Plant Response as a Means of Physiological Investigation

reversed in sign by unilateral stimulus of light. A constant current, flowing along the length of the organ, often retards of gravity also has the effect of retarding or reversing normal torsional movement. The effective intensity of this stimulus Comparison between nyctitropic and autonomous pulsations — Diurnal movement of plagiotropic stem — Supposed distinction between nyctitropic and other movements of response to stimulus of light — Diurnal response of leaf of Biophytum — Diurnal response of primary petiole of Mimosa — Periodic impulses acting on the leaf — Periodic impulses contributed by the plant as a whole — Other modes of exhibition of diurnal periodicity of hydrostatic tension — Forced vibration and its periodic after-effects — Physical analogue — Impressed periodic vibrations in organ originally radial.

IN addition to the other effects of light which we have been studying, there is also a periodic movement, induced by the alternation of day and night, which consists of a pulsation executed by the responding organs in the very long period of twenty-four hours. In its pulsatory character this movement resembles the so-called autonomous pulsations of such leaflets as those of Desmodium gyrans. But besides the fact that it has the long and definite duration of twenty-four hours, whereas the autonomous pulsations of the leaflets of Desmodium are short and variable in period, there are other differences between the two.

Comparison between nyctitropic and autonomous pulsations. — In the first place it will be noticed with regard to the daily periodic movement that at any given time all the motile organs of the same plant are going through the same phase. This rhythm therefore is in a manner controlled by the plant as a whole. In the case of autonomous movements so called, on the other hand, the seat of rhythm is localised in the motile organ of that particular leaflet whose pulsations are being observed, and there is no necessary coincidence of period, as between any two leaflets on the same plant. The

period of autonomous vibration is much modified, again, by rise or fall of temperature, and other conditions ; whereas the daily period is unaffected by these circumstances. This is due to the fact that the autonomous pulsations are akin to free or natural vibrations, whereas the daily periodic movements are, as we shall see, of the nature of forced vibrations. As the nyctitropic movement in the primary petiole of Mimosa is of a simple character, free from those other complicating considerations which have to be taken into account in the case of the leaflets, this organ has been chosen as the typical specimen by many investigators. We shall therefore consider in detail all its peculiarities in regard to this movement, but I shall at the same time endeavour by the use of the comparative method to trace the evolution of the movement, as first seen in plagiotropic stems, more clearly manifested in certain dorsi-ventral organs like ordinary leaves, and strongly exhibited in pulvinated leaves, such as those of Mimosa.

Supposed distinction between nyctitropic and other movements of response to stimulus of light. — This nyctitropic movement has been sharply distinguished from ordinary response to the stimulation of light, by the statement that while the latter depends on the direction, this is determined only by the varying intensity of illumination. As a further distinction, it is also insisted that, whereas the responsive curvature induced by light takes place in all directions, nyctitropic movement always occurs in a single definite plane.

But whenever we subject the pulvinus of Mimosa to the action of strong light, we obtain response by a greater or smaller depression or fall, which occurs in a definite vertical plane, and is precisely the same in character as that which is exhibited slowly during the course of the whole day, and attains its maximum in the evening. This response to strong light, moreover, is always the same, on whichever side of the pulvinus the stimulus may act — that is to say, it is independent of the direction of light. In analysing this case

further (p. 635) we saw that it constituted a true instance of phototropic reaction, and that this particular movement of fall in a definite direction was due to the stimulus being diffused, whether internally or externally, and by differential action inducing concavity of the lower and more excitable half of the pulvinus. In the assumption by the leaf at evening, then, of its lowest position, we have nothing which is specifically different from this responsive movement, as caused by the action of light on the anisotropic organ ; but this same fall, when it attains its maximum in the evening, as a phase of the nyctitropic movement, is usually said to be due, not to the stimulus of light, but to the variation in that stimulus, or to the effect of on-coming darkness. Now, if it had been true that the diminution of light, or on-coming of darkness in the evening, had acted in some unknown manner as a stimulus to bring about the fall, then we should have found that such a fall was at that time extremely rapid, and that it persisted on the withdrawal of light during the night. As a matter of fact, however, it is found that this movement of the petiole downwards has been taking place progressively throughout the day, and that the fall of the leaf at evening is merely a continuation of this previous movement, and not markedly more rapid at that time than before, if the increased mechanical moment due to the closing of the secondary petioles and their leaflets be eliminated by their removal. Nor does the petiole remain in this depressed position, but begins after an interval to erect itself, till in the morning, or earlier, it has attained its highest degree of erection. So far, again, from darkness being efficient to cause the depression of the leaf, it is well known that on artificially darkening the Mimosa plant the leaves respond by erection.

It is thus clear that we must seek some other explanation of the nyctitropic movement, and as it is known that such periodic movement persists for a time, even when the plant is kept for days in continuous darkness, the inquiry resolves itself into the two questions: (1) how are these diurnal motile impulses generated ? and (2) how is it that the movement will still persist in the absence of a periodically exciting cause ? Diurnal movement of plagiotropic stem.— In order to understand clearly the periodic action of light, in inducing diurnal movements, it is best to take as our startingpoint the simplest type of anisotropic organ, in which this effect may be observed, and for this we may select the plagiotropic stem of, say, Cucurbita. This creeping stem is acted on, under natural conditions, by vertical light from above, the excitatory effect of which, by long-continued action, reaches the lower and more excitable side. The stimulus thus becomes internally diffused, and there is also a certain amount of externally diffused light from the environment. Hence it happens that, by the contraction of the more excitable lower half, the free end of the stem becomes progressively depressed, under the continued action of light, during the course of the day (p. 627). We have seen that in consequence of this the greatest depression occurs about the time of evening, and that on the cessation of the stimulus of light there is a recovery, the stem erecting itself more and more as the night advances (fig. 252). Thus this nyctitropic depression at nightfall is not due to the on-coming of darkness, but represents the cumulative responsive effect oj the day's illumination. In fact we may regard the responsive movement down, and recovery up, which are executed in the course of the diurnal period, as parallel to that phenomenon with which we are already familiar, tof a "single response to a single transient stimulus. The only difference lies in the fact that, while in this latter case the whole process is completed in a few minutes, in the former the stimulus acts continuously during something like twelve hours, and the recovery is allowed to take place during the course of the night.

Thus, in the ordinary case of records of uniform responses to uniform stimuli, the successive stimuli, each lasting for a few seconds, are given at intervals of some minutes, but in the case of the diurnal responses we have a series of stimuli, each lasting for twelve hours, and the beginning of each separated from the next by a period of twenty-four hours. Diurnal responses of leaf of Biophytum. — From this simple instance of anisotropy we pass on to a more highly differentiated case of dorsi-ventrality, as seen in leaves. We have already seen that the petiole of Biophytum is only provided with a moderately developed pulvinus, and that the petiole also, as a whole, acts as a diffuse pulvinoid. In this

case also, by the internally and externally diffused action of light during the course of the day, the petiole is progressively depressed, from its position of highest erection in the morning to its lowest depression in the evening. Recovery takes place again in the absence of the stimulus of light, and the leaf once more assumes its highest position by morning. If, however, in the course of the afternoon, say, at five o'clock, the plant be taken to a dark room, then, owing to the positive after-effect, the leaf will still continue to fall for about an hour, and then begin to erect itself. Owing to the daily periodicity impressed on the plant, of which we shall presently speak in greater detail, the leaves continue to exhibit the diurnal movement even when kept in continuous darkness. I give here (fig. 273) an interesting photographic record of the diurnal movement of the leaf of Biophytum from 5 P.M., when it was brought into the dark room, till 9 A.M. next morning. It will here be seen that, owing to the

FlG. 273. Photographic Record of Diurnal Movement of Petiole of Biophytum from 5 p.m. till 9 A.M. First part of record shows maximum depression to be reached at 6 p.m. and maximum erection at 7 a.m. After the latter hour the leaf again begins to exhibit its usual daytime depression. positive after-effect, the leaf continued to fall till 6 P.M., and that after this it erected itself by a series of five pulsations, until its highest position was attained at about y A.M. It next began to exhibit the impressed effect of day, and the leaf then fell rapidly.

Diurnal response of primary petiole of Mimosa. — In Mimosa we merely find the repetition of those movements whose evolution we have already traced through the plagiotropic stem and the dorsi-ventral petiole. This is made very FlG. 274. Continuous Records of the Diurnal Movement during Thirty six Hours in Two Specimens of Mimosa The continuous line represents the movement of the one-year old, and the dotted line that of the six-months old specimen. The maximum depression is seen to take place at six in the evening, and the recovery nearly completed by midnight. This maximum erection is slightly augmented at dawn.

apparent in the continuous records which I took of the diurnal movement in two specimens of Mimosa, the records being continued during a period of thirty-six hours (fig. 274). The two plants were placed in an open verandah, and long light indices made, as already described, of peacock's quills, were so attached to the petiole as to form prolongations. Records were taken on a vertical revolving drum. A vertical thread was suspended in front of the drum, and the point at which the moving index cut this vertical line was marked at every fifteen minutes. Thus the record gives

a measure of the angular movement at different parts of the day. The dotted line shows the diurnal movement of the younger of the two plants, which was six months old, and the continuous line that of the other, which was one year old. The two, as will be seen, are practically the same. It should be mentioned here that, when this record was taken, 6 A.M. and 6 P.M. were the hours of sunrise and sunset, there being no twilight. The records show that the leaves exhibited the erectile or recovery effect with great rapidity during the first part of the night, this movement being almost completed by I A.M. After this there was but slight upward movement, until about y A.M., from which time onwards, during twelve hours, they fell continuously. The maximum depression was reached at almost exactly 6 P.M., and then the leaves again rose, repeating their former movement of recovery.

Periodic impulses acting on the leaf. — The periodic movements down during the day, and up during the night, are thus seen to be due to periodic impulses of stimulus and recovery acting on the responding leaves. It must be pointed out here that the process of recovery is not altogether passive. We saw in the records of Mimosa under light (fig. 256) that, on the cessation of light, the responding leaf continued to move down for a while owing to the positive after-effect. Later, however, owing to the latent energy which it had acquired by the absorption of light, it exhibited the negative after-effect in an erection which carried it beyond the original position. Hence we can see that the increased internal energy due to previous absorption of light plays an important part in that movement of erection which is initiated shortly after nightfall. We may therefore say that the diurnal movements of the leaves are brought about by two alternate periodic impulses, those, namely, of external stimulus and internal energy. The persistence of the effect of each of these impulses will depend on the intensity of the two factors respectively, and also on the capacity of the tissue for absorbing stimulus. For example, the positive after-effect by which the leaf continues

to be depressed, even on the cessation of the stimulating daylight, may, in some cases, be short-lived, and in others may continue for a considerable period. The return movement will in the latter case be somewhat delayed. Again, the internal energy which hastens recovery may, in certain cases, bring about the utmost erection of which the leaf is capable at'some time earlier than the ensuing morning. Thus, for example, in the response of Biophytum, the highest position (fig. 273) was attained at about 7 a.m., while in the records of these particular Mimosa leaves the corresponding point was reached very much earlier, that is to say at 1 P.M.

Periodic impulses contributed by the plant as a whole. — In addition, however, to those periodic impulses whose seat is in the responding leaf, there is another concordant impulse contributed by the plant as a whole which intensifies the diurnal movement. The plant is subjected during the day to the stimulus of light, which causes contraction of the exterior tissues, and thereby tends to drive the water inwards. The turgidity of the cortex is thus progressively diminished during daylight. But at night the water which has thus been driven into the central reservoir of the plant will flow outwards. In this also the absorbed energy will play an important part. The rhythmic activity of the cortical tissues being thereby increased, they will suck water outwards from the central reservoir, just as the rootlets suck it from the ground. The result of this alternation of external stimulus during the day, and internal stimulus during the night, will be a periodic inflow and outflow — a diminution and increase of tension — the first half of the cycle being completed in the daytime, and the second half in the night.

I have shown (p. 46) that the leaf of Mimosa is erected whenever the internal hydrostatic tension is artificially increased, and depressed when it is diminished ; and we have seen how such variations are induced by the alternation of day and night, not only in individual petioles, but throughout the plant as a whole. The alternate ebb and flow of the water, from the central reservoir, will thus be indicated by the periodic erection and depression of all the leaves synchronously, which will thus act, so to speak, as signal-flags.

Other modes of exhibition of diurnal periodicity of hydrostatic tension. — There are also other modes besides that of mechanical response of the leaves by which this diurnal periodicity of tension can be detected ; and Millardet has shown that the maximum tension in Mimosa occurs at dawn, when the primary petiole is in its most erect position. The minimum tension, again, occurs in the evening, when the leaf occupies its most depressed position. Kraus, further, has found the organs of the plant diminish in bulk from morning till afternoon, and that the reverse process takes place from afternoon till morning. Growth itself, again, is well known to exhibit a diurnal periodicity. It is interesting, however, to realise that this is simply the mode by which a radial organ exhibits, in a form of longitudinal response, what was otherwise exhibited in Mimosa as lateral response. The periodic variation of tension induced by the diurnal period has been seen to manifest itself in Mimosa in two alternate movements, positive and negative. Similarly we shall find, in growthresponse, positive and negative variations above and below the normal or average rate. If we take a balanced record of growth, representing its average rate, a downward line will indicate retardation or negative response, while an upward line will indicate an acceleration of growth, or positive response ; and in thus recording variations of the rate of growth, by the balanced method, for a period of twenty-four hours, we obtain, as we have seen (fig. 191), a curve which closely resembles that of the nyctitropic movement.

Forced vibration and its periodic after-effects. — We have thus traced out the two alternating impulses — the direct effect of external stimulus and its direct after-effect, and the internal stimulus with the negative after-effect — which induce the forced vibration of the responding organ. The periodic effects of protoplasmic contraction and expansion, induced alternately in the plant under alternating light and darkness, thus leave a molecular impress, and this impression is deepened by repetition, finding subsequent expression even when the primary alternating cause is absent. The length of time during which such after-oscillation persists will depend, amongst other things, on the depth of the molecular impression.

Physical analogue.— An analogous physical phenomenon will make the point clearer. If a wire be taken and subjected to alternating molecular strains, say by giving it alternate positive and negative twists, the wire being held in each of these twisted positions for a time, then, even after stoppage of such alternate twisting movements, the released wire will continue to vibrate to and fro in expression of the release of impressed latent strains, consequent on previous forced vibrations.

That such forced vibrations may persist in a plant has been shown by F. Darwin and D. Pertz, who subjected a plant to alternating geotropic stimuli, by which a rhythmic movement was found to persist for a time, even on the stoppage of stimulation. Czapek and Wiesner obtained similar after-vibrations with alternating phototropic stimuli. Impressed periodic vibrations in organ originally radial. — I shall here give a very remarkable instance of such forced periodic vibrations, as induced in an originally radial organ. I had a row of sunflowers planted, in a line which ran accurately east and west, at the season when the sun moved daily in an almost vertical plane. The plants were thus stimulated in the morning from the east, and in the afternoon from the west. All these plants, then, by heliotropic action, followed the path of the sun during the course of the day, and during the course of the night, again, there was recovery. At first the diurnal swing east and west was only through a small angle, but under the action of these repeated periodic impulses it became larger and larger, like that of a pendulum under regular and repeated blows. When the plants had grown to a height of one metre, it was

a remarkable sight in the early mornings to see all the six, with their upper halves bent over equally to the east, and in the evenings equally to the west. One curious phenomenon connected with this consisted in the fact that not only was the nightly recovery completed by I A.M., but the upper part of the shoot was already carried over to the east, just as we found the Mimosa leaf to be erected to the highest position by midnight in consequence of the presence of internal energy.

Analysis of constituent impulses causing nyctitropic movement. — Nyctitropic movements are thus brought about by two different periodic factors, themselves induced by the periodic action of light and darkness. These periodic forces acting on the pulvinus are : 1. The differential heliotropic effect on the pulvinus itself. By the stimulus of light, externally and internally diffused, the dorsi-ventral leaf is progressively depressed during the day. The reverse process takes place at night, by means of natural recovery, aided by internal energy, which gives an impulse opposite to that of external stimulus.

2. A periodic inflow and outflow of water taking place in the plant as a whole, by the recurrent action of light and darkness. This, acting on the dorsi-ventral pulvinus, causes periodic movement of the petiole. 3. Both these periodic forces are concordant in their action on the pulvinated organ, and give rise to periodic movements of large amplitude. The nyctitropic movement of such leaves as that of Mimosa is believed to be distinguished from heliotropic action proper by the facts that (1) it takes place in a definite plane, and (2) it is caused, not by light as a constant force, but by its variation of intensity, the fall of the petiole in the evening being thus ascribed to the on-coming of darkness.

With regard to the first of these points, I have shown that, even under heliotropic action proper, all dorsi-ventral organs move in a definite plane ; and as regards the second, it has been shown that the fall of the leaf in the evening is not due to the action of on-coming darkness, but to the cumulative stimulation of the day's illumination — that is to say, to the action of light as a constant force. It is possible to trace out the gradual evolution of this periodic diurnal movement, taking as the simplest type that of the plagiotropic stem. Under the action of stimulus, which is internally or externally diffused, the lower and more excitable side of such a stem undergoes progressive concavity, the lowest position being attained in the evening. »At night, however, on removal of the stimulus of light, recovery takes place by erection of the stem. A diurnal up- and-down movement is thus induced.

In the leaf of Mimosa the action is precisely the same. Here, owing to the direct action, and the positive aftereffect of light, the leaf is depressed progressively till evening. At night, however, recovery takes place by an erectile movement. This is not due to a passive recovery merely, but is aided by the negative after-effect, consequent on the storage of internal energy by the day's illumination. By this active internal impulse the leaf attains its highest position some time before dawn. The alternate impulses acting periodically on the leaf are thus : (1) the direct effect of continuous stimulation of light, and (2) the opposite impulse due to internal energy.

In addition to these alternate impulses the plant as a whole contributes periodic impulses, which are concordant with the periodic impulses in the leaf. The external tissues of the plant, acted on by light stimulus, contract and drive the water inwards, into the central reservoir. At night a reverse movement of water takes place. As a result of this alternation of external stimulus during the day, and internal stimulus during the night, there is a periodic inflow and outflow, a diminution and increase of tension, and these variations of tension are indicated by the periodic depression and erection of all motile leaves synchronously.

The fact that these periodic forces in leaves and in plant act concordantly causes the periodic movements to be of large amplitude. The periodic diurnal variations of internal hydrostatic tension are also exhibited by periodic variations in the rate of growth. The curves of the diurnal periodicity of growth and of the nyctitropic movement of the leaf are therefore similar. This forced diurnal vibration, being often repeated, gives rise to periodic after-effects, which persist for a time even on the cessation of the periodically exciting cause.

Investigations on the influence of light on the lateral leaflet of Desmodium gyrans : (a) in sub-tonic condition ; (&) in normal tonic condition - Changes induced in existing anisotropy of Desmodium leaflets— Reversal under intense stimulation seen in all forms of response— The swimming movements of ciliated organisms — Fundamental resemblance between the swimming responses and the ordinary heliotropic responses in radial organs— Phototactic movements : {a) Two natural types of responsive movements ; {b) Responsive movement positive, negative, or intermediate, according to intensity of stimulation — Directive action of light— Thermotaxis— Galvanotaxis — Chemotaxis.