Life Movements in Plants, Volume II
PART IV.
NIGHT AND DAY MOVEMENTS IN PLANTS.
XLVI.--DIURNAL MOVEMENTS IN PLANTS
_By_
SIR J. C. BOSE.
The subject has long been a perplexing one, and its literature is copious. After a good many years of experimental investigation, I have succeeded in analysing the main factors concerned in the many phenomena which have been described as Nyctitropism. The results of the researches are given in a sequence of five papers, which may be read separately, yet will be seen as so many chapters of what has been a single though varied investigation.
The different chapters are:
1. Daily movements in relation to Light and Darkness.
2. Daily movements due to Variation of Temperature affecting Growth.
3. Daily movements due to Variation of Temperature affecting Geotropic Curvature.
4. The Immediate and After-effect of Light.
5. Diurnal Movement of the leaf of _Mimosa_ due to combined effects of various factors.
Nyctitropic movements are thus described by Jost[39]:
"Many plant organs, especially foliage and floral leaves take up, towards evening, positions other than those they occupy by day. Petals and perianth leaves, for example, bend outwards by day so as to open the flower, and inwards at night so as to close it.... Many foliage leaves also may be said to exhibit opening and closing movements, not merely when they open and close in the bud but also when arranged in pairs on an axis, they exhibit movements towards and away from each other. In other cases, speaking generally, we may employ the terms _night position_ and _day position_ for the closed and open conditions respectively. The night position may also be described as the _sleep position_." After reviewing the various theories proposed, he proceeds to say "that a completely satisfactory theory of nyctitropic pulvinus movements is not yet forthcoming. Such a theory can only be established after new and exhaustive experimental research."
[39] Jost--_Ibid_, p. 500.
The difficulties of the experimental reinvestigation here called for towards clearing up and explanation of the subject are sufficiently great; they are further increased by the fact that these diurnal movements may be brought about by different agencies independent of each other. Thus in _Crocus_ and in _Tulip_, the movement of opening during rise of temperature has been shown by Pfeffer to be due to differential growth in the inner and outer halves of the perianth. I shall in this connection show that a precisely opposite movement of closing is induced in _Nymphæa_ under similar rise of temperature. I shall for convenience distinguish the differential growth under temperature variation as _Thermonasty_ proper. Again certain leaflets open in light, and close in darkness in the so-called sleep position. Intense light, however, produces the 'midday sleep'--an effect which is apparently similar to that of darkness. The determining factor of these movements is the variation of light.
There are other instances of diurnal movement, far more numerous, which cannot be explained from considerations given above. It has therefore been suggested that the "Day and night positions may arise by the combined action of geotropism and heliotropism. Thus Vochting (1888) observed in the case of _Malva verticillatta_, that the leaves, when illuminated from below, turned their laminæ downwards during the day, but during the night became erect geotropically. The sleep movements in leaves and flowers, referred to above, cannot however be explained by assuming such a combination of heliotropism and geotropism."[40]
[40] For further information on the subject of Nyctitropism, _cf._-- Pfeffer--_Ibid_, Vol. II (1903), p. 112; Jost--_Ibid_, pp. 500, 507; Vines--Physiology of Plants (1886), pp. 406, 543.
I commenced my investigation on nyctitropism five years ago, after having perfected an apparatus for continuous record of the movements of plants throughout day and night. A contrivance, described further on, has been devised for obtaining a record of diurnal variation of temperature. I have also succeeded recently, in perfecting a device for automatic record of variation of intensity of light. It has thus been possible not only to obtain a continuous record of the diurnal movement of the plant, but also obtain simultaneous record of those changes in the environment which might have an influence on the daily movement. I have in this way collected several hundred autographs of different plants throughout all seasons of the year. The records thus obtained were extremely diverse, and it was at first impossible to discover any fundamental reaction which would explain the phenomenon. While in this perplexity my attention was directed two years ago to the extraordinary performances of the "Praying Palm" of Faridpore, in which the geotropic curvature of the tree underwent an accentuation during fall of temperature, and a diminution during rise of temperature.
The discovery of this new phenomenon led me to the inquiry whether Thermo-geotropic reaction, as I may call it, was exerted only on Palm trees, or whether it was a phenomenon of universal occurrence. I therefore extended my investigation on various geotropically curved procumbent stems of _Ipoemia_, _Basella_, and of _Tropæolum majus_. Here also I found that diurnal variation of temperature induced a periodic movement exactly similar to that in Palm trees.
I next wished to find whether the Thermo-geotropic reaction observed in stems was also exhibited by lateral organs such as leaves, which being spread out in a horizontal direction are subjected to the stimulus of gravity. I found that in a large number of typical cases, a periodic movement took place which was exactly similar to that given by rigid trees and trailing stems. A standard curve was thus obtained which was found to be characteristic not only of trees and herbs, but also of leaves. The stem and leaves _fell_ continuously with the rise of temperature, from the minimum at about 6 in the morning to the maximum at about 2 p.m. They erected themselves with falling temperature from 2 p.m. to 6 a.m. next morning.
In the diurnal record of _Mimosa_ I met, however, with an unaccountable deviation from the standard curve, for which I could not for a long time find an adequate explanation. Subsequent investigations showed that the deviation was due to the introduction of additional factors of variation, namely of immediate and after-effects of light.
COMPLEXITY OF THE PROBLEM.
I have already referred to the great difficulty of explanation of nyctitropism from the fact that the diurnal movements may be brought about by different agencies independent of each other. It is, moreover, not easy to discriminate the effect of one agency from that of the other.
The combined effects of different factors will evidently be very numerous. This will be understood from consideration of the number of possible combinations with only two variables, geotropism and phototropism. The effect of geotropism may be strong _G_, or feeble, _g_. Similarly we may have strong effect of light _L_, or feeble effect of light _l_. Light may exert positive phototropic action +_L_ or negative action -_L_. Thus from two variables we obtain the following eight combinations:
_G_ + _L_; _G_ - _L_; _G_ + _l_; _G_ - _l_; _g_ + _L_; _g_ - _L_; _g_ + _l_; _g_ - _l_.
The number of possible variables are, however, far more numerous as will be seen from the following:
_Geotropism._--The effect of geotropic stimulus on horizontally placed organs is one of erection. But this stimulus, which is constant, cannot by itself give rise to periodic movements. It has however been shown that variation of temperature has a modifying influence on geotropic curvature (p. 519).
_Phototropism._--The action of unilateral light is to induce a tropic curvature, which in some cases is positive, in others negative (p. 386). In addition to these effects induced during the incidence of light, we have to take account of the after-effects on the cessation of light.
_After-effects of light._--I find two very different effects, depending on the intensity and duration of previous illumination. Of these the most important is the phenomenon of 'overshooting' which occurs on the cessation of light of long duration. This particular reaction, to be fully described, will be found to offer an explanation of certain anomalous effects in diurnal movement.
_Periodic variation of turgor._--I have shown (p. 39) that artificial enhancement of turgor in the plant induces an erectile movement of the leaf of _Mimosa_, diminution of turgor inducing the opposite movement of fall. Kraus and Millardet have shown that a diurnal variation of tension takes place in the shoot of all plants, which is presumably indicative of variation of turgor. This variation of turgor in the shoot must have some effect on the lateral leaves. But the leaves are subjected to conditions which are absent in the stem. The erect stem is, for example, free from geotropic action, whereas the lateral leaf is subject to it. The effect of turgor variation in the shoot on the movement of leaves may be, and often is, overpowered by the predominant geotropic action. I shall, later on, refer to this question in greater detail.
_Autonomous movements: Experiment 202._--The lateral organ, say the leaf or leaflet, may have an autonomous movement of its own. In some, the autonomous movement may be relatively quick; the complete pulsation in _Desmodium gyrans_ may be as short as a minute or so. I find that this autonomous movement becomes modified or even arrested by the paratonic effect of light. This is seen in figure 188, where light applied from above is seen to arrest the pulsation; the normal activity is, however, restored on the stoppage of light.
_Epinasty and Hyponasty: Experiment 203._--There are other autonomous movements which are relatively slow. Even in an erect stem there may be a to and fro oscillation. In such a case the effect of an external stimulus, say of light, is one of algebraical summation. The following is the summary of results of unilateral action of light on the nutating hypocotyl of a pea seedling:
+--------------------------------------------------------------------+ | Natural movement. | Effect of light applied on the right side.| +------------------------+-------------------------------------------+ |Movement to the right |Acceleration of existing movement. | |Movement to the left |Retardation, arrest or reversal of natural | | | movement. | +--------------------------------------------------------------------+
Figure 189 exhibits the effect of light applied alternately above or below the cotyledon of _Cucurbita Pepo_. On account of the more rapid growth of the lower side, the cotyledon was exhibiting a hyponastic up-movement. Application of light from above enhanced the existing rate of movement, whereas light acting from below retarded the movement. Here we have instances of photo-hyponastic modification of natural movement. Similarly epinastic organs will, normally speaking, have their natural down movement retarded by light from above, and accelerated by light from below. If the periodicity of the autonomous movements coincides with the periodicity of the external stimulus, the resulting movement will be determined by algebraical summation; it will be very pronounced when the two effects are concordant. If the two periodicities do not agree, the interference effects will become extremely complicated.
_Positive thermonasty._--Rise of temperature inducing differential growth brings about the _closure_ of the flower. Fall of temperature on the other hand induces a movement of opening. Example of this has already been given in the responsive movement of _Nymphæa_.
_Negative thermonasty._--The opposite type of movement is exhibited by _Crocus_ and _Tulip_. Pfeffer has shown that a rise of temperature induces in these flowers, a quicker rate of growth of the inner side of the perianth. Rise of temperature thus induces a movement of opening, and a fall of temperature brings about the opposite movement of closure. I shall presently describe the effects of both positive and negative thermonasty, in diurnal movements of flowers.
_Thermo-geotropism._--I have already described the accentuation of geotropic curvature during the fall, and a flattening of curvature during the rise of temperature (p. 519). The influence of this factor on diurnal movement will presently be treated in fuller detail.
There are thus more than ten variables, and the resulting effect due to their combinations will exceed a thousand. This will explain why attempts at explanation of the phenomenon of nyctitropism had hitherto proved so baffling. It is indeed a difficult task to disentangle the full explanation of each given case in the vast complexity. It is, however, possible, by a process of judicious elimination, to reduce the difficulties which at first appear to be insurmountable.
In the periodic movement of plants there are several factors which are predominant, others being of minor importance. The important factors are the effects of light and darkness, of variation of temperature on differential growth, and of thermal variation on geotropic curvature.
For facility of treatment, I shall first take the three ideal types: (1) where the variation of light is the important factor, (2) where the movement is due to differential growth under variation of temperature, and (3) where thermal variation induces changes in geotropic curvature. I shall then take up the movement of the leaf of _Mimosa_ where the combined effects of numerous factors give rise to a highly complex diurnal curve. There remains now the difficulty of discriminating the three types which approximate to the ideal.
DISCRIMINATING TESTS FOR CLASSIFICATION.
_Predominant effect of light and darkness._--Turning first to the case where light exerts a predominant influence, the obvious test of keeping the plant in continuous darkness or continuous light is not practicable. One would think that if the movement was due to periodic variation of light, such movement would disappear under constant light or darkness. But owing to the persistence of after-effect, the periodic movement previously acquired is continued for a long time.
There is, however, another possibility of discrimination. The effect of variation of light will be most marked at the two periods, early in the morning when the light appears, and in the evening when it disappears. In the tropics there is little twilight; in Calcutta, the sun rises in summer at about 5-30 a.m., and sets at 6-30 p.m. In winter the sun rises an hour later, and the sunset is an hour earlier. The average dawn may therefore be taken approximately at 6 a.m., and the average sunset at 6 p.m. Unlike the diurnal variation of temperature which is gradual, the change from light to darkness or from darkness to light is very abrupt. If we succeed next in obtaining a continuous curve of the diurnal movement of the plant, the phototropic action would be evidenced by some flexures of the curve in the morning and towards evening.
The other two types of daily movement depend on the diurnal variation of temperature, and there is some difficulty in distinguishing the effect of variation of light from that of temperature, since both are connected with the appearance and disappearance of the sun.
_Diurnal variation of light and of temperature._--There are certain differences, however, which enable us to distinguish the two variations. Light appears in the morning, say at 6 a.m., becomes most intense at noon; after 4 p.m. the light wanes, and darkness sets in quickly after 5 p.m. and remains persistent till next morning. The course of variation of temperature is somewhat different. The minimum temperature is attained in my green house at about 5 a.m. in summer, and at about 7 a.m. in winter. The maximum temperature is reached at about 3 p.m. in summer, and about 1 p.m. in winter. The range of daily variation in summer may be taken to be from about 23° C. to 34° C.; in winter it is from 16° C. to about 29° C. The above gives the normal variation and not the sudden fluctuations that occur during uncertain weather conditions.
The temperature remains constant for nearly an hour during the period of transition from falling to rising temperature, and _vice versâ_. The average period of minimum temperature may be taken at 6 a.m., which I shall distinguish as the _thermal-dawn_. The average period for maximum temperature, the _thermal-noon_, is at 2 p.m. Variations from these average periods at different seasons do not amount to more than an hour.
The light-dawn and thermal-dawn are more or less coincident, while the thermal-noon is two hours later than the light-noon. A change in the diurnal curve of movement due to thermal variation will thus be detected at about 2 p.m. If the curve of daily movement of the plant-organ closely resemble the diurnal thermographic curve, there can then be no doubt of the causal relation of variation of temperature in the production of the periodic movement. Two different classes of phenomena, as already stated, arise however from the variation of temperature, _thermonasty_ and _thermo-geotropism_. In the former, the movement is autonomous, and determined in relation to the plant; in the latter, the movement is related to the direction of external stimulus of gravity. Further tests will be given later, to distinguish the phenomenon of Thermonasty from that of Thermo-geotropism.
I shall in the succeeding papers describe the principal types of diurnal movements as sketched above. The success of the investigation greatly depends on the elaboration of automatic apparatus of precision, which gives a continuous record of the diurnal movement of different plant organs. The description of this Nyctitropic Recorder will be given in the next paper.
SUMMARY.
The obscurities in the nyctitropic movement of plants arises from the presence of numerous complicating factors.
In the diurnal movement of plants the most important factors are the effects of light and darkness, of variation of temperature on differential growth, and of thermal variation on geotropic curvature.
These three classes of phenomena may be discriminated from each other by the following tests. The effects of light and darkness are most pronounced in the morning when light appears, and in the evening when light disappears. A pronounced flexure in the diurnal curve at these periods indicates the dominant character of the phototropic action. The effect of light can also be distinguished from that of temperature from the fact that the period of maximum intensity of light, or _light-noon_, is about two hours earlier than the _thermal-noon_, at which the temperature is maximum.
A flexure of the diurnal curve about thermal noon, at which an inversion takes place from rise to fall of temperature, indicates the effect of temperature. The additional test of the effect of temperature is furnished by the close resemblance of the diurnal curve of the plant with the thermographic record for 24 hours.
Two different classes of phenomena arise from variation of temperature--Thermonasty and Thermo-geotropism. In the former the movement is autonomous and determined by the differential growth-activity of the two sides of an anisotropic organ. In the latter the movement is not in relation to the plant but directed by the external stimulus of gravity.
XLVII.--DIURNAL MOVEMENT DUE TO ALTERNATION OF LIGHT AND DARKNESS
_By_
SIR J. C. BOSE,
_Assisted by_
LALIT MOHAN MUKHERJI, B.Sc. (_Nawroji Scholar_).
The nyctitropic movements of the leaflet of _Cassia alata_ and of the terminal leaflet of _Desmodium gyrans_ furnish us with typical examples of the recurrent effects of light and darkness. The petiole of _Cassia_ contains a number of paired leaflets each of which is about 5 cm. long and 2·5 cm. broad. The leaflets are extremely sensitive to light; at night each pair of leaflets fold themselves in a forward direction (see Fig. 150). With the appearance of light they open at first in a lateral direction; later on there is a twist of the pulvinus by which the inner surface of the leaflets faces light coming from above (p. 405). I shall show that the diurnal movements of the leaflets are predominantly due to phototropic action.
Before proceeding further it will be necessary to give a general description of the experimental method employed, and of the apparatus by which diurnal movements are recorded.
EXPERIMENTAL ARRANGEMENTS.
The diurnal record is often taken continuously for several days, and it is therefore necessary to take precautions against the disturbing effect of watering the plant. The record is also liable to be affected by the twist induced by light when it acts on one side of the organ.
_Irrigation._--There is, as is well known, a periodic variation of turgor in the plant. This normal variation is, however, disturbed by watering the plant at irregular intervals. Precaution against this was taken by placing the three flower pots on a long trough filled with water (Fig. 190). The height of water in the trough is always maintained constant by a syphon.
_Vertical illumination._--The direction of sunlight changes from morning to evening, and the leaves exhibit appropriate phototropic movements or torsions under changing directions of lateral light. In order to obviate this, a special chamber was constructed, which allowed light from the sky to fall vertically on the plant through a sheet of ground glass which covered the roof. The sides and the base of the chamber are impervious to light. A narrow slit covered with red glass allows inspection of the curve during the process of record.
_The Ventilator._--A revolving ventilator, acted on by the wind, sucks the air away from the chamber, thus ensuring constant supply of fresh air, without causing any disturbances of the record.
_The Recorder._--The Oscillating Recorder employed is of the quadruplex type carrying four recording plates (Fig. 190). The first lever records the daily variation of temperature. The other three are attached to three different specimens of the same plant, or to three different plants. In the former case, three records are obtained of the same species of plant, under identical external condition. If they agree in all essentials, the periodic curve may be taken as characteristic of the given plant. A very great saving of time is thus ensured, and it is thus possible to obtain characteristic curves of numbers of different species of plants within the short period of a season. The quadruplex recorder enables us also to obtain simultaneous records under identical external condition of leaves of different age of the same plant, or of leaves of three different species of plant. I have for the last five years taken records of numerous plants at all seasons of the year. The autograph of the plant is often so characteristic that it is possible to name it by mere inspection of its daily record.
_Thermograph._--For obtaining a continuous record of diurnal variation of temperature, I use a compound strip, T, made of brass and steel. Variation of temperature induces a curvature of the compound strip which is recorded by means of the attached lever. The oscillation of the plate takes place once in fifteen minutes, and the successive dots thus produced give time records of the diurnal curve. The record thus consists of a series of dots. An additional device makes the plate oscillate three times in rapid succession at the end of each hour; the hourly dot is thus thicker than others. The movement of the plant, corresponding to the particular variation of temperature at any period, may thus be easily determined. I shall now give a typical example of diurnal movement induced by variation of light and darkness.
DIURNAL MOVEMENT OF THE LEAFLET OF _Cassia alata_.
The leaflet of _Cassia alata_ exhibits a movement of opening in the morning, and it remains outspread throughout the day. It then begins to close before evening and remains closed throughout the night. The problem before us is to find out the relative importance of variation of temperature and of light in the diurnal movement of the leaflets.
In the daytime the light is increasing till midday; there is, on the other hand, a rapid decline of light after 5 p.m. and uninterrupted darkness at night. As regards temperature there is a continuous rise from morning till the thermal noon at 2 p.m., after which the fall of temperature is continuous till next morning. The opening of the leaflets in the daytime may therefore be due to the summated effects of rising temperature and increasing light, the closure, on the other hand, being due to falling temperature, and to darkness. The individual effect of each of these factors is not known and it is therefore necessary to determine the effects of variation of temperature and of light.
EFFECT OF VARIATION OF TEMPERATURE.
_Experiment 204._--The plant was enclosed in a glass chamber and exposed to diffuse light. The experiment was commenced at midday, when the leaflets were open; the light was kept uniform while temperature was artificially increased by means of an electric heater placed in the chamber, and decreased by introducing cold air into the plant chamber. One of the leaflets was attached to the recording lever and its movement, up or down, indicated the movement of opening or closure. The records showed that rise of temperature induces a movement of closure, while that of fall brings about the movement of opening.
EFFECT OF VARIATION OF LIGHT.
_Experiment 205._--This experiment was also carried out at midday, when the leaflets were open. The horizontal record in figure 191 represents the stationary expanded condition of the leaflet; a black cloth was put over the glass chamber at 1 p.m., and the effect of darkness was recorded for one hour. Darkness is seen to initiate a movement of closure which increased at a rapid rate; the black cloth was removed after an hour, and the movement of opening under light was completed in the course of five quarters of an hour. It is thus seen that the leaflets are extremely sensitive to the action of light.
The experiments that have just been described on the effects of rise of temperature, and of light, show that they are antagonistic to each other. In the forenoon the opening movement under light has to be carried out against the closure movement due to rise of temperature. Light, therefore, is the predominant factor in the diurnal movement of the leaflets of _Cassia_. The closure effect of darkness at night, on the other hand, overpowers the tendency of movement of opening due to fall of temperature.
DIURNAL MOVEMENT OF THE LEAFLET OF _Cassia alata_.
_Experiment 206._--I next obtained the diurnal record of the leaflet, from 4 p.m. till 1 p.m. next day. The leaflets remain open from 1 p.m. to 4 p.m. and the record of this period is therefore omitted. In the diurnal record (Fig. 192) the first thick dot was made at 4 p.m. and successive thick dots are at intervals of an hour, the thinner dots being at intervals of 15 minutes. It will be seen that a rapid movement of closure was initiated at 5 p.m. when the light is undergoing a rapid diminution. The movement of closure is completed at about 9 p.m. The leaflets remain closed till 5 a.m. next morning, after which they begin to open; this opening may commence even an hour earlier. It should be borne in mind in this connection, that since light and rise of temperature are antagonistic in their reactions, the effects of light and fall of temperature would be concordant; and the opening in the early hours may possibly be hastened by the low temperature in the morning. The leaflets open to their utmost by 9 a.m., and they remain open till the afternoon. The plant is so extremely sensitive to light that any slight fluctuation is followed by responsive movement of the leaflet. Thus the transitory passage of a cloud is marked in the record by a short-lived closure movement.
DIURNAL MOVEMENT OF THE TERMINAL LEAFLET OF _Desmodium gyrans_.
Both the petiole, and the terminal leaflet of this plant exhibit very marked nyctitropic movement. The petiole is raised and becomes almost erect in the evening, while the pulvinus of the terminal leaflet exhibits a sharp curvature downwards (Fig. 193).
_Experiment 207._--The petiole was held fixed, and the terminal leaflet attached to the recording lever. I have already explained that light falling on the pulvinus from above, induces an up-movement of the leaflet, which is thus erected under light of moderate intensity. If the light be strong, the transversely conducted excitation induces a partial neutralisation; very intense light may even cause a reversal into down-movement. Under natural conditions, day-light acting from above induces an up-movement; darkness, on the other hand, induces a rapid movement of fall. The leaflets sometimes exhibit autonomous pulsations; but the diurnal movement is very strong and the daily curve appears as a single large pulse on which smaller autonomous pulsations may become superposed.
The diurnal curve (Fig. 194) exhibits a sudden flexure at about 5 p.m. on the rapid waning of afternoon light till, by 6-30 p.m., it becomes closely pressed against the petiole, by the rapid fall of the leaflet. The discriminating test, between effects of variation of temperature and of light, lies in the fact that the flexure of the diurnal curve takes place in the former at about 2 p.m. when temperature undergoes change from ascent to descent; in the case of light, the change in the intensity of light begins to be marked about three hours later. In the diurnal curve of _Desmodium_ the record shows little change at 2 p.m., showing that the leaflet is not affected to any great extent by the variation of temperature; it is, however, strongly affected by change in light as seen in the rapid closure movement about 5 p.m. The leaflet remains tightly closed throughout the night and begins to open and spread out early in the morning at about 5 a.m. This up-movement is also very rapid and the leaflet assumes the fullest outspread position by 7 a.m. It remains in this position till the afternoon, after which the cycle becomes repeated. As the leaflet is very sensitive to light, the position of equilibrium of the leaflet is liable to be disturbed by the slightest variation of light and the fluctuation of light from the sky often gives rise to a wavy outline in the record. The leaflet, moreover, has a tendency to exhibit rhythmic pulsations.
In the leaflets of _Cassia_ and _Desmodium_, the daily movement is thus brought about by the predominant action of recurrent light and darkness.
MIDDAY SLEEP.
I shall here briefly recapitulate the results given in greater detail in an earlier paper (p. 352). I have shown that the midday closure of leaflets is brought about by the excitatory action of strong sunlight. The responsive movement of motile pulvinus under diffuse stimulus is determined by the greater contraction of the more excitable half of the organ. Under the action of the midday sun the leaflets of _Mimosa_ undergo a folding upwards, whereas the leaflets of _Averrhoa carambola_ a folding downwards. The explanation of the difference lies in the fact that in the leaflets of _Mimosa_ it is the upper half, and in _Averrhoa_ it is the lower half of the pulvinule, that is the more excitable. This difference may be demonstrated by the action of diffuse electric shock under which the leaflets of _Mimosa_ exhibit an upward, and those of _Averrhoa_ a downward, closure. I have also shown that conduction of excitation takes place across the pulvinule; hence the strong excitation caused by sunlight becomes internally diffused, and brings about the responsive movements, the direction of which is determined by the more excitable half of the pulvinule.
SUMMARY.
Rise of temperature induces a movement of closure of the leaflet of _Cassia_, fall of temperature inducing the opposite movement.
Artificial darkness induces a closure of the leaflets, the closure being completed in the course of an hour. On readmission of light, the leaflets become fully expanded in the course of one hour and a quarter. The leaflets are extremely sensitive to light, closure movement being induced by the transitory passage of a cloud.
The effect of rise of temperature is antagonistic to the action of light. The movement of opening during the course of the day is due to the effect of light overpowering the effect of rise of temperature.
Under daily variation of light and darkness, the movement of closure is initiated at about 5 p.m., when the light is undergoing a rapid diminution. The movement of closure is complete by 9 p.m. The leaflets remain closed till about 5 a.m. next morning, after which they begin to open and become fully expanded by 9 a.m.
The terminal leaflet of _Desmodium gyrans_ exhibits a diurnal movement which is very similar to that of _Cassia_. It begins to open early in the morning and remains outspread during the whole day; the leaflet exhibits a rapid down-movement after 5 p.m. and becomes closely pressed against the petiole in the course of about two hours.
The midday sleep of leaflets of _Mimosa_ and _Averrhoa_ is due to the excitatory action of strong sunlight on the pulvinule, the more excitable half becoming contracted under excitation. In _Mimosa_ leaflets it is the upper, and in _Averrhoa_, it is the lower half of the pulvinule that is the more excitable. It is in consequence of this that the diffuse excitation of strong sunlight causes the leaflets of _Mimosa_ to fold upwards, those of _Averrhoa_ to fold downwards.
XLVIII.--DIURNAL MOVEMENT DUE TO VARIATION OF TEMPERATURE AFFECTING GROWTH
_By_
SIR J. C. BOSE,
_Assisted by_
LALIT MOHAN MUKERJEE.
It has been stated that there are two classes of diurnal movements caused by variation of temperature; one of these is due to differential growth induced on two sides of the organ, and the other is brought about by the induced variation of geotropic curvature. The former may be distinguished as _Thermonastic_, and the latter as _Thermo-geotropic_ movement. Before laying down the criteria to distinguish the one class of phenomenon from the other, it would be advisable to refer to the somewhat arbitrary distinction that has been made between nastic and tropic reactions.
TROPIC AND NASTIC MOVEMENTS.
The explanation, which I shall offer about the night and day movements in plants, has been reached through the study not only of pulvinated, but also of growing and fully grown organs. A distinction is made between the movement due to growth, and the 'variation movement' due to change of turgor. I have shown (p. 239) that the same diminution of turgor which induces a contraction in a pulvinus, also induces in a growing organ an incipient contraction, and retardation of growth. Enhancement of turgor, on the other hand, induces in both the opposite effect of expansion. Unilateral stimulus induces curvature, and there is no essential difference in the production of such curvatures in pulvinated, growing, and fully grown organs. The exhibition of nyctitropic movement by the fully grown, and rigid 'Praying Palm' is a striking demonstration of the unity of response of all plant organs.
As regards the distinction between the tropic and nastic movements, it will be found that there is no sharp line of demarcation between the two. A movement is said to be _tropic_, when unilateral stimulus acts on an organ and induces in it a directive movement. Curvature induced by diffused stimulus on a dorsiventral or anisotropic organ (with differential excitabilities of the two halves) is termed _nastic_. Daylight is supposed to act diffusely (_i.e._, equally on all sides) on leaves; this is, however, not strictly true, since the light from sky above is stronger than from ground below. Moreover, the tropic action of unilateral light may become nastic by internal diffusion of excitation. This is seen in the response of the pulvinus of _Mimosa_ to light acting from above. The leaf at first moves upwards towards the stimulus, the response being positively phototropic. But under the continued action of light, excitation becomes internally diffused, and the leaf undergoes a fall by the greater contraction of the more excitable lower half of the organ (p. 331). No sharp distinction can therefore be made between the movements of growth and of variation, between tropic and nastic curvatures.
The employment of the term 'nastic' is, however, convenient when used in a well-defined and restricted sense. "We speak of tropism when the organ takes up a resting position definitely _related to the effective stimulus_. Nastic movements, on the other hand, are curvatures which bring about a particular position _in relation to the plant_, and not to the direction of the stimulus".[41] It will sometimes be necessary, in the course of this paper, to discriminate the movements which are autonomous from others which are paratonic, _i.e._, brought about by external stimulus, to the former class belongs a large number of automatic activities ranging from the quick pulsations of _Desmodium gyrans_ to the slow movements, exhibited by epinastic and hyponastic organs. Under the category of nastic movements may also be included those of the flower of _Crocus_ and _Tulip_, in which variation of temperature induces differential growth on two sides of the organ. The direction of the movement, though initiated by change of temperature, is determined by the difference of growth-activity on the two sides. In these instances of nastic movement, the induced curvature is in relation of the plant; the opening of the flower due to rise of temperature will remain the same, whether the flower be kept in an erect or in an inverted position. Had the movement, on the other hand, been paratonic, that is to say, due to the external stimulus of gravity, the responsive movement would have been determined not in relation to the plant but to the direction of external force of gravity.
[41] Strasburger--"Text-book of Botany" (1912), p. 300.
In the description of direction of responsive movements, confusion is likely to arise unless the point of view be carefully defined. An up-movement of a leaf or a petal means approach towards the growing point of the axis. This may be variously described as movement of closure or of folding. A down-movement may, on the other hand, be described as a movement of opening or of unfolding. If the plant be held inverted, two different effects will be noticed depending on the character of the movement, whether nastic or tropic. In the case of nastic movement, the former up-movement in erect position would appear, on inversion of the plant, to be a down-movement; but in relation to the plant the closure movement will remain closure movement, whether the plant be held in the normal position or upside down. If, on the other hand, the direction of movement be determined by the paratonic effect of external stimulus, gravity for example, an up-movement due to fall of temperature will continue to be an up-movement, whether the plant be held in its normal or inverted position. The responsive movement in relation to the plant will, however, be different; the closure movement will, on inversion, be reversed into a movement of opening. The reversal of closure into an opening movement or _vice versâ_ will thus be a test of the paratonic effect of external stimulus.
We may thus distinguish thermonastic from thermo-geotropic action by the following tests:
1. Thermonastic movements are, generally speaking, due to differential growth, and are therefore characteristically present in growing organs. Thermo-geotropic action is independent of growth.
2. Thermonastic movements take place in relation to the plant, and is not determined by external force of a directive nature. Opening or closing movement will remain unchanged after inversion of the plant. But thermo-geotropic reaction being determined by the external stimulus of gravity, becomes reversed on inversion of the plant. Closure movement is thus converted into opening movement, and _vice versâ_.
I shall now take up the diurnal movement due to variation of growth induced by change of temperature. Of this the flower of _Nymphæa_ furnishes an example.
DIURNAL MOVEMENTS OF _Nymphæa_.
The flower of _Nymphæa_ remains closed during the day and opens at night. Figures 195 and 196 are from photographs of the day and night positions of the flower. The closure and opening movements of this flower have been regarded as being mainly due to recurrent variations of light and darkness.[42] If the opening be due to darkness, closure of the flower should take place in the morning with the appearance of light. But the flowers often remain open till ten or eleven in the forenoon. I have sometimes succeeded in keeping the flower open for greater part of the day by lowering the temperature of the plant-chamber. The movement of the flower thus appeared to be associated with variation of temperature rather than of light.
[42] Pfeffer--Ibid, Vol. III. p. 122.
_Action of light: Experiment 208._--I investigated the effect of light on the movement of opening or of closing of the flower. One of the petals was attached to the recording lever; light from an arc lamp was made to act diffusely on the petal; this was done by means of two inclined mirrors by which the divergent horizontal beam of light was thrown on the upper and lower sides. The record in figure 197 shows that light induced a movement of opening, followed by closure in darkness. Since light induces a movement of opening, and darkness brings about a closure, the opening of the flower at night could not be due to darkness. We have therefore to look for a different cause for the diurnal movement of the flower.
_Effect of variation of temperature._--I have already described an experiment which proves that rise of temperature induces a movement of closure of the floral leaves of _Nymphæa_, lowering of temperature producing the opposite effect (p. 311).
From the study of the action of light and of variation of temperature, it will be seen that the flower of _Nymphæa_ is acted on in the evening by two antagonistic forces; darkness induces a movement of closure, and fall of temperature gives rise to a movement of opening. Since the flower opens in the evening, the predominant effect is that of falling temperature.
The above conclusions are fully borne out by the diurnal record which I obtained with _Nymphæa_.
_Experiment 209._--One of the perianth leaves was attached to one of the recording levers, the differential thermometer being attached to the other. It will be seen (Fig. 198) that the movement of the flower follows very closely the curve of variation of temperature. The flower was tightly closed in the day time; and the perianth leaves began to open out in the evening at first slowly, then very rapidly, and the flower becoming fully expanded by 10 p.m. at night. Though the temperature continued to fall, there was no possibility of further expansion beyond the maximum. The temperature began to rise after passing through the minimum at 6 a.m., and the movement of closure set in with rising temperature, the flower becoming completely closed by 10 a.m. That geotropism has little effect is seen from the fact that the inversion of flower does not interfere with the normal opening or closing of the flower.
The phenomenon of diurnal movement of _Nymphæa_ is therefore thermonastic, the floral leaves exhibiting movement of opening at night owing to fall of temperature. _Luffa acutangula_, which opens in the afternoon, and closes early in the morning, gives a diurnal record similar to that of _Nymphæa_.
SUMMARY.
The flower of _Nymphæa_ exhibits a movement of closure during rise of temperature, and of opening during fall of temperature.
It is shown further that the effects of light and of rise of temperature are antagonistic to each other. Light is shown to induce in _Nymphæa_ the movement of opening, and darkness to cause the movement of closure. The diurnal movement of _Nymphæa_ is not therefore due to periodic variation of light and darkness, but to the predominant effect of variation of temperature.
The diurnal record shows that the perianth leaves begin to open in the evening with falling temperature, and the flower becomes fully expanded by 10 p.m. The movement of closure sets in with rising temperature in the morning, and the flower becomes fully closed by 10 a.m.
XLIX.--DAILY MOVEMENT IN PLANTS DUE TO THERMO-GEOTROPISM
_By_
SIR J. C. BOSE,
_Assisted by_
LALIT MOHAN MUKHERJI.
Of the vast number of daily movements perhaps the largest proportion is due to thermo-geotropic reaction and its modifications. Thermo-geotropic movements have the following characteristics:
1. The organs are sensitive to the stimulus of gravity and the periodic movements are brought about by variation of geotropic curvature under change of temperature.
2. The movement is not confined to growing organs, but is also exhibited by organs which are fully grown and even by rigid trees.
3. The periodic movement is closely related to the diurnal variation of temperature. Fall of temperature from thermal-noon (about 2 p.m.) to thermal-dawn (about 6 a.m.) is attended by a movement of erection; rise of temperature from thermal-dawn to thermal-noon is followed, on the other hand, by a reverse movement of fall.
That the movement is primarily due to variation of temperature will be demonstrated in two different ways:
(_a_) by the change of normal rhythm of movement by artificial transpositions of periods of maximum and minimum temperature, and
(_b_) by the abolition of periodic movement through maintenance of constant temperature.
That the phenomenon is not nastic, but paratonic will be demonstrated:--
(_a_) by the reversal of closure into opening movement and _vice 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.
DIURNAL MOVEMENT OF PALM TREES.
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 and _vice 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 rose after 6 a.m. and the movement of the tree became reversed from ascent to descent.
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; thus _Ipomoea 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 of _Basella 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.
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, on the 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.
DIURNAL MOVEMENT OF PROCUMBENT STEMS AND OF LEAVES.
_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 assumed the stable position of geotropic equilibrium. In figure 200 is given records of the inclined palm tree, of procumbent stem of _Tropæolum_, 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 of _Dahlia_, _Papaya_ and _Croton_ (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.
CONTINUOUS DIURNAL RECORD FOR SUCCESSIVE THERMAL NOONS.
_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 from _thermal-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 of _Tropæolum_ and the leaf of _Dahlia_ for two days in succession.
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.
MODIFICATION OF THE DIURNAL CURVE.
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 or _vice versâ_ takes 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 thermal variation 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 of _Mimosa_, 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 of _Croton_ (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 of _Nymphæa_ the 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 in day-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 of _Nicotina_ is 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 of _Mimosa_ in 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 of _Mimosa_ will 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 plants demonstrate the causal relation between temperature variation and diurnal movement, of which the two additional tests described below offer further confirmation.
REVERSAL OF NORMAL RHYTHM.
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 of _Basella_ by 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).
EFFECT OF CONSTANT TEMPERATURE.
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 a hundredth 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.
_Diurnal record of_ Tropæolum _under constant temperature: Experiment 212._--The normal record of geotropically curved _Tropæolum_ is 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 normal diurnal 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.
DIURNAL MOVEMENT IN INVERTED POSITION.
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[42]; 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.
[42] By closure is meant movement of opposite pairs of leaf-organs towards each other.
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 of this 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.
_Experiment 213._--A still more striking result exhibiting the phase of transition was given by the geotropically curved stem of _Tropæolum_. 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.
SUMMARY.
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.
L.--THE AFTER-EFFECT OF LIGHT
_By_
SIR J. C. BOSE,
_Assisted by_
SURENDRA CHANDRA DAS.
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.[43] I shall here refer only to one or two characteristic results which have immediate bearing on the present subject.
[43] "Comparative Electro-Physiology"--p. 392.
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 continuous stimulation 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.
ELECTRIC AFTER-EFFECT.
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 of _Bryophyllum_ under 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.
_After-effect of pre-maximum stimulation: Experiment 214._--Light is applied at arrow and stopped in different experiments at _a_, _b_, and _c_ (Fig. 106). Continuous stimulation induces increasing galvanometric negativity; when stimulus is stopped at _a_ before 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 at _b_. 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 at _c_ causes an over-shooting at a rapid rate far _below_ the zero line; and it is after a considerable period that the curve returns to the zero line of equilibrium.
The condition at post-maximum _c_ is 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-shooting _downward_ below the zero line.
TROPIC RESPONSE UNDER LIGHT AND ITS AFTER-EFFECT.
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 of _Desmodium 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-movement _i.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.
_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 the upper 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 overshooting _below_ the 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.
SUMMARY.
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.
LI.--THE DIURNAL MOVEMENT OF THE LEAF OF _MIMOSA_
_By_
SIR J. C. BOSE.
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 the _Mimosa_ leaf appears, however, to be totally different.
_Experiment 220._--I obtained the diurnal record of _Mimosa_ (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 years ago do not differ in any way from those obtained this year. The record may conveniently be divided into four phases.
_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 of _Mimosa_ is 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 in _Mimosa_ itself, 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 of _Mimosa_. 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 of _Mimosa_ leaf 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 in _Mimosa_ arises 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 the algebraical 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 of _Mimosa_ has, 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 of _Mimosa_.
2. _Autonomous pulsation of Mimosa._--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 of _Mimosa_ towards 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 of _Mimosa_. 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 on the 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.
DIURNAL VARIATION OF GEOTROPIC TORSION.
I have shown that the pulvinus of _Mimosa_, subjected laterally to the action of stimulus of gravity, exhibits a torsional response. When the _Mimosa_ plant 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.
_Experiment 221._--I obtained 24 hours' record of variation of torsional response of _Mimosa_, 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 of _Mimosa_. 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 by _Mimosa_. The torsion undergoes a periodic 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 of Mimosa._--The occurrence of the pulsatory response in the morning record of _Mimosa_ led me to search for multiple activity in the response of the pulvinus. I have in my previous investigation on the electric response of _Mimosa_ obtained multiple series of responses to a single strong stimulus. Blackman and Paine have recently shown that an isolated pulvinus of _Mimosa_ exhibit multiple mechanical twitches under excitation.[44]
[44] Blackman and Paine--"Annals of Botany" January 1918.
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 of _Desmodium gyrans_ when the leaflet was pulled up by the action of light (cf. Fig. 188). The leaf of _Mimosa_ executes 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 exhibit its 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.
One of the factors that determines the diurnal movement of the leaf is the immediate and after-effect of light. The movement under the action of light, is modified by the intensity and duration of illumination. The experimental investigation of the subject offers many difficulties, principally owing to the absence of any reliable indicator for the varying intensity of light during the course of the day.
THE PHOTOMETRIC RECORDER.
This difficulty I have been able to overcome by the automatic device for continuous record of the variation of light. The electric resistance of a selenium cell undergoes diminution with the intensity of light that falls on it. The photo-sensitive cell was made the fourth arm of a Wheatstone bridge, the resistance of the cell being exactly balanced when the shutter of the sensitive cell was closed. The selenium receiver was pointed upwards against the sky. Precaution was taken that it was protected from the direct action of sunlight. On opening the shutter a deflection of the index of a sensitive galvanometer was produced, and the deflection increased with increasing intensity of diffuse skylight. The special difficulty was in securing automatic record of the galvanometer deflections. This was obtained by a special contrivance of an oscillating smoked glass plate, the up and down oscillation being at intervals of 30 minutes. A detailed account of this apparatus will, with its possibilities for meteorology, be given in a future paper. I reproduce the record obtained in my greenhouse on the 5th March (1919), which gives a general idea of the variation of the light from morning to evening (Fig. 213). The record shows that the light began to be perceptible at 5-30 a.m., and that the intensity increased rapidly and continuously till it reached a climax at noon, after which it began to decline slowly. The decline of intensity of light was very abrupt after 5 p.m., the effect being reduced to zero at 6-30 p.m.
THE EFFECT OF DIRECT LIGHT.
Under natural conditions, the leaf of _Mimosa_ is acted on by light from above, and it is generally supposed that the pulvinus is positively phototropic, that is to say, it curves upwards till the leaf is placed at right angles to the direction of light. My investigations show, however, that the phototropic effects vary from positive to negative through an intermediate stage of neutralisation, these depending on the intensity and duration of exposure. When light acts continuously on the upper half of the pulvinus, there follows the following sequences of reaction:
(1) The leaf is at first erected by the contraction of the upper half of the pulvinus due to direct action of light acting from above.
(2) Under continuous stimulation of the upper half of the pulvinus by light, the excitation is slowly conducted to the lower half across the pulvinus. In consequence of this transmitted excitation, the lower half begins to contract and thus neutralises the first effect of erection. The upper half of the pulvinus is less contractile than the lower half, and the neutralisation is due to the full contraction of the upper half antagonised by slight contraction of the lower half. The horizontal position of the leaf under light is therefore the result of balance of the two antagonistic reactions. If the incident light be very strong, the more intense transmitted excitation induces greater contraction of the lower half, and bring about a resultant down-movement (_cf._ p. 331).
Let us consider the effect of daily variation of light on _Mimosa_; we have here to take account both of intensity and duration. The intensity of light is seen to undergo a continuous increase which reaches a climax at noon; it then begins to decline slowly and the diminution of intensity of light is very abrupt after 5 p.m.
Under natural conditions the following phototropic effects are observed during the course of the day: light acting from above induces an up-movement of the leaf; but this is opposed by the thermo-geotropic fall of the leaf due to rise of temperature. As the two opposing effects are nearly balanced, any fluctuation of the relative intensity of the two gives rise to the pulsatory movements often seen in the forenoon; the _Mimosa_ leaf has moreover an autonomous movement of its own. Under continued action of light neutralisation begins to take place after 1 p.m. (_cf._ _Expt._ 135). Later in the day the phototropic effect may become negative; reversal into this negative takes place under the joint action of intensity and duration of light; it takes place earlier under strong, and later under feeble, light.
THE EVENING SPASMODIC FALL OF THE LEAF.
I shall now deal with the difficult problem of the sudden fall of the leaf after 5 p.m. Pfeffer regarded this sudden fall in the evening as due to the increased mechanical moment of the secondary petioles moving forward on the withdrawal of light. But the following experiment shows that the increased mechanical moment cannot be the true explanation of the fall.
_Diurnal movement of the amputated petiole: Experiment 223._--In my present experiment the possibility of variation of mechanical movement was obviated by cutting off the end of the petiole, which carried the sub-petioles. The cut end was coated with collodion flexile to prevent evaporation. The intense stimulus caused by amputation induced the excitatory fall of the leaf, but it recovered its normal activity after a period of three hours or so. The diurnal record of the leaf was commenced shortly after 1 p.m.; it will be noticed that the leaf, though deprived of the weight of its sub-petioles, still exhibited a sudden fall at about 5 p.m. (Fig. 214). The fall of the leaf cannot therefore be due to increased mechanical moment. The effect of weight was, moreover, eliminated in torsional response (_Expt._ 221). In spite of this the leaf exhibited a sudden movement after 5 p.m.
Pfeffer has in his 'Entstehung der Schlafbewegung' (1907) offered another explanation of the sudden fall of the leaf of _Mimosa_. This, according to him, is not the direct effect of diminished intensity of light in the evening, but is due to the release of the leaf from the phototropic action of light, which, so long as it is sufficiently intense, holds the leaf in the normal position with its upper surface at right angles to the incident rays. Thus, on being set free from the strong action of light, the leaf moves in accordance with the preceding condition of tension; and as this is low the leaf falls, soon to rise again as the tension increases in prolonged darkness.
The above explanation presupposes: (1) that the tension was continuously decreasing till the evening, and (2) that as soon as the phototropic restraint which held the leaf up was removed it fell down in accordance with the prevailing diminished tension.
Referring to the first point, an inspection of the diurnal curve of _Mimosa_ shows that the leaf had no natural tendency to fall towards the evening. There was on the contrary a movement of erection, on account of fall of temperature after the thermal-noon (Fig. 210). As the natural tendency of the leaf was to erect itself, the removal of phototropic restraint cannot therefore induce a movement of fall.
As regards the factor of light, the effect in the afternoon is a down-movement on account of transverse conduction of excitation; but the leaf is prevented from exhibiting this down-movement by the thermo-geotropic up-movement due to fall of temperature after the thermal noon. I shall presently describe experiments on the pure effect of light, which will show that the action of continued photic stimulus induces a down-movement of the leaf in the afternoon.
The results of experiments that have been described show that the sudden fall of the leaf in the evening could not be due to:
(1) increased mechanical moment,
(2) the natural tendency of the leaf to fall towards evening against phototropic action by which the leaf is held up.
The above explanations having proved unsatisfactory we have to search for other factors to account for the fall of the leaf on the cessation of light. In this connection I was struck by the extraordinary similarity of the diurnal curve of the petiole of _Cassia alata_ with that of _Mimosa_.
DIURNAL CURVE OF THE PETIOLE OF _Cassia alata_.
_Experiment 224_.--The leaf of _Cassia_ exhibits as in the leaf of _Mimosa_ a slight erectile movement after the thermal-noon at 2 p.m., there is next a sudden fall after 5 p.m., which continues about 9 p.m.; after this the leaf exhibits a continuous rise with the fall of temperature, till the climax is reached about 6 a.m. in the morning; the leaf then undergoes a fall with rise of temperature, there being a number of pulsatory movements in the forenoon, evidently due to unstable balance under the opposing effects of light and of rise of temperature (Fig. 215).
The reason of this similarity between the records of _Cassia_ and _Mimosa_ was found in the fact:
(1) That the main pulvinus of the leaf of _Cassia_ is, like the pulvinus of _Mimosa_, differentially excitable, the lower half being more excitable than the upper. This is demonstrated by sending a diffuse electric shock through the leaf, the response being by a fall of the leaf due to the greater contraction of the lower half of the pulvinus. The leaf recovered after an interval of 20 minutes, the curve of response being similar to that of _Mimosa_. The only difference between the two organs is in the lesser excitability of the pulvinus of _Cassia_, on account of which a greater intensity of shock is necessary for producing the responsive fall.
(2) The responses to light are the same in both as will be seen in the following experiment.
_Experiment 225._--In _Cassia_, as in _Mimosa_, light acting from above induces at first an erectile movement which reaches a maximum; after this there is a neutralisation and reversal. In the record given in figure 216, light from a small arc lamp acting on the upper half of the pulvinus for 48 minutes gave the maximum positive curvature; this was completely neutralised by further exposure to light for 20 minutes. Light was cut off at neutralisation and there was a sudden fall beyond the equilibrium position, which was more rapid than the movement under light. The after-effect of prolonged exposure is thus an 'over-shooting' beyond the normal position of equilibrium.
RESPONSE OF _Mimosa_ TO DARKNESS AT DIFFERENT PARTS OF THE DAY.
I now tried the effect of darkness on the movement of _Mimosa_, and was surprised to find that while artificial darkness caused a sudden fall of the leaf in the afternoon, it had no such effect in the forenoon.
_Experiment 226._--Successive records were taken of the effect of artificial darkness for two hours, alternating with exposure to light for two hours. The plant was subjected to darkness by placing a piece of black cloth over the glass cover from 12 to 2 p.m., it was exposed to light from 2 to 4 p.m. and darkened once more from 4 to 6 p.m.
The record given in figure 217 shows that the leaf had been moving upwards under the action of light (positive phototropism); darkness commenced at the point marked with a thick dot. _The after-effect on the stoppage of light is seen to be in the same direction as under light_; this persisted for ten minutes followed by recovery which was complete by 2 p.m., as seen in the horizontal character of the curve. On restoration of light (at the point marked with the second thick dot) the leaf moved upwards till the positive phototropic movement attained a maximum in the course of an hour and twenty minutes, after which neutralisation set in, and by 4 p.m. the positive phototropic effect had become partially neutralised. Artificial darkness at the third thick dot caused a rapid down-movement which overshot the position of equilibrium. The difference of after-effect in the forenoon and in the afternoon lies in the fact that in the first case it was the pre-maximum after-effect; but in the second case the after-effect was post-maximum. I have already shown in the previous chapter that the pre-maximum after-effect of light is a short-lived movement in the same direction as under light, while post-maximum after-effect was a rapid over-shooting downwards beyond the equilibrium position. These characteristics are also found in the after-effects of light in _Mimosa_.
The responses of _Mimosa_ on the cessation of light described above took place in the course of experiments which lasted for more than six hours. Objection may be raised that during this long period the temperature variation must have produced certain effects on the response. In order to meet this difficulty, I carried out the following experiments which were completed in a relatively short time. I have already explained how the period of experiment could be shortened by suitable increase of the intensity of light. The experiment was commenced inside a room at noon and completed by 2 p.m.; the temperature variation during this period was less than 1°C.
_After-effect at pre-maximum: Experiment 227._--Light from an 100 c.p. incandescent lamp was focussed on the upper half of the pulvinus of _Mimosa_ for 8 minutes, after which the light was turned off. The after-effect was a persistence of previous movement followed by recovery (Fig. 218).
_After-effect at maximum: Experiment 228._--Continued action of light for 18 minutes induced maximum positive curvature as seen in the upper part of the curve becoming horizontal. On the stoppage of light, there was a recovery to the original position of equilibrium (Fig. 219).
_After-effect at post-maximum: Experiment 229._--A fresh specimen of plant was taken for this experiment; it exhibited maximum positive curvature after an exposure of 20 minutes; continuation of light for a further period of 17 minutes produced complete neutralisation. Stoppage of light at this point, gave rise to a rapid down-movement (Fig. 220) below the equilibrium position.
The experiments that have been described show that the rapid fall of the leaf of _Mimosa_ in the afternoon is due to 'over-shooting' which is the after-effect of prolonged action of light.
We are now in a position to give a full explanation of the different phases of diurnal movement of the leaf of _Mimosa_. The fall of the leaf commences from its highest position at thermal-dawn at 6 a.m. in the morning and continued till the thermal-noon at 2 p.m. This is the thermo-geotropic reaction due to rise of temperature. In the forenoon the phototropic action is positive, and the fall of the leaf, due to rise of temperature, is brought about in opposition to the action of light. The temperature begins to fall after 2 p.m. and the leaf begins to erect itself, and in the absence of any disturbing factor would have continued its up-movement till next morning. But light undergoes a rapid diminution after 5 p.m. and the after-effect of light is an 'over-shooting' in a downward direction. This fall continues till about 9 p.m., after which the leaf erects itself under thermo-geotropic action of falling temperature, the maximum erection being attained at the thermal-dawn at about 6 a.m.
SUMMARY.
The very complex type of nyctitropic movement of the primary petiole of _Mimosa_ results from the combined effects of thermo-geotropism and phototropism.
With the exception of a small portion of the curve in the evening, the diurnal curve of _Mimosa_ is similar to the standard thermo-geotropic curve, where the leaf exhibits an erectile movement from thermal-noon to thermal-dawn, and a fall from thermal-dawn to thermal-noon.
Investigations show that the leaf of _Mimosa_ has an autonomous movement of its own, which persists throughout twenty-four hours.
The torsional response of _Mimosa_ exhibits a diurnal variation similar to that exhibited by the leaf in normal position.
The leaf of _Cassia alata_ exhibits a diurnal movement of the same type as that of _Mimosa_.
The spasmodic fall of the leaf towards evening is not due to the increased mechanical moment caused by the forward position of the sub-petioles. The record of the leaf with amputated sub-petioles exhibits the sudden fall in the evening as that of the intact leaf.
The evening fall of the leaf of _Mimosa_ is shown to be due to the post-maximum after-effect of light, which causes an 'over-shooting', the leaf undergoing a fall below the position of equilibrium.
B. S. Press--5-11-1919--19754J--750--R. D'S.