(a) by the reversal of closure into opening movement andvice versâ, in consequence of inversion of the plant upside down, and(b) by the diurnal variation of torsional movement, the direction of which is dependent on the directive action of the stimulus of gravity.
(a) by the reversal of closure into opening movement andvice versâ, in consequence of inversion of the plant upside down, and
(b) by the diurnal variation of torsional movement, the direction of which is dependent on the directive action of the stimulus of gravity.
I shall now describe the diurnal movement of various geotropically curved plant-organs; the most striking example of this is furnished by the 'Praying' Palm of Faridpore, already described. I shall here recapitulate some of the important features connected with the phenomenon.
Movements similar to that of the Faridpore Palm (p. 12) are found in other Palm trees growing at an inclination from the vertical. I reproduce once more the diurnal curve given by the Sijberia Palm together with the curve of daily thermal variation (Fig. 199). It will be seen that the two curves resemble each other so closely that the curve of movement of the tree is practically a replica of the thermographic record. There can therefore be no doubt of the movement being brought about by variation of temperature; rise of temperature is attended by the movement of fall of the tree andvice versâ. The record was commenced at noon; the temperature rose till the maximum was reached at about 3 p.m. and the tree also reached its lowest position at 3-45 p.m., the lag being 45 minutes. The temperature fell continuously after the maximum at 3 p.m., to the minimum at 6 a.m. next morning. In response to the falling temperature, the tree exhibited a movement of erection. The temperature roseafter 6 a.m. and the movement of the tree became reversed from ascent to descent.
Fig. 199.Fig. 199.—Diurnal record of the Sijberia Palm. Upper curve gives variation of temperature, and the lower curve the movement of the tree.
Fig. 199.—Diurnal record of the Sijberia Palm. Upper curve gives variation of temperature, and the lower curve the movement of the tree.
I have already shown: (1) that the diurnal movement just described is due to physiological reaction, and that the movement is abolished at the death of the plant; (2) that light has little or no effect, since the thick bark and bases of leaves screen the living tissue from the action of light; (3) that transpiration has practically no effect on the periodic movement, since such movement takes place in other plants completely immersed under water; thusIpomœa aquatica, a water plant, kept under water, gave the normal diurnal curve similar to that of the palm. The modifying effect of transpiration was in this case, completely excluded. I obtained similar effect with geotropically curved stem ofBasella cordifolia(p. 25);(4) that the weight of the plant-organ as such, has little effect on the diurnal curve, since an inverted plant continues for a few days to exhibit the periodic movement, in spite of the antagonistic effect of weight. A different experiment will be described (see p. 582) where the effect of weight was completely neutralised and the plant-organ gave, nevertheless, the normal diurnal curve.
Fig. 200.Fig. 200.—Diurnal record of inclined palm tree, of geotropically curved procumbent stem ofTropæolumand the dia-geotropic leaf of palm. Note general similarity between diurnal curve of plants and the thermographic record.
Fig. 200.—Diurnal record of inclined palm tree, of geotropically curved procumbent stem ofTropæolumand the dia-geotropic leaf of palm. Note general similarity between diurnal curve of plants and the thermographic record.
I have also shown that the diurnal movement is determined by the modifying influence of temperature on geotropic curvature. Rise of temperature opposes or neutralises the geotropic curvature; fall of temperature, onthe other hand, accentuates it. The particular diurnal movement was not confined to the palm trees, but was exhibited by all plant-organs subjected to the stimulus of gravity.
Fig. 201.Fig. 201.—-Diurnal records of leaves ofDahlia,PapayaandCroton.
Fig. 201.—-Diurnal records of leaves ofDahlia,PapayaandCroton.
Experiment 210.—In order to demonstrate the continuity of the phenomenon of diurnal movement I took various stems growing in water or land for my experiment. The plants were laid horizontally, till the stems bent up and assumedthe stable position of geotropic equilibrium. In figure 200 is given records of the inclined palm tree, of procumbent stem ofTropæolum, and the leaf of the palm tree. The very close relation between the temperature-variation and the movement of different plant-organs is sufficiently obvious.
I shall next give a series of diurnal records of leaves of different plants such as those ofDahlia,PapayaandCroton(Fig. 201). In these also fall of temperature induces an up-movement while rise of temperature causes a fall of the leaf. I shall presently refer to the 'personal equation' by which the record of one plant is distinguished from another.
Experiment 211.—The diurnal record given above, was taken from ordinary noon at 12 o'clock to noon next day. The diurnal curve becomes much simplified if the record be taken fromthermal-noon(at about 2 p.m.) to the thermal noon next day. The plant-organ becomes erected during falling temperature from thermal-noon to thermal-dawn next morning, and undergoes a fall during rise of temperature from thermal-dawn to thermal-noon. The subsequent diurnal records will therefore be given for 24 hours commencing with 2 p.m. In figure 202 is given diurnal records of geotropically curved stem ofTropæolumand the leaf ofDahliafor two days in succession.
Fig. 202.Fig. 202.—Diurnal curve of the procumbent stem ofTropæolum majus, and the leaf ofDahliafor two successive days. In the thermographic record the up-curve represents fall, and down-curve rise of temperature.
Fig. 202.—Diurnal curve of the procumbent stem ofTropæolum majus, and the leaf ofDahliafor two successive days. In the thermographic record the up-curve represents fall, and down-curve rise of temperature.
The thermal record shows that there was a continuous fall of temperature from thermal-noon at 2 p.m. to the thermal-dawn at 6 a.m. next morning, that is to say, for 16 hours. Rise of temperature through the same range occurred in 8 hours from 6 a.m. till 2 p.m. The average rate of rise of temperature was thus twice as quick as that of fall. This is clearly seen from the slopes of thermal curve during thermal ascent and descent. The record of the movement of the plant shows a striking parallelism; the different plant-organs became erected from thermal-noon to thermal-dawn, and underwent a fall from thermal-dawn to thermal-noon. The descent of the curve is, as in the case of thermal curve, relatively more abrupt. The records on two successive days are very similar, the slight difference being due to the physiological depression consequent on prolonged maintenance of the plants in a closed chamber.
I shall now proceed to explain the modifications that may occur in the standard thermo-geotropic curve.
Turning points.—In the bulky Palm, the reversal of movement from fall to rise orvice versâtakes place about an hour after the thermal inversion. This lag is partly due to the time taken by a mass of tissue to assume the temperature of the surrounding air. There is, moreover, the question of physiological inertia which delays the reaction. In leaves this lag may be considerably less or even absent. In certain cases the reversal of movement may take place a little earlier than the temperature inversion. It should be remembered in this connection, that in response to temperature change, the leaf is often displaced to a considerable extent from its 'mean position of equilibrium'; moreover the force of recovery is greatest at the two extreme positions. These considerations probably explain the quick return of the leaf to equilibrium position. The slow autonomous movement of the leaf may sometimes prove to be a contributory factor.
Effect of irregular fluctuation of temperature.—In settled weather the diurnal rise and fall of temperature is very regular. But under less settled condition, owing to the change of direction of the wind, the temperature curve shows one or more fluctuations, specially in the forenoon. It was a matter of surprise to me to find the plant-record repeating the fluctuations of thermal record with astonishing fidelity. This common twitch in the two records is seen in the record of the Sijberia Palm (Fig. 199). Certain plants are extremely sensitive to variation of temperature; so much so that these physiological indicators of thermalvariation are far more delicate than ordinary thermometers.
Effect of restricted pliability of the organ.—A leaf is more pliable in one direction than in the other. The pulvinus ofMimosa, for example, allows a greater amount of bending downwards than upwards; in consequence of this the leaf in its fall becomes almost parallel to the internode below; the up-movement is, however, far more restricted. The leaf in its most erect position still makes a considerable angle with the internode of the stem above it. If the leaf-stalk of a plant be restricted in its rise the erectile movement at night will reach a limit, and the top of the curve will remain flat. This is seen, illustrated in the record of the leaf ofCroton(Fig. 202), which attains its maximum erection at 9 p.m. and the subsequent curve remains flattened till 7 a.m.; after this the leaf begins to execute its downward movement. In other cases, the range of up-movement is very great and the plant-organ erects itself continuously till morning. In certain cases the impulse of up-movement carries the organ beyond the stable position of equilibrium; after this the leaf begins to retrace its path slowly; the down-movement due to rise of temperature is, however, far more abrupt, and easily distinguishable from the previous slow return.
It will thus be seen that though the diurnal record consists of an alternating up and down curve, yet these minor characteristics or 'personal equation' of the plant confers on the record a certain stamp of individuality.
Effect of age.—In the floral leaves ofNymphæathe thermonastic movement is of positive sign; that is to say, an erection of the petal during rise, and a fall during the lowering of temperature. The corresponding movement of leaves would therefore be an erection of the leaf inday-time, and a fall of the leaf at night. The periodic curve of such leaves would be of opposite sign to the standard thermo-geotropic curves given above. The leaf ofNicotinais adduced as an example of a leaf which exhibits a movement of fall at night. But the fully grown and horizontally spread leaf I find that gives the normal record. The very young growing leaves give a different and somewhat erratic curve. The difference between growing and fully grown leaves is explained by the fact that the former would be affected by thermotropism, and the latter by thermo-geotropism. Young leaves exhibit moreover a pronounced hyponasty or epinasty, which would naturally modify the diurnal curve.
Certain interesting variation is met with in the diurnal record of sprouting leaves ofMimosain spring. The movements of leaves grown later in the season, as will be explained in a later chapter, are very definite and characteristic. But the young leaves in spring exhibit no definite diurnal curve, but a series of automatic pulsations, the unsuspected presence of which in all leaves ofMimosawill be demonstrated in a subsequent chapter. Later in the season, the leaf becomes tuned, as it were, to the periodic variation of the environment; the automatic movements become suppressed, and the diurnal periodicity becomes deeply impressed on the organism.
Effect of season.—The diurnal curve may also be modified by the seasonal variation of any one of the effective factors.Tropæolum majus, for example, exhibits positive phototropic action in one season and a negative reaction in a different season. These seasonal variations must necessarily modify the diurnal curve.
I shall now proceed to demonstrate the determining influence of thermal variation, and of stimulus of gravity on the thermo-geotropic movements. The striking similarity of the thermograph, and the record of movement of plantsdemonstrate the causal relation between temperature variation and diurnal movement, of which the two additional tests described below offer further confirmation.
The normal diurnal movement is, as we have seen, a fall during rise of temperature from morning to afternoon, and a rise from afternoon till next morning. I succeeded in reversing the normal rhythm ofBasellaby reversing the normal variation of temperature at the two turning points, in the morning and in the afternoon. The plant was subjected to falling temperature in the morning and to rising temperature in the afternoon. The normal movement now became reversed,i.e., an erection instead of fall in the forenoon and a fall instead of rise in the afternoon (p. 28).
The second test which I shall employ is the effect of maintenance of constant temperature, which should wipe off, as it were, traces of periodic movement. It was necessary for this investigation to maintain the plant chamber at constant temperature throughout day and night. The usual thermostat is virtually a recess in a double-walled chamber filled with water, the chamber being covered with a heat insulating material. But this contrivance is unsuitable for the plant chamber which is to contain good sized plants, and the recording apparatus. The problem of maintaining a large air-chamber at constant temperature presented many difficulties which were ultimately overcome by the device of an extremely sensitive thermal regulator.
The Thermal Regulator.—I shall in a future paper give a complete account of the large thermostatic air-chamber. The important part of the apparatus is an electro-thermic regulator which interrupts the heating electric current as soon as the temperature of the chamber is raised ahundredth part of a degree above the predetermined temperature. The automatic make and break of the current takes place in rapid succession, and the temperature of the chamber is thus maintained constant within tenth of a degree, throughout day and night.
Fig. 203.Fig. 203.—Abolition of diurnal movement inTropæolumunder constant temperature, and its restoration under normal daily fluctuation. The upper record is of temperature and the lower of plant movement.
Fig. 203.—Abolition of diurnal movement inTropæolumunder constant temperature, and its restoration under normal daily fluctuation. The upper record is of temperature and the lower of plant movement.
Diurnal record ofTropæolumunder constant temperature: Experiment 212.—The normal record of geotropically curvedTropæolumis already given in figure 202. In repeating the record I maintained the plant at constant temperature for 24 hours; the result of this is seen in the first part of the record (Fig. 203). The thermal record is practically horizontal, and the diurnal record of the plant shows no periodic movement. The thermal regulator was on the next day put out of operation, thus restoring the normaldiurnal variation of temperature. The record of the plant is seen to exhibit once more its normal periodic movement.
I have in the chapter on thermo-geotropism (p. 515) shown that the diurnal movement of a geotropically curved organ is determined in reference to the direction of force of gravity. This will be seen demonstrated in an interesting manner in the two following experiments on the effect of inversion of the plant on daily movement.
I have already referred to the distinction that is made between nastic and paratonic movements. In the former the movement is autonomous and in relation to the plant, and in the latter it is due to an external force which determines the direction of movement. In nastic reaction, closure movement would persist as a closure movement[43]; but should the direction of movement be determined by the stimulus of gravity, closure movement would, on inversion, be reversed into an opening movement. Viewed from an external point of view an up-movement in the latter case would, after readjustment on inversion, become an up-movement, though in so doing, the expansion should be transferred from the upper to the lower side of the organ. It is to be understood in this connection, that some time must lapse before this readjustment is possible, and that the former movement may continue, in certain cases, as a persistence of after-effect.
I succeeded in demonstrating the paratonic effect of geotropic stimulus on the periodic movement of the palm leaf, by holding the plant in an inverted position (p. 24). On the first day of inversion, the diurnal record was erratic, but in the course of 24 hours, the leaf readjusted itself to its unaccustomed position, and became somewhat erected under geotropic action. After the attainment ofthis new state of geotropic equilibrium, the leaf gave the record of down-movement during rise, and up-movement during fall of temperature, movements which in reference to the plant are the very opposite to those in a normal position. But seen from an external point of view, rise of temperature caused in both normal and inverted positions, a down-movement indicative of diminished geotropic curvature; fall of temperature, on the other hand, brought about an erectile movement, thus exhibiting enhancement of geotropic curvature.
Fig. 204.Fig. 204.—Effect of inversion of the plant on diurnal movement. (a) Normal record, (b) record 24 hours after inversion and (c) after 48 hours (Tropæolum).
Fig. 204.—Effect of inversion of the plant on diurnal movement. (a) Normal record, (b) record 24 hours after inversion and (c) after 48 hours (Tropæolum).
Experiment 213.—A still more striking result exhibiting the phase of transition was given by the geotropically curved stem ofTropæolum. Its diurnal curve and the subsequent changes after inversion are given in figure 204. In(a) is seen the normal diurnal curve; the specimen was inverted, and it took an entire day for the plant to readjust itself to the new geotropic condition. The record (b) was recommenced on the second day after inversion; the persistence of previous movement is seen in the reversed curve during the first half of the second day; but in the second half the record became true, and the third day the inverted plant gave a record which, from an external point of view, was similar to that given by the plant in the normal position.
A continuity is shown to exist between the thermo-geotropic response of rigid trees, stems, and leaves of plants.
The diurnal record exhibits an erectile movement from thermal-noon to thermal-dawn, and a movement of fall from thermal-dawn to thermal-noon.
In contrast with thermonastic movement which takes place in growing organs, thermo-geotropic movement takes place in fully grown organs including rigid trees. The thermonastic movement is independent of the direction of gravity, while in thermo-geotropic reaction, the stimulus of gravity exerts a directive action.
The effect of variation of temperature on the diurnal movement is demonstrated by induced change of normal rhythm, by artificial transposition of periods of thermal inversion, and by the abolition of periodic movement under constant temperature.
The effect of stimulus of gravity on the diurnal movement is demonstrated by the effect induced on holding the plant upside down. The direction of the daily movement is found to be determined by the directive action of the stimulus of gravity.
[43]By closure is meant movement of opposite pairs of leaf-organs towards each other.
[43]By closure is meant movement of opposite pairs of leaf-organs towards each other.
We have considered two types of diurnal movement, one due to the predominant action of variation of light, and the other, to that of changing temperature. There are, however, other organs which are sensitive to variations both of light and of temperature. The effect of light is, generally speaking, antagonistic to that of rise of temperature; hence the resultant of the two becomes highly complex.
Still greater complexity is introduced by the different factors of immediate and after-effect of light. This latter phenomenon is very obscure, and I attempted to determine its characteristics by electrical method of investigation. A fuller account of after-effect of light on the response of various plant-organs and of animal retinæ will be found elsewhere.[44]I shall here refer only to one or two characteristic results which have immediate bearing on the present subject.
Direct stimulation under light induces excitatory reaction, which is mechanically exhibited by contraction, and electrically by induced galvanometric negativity. Under continuous stimulation, the excitatory effect, either of positive curvature or of induced galvanometric negativity, is found to attain a maximum. This is often found to undergo a decline and reversal; for under continuousstimulation there is a fatigue-decline, as seen in the relaxation following normal contraction in animal muscle. The positive tropic curvature, and the induced galvanometric negativity may thus undergo a decline, and neutralisation. This neutralisation is also favoured, in certain cases, by transverse conduction of excitation to the distal side.
The character of the after-effect will presently be shown to be modified by the duration of previous stimulation, the different phases of which will for convenience, be distinguished as pre-maximum, maximum and post-maximum. Since stimulus simultaneously induces positive "A" and the negative "D" changes (p. 143), their intensities will undergo relative variation during the continuance and cessation of stimulus. The after-effect will therefore exhibit unequal persistence of the expansive "A" and contractile "D" reaction at different phases of stimulation.
Confining our attention to the electric response, it is found that under continued action of light the excitatory galvanometric negativity increases to a maximum, after which there is a decline, and neutralisation. Figure 205 gives the galvanographic record of the electric response of the leaf stalk ofBryophyllumunder light; the up-curve represents increasing negativity which, after attaining a maximum, undergoes neutralisation as seen in the down-curve. I shall, with the help of the diagram given in the next figure, describe and explain the various after-effects I observed on sudden stoppage of light: before the attainment of maximum, at the maximum, and after the maximum.
Fig. 205, 206.Fig. 205.Fig. 206.Fig. 205.—Electric response of the leaf-stalk ofBryophyllumunder continuous photic stimulation. Increasing negativity represented by up-curve; neutralisation by down-curve.Fig. 206.—Diagrammatic representation of electric after-effect of stimulation. Pre-maximal stimulation produced by stoppage of light ata, gives rise to continuation of previous response followed by recovery. Stoppage of light at maximumbgives rise to recovery to equilibrium position. Stoppage of light at post-maximumc, gives rise to over-shooting below zero line.
Fig. 205.Fig. 206.
Fig. 205.—Electric response of the leaf-stalk ofBryophyllumunder continuous photic stimulation. Increasing negativity represented by up-curve; neutralisation by down-curve.
Fig. 206.—Diagrammatic representation of electric after-effect of stimulation. Pre-maximal stimulation produced by stoppage of light ata, gives rise to continuation of previous response followed by recovery. Stoppage of light at maximumbgives rise to recovery to equilibrium position. Stoppage of light at post-maximumc, gives rise to over-shooting below zero line.
After-effect of pre-maximum stimulation: Experiment 214.—Light is applied at arrow and stopped in different experiments ata,b, andc(Fig. 106). Continuous stimulation induces increasing galvanometric negativity; when stimulus is stopped atabefore the maximum, the after-effect is a persistence of excitatory galvanometric negativity,which carries the response record higher up; after a certain interval recovery takes place and the record returns to the zero line of normal equilibrium. The after-effect of pre-maximum stimulation is thus a short-lived continuance of response followed by recovery.
After-effect at maximum: Experiment 215.—In this the photic stimulus was continued till the attainment of maximum, when light was suddenly removed atb. The after-effect was no longer a persistence of responsive movement, but disappearance of negativity and recovery to zero line of equilibrium.
Post-maximum after-effect: Experiment 216.—In this light was continued till there was a complete neutralisation, the curve of response returning to zero line; to all outer seeming the responsive indication of the tissue is the same as before excitation. But stoppage of stimulus atccauses an over-shooting at a rapid rate farbelowthe zero line; and it is after a considerable period that the curve returns to the zero line of equilibrium.
The condition at post-maximumcis thus one of dynamic equilibrium where two opposite activities, "A" and "D," balance each other; for had the condition of the 'neutralised' tissue been exactly the same when fresh, cessation of stimulus would have kept the galvanometric spot of light at the zero position.
The electric investigation described above shows that the after-effect is modified by duration of stimulation, and that:
(1) the after-effect of pre-maximum stimulation is the continuation of response in the original direction (upward, and away from zero line), followed by recovery,(2) the after-effect of the maximum is an electric recovery towards zero position, and(3) the after-effect of post-maximum stimulation is an over-shootingdownwardbelow the zero line.
(1) the after-effect of pre-maximum stimulation is the continuation of response in the original direction (upward, and away from zero line), followed by recovery,
(2) the after-effect of the maximum is an electric recovery towards zero position, and
(3) the after-effect of post-maximum stimulation is an over-shootingdownwardbelow the zero line.
I shall now describe the after-effect of light as seen in mechanical response, and the results will be found parallel to those given by the electric response. The specimen employed is the terminal leaflet ofDesmodium gyrans, the pulvinus of which is very sensitive to light. Pulvinated organs, generally speaking, exhibit a diurnal variation of turgor in consequence of which the position of equilibrium of the leaf or leaflet undergoes a periodic change. But this equilibrium position of the organ remains fairly constant for nearly two hours about midday, the variation of temperature at this period being slight. We may therefore obtain the pure effect of light by carrying out the experiment at this period, and completing it within a short time to avoid complication arising from the autonomous variation of turgor.
The period of experiment of the plant may be shortened by a choice of suitable intensity of light; a given tropic effect induced by prolonged feeble light may thus be obtained by short exposure to stronger light. The source of light for the following experiment was a 50 c.p. incandescent lamp. The intensity was increased to a suitable value by focussing light on the upper half of the pulvinus by means of a lens. The intensity was so adjusted that the maximum positive curvature was attained in the course of about 6 minutes, and complete neutralisation after an exposure of 17 minutes.
Pre-maximum after-effect: Experiment 217.—Light was allowed to act on the upper half of the pulvinus for two minutes and twenty seconds; this induced an up-movementi.e., a positive curvature. On the stoppage of light the up-movement continued for one minute and twenty seconds, after which the down-movement of recovery was completed in six minutes and twenty seconds (Fig. 207). The immediate after-effect is thus a movement upward, away from the zero line of equilibrium. The result is seen to be the same as the electric after-effect of pre-maximum stimulation.
Fig. 207, 208, 209.Fig. 207.Fig. 208.Fig. 209.Fig. 207.—Light applied at arrow, and stopped at the second arrow within a circle. After-effect of pre-maximum stimulation is continuation of positive curvature followed by recovery.Fig. 208.—After-effect at maximum; recovery towards zero position of equilibrium.Fig. 209.—After-effect at post-maximum is a rapid overshooting below the position of equilibrium. Light was applied in all cases on upper half of pulvinus of terminal leaflet ofDesmodium gyrans.
Fig. 207.Fig. 208.Fig. 209.
Fig. 207.—Light applied at arrow, and stopped at the second arrow within a circle. After-effect of pre-maximum stimulation is continuation of positive curvature followed by recovery.
Fig. 208.—After-effect at maximum; recovery towards zero position of equilibrium.
Fig. 209.—After-effect at post-maximum is a rapid overshooting below the position of equilibrium. Light was applied in all cases on upper half of pulvinus of terminal leaflet ofDesmodium gyrans.
After-effect at maximum: Experiment 218.—Application of light for 5 minutes and twenty seconds induced a maximum positive curvature. Stoppage of light was followed at once by recovery which was completed in about 10 minutes (Fig. 208).
After-effect at post-maximum: Experiment 219.—As the plant was fatigued by previous experiments, a fresh specimen was taken and light was applied continuously on theupper half of the pulvinus. This gave rise first to a maximum positive curvature, subsequently diminished by transverse transmission of excitation. Neutralisation took place after application of light for 17 minutes. On the stoppage of light, there was a sudden overshootingbelowthe zero line (Fig. 209), and the rate of the movement on the cessation of light was nearly twice as quick as during the process of neutralisation.
The after-effect of light is modified by the duration of exposure to light.
Under continued action of light, the electric response of galvanometric negativity in plants attains a maximum after which it undergoes decline, and neutralisation.
The electrical after-effect exhibits characteristic differences depending on the duration of previous exposure to light.
The pre-maximal after-effect is a temporary continuation of response under light followed by recovery.
The after-effect at the maximum is a recovery to the normal equilibrium.
The after-effect at post-maximum is an 'overshooting' below the position of equilibrium.
The immediate and after-tropic response of light are similar to the corresponding photo-electric effects.
The pre-maximum after-effect is a continuation of positive tropic movement followed by recovery; the after-effect at maximum is a recovery to the normal equilibrium position of the organ. The post-maximum after-effect is an overshooting below the position of normal equilibrium.
[44]"Comparative Electro-Physiology"—p. 392.
[44]"Comparative Electro-Physiology"—p. 392.
In the standard curve of nyctitropic movement under thermo-geotropism described in a previous paper, the diurnal record consisted of an up-curve from thermal-noon to thermal-dawn, and a down-curve from the thermal-dawn to thermal-noon. The responding organ, which may be an inclined stem or a horizontally spread petiole, underwent an erection during the decline of temperature, and a fall with the rise of temperature. The diurnal record of theMimosaleaf appears, however, to be totally different.
Experiment 220.—I obtained the diurnal record ofMimosa(Fig. 210) for twenty-four hours commencing at 2 p.m. which is the thermal-noon. The summer and winter records are essentially the same; the only difference is in the greater vigour of movement exhibited by summer specimens. The diurnal movement of the leaf is very definite and characteristic; for the curves taken five yearsago do not differ in any way from those obtained this year. The record may conveniently be divided into four phases.
Fig. 210.Fig. 210.—Diurnal record ofMimosain summer, and in winter. Leaf rises from 2 to 5 p.m., when there is a spasmodic fall. Leaf re-erects itself from 9 p.m. to 6 a.m. after which there is a gradual fall till 2 p.m. with pulsations. The upper-most record gives temperature variation, up-curve representing fall of temperature andvice versâ.
Fig. 210.—Diurnal record ofMimosain summer, and in winter. Leaf rises from 2 to 5 p.m., when there is a spasmodic fall. Leaf re-erects itself from 9 p.m. to 6 a.m. after which there is a gradual fall till 2 p.m. with pulsations. The upper-most record gives temperature variation, up-curve representing fall of temperature andvice versâ.
First phase.—The leaf erects itself after the thermal-noon up to 5 or 5-30 p.m. The temperature, it should be remembered, is undergoing a fall during this period.
Second phase.—There is a sudden fall of the leaf in the evening which continues till 9 p.m. or thereabout.
Third phase.—The leaf erects itself till thermal-dawn at about 6 a.m. next morning.
Fourth phase.—There is a fall of the leaf during the rise of temperature from thermal-dawn to thermal-noon. The uniformity of the fall is, however, interrupted by one or more pulsations in the forenoon. These pulsations are more frequent in summer than in winter.
It will thus be seen that the difference between the normal thermo-geotropic curve, and the curve ofMimosais not so great as appears at first sight. With the exception of the spasmodic fall in the evening, the diurnal curve shows an erectile movement during lowering of temperature, and a movement of fall during rise of temperature. I shall presently explain the reason of the sudden fall in the evening, and of the multiple pulsations in the forenoon.
I have, moreover, been able to trace a continuity inMimosaitself, between the standard thermo-geotropic reactions and the modification of it by the action of light. The young leaves which sprout out at the beginning of spring take some time to become adjusted to the diurnal variation. There are two intermediate stages through which the leaves pass before they exhibit their characteristic diurnal curve. Slow rhythmic pulsations are at first seen to occur during day and night. At the next stage the leaves exhibit the diurnal movement of fall from thermal-dawn to thermal-noon, and movement of erection from thermal-noon to thermal-dawn next morning, the record being in every way similar to the standard thermo-geotropic curve. It is only at the final stage that there is a spasmodic fall in the evening which we shall find is the characteristic after-effect of light.
Before proceeding further I shall refer briefly to the theory of Millardet in explanation of the diurnal movement of the leaf ofMimosa. He found that the tension in stems, and presumably its turgor, is increased with rise and decreased with fall of temperature. The movement of the lateral leaf may, therefore, be due to the induced variation of tension in the main axis. Had this been the case the minimum tension would have occurred at the minimum temperature in the morning, and the leaf should have undergone a maximum fall. The maximum temperature attained in the afternoon should have, on the other hand, brought about the maximum erection. The observed facts are, however, the very opposite to these. Kraus and Millardet also found that light and darkness had great influence on the tension, which increases in darkness and diminishes in light. The tension at dawn may therefore be a resultant of the depressing effect of low temperature opposed by the promoting effect of darkness, the latter being the predominant factor. The erect position ofMimosaleaf in the morning may thus be accounted for by the resultant increase of tension of the stem. The explanation of the movements of the leaves is thus to be attributed to the variation of tension in the main axis to which the leaves are attached; this leads to the conclusion that the leaf movement should be determined in relation to the plant, and not in relation to the external stimulus. I shall, however, describe a crucial experiment in the course of this paper, which will show that the direction of stimulus of gravity has a determining influence on the periodic movement. The sudden fall of the leaf before evening is again inexplicable from the theory of periodic variation of tension.
The complexity in the diurnal movement inMimosaarises from the fact that there are three factors whose fluctuating effects are different at different parts of the day. The effect at any particular hour results from thealgebraical summation of the following factors: (1) the thermo-geotropic action, (2) the immediate effect of photic stimulus and (3) the after-effect of light. The leaf ofMimosahas, moreover, as I shall show, an autonomous movement of its own. I shall take up the full consideration of the subject in the following order:
1.The thermo-geotropic reaction.—A crucial experiment will be described which demonstrates the effect of thermo-geotropism in the diurnal movement of the leaf ofMimosa.
2.Autonomous pulsation ofMimosa.—The natural pulsation of the plant is obscured by the paratonic effect of external stimuli. I shall explain the method by which the natural pulsation of the leaf becomes fully revealed.
3.The immediate effect of light.—This is not constant, but will be shown to undergo a definite variation with the intensity and duration of light. A very great difficulty in the study of effect of daylight at different parts of the day is introduced on account of the absence of any reliable recorder for measurement of fluctuation of light. I shall describe a device which gives a continuous record of photic variation for the whole day.
4.The after-effect of light.—The spasmodic fall of the leaf ofMimosatowards the evening presents the most difficult problem for solution. I shall first describe the diurnal movement of another plant which presents characteristics similar to those ofMimosa. I shall also demonstrate the various after-effects of light at different parts of the day. These results will offer the fullest explanation of the sudden fall of the leaf towards evening.
As regards the sudden fall of the leaf about evening, Pfeffer regarded it as due to increased mechanical moment of the secondary petioles moving forward onthe withdrawal of light. I shall, however, in the course of this paper show, that the characteristic movements occur even after complete removal of the sub-petioles. In the following experiment, carried out with the intact plant, the effect of possible variation of weight is completely eliminated. In spite of this, the diurnal movement exhibited its characteristic phases including sudden movement in the evening.
The experiment I am going to describe will exhibit the diurnal curve obtained by an entirely different method, and will clearly exhibit the thermo-geotropic effect, as well as the immediate and after-effect of light.
I have shown that the pulvinus ofMimosa, subjected laterally to the action of stimulus of gravity, exhibits a torsional response. When theMimosaplant is laid sideways, so that the plane of separation of the upper and lower halves of the pulvinus is vertical, geotropic stimulus acts laterally on the two halves of the differentially excitable pulvinus. When the less excitable upper half is to the left of the observer (see Fig. 179), the responsive torsion under geotropic stimulus will be clock-wise, the less excitable upper half of the pulvinus being thereby made to face the vertical lines of gravity. When the plant is turned over to the other side (the less excitable upper half being now to the right of the observer) the induced torsion will be counter clock-wise. The response is therefore determined by the directive action of stimulus of gravity. Light has also been shown to give rise to torsion (p. 400). Light acting in the same direction as the stimulus of gravity,i.e., from above, enhances the rate of torsion, the curve of response being due to the joint effects of light and gravity.
Fig. 211.Fig. 211.—Record of diurnal variation of torsion inMimosaleaf. Up-curve represents increase and down-curve decrease of geotropic torsion.
Fig. 211.—Record of diurnal variation of torsion inMimosaleaf. Up-curve represents increase and down-curve decrease of geotropic torsion.
Experiment 221.—I obtained 24 hours' record of variation of torsional response ofMimosa, commencing with thermal-noon at 2 p.m. It is to be borne in mind that increase of torsion indicates increase of geotropic action, just as the erectile movement of the leaf in the normal position indicates the enhanced geotropic effect. Inspection of figure 211 shows that the fall of temperature after thermal-noon was attended by increase of torsion. The curve went up till about 5 p.m., as in the ordinary record ofMimosa. The torsion suddenly decreased with the rapid diminution of light after 5 p.m. The torsion then increased with falling temperature from 9 p.m. till thermal-dawn next morning. After 6 a.m. there is a continuous diminution of torsion till 5 p.m.
We may now summarise the diurnal variation of torsion exhibited byMimosa. The torsion undergoes aperiodic increase during the fall of temperature from afternoon till next morning, and a diminution during rising temperature from morning till afternoon. A sudden diminution of torsion occurs at about 5 p.m. due to the disappearance of light. The torsional record is, to all intents and purposes, a replica of the record of periodic up and down movements of the leaf.
This method of torsion has several advantages over the ordinary method. First, the petiole being supported by the loop of wire, the weight of the leaf has no effect on the curve of response. In the second place, the periodic variation of turgor of the stem, as suggested by Millardet, will not in any way affect the record. Variation of turgor can only cause a swing to and fro, in a direction perpendicular to the plane which divides the pulvinus into upper and lower halves; it can in no way induce a torsional movement, or a variation of the rate of that movement.
The automatic pulsation of the leaf ofMimosa.—The occurrence of the pulsatory response in the morning record ofMimosaled me to search for multiple activity in the response of the pulvinus. I have in my previous investigation on the electric response ofMimosaobtained multiple series of responses to a single strong stimulus. Blackman and Paine have recently shown that an isolated pulvinus ofMimosaexhibit multiple mechanical twitches under excitation.[45]
Even under normal conditions, the sprouting young leaves in March, as already stated, exhibit automatic pulsations throughout the day and night; in older leaves tuned to diurnal periodic movements, these natural pulsations are more or less suppressed. But in the forenoon, several pulsations are exhibited even by the old leaves.
The question may now be asked: Why should the pulsations occur preferably in the morning? In connection with this I shall refer to the suppression of the pulsatory activity ofDesmodium gyranswhen the leaflet was pulled up by the action of light (cf. Fig. 188). The leaf ofMimosaexecutes a very rapid movement of erection at night, and the natural pulsations are thereby rendered very inconspicuous. These pulsations may, however, be found in the night record of young leaves. The general occurrence of pulsations in the forenoon is probably due to the fact that the resultant force which causes the down-movement is at the time relatively feeble—the operative factors being: (1) the action of the rising temperature which induces down-movement, and (2) the action of light which in the forenoon opposes this movement. It will thus be seen that the forces in operation in the forenoon are more or less in a state of balance, hence conditions for exhibition of natural pulsations are more favourable in the morning than in other parts of the day.
Experiment 222.—I next tried to discover conditions under which the plant would exhibit its normal rhythmic activity during the whole course of 24 hours. The external stimuli which may interfere with the exhibition of its automatic pulsations are those due to gravity and light. They act most effectively on the pulvinus, when that organ is more or less horizontal and therefore at right angles to the direction of the incident stimulus; they act least effectively on the pulvinus when the organ is parallel to the direction of the external force. This latter condition may be secured by holding the plant upside down, when the pulvinus bends up and the leaf becomes erect and almost parallel to the vertical lines of gravity and to vertical light from above. The leaf, now relatively free from the effects of external stimulus, was found to exhibitits autonomous pulsations for more than seven days. I reproduce two sets of records (Fig. 212) for 24 hours each, obtained on the first and the third day. The average period of a single pulsation is slightly less than six hours; but this is likely to be modified by the age of the specimen and the temperature of the environment.