Plant Response as a Means of Physiological Investigation

In such fashion, work is performed continuously by the organism, as if by a machine, and the magnitude of the work performed is often very considerable, as is seen, for instance, when sprouting seedlings break through a pavement. The few instances enumerated by no means exhaust the activities that go on in the living machine of the plant ; the) only suffice to give us a glimpse into the complexity of ite functions. We can arrive at a comprehensive idea of such multifarious and obscure phenomena only by coming tc understand the machine itself, and trying to disentangle the processes by which the various stimuli supplied by the environment bring about the appropriate responsive movements in the organism. This is difficult to do, inasmuch as the intricate internal machinery is hidden from our view.

Indicator-diagrams.— Though the interior be thus concealed, however, it is still not impossible, by careful observatior of external actions, to gain some conception of the hidden mechanism. Let us take, for example, the analogous case of a steam-engine. That we may be able to infer at any moment the efficiency of various hidden parts of the machine we attach, to the moving piston, a recording apparatus, and from the diagram thus obtained we are able to judge of the working efficiency of the engine. The upstroke is followed by a downstroke, and a recording pen traces for us, on moving paper, the responsive movements. But an irregularity may suddenly take place in the curve. This is due to some internal obstruction. On removal of the cause, the amplitude of the record is restored, and the pulsating strokes resume their normal frequency.

In dealing with living machines also, we may use similar contrivances, in order to gain some indication ol their efficiency ; and by means of indicator-diagrams, or 1 response-curves,' thus obtained, we are able to gather much information as to the physiological perfection or imperfection of the living machine. Pulse-records as indicators of physiological efficiency. We shall first take as an example that responsive pulsation with which we are most familiar, our own heart-beat. As in the steam-engine the energy of heat brings about the responsive movement of the piston, so in the heart, some internal stimulus brings about responsive pulsations. A

sudden contractile movement is followed by relaxation. By a series of these, the blood is forced in a pulsating manner through the arteries, and we perceive the pulsatory movement at the wrist. Physicians by feeling such pulse-throbbings are able to pronounce on the condition of a patient. Or the movements may be recorded by means of a leverarrangement, in which the short arm of the lever rests on the throbbing pulse. Its longer arm, provided with a tracing point, records these pulsatory movements on a travelling band of paper which is moved by clock-work at a uniform rate. It is on this principle that the instruments known as sphygmographs are constructed, and the response-records, or sphygmograms, reveal the physiological condition of the individual at the time (figs. 1 and 3).

Fig. 1. Record of (a) Healthy Adult and (b) Senile Human Pulse (Broadbent) The heart-record, however, has been still more directly obtained, in the case of the lower animals, by attaching one end of the lever to the apex of the heart itself. Each contraction and subsequent recovery is now recorded, in the manner which has been indicated. If we know the rate at which the recording surface is travelling, or if we make timemarks at regular intervals, we are able to determine the frequency of pulsation. The record also gives us the amplitude of each pulse.

If now these records are to furnish reliable indications of the internal condition of the living machine, then, any circumstance which affects this internal condition must reveal itself in the external record. And this is found to be the case. For example, the effects of age are seen in the accompanying record (fig. 1) ; and that of poison by the gradual waning of pulsation, culminating in arrest at the moment of death (fig. 2). Similarly, changes of heat

and cold, and the influence of various drugs (fig. 3), are all discernible from the modifications which they induce in the pulse-record. For the purpose of studying the actions by which the plant responds to the various stimuli of its environment, I have been able to devise apparatus, by means of which records of its responsive pulsations may be made. In the matter of automatic pulsations, we have in plants many instances which have not hitherto been recognised ; but in one case which is well known, that of Desmodium gyrans — Hedysarum gyrans, the telegraph-plant — we observe pulsatory movements of its lateral leaflets, which, as I shall

Fig. 2. Effect of Muscarin in arresting Pulsation of Frog's Ventricle (Cushing) The arrow indicates the moment of application of reagent in this and following. Fig. 3. Record of Human Pulse [a) before and (d) after Inhalation of Nitrite of Amyl. (Broadbent. ) elsewhere show, exhibit a resemblance to those of the animal heart, a resemblance which is not merely superficial, but is the result of causes fundamentally the same. This telegraph-plant grows wild on the Gangetic plain, where its Indian name is Bon Charal or 'outcast of the forests,' and where the peasant belief is that it dances to the clapping of the hand. It is a papilionaceous plant with trifoliate leaves, of which the terminal leaflet is large, and the two lateral very small. Each of the latter is inserted on the petiole by means of a motile organ known as a pulvinus.

These lateral leaflets, when in normal condition, go on continuously, and apparently spontaneously, executing approximately up and down movements, each of which takes from two to four minutes to complete. The great difficulty in recording the pulsatory movements of Desmodium arises from the extreme slenderness of these lateral leaflets. This is such that in attaching to them a recording lever, however light, its weight, and the friction of the writing-point, are sufficient to bring their movements to a stop. I have, however, succeeded in overcoming this difficulty by devising a recording Optical Lever.

This lever consists of a very light aluminium wire, or, which is still better, the stripped quill of a peacock's tail feather, this being extremely light, and sufficiently rigid for the purpose. The two arms of the lever are unequal. The fulcrum rod rests on frictionless supports of glass or agate. The same rod carries a light mirror. A thread of cocoon silk is stuck to the motile leaflet by a minute drop of shellac varnish. The far end of the thread is looped, and fixed at any suitable notch on the arm B of the lever. The other arm of the lever has a light sliding counterpoise. It will thus be seen that by gradually shifting the silk loop nearer the fulcrum, the magnification may be increased. When the automatically moving leaflet executes a downward movement, the arm B is pulled down, and there is a rotation of the fulcrum rod with its attached mirror. A spot of light reflected from the mirror is thus suddenly moved downwards from its original position. It will be observed that by moving the recording surface further away, the magnification may be still more enhanced. A wide latitude of magnification may thus be obtained, by changes in the effective length of arm of the lever, and by variation of the distance of the recording surface. Thus, for example, for purposes of demonstration, with a screen at a distance of five metres, it is easy to exhibit a pulsatory movement magnified to as much as one metre in amplitude. But in the case of the illustrations in the present book, it has not been found necessary to have any magnifica-

tion whatever, since the movement of the leaflet is itself considerable. A very light counterpoise is used, as will be seen later, to exert a slight pull on the leaflet in an upward direction when necessary, the sliding arrangement enabling us to vary the amount of this tension. It will thus be seen that the leaflet is practically free from constraint, and any movement, however slight, is easily detected. When the leaflet falls, the spot of light moves, say downwards, and vice versa. The record of the entire response — down movement followed by up — may thus be made on a vertical revolving drum, whose speed is regulated by clock-work. The magnification of the record having been determined previously, and the speed of the drum being known, the response-curve gives the absolute movement and the time-relations of such movement.

Instead of using a vertical drum, it is more convenient to record on the revolving surface of a horizontal drum. The up and down movement of the spot of light may now be converted into lateral, or left and right movement, by means of a second mirror suitably inclined. The finer adjustment of the reflected spot of light may be brought about by means of a milled head with which the second mirror is provided. A photographic record may be obtained by wrapping over the drum surface a sensitised roll-film. But since these movements are comparatively slow, it is easy to obtain the record more simply by following the spot of light with a recording pen, which slides on a horizontal guide-bar, parallel with its movements.

These response-records can be traced on a large scale in the presence of an audience, by the use of the Demonstration Recorder, which consists of a twin-drum, over which is wrapped an endless band of paper to serve as the recording surface (fig. 4). Elastic bands pass over the two drums, one of which is kept revolving by clock-work. The excursion of the spot of light is now followed by means of a sliding ink-well, from which projects the ink-sponge. By this means, the tracing of the response-curve, and its various modifications under the

action of different influences, can be made visible to the whole audience. It is thus possible to obtain records of these pulsatory movements, by attaching the Optical Lever to the leaflet of an intact plant. Or we may detach a petiole, B, Arm of Optical Lever, attached to moving leaflet ; L, Ray of light, which after two reflections from the two mirrors falls on the recorder ; c, Clock, which keeps twin-drum— on which is wrapped the recording paper — revolving ; H, Horizontal guide-bar ; K, Ink-well, with projecting sponge.

carrying the leaf, and place it in water, in which case it will remain alive as long as a couple of days, executing its accustomed pulsatory movements during a considerable time. The effect of any given agency, say poison, on the living machinery may now be observed, as graphically indicated in the waning and final arrest of the pulse-record (fig. 5). Or the plant may be killed by passing through it excessively strong electric shocks, after which the occurrence of death will be indicated by the arrest of pulsation (fig. 6).

Thus we see not only the similarity between the pulsations of Desmodium and those of cardiac muscle, but also how similarly both are affected by external agencies, such as poison. Later, we shall study the effects of other physiological in- Fig. 6. Death, and Arrest of Pulsation, in Leaflet of Desmodium by Strong Electric Shock fluences on both. In the present chapter, however, it has been my aim to show that these pulse-records give us a reliable indication of the very obscure modifications of the life-processes initiated in the living tissues by various external factors. Speaking generally, we may say that an exciting reagent exalts the pulse, a depressing reagent reduces the amplitude of pulsation, and a poison arrests it permanently, this arrest being death.

In the cases which we have chosen as examples, there is the advantage of a store of latent energy, which maintains the pulsation by providing an internal source of stimulus. This internal stimulation, as will be shown later, is really derived from external sources, the absorbed energy having been held latent in the plant. We shall in the next chapter take up a very much simpler case, in which the plant has no such reserve, but responds immediately to external stimulus.

A plant, like a machine, responds either to the impact of external forces, or to energy that is latent within. As the working efficiency of an engine is exhibited by indicator-diagrams, so the physiological efficiency of a living machine may be inferred from the character of its pulserecords. Agencies which depress the physiological condition of a tissue, also depress its responsive pulsation. At the death of a tissue there is a permanent arrest of pulsation.

Molecular derangement caused by stimulus — Expression in change of form, contraction — Mechanical model — Myograph— Response by differential contraction in pulvinated plant-organs— Longitudinal response in plants — Response of plant to all forms of stimulus — Plant chamber — Practicable forms of graduated stimulus — Electro-thermic stimulator — Stimulation by condenser discharge — Response-recorder — Advantage of counterpoise — Response of Biophytum to thermal stimulation — Response to condenser discharge — Absolute measurements of motile effect and of work performed — Effect of load — Definite determination of threshold of response — Determination of variation of excitability by measurement of minimally effective stimulus.

FEW of the phenomena" of plant-life are so striking as the conspicuous mechanical movements of certain plants, like Mimosa, commonly known as ' sensitive ' in contradistinction to * ordinary ' plants. These movements take place in response to various forms of stimulation, such as is caused by mechanical touch or application of heat. It will be shown, however, in the course of the present book that this division of plants into sensitive and ordinary is arbitrary, since all plants are sensitive — that is to say, react to stimulus. The plant, throughout its life, is constantly responding to stimuli, external and internal. Some of its responses are manifested in mechanical movements which are too striking to be overlooked. Others, not so obvious, have passed hitherto unnoticed. But in both these cases changes of form occur in the tissue, in consequence of stimulation. In some instances, owing to conditions which will be explained later, these changes produce little visible effect. In others, the responsive change of form is displayed in a striking manner, owing to certain advantageous circumstances of structure, and to the possession of a magnifying arrangement.

The shock of stimulus causes molecular derangement in the tissue of the plant, and it is this fundamental molecular change that finds expression in mechanical movement. It finds independent expression also in electrical movement. For the conspicuous display of mechanical response certain peculiar structural arrangements are, as has been said, advantageous ; but for the exhibition of electrical response, the molecular change itself, which is concomitant to excitation, is the only condition. This subject of the electrical response of plants, however, I treat in detail elsewhere.1 For the present we are concerned only with the question of mechanical response to stimulus. We have not only to determine the existence of such response, but also to ascertain under what conditions it occurs, and by what means it is brought about.

The whole sequence of molecular events initiated by stimulus and expressed as mechanical response, may be very simply illustrated by means of an india-rubber model. We take a piece of stretched india-rubber, attached to a recording lever. The rubber is enclosed in a tube in which there is also enclosed a spiral of thin German-silver wire, by which the indiarubber may be subjected to the momentary action of heat. The quantity of heat generated is regulated by the strength and duration of an electrical current flowing through the heating wire. This application may be uniform for successive experiments, or increased at will.

Longitudinal response. — The thermal stimulus causes a molecular rearrangement in the substance of the india-rubber, in consequence of which the piece becomes shorter and broader. This sudden longitudinal shortening is recorded by the lever as the first half of the responsive movement. As the substance gradually recovers from the effect of the momentary stimulation, the molecules return to their normal position, with a concomitant restoration of the india-rubber to its original form. During this second half of the process, we

1 Bose, Response in the Living and Non-Living (Messrs. Longmans, Green & Co.). Bose, Electro-Physiology of Plants. obtain the curve of recovery.1 If we apply similar stimuli successively, we obtain successive responses which are alike (fig. 7). But if stronger stimulus be applied, by means of stronger heating current, the amplitude of response will be correspondingly increased. In the simple instance which we have considered, the response-record was obtained by taking advantage of the sudden contraction of the indiarubber. In the response of contractile animal muscle, we obtain response-records in exactly the same manner (fig. 8), and such records are known as myographs

Similar contraction in length, or LONGITUDINAL RESPONSE under the action of stimulus, has been shown byl Pfeffer to occur in the The muscle M with the attached bone is securely held at one end, the other end being connected with the writing lever. Under the action of stimulus the contracting muscle pulls the lever, and moves the tracing point to the right over the travelling recording surface p. When the muscle recovers from contraction the tracing point returns to its original position. See on P the record of muscle-curve.

filament of the sensitive stamens of Cynerece. I shall, however, show in Chapter IV. that such longitudinal contraction under stimulus is not unique, but a phenomenon very extensively exhibited by plant-tissues, as seen in the series of uniform responses to stimulation, obtained from the stamen of an ordinary plant, which is here given (fig. 10). 1 Such models made of catgut and stretched caoutchouc have been used by Engelmann for explaining muscle response.

Differential response. — But the responsive movement in plants is more generally produced by differential contractile movement, and a mechanical model again will clearly show how such movements are brought about. We take two equal strips of unequally contracting substances, which are glued together throughout their length. The two strips consist of ebonite and the relatively more contractile caoutchouc. Fig. 10. Photographic Record of Longitudinal Contractile Kesponse in ordinary Stamens {Brownia ariza)

If such a compound strip be held horizontally, with the more contractile element below, and if we subject it to thermal stimulation in the manner described above, the result will be a responsive curvature downwards, the more contractile caoutchouc forming the concave surface. Thermal Fig. 11. Differential Lateral Response of Compound Strip Thermal stimuli applied at intervals of three minutes. stimulus may be applied, as in the last case, by sending a momentarv heating current through an enclosing spiral of German-silver wire, the responses being recorded in the usual manner (fig. 1 1).

unequally contractile, we obtain a DIFFERENTIAL RESPONSE, the more contractile becoming concave ; and it is evident that such movements must take place in a direction perpendicular to the plane of separation. Typical cases of mechanical response in plants are obtained from pulvinated organs. A good example of this is found at the insertion of the petiole in Mimosa pudica. When such an organ is stimulated, it is the lower half that undergoes the greater contraction, and the leaf is depressed by the concavity thus produced. It is generally assumed that the upper half of the pulvinus is not excitable, but this, as I shall show later, is an error. The responsive movement, however, is due to the differential contraction of the two halves, and, as already explained, takes place in a direction perpendicular to the plane which separates them. Such differential response will be found characteristic of all organs possessing dorsi-ventral differentiation.

Whenever the plant is subjected to any sudden disturbance, the sensitive leaf reacts by a fall, which is brought about by the hinge-like mechanism at the pulvinus. The sudden disturbance which induces the fall constitutes the stimulus. The leaf responds when it is shaken, or cut, or when a prick is applied to it, or when a sudden variation of temperature is produced, as by touching it with a hot wire, or with ice, or when an electrical shock is passed through it, or if it be acted on by certain chemical reagents, or a beam of strong light be thrown on it. All these constitute the various forms of stimuli — mechanical, thermal, electrical, chemical, and photic.