an optimum condition, was ton times more sensitive to an electric shock than a human being. It will further be shown that a shock individually ineffective becomes effective on repetition. The plant may thus be stimulated by subjecting it to a number of feeble shocks, which cause no injury to the tissue The intensity ot the shock can be gradually increased by bringing the primary coil nearer the secondary (fig. 3). Instead of a single make-or-break shock, alternating

induction-shocks can be produced by an automatic spring- interrupter, included in the primary circuit. 1 he duration of stimulation is adjusted bv a metronome which completes the primary circuit for a definite length of time. The intensity of the exciting shock which 1 have adopted as the unit, is one which barely induces a perceptible sen¬ sation in man. The observer dips two fingers, one of each hand, into two troughs of saline solution, winch are in series with the experimental Mimosa and the secondary coil. This coil, at first placed at a great distance, is gradually brought nearer the primary till, at a certain scale-reading, the observer just begins to perceive the shock. This scale division is marked as 1. The other positions of the scale are marked in terms of this unit. The calibration is

carried out by means of a ballistic galvanometer. The scale readings indicate intensities of stimulus 0*1. 0*5, I, 2, 3, 4, and so on. In a highly excitable specimen of Mimosa the intensity of shock which causes the fall of the leaf is found to be as low as o*i unit. In a nerve-and-muscie preparation the stimulus may be applied either directly on the muscle or on the distant end of the attached nerve. The excitation transmitted by the conducting nerve reaches the muscle and induces it to con¬ tract. Similarly, the pulvinus of Mimosa may be stimulated either directly or indirectly ; in the latter case the stimulu : is applied on the petiole, and the resulting excitation is transmitted along the conducting tissue in the petiole to the pulvinus and causes the fall of the leaf.

1 have shown elsewhere that the conducting tissue is the strand of bast or phloem, which functions as a nerve for the rapid conduction of excitation to a distance, it is to be borne in mind that ho tissue is a perfect conductor, nor is any a non-conductor of excitation, the difference being a matter of degree. When stimulus is applied at the surface of the plant, the excitation traverses the thin layers of intervening semi-conducting tissue and reaches the phloem, which is an efficient conductor.

t he next problem is the accurate record of Uk movement in response to stimulation. A diagrammatic representation of the recorder used is shown in fig. 4. The leal is attached by means of a fine thread to one end of the lever, which is pivoted on jewel bearings. The other end of the lever carries a small weight, by which the thread is kept taut. From the middle of the lever and at right angles tc it, there extends a thin wire with a curved end serving as the writer. The tip of the wire just touches a smoked-glass plate on which the record is made, the plate being allowed to fall under the action of gravity at a rate regulated by clockwork.

When the leaf executes ar abrupt fall on excitation, it pulls down the right end of the lever, so that the writer moves to the left, marking an up-line. The leaf slowly recovers from the effect of the shock and again becomes erect, producing the down-curve of recovery. This record of response, the phytogram, supplies in fact all the informa¬ tion that the myogram affords in the case of the animal muscle. The experimental difficulties in the case of the plant are, however, very great. The pull of the contracting animal muscle is so considerable that the friction offered by the recording surface constitutes no essential obstacle, though even here the time-relations of the curve are rendered unreliable on account of the friction. But in the plant the contractile movement is, generally speaking, only feeble : hence the friction offered, even by the smoked- glass surface, introduces serious error in the record of 4 the amplitude and time-relations of the response.

I have succeeded in eliminating the frictional error by tne employment of different devices, each of which Response Recorder. possesses a certain advantage for the requirements of a particular investiga¬ tion. T he different devices are : 5- the Electro- Oscillating Recorder. 6. The Clockwork Oscillator. The responsive movement can be mognined even to ten thousand times or more by an optical method. The petiole of Mimosa, or the leaflet of any sensitive plant, is attached by a cocoon-thread to the lever. A small reflecting mirror

i’, plant; 1, lever passing through fulcrum rod f with counter¬ poise c ; m, mirror attached to fulcrum-rod ; l, electric lamp : M, mirror for transforming vertical into horizontal movement of spot of light ; m', throws down light on revolving drum d, excursion of which is followed by writing-pen w. is attached to the fulcrum-rod, the rotation of which, under pull of the falling leaf or leaflet, is highly magnified by the light reflected from the mirror, the magnification depending on the distance of the recording surface. The spot of light falls on a vertical revolving drum, round which is wrapped a length of sensitive bromide paper ; the curve of response and recovery is thus obtained by subsequent photographic development. The record can also be taken on a hori¬ zontal drum, by following the moving spot of liglf with

a pencil. In this case the vertical up- or down-movement is 'converted into a right or left movement by a second reflection from another mirror, suitably inclined (fig. 5). Taking a record by the photographic method necessitates the employment of a dark room : but many plants lose their sensibility in the dark. In order to obviate this difficulty and also the discomfoft of working in a dark room, I devised "several types of direct mechanical recorders in which the response is inscribed on a moving smoked-glass plate, the error of friction being avoided ■ by making an intermittent, instead of a permanent, contact, the record consisting of a series of dots instead of a continuous line. Intermittent contact ran be produced either by making the writing-lever tap dots on the recording plate, or Ivy making the recording plate itself move to-and-fro, so as to come periodically in contact With the writing-lever. The first type is designated the Tapping Recorder and the second the Oscillating Recorder. These instruments can be employed in recording the movements not only of sensitive but also of ordinary plants. c

the principle and construction of this apparatus will be understood from fig. 6. A leaf is attached to the short arm of a light horizontal lever, which records its movements on a smoked-glass plate, kept moving from tight to left. This latter movement is produced by the action of a spring barred which pulls the plate-carrier to the left, the rate of release and resulting movement of the plate being adjusted by clockwork ; a thread attached to the plate-carrier is wound round the wheel of a clock, the size and rate of rotation of which modify the speed of the recording plate. The rate of rotation of the clock wheel can be continuously adjusted by 1 special governor, so that the apparatus can be used for records of either short < r long duration. The tip of the lever is not in contact With the smoked glass, but about 2 mm. away from it ; it is made to press periodically

against the recording surface by means of the striker S, attached to a sliding de\ ice. 1 he striker is at a short distance from the lever and at right angles to it. A momentary pull oi Riant placed with root in water- vessel c. Leaf attached to recording-lever which is periodically pressed against smoked- glass plate g, by means of striker s, actuated by pull on the ring r {see text). the ring R with its attached string makes the slide move and press the striker S against the recording-lever which marks a dot on the smoked-glass plate. The successive dots may thus be made at any interval of, say, 2 to io seconds This method of record is relatively simple in working and requires few adjustments.

It is, how ^ vrer. far more convenient to have the record rendered perfectly automatic. The Phvtograph which has Leaf attached to hinged recording-lever, the steel wire in which is periodically attracted by electro-magnet m. Intermittent closure of electric circuit made by contact-maker c, actuated by toothed wheel, b, box containing dry cell. Thread t, attached to plate-carrier, is wound round clock-wheel, rotation of which is regulated by governor. Two cups (one marked a) containing water or other liquid can be rapidly applied ps required to the cut end of stem {see text).

been devised for the purpose is very reliable and sensitive. The leal is attached by a thin silk thread to the recording- lever, which magnifies the movement of the leaf from 10 to 200 . nes. The lever is hinged and a short piece of steel wire attached to the recording-lever in Iront of a small electi -magnet M ; the steel wire, periodically attracted at equal :ntervals by an intermittent electric current sent through the electro-magnet, causes the recording-lever to make a series of dot-marks on the smoked plate. The inter¬ mission of the current is produced by a contact-maker C, actuated by clockwork (fig. 7). The current required for working the electro-magnet is extremely feeble. One small dry cell out of the three employed in an electric torch has been found to last for more than a fortnight for working the apparatus. The electric cell is enclosed in the box B.

In the different types of recorders described above, the error arising from friction is completely removed by the method of intermittent dots for the record. The successive dots, moreover, measure definite intervals of time ; the phytograin is thus its own chronogram. The interval - between *he dots can be made to vary from 1 to 20 seconds, which is sufficient for measurement of the rate of responsive movements of sluggish plants. But in the highly excitable Mimosa the responsive contraction is extremely rapid, and it is therefore necessary to record time-intervals as short as a tenth, a hundredth, or a. thousandth of a second. I will now describe the special Resonant Recorder that has been devised for the purpose of recording such rapid movements.

A diagrammatic representation of this Recorder is given in fig. 8. The writer W is a fine steel wire which lias to be maintained in a state of resonant vibration, and is exactly tuned to vibrate, say, ten times in a second ; it is supported on jewel bearings at the centre of one pole of a circular electro-magnet. The magnetising coil is in circuit with a storage-cell B, and a vibrating reed V which periodically completes the electric circuit. When the reed is exactly

tuned to vibrate ten tipies in a second, the recording- writer is thrown into sympathetic vibration and strikes the smoked- glass plate ones every .tenth of a second. With liner recorders it is possible to measure as short a r time as a thousandth Fig. 8. diagrammatic representation of Resonant Recorder. m, electro-magnet ; \v, resonant writer ; b, battery ; v, vibrating reed. part of a second. A photograph of the upper part of the Resonant Recorder is given in fig. 9.

The records of response of Mimosa given in this chapter were taken with The Resonant Recorder. The complete apparatus is illustrated in tig. 10.^ In order that the results obtained should not bednfluenqed by the personal equation, arrangements were made that thq plant attached to the recording apparatus should be automatically excited by an absolutely constant electric stimulus, and should make its own responsive records, go through its period of recovery, and repeat the same cycle without assistance rr interference at any point on the part of the observer.

that vibrates and taps the record. In the Oscillating Recorder the plate is made to move to-and-iro, and thus to come in intermittent contact with the tip of the writer. 'he smoked-glass carrier is mounted -on a ball-hearing slide for oscillation to-and-iro at right angles to the tip of the writer. I he oscillation of the plate-carrier is secured by made to oscillate. The arrangement of the two pairs of coils is such that the®N and S poles of the moving coils face respectively the S and N poles of the fixed coils. 1 he two pairs m co.ils are slightly separated from each other by a light spring. An electric current .periodically sent through the two coils by a contact-maker activates the coils, and

two pairs of small electro-magnets, periodically activated by the passage of an electric current through them. One pair of coils is fixed, and the second pair is movable, being attached to the slide on ball-bearings by which the plat e-carrier is the electro-magnetic attraction moves the elide forwards, bringing the smoked-glass plate in brief contact with the tip of the writing-lever which thus makes a dot (fig. 11). Fig. 11. Diagrammatic representation of the Electro-Oscillating Record' r. Electro-magnetic oscillating arrangement alone shown m the figuii.

On the cessation of the electric current the slide recedes, and another dot is produced at the next forward move¬ ment of the slide. © In this an eccentric actuated by clockwork produces a periodic to-and-fro movement of the carrier of the smoked plate. A detailed description of this apparatus will be given later. Having explained the, method of uniform stimulation and the accurate means of reconi, 1 will describe the responses of various pul/inated organs which exhibit very interesting and characteristic differences.

Experiment 2 - The record is a horizontal line when the leaf is in the normal outspread position. Electric stimulation of intensity o-i was applied at a ; the specimen was highly excitable and the stimulation proved to be quite effective. The movement of excitatory fall is represented by the up-curve, and the erectile movement of recovery by the The vertical lines below the record indicate intervals of j minute each. down-curve. The contractile reaction was very rapid, ana the successive dots in the record are correspondingly separated from each other. The recovery was very slow, hence the superposed dots appear as a continuous line (tig. 12). The period of maximum fall b is often as short as 1 • 1 second, while in less vigorous specimens it may be as long as 3 seconds ; the period of complete recovery b c is, however, comparatively long, varying under different cir¬ cumstances from 8 to 20 minutes. The maximum rate of the contractile fall of the leaf is about 24 mm., while that of the erectile recovery is 0-09 iwm. per second.

Experiment 3. — Biophytum is also highly excitable, a responsive contrac¬ tion taking place u 11 d er corn par at i vely feeble stimulation ; the recovery is quicker than in Mimosa (fig. 13). The response and recovery were com¬ pleted in the course of only 3 minutes. Successive and uni¬ form responses can therefore be obtained with this plant at shorter intervals than with Mimosa. Experiment 4. — In contrast with the high excitability and rapid contraction of Mimosa is the comparatively feeble excitability and sluggish response of Neptunia oleracca. The