PART I. RESPONSE OF PLANT ORGANS.

I.—THE PROBLEM OF MOVEMENT IN PLANTS

By

Prof. Sir J. C. Bose.

The phenomenon of movement in plants under the action of external stimuli presents innumerable difficulties and complications. The responding organs are very different: they may be the pulvini of the ‘sensitive’ or those of the less excitable leguminous plants; the petioles of leaves, which often act as pulvinoids; and organs of plants in a state of active growth.

Taking first the case of the pulvinus of Mimosa, we find that it responds to mechanical stimulation, to constant electric current, to induction shock, to the action of chemical agents, to light, and to warmth as differentiated from thermal radiation. The reactions induced by these agents may be similar or dissimilar. An identical agent, again, may give rise to movements which are not merely different, but sometimes even of diametrically opposite characters. Certain organs, for example, direct themselves towards light, others away from it. Some plants close their leaflets on the approach of darkness, in the so-called position of ‘sleep’; apparently similar ‘sleep’ movement is induced in others by the action of the midday sun.

In Mimosa, the responsive movement is brought about by a sudden diminution of turgor in the pulvinus. But very little is definitely known about the responsive reaction in growing organs. Thus in a tendril, one-sided contraction causes a shortening of the concave side and a sudden increase of growth on the convex. No explanation of this difference has hitherto been forthcoming. Under the action of light of different intensities a growing organ may approach the source of light, or place itself at right angles or move away from it. Again under the identical stimulus of gravity, the root moves downwards, and the shoot upwards. The sign of response in different organs thus changes, apparently without any reason. It is thus seen, that there is hardly any responsive movement that has been observed of which an example directly to the contrary may not be found. For this reason it has appeared hopeless to unify these very diverse phenomena, and there has been a tendency towards a belief that it was not any definite physio­logic­al reaction, but the individuality of the plant that determines the choice of its movement.

The complexities which baffle us may, however, arise from the combination of factors whose individual reactions are unknown to us. I shall show, for example, how the movement of a pulvinus under a given stimulus is determined by the point of application, direct stimulus producing one effect, and indirect the diametrically opposite. The normal reaction is again modified by the tonic condition of the plant. There is again the likelihood of the presence of other modifying factors. It is clear how very different the results would become by the permutation and combination of these diverse factors.

For a comprehensive study of the phenomenon of plant movement, it is therefore necessary to investigate in detail the effect of a given stimulus under definite changes of the environmental condition. With regard to a given stimulus we have to determine the effects of intensity, of duration, and of the point of application. The in­ves­ti­ga­tion has to include the effects exhibited not merely by the pulvinated but also by growing organs. As a result of such a comprehensive study, it may perhaps be possible to discover some fundamental reaction operative in bringing about the responsive movement in all plant organs.

I shall in the course of the following series of Papers, describe the different apparatus by which the movement of pulvinated organ and its time-relations are automatically recorded. In a growing organ the induced movement under stimulus is brought about by the change in its rate of growth. That the change is solely due to the particular stimulus can only be assured by strict maintenance of constancy of external conditions, during the period of experiment; this constancy can, in practice, be secured only for a short time. The necessity for shortening the period of experiment also arises from a different consideration; for numerous and varied are the stimulating and mechanical interactions between neighbouring organs. These effects, however, come into play after a certain lapse of time. They may be eliminated by reduction of the period of experiment.

In order to shorten the period of experiment for the study of growth movements, the rate of growth has to be very highly magnified, so as to determine the absolute rate and its variations in the course of a minute or so. I shall in a subsequent Paper give full account of an apparatus I have been able to devise, by which it is possible to record automatically the rate of growth magnified many thousand times.

I stated that anomalies of plant movements would disappear, if we succeeded in carrying out in detail in­ves­ti­ga­tions of effects of the different individual factors in operation. In illustration of this I shall, in the first Paper of the series, give an account of the mysterious movement of the ‘Praying’ Palm of Faridpur, and describe the in­ves­ti­ga­tions by which the problem found its solution.

II.—THE “PRAYING” PALM TREE

By

Sir J. Bose,

Assisted by

Narendra Nath Neogi, M.Sc.

Perhaps no phenomenon is so remarkable and shrouded with greater mystery as the performances of a particular Date Palm near Faridpur in Bengal. In the evening, while the temple bells ring calling upon people to prayer, this tree bows down as if to prostrate itself. It erects its head again in the morning, and this process is repeated every day of the year. This extraordinary phenomenon has been regarded as miraculous, and pilgrims have been attracted in large numbers. It is alleged that offerings made to the tree have been the means of effecting marvellous cures. It is not necessary to pronounce any opinion on the subject; these cures may be taken as effective as other faith-cures now prevalent in the West.

This particular Date Palm, Phœnix dactylifera, is a full-grown rigid tree, its trunk being 5 metres in length and 25 cm. in diameter. It must have been displaced by storm from the vertical and is now at an inclination of about 60° to the vertical. In consequence of the diurnal movement, the trunk throughout its entire length is erected in the morning, and depressed in the afternoon. The highest point of the trunk thus moves up and down through one metre; the ‘neck,’ above the trunk, is concave to the sky in the morning; in the afternoon the curvature disappears, or is even slightly reversed. The large leaves which point high up against the sky in the morning are thus swung round in the afternoon through a vertical distance of about five metres. To the popular imagination the tree appears like a living giant, more than twice the height of a human being, which leans forward in the evening from its towering height and bends its neck till the crown of leaves press against the ground in an apparent attitude of devotion (Fig. 1). Two vertical stakes, each one metre high, give a general idea of the size of the tree and movements of the different parts of the trunk.

Fig. 1. The Faridpur ‘Praying’ Palm; the upper photograph shows position in the morning; the lower, position in the afternoon. The two fixed stakes are one metre in height. In front is seen erect trunk of a different Palm.

For an in­ves­ti­ga­tion in elucidation of this phenomenon it was necessary:—

1. To obtain an accurate record of the movement of the tree day and night, and determine the time of its maximum erection and fall.

2. To find whether this particular instance of movement was unique, or whether the phenomenon was universal.

3. To discover the cause of the periodic movement of the tree.

4. To find the reason of the remarkable similarity between the diurnal movement of the tree, and the diurnal variation of moto-ex­cit­abil­ity in Mimosa pudica.

5. To determine the relative effects of light and temperature on the movement.

6. To demonstrate the physio­logic­al character of the movement of the tree.

7. To discover the physio­logic­al factor whose variation determines the directive movement.

THE RECORDING APPARATUS.

I shall now describe the principle and construction of my recording apparatus (Fig. 2) seen attached to a horizontally growing stem of Mimosa pudica. When used to trace the movement of the palm tree, a reducing device is employed to keep the record within the plate. A lever, R′, records the movement of the attached tree or plant on a moving plate of smoked glass. The plate is not in contact with the tip of the recording lever, but separated from it by a distance of about 3 mm. A special oscillating device, actuated by clock-work, C, makes the plate move forwards and backwards. The forward movement brings about a momentary contact of the recording tip with the smoked plate inscribing a dot. These single dots are made at intervals of 15 minutes; at the expiration of the hour, however, contact is made three times in rapid succession, printing a thick dot. It is thus easy to determine the movement of the tree at all times of the day and night. A second lever, R, placed above, gives on the same plate, thermographic record of the diurnal variation of temperature. For this I use a differential thermometer, T, made of a compound strip of brass and steel. Curvature is induced by the differential expansion of the two pieces of metal. The up or down movement of the free end of the compound strip is further magnified by the recording lever. This arrangement was extremely sensitive and gave accurate record of variation of temperature. By the forward movement of the oscillating plate two dots are made at the same time,—one for the temperature and the other for the corresponding movement of the tree. As the two recorders do not move vertically up or down, but describe a circle, the dots vertically one above the other may not correspond as regards time. Any possibility of error in calculation is obviated by the fact that the thick dots in both the records are made every hour, and the subsequent thin dots at intervals of 15 minutes.

Fig. 2. Apparatus for automatic record of movement of trees and plants; T, differential metallic thermometer; R, recording lever for temperature; R′, for recording plant movement; C, clock-work for oscillation of recording plate. The same clock-work moves plate laterally in 24 hours.

A difficulty arose at the beginning in obtaining sanction of the proprietor to attach the recorder to the tree. He was apprehensive that its miraculous power might disappear by profane contact with foreign-looking instruments. His misgivings were removed on the assurance that the instrument was made in my laboratory in India, and that it would be attached to the tree by one of my assistants, who was the son of a priest.

From results of observation it is found that the tree moves through its entire length; the fall of the highest point of the trunk is one metre. The movement is not passive, but an active force is exerted; the force necessary to counteract this movement is equivalent to the weight of 47 kilograms: in other words, the force is sufficient to lift a man off the ground. But far greater force would be required to restrain the change of curvature of the neck of the hard and rigid tree.

Fig. 3. Record of diurnal movement of the ‘Praying’ Palm (Phœnix dactylifera). Thermographic curve for 24 hours commencing at 9 in the evening is given in the upper record; the corresponding diurnal curve of movement of the tree is given in the lower. Successive dots at intervals of 15 minutes; thick dots at intervals of an hour.

Before entering into the in­ves­ti­ga­tion of the cause of periodic movement I shall give a general account of its characteristics. A casual observation would lead one to conclude that the tree lifted itself at sunrise and prostrated at sunset. But continuous record obtained with my recorder attached to the upper part of the trunk shows that the tree was never at rest, but in a state of continuous movement, which underwent periodic reversals (Fig. 3). The tree attained its maximum erection at 7 in the morning, after which there is a rapid movement of fall. The down movement reached its maximum at 3-15 P.M., after which it was reversed and the tree erected itself to its greatest height at 7 next morning. This diurnal periodicity was maintained day after day.

UNIVERSALITY OF TREE MOVEMENT.

The next question which I wished to investigate was whether the movement of the particular Faridpur tree was a unique phenomenon. It appeared more likely that similar movement would, under careful observation, be detected in all trees. The particular palm tree was growing at a considerable inclination to the vertical; the movement of the tree and its leaves became easily noticeable, since the ground afforded a fixed and striking object of reference. In a tree growing more or less erect, the movement, if any, would escape notice, since such movements would be executed with only the empty space as the background.

Fig. 4. Record of the Sijbaria Palm from noon for 24 hours. Successive dots, at intervals of 15 minutes.

Experiment 1.—Believing the phenomenon to be universal I experimented with a different Date Palm that was growing at my research station at Sijbaria on the Ganges, situated at a distance of about 200 miles from Faridpur. The surrounding conditions were very different. The tree was much younger; it was 2 metres in height and inclined 20° to the vertical. The curve obtained with this tree (Fig. 4) was very similar to that of the Faridpur Palm, though this extent to the movement was much reduced. The tree attained the highest erect position at 7-15 A.M. and the lowest at 3-45 P.M. Hence the movement of the Faridpur Palm is not a solitary phenomenon.

THE CAUSE OF PERIODIC MOVEMENT.

The recurrent daily movement of the tree must be due to some diurnal changes in the environment,—either the recurrent changes of light and darkness, or the diurnal changes of temperature. These changes synchronise to a certain extent; for, as the sun rises, light appears and the temperature begins to rise. It is therefore difficult to discriminate the effect of light from that of temperature. The only satisfactory method of discrimination would have been in the erection of a large structure with screens to cut off light. The effect of fluctuation of temperature under constant darkness would have demonstrated the effect of one agent without complication arising from the other. Unfortunately screening the tree was impracticable. I shall presently describe other experiments where the action of light was completely excluded.

The curve of movement of the tree, however, affords us material for correct inference as regards the relative effects of light and temperature. The experiment was commenced in March; light appeared at about 5 A.M., the sunrise being at 6-15 A.M.; the sun set at 6-15 P.M., and it became dark by 7 P.M. The incident light would be the most intense at about noon; after this it would decline continuously till night time. If the movement was due to light, its climax, either in up or down movement, would be reached at or about noon, and the opposite climax at midnight. But instead of this we find (Fig. 3) the up-movement reaching its highest point not at noon, but at 7 in the morning; after this the fall is rapid and continuous, and the lowest position was reached not in the evening but at 3-15 P.M. The fluctuation of light has, therefore, little to do with the movement of the tree.

Turning next to the element of variation of temperature we are at once struck by the fact that the curve of movement of the tree is practically a replica of the thermographic curve (Fig. 3). The fall of temperature is seen to induce a rise in the tree and vice versâ. There is a lag in the turning points of the two curves; thus while temperature began to rise at 6 A.M., the tree did not begin to fall till 7 A.M. There is in this case a lag of an hour; but the latent period may, sometimes, be as long as three hours. The delay is due to two reasons; it must take some time for the thick trunk of the tree to attain the temperature of the surrounding, and secondly, the physio­logic­al inertia will delay the reaction. As a result of other in­ves­ti­ga­tions, I find that the induced effect always lags behind the inducing cause. It is interesting in this connection to draw attention to the parallel phenomenon, which is described below, of lag in the variation of sensibility of Mimosa in response to variation of temperature. In this case the lag was found to be about three hours. Returning to the Palm, the tree continues to fall in the forenoon with rising temperature. At about 2-30 P.M. the temperature was at its maximum after which it began to decline; the movement of the tree was not reversed into erection till after 3-15 P.M., the lag being now 45 minutes nearly.

I may state here that the movement of the tree is not primarily affected by the periodicity of day and night, but by variation of temperature. In spring and in early summer the rise of temperature during the early part of the day and the fall of the temperature from afternoon to next morning, are regular and continuous; the corresponding movements of the tree are also regular. But at other seasons, owing to the sudden change of direction of the wind, the fluctuations of temperature are irregular. Thus at night there may be a sudden rise, and in the earlier part of the day sudden fall of temperature. And the record of movement of the tree is found to follow these fluctuations with astonishing fidelity, the rise of temperature being followed by a fall of the tree and vice versâ. That the movement is determined by the temperature variation is exhibited in a striking manner in Fig. 4, where, between 8 and 9 A.M., a common twitch will be noticed in the two curves.

While trying to obtain some clue to the mysterious movement of the tree, my attention was strongly attracted by certain striking similarities which the record of the movement of the tree showed to the curve of the diurnal variation of moto-ex­cit­abil­ity, of the pulvinus of Mimosa pudica, an account of which will be found in a subsequent Paper of the series.[A]

PERIODIC MOVEMENT OF TREES AND DIURNAL VARIATION OF MOTO-EXCITABILITY IN MIMOSA PUDICA.

The ex­cit­abil­ity of the main pulvinus of Mimosa pudica I find does not remain constant during the 24 hours, but undergoes a striking periodic change. At certain hours of the day, the ex­cit­abil­ity is at its maximum; at a different period it practically disappears. The period of insensibility is about 7 A.M., which, strangely enough, is also the time when the palm tree attains its maximum height. At about 3 in the afternoon the ex­cit­abil­ity of Mimosa reaches its climax, and this is the time when the head of the palm tree bends down to its lowest position. For the determination of the periodic variation of ex­cit­abil­ity of Mimosa I devised a special apparatus by which an electric stimulus of constant intensity was automatically applied to the plant every hour of the day and night, the responsive moment being recorded at the same time. The amplitude of responsive fall of leaf under uniform stimulus gave a measure of ex­cit­abil­ity of the leaf at any particular moment. In the lower curve of Fig. 5 is given the record of diurnal variation of ex­cit­abil­ity of Mimosa. Comparison of this figure with Figs. 3 and 4, will show the remarkable resemblance between the curves of diurnal movement of the Palm tree, and of diurnal variation of moto-ex­cit­abil­ity of Mimosa. The ex­cit­abil­ity of Mimosa reached its maximum at about 3 in the afternoon, when the Palm was at its lowest position. After this hour ex­cit­abil­ity fell continuously till 7 or 8 next morning. Corresponding to this is the continuous erection of the Palm from its lowest position at 3 P.M. to the highest between 7 and 8 A.M. Still more remarkable is the modifying influence of variation of temperature on the diurnal curve of ex­cit­abil­ity in Mimosa, and the diurnal curve of movement of the Palm. This will be quite evident from the inspection of the temperature curves in Figs. 4 and 5.

Fig. 5. Curve of variation of moto-ex­cit­abil­ity of Mimosa pudica. The upper curve gives variation of temperature and the lower, the corresponding variation of ex­cit­abil­ity.

I have shown elsewhere[B] that the variation of moto-ex­cit­abil­ity of the pulvinus of Mimosa is a physio­logic­al function of temperature. The remarkable similarity between the diurnal variation of moto-ex­cit­abil­ity of Mimosa and diurnal movement of the Palm is due to the fact that both are determined by the physio­logic­al action of temperature. I shall presently describe experiments, which will establish the physio­logic­al character of the movement of the tree in response to changes of temperature.

The records that have been given show that it is the diurnal variation of temperature, and not of light that is effective in inducing the periodic movement of the tree. Further experiments will be given in support of this conclusion.

RELATIVE EFFECTS OF LIGHT AND TEMPERATURE.

As regards the possibility of light exerting any marked influence on the movement of the Palm tree, I have shown from study of time-relations of the movement, that this could not be the case. Moreover, it is impossible for light to reach the living tissue through the thick layer of bark that surrounds the tree. That the effect of light is negligible will appear from the accounts of following experiments, where the possibility of the effect of changing intensity of light is excluded by maintaining the plant in constant darkness, or in constant light.

The employment of the large Palm was obviously impracticable in these in­ves­ti­ga­tions. I, therefore, searched for other plant-organs in which the movement under variation of temperature was similar to that of the Date Palm. I found that the horizontally spread leaves of vigorous specimens of Arenga saccharifera growing in a flower pot executed movements which were practically the same as that of the Faridpur tree. The leaf moved downwards with rise of temperature and vice versâ.

There are many practical advantages in working with a small specimen. It can easily be placed under glass cover or taken to a glass house, thus completely eliminating the troublesome disturbance caused by the wind.

Diurnal movement in continued darkness: Experiment 2.—The plant was placed in a dark room and records taken continuously for three days. These did not differ in any way from the normal records taken in a glass house under daily variation of light and darkness. Exposure of plant to darkness for the very prolonged period of a week or more, undoubtedly interferes with the healthy photo-tonic condition of the plant. But such unhealthy condition did not make its appearance in the first few days.

PHYSIOLOGICAL CHARACTER OF THE MOVEMENT.

There may be a misgiving that the movement of the tree might be due to physical effect of temperature. If the upper strip of a differential thermometer be made of the more expansible brass and the lower of iron, the compound strip bends down with the rise of temperature. Similarly the movement of the tree might be due to the upper half being physically more expansible. It would have been possible to discriminate the physical from the physio­logic­al action by causing the death of the tree; in that case physical movement would have persisted, while the physio­logic­al action would have disappeared. As this test was not practicable, I tried the effect of physio­logic­al depression on the periodic movement of the leaf of Arenga saccharifera.

Fig. 6. Effect of physio­logic­al depression on diurnal movement of the petiole of Arenga saccharifera. The uppermost curve exhibits variation of temperature, (a), normal diurnal curve, (b), modi­fi­ca­tion after 3 days’ and (c) after 7 days’ withholding of water.

Effect of Drought: Experiment 3.—In Fig. 6 is given a series of records of movement of the leaf-stalk of Arenga, first under normal condition, afterwards under increasing drought, brought about by withholding water. The uppermost is the thermographic record which remained practically the same for successive days. Below this are records of movement of the leaf (a) under normal condition, (b) after withholding water for three days, and (c) after deprivation for seven days. It will be noticed how the extent of movement is diminished under increasing physio­logic­al depression brought on by drought. On the seventh day, the responsive movement disappeared, there being now a mere fall of the leaf, which was slow and continuous. After this I supplied the plant with water and the periodic movement was in consequence nearly restored to its original vigour.

Effect of poison: Experiment 4.—In another experiment the normal diurnal record with the leaf was taken and the plant was afterwards killed by application of poisonous solution of potassium cyanide. The diurnal movement was found permanently abolished at the death of the plant.

These experiments conclusively prove that the periodic movement of the leaf-stalk induced by variation of temperature is a physio­logic­al phenomenon, and from analogy we are justified in drawing the inference that the movement of the Faridpur tree is also physio­logic­al. The question, however, was finally settled by the unfortunate death of the tree which occurred the other day, nearly a year after I commenced my in­ves­ti­ga­tions. While presiding at my lecture on the subject, His Excellency Lord Ronaldshay, the Governor of Bengal, announced that a telegram had just reached him from his officer at Faridpur that “the palm tree was dead, and that its movements had ceased.”

Since my in­ves­ti­ga­tion with the Faridpur ‘Praying’ Palm, I have received information regarding other Palms, which exhibit movements equally striking. One of the trees is growing by the side of a tank, the trunk of the tree being inclined towards it. The up-lifted leaves of this tree are swung round in the afternoon and dipped into the water of the tank.

The movement of the tree has been shown to be brought about by the physio­logic­al action of temperature variation; in other words the diurnal movement of the ‘Praying’ Palm is a THERMONASTIC PHENOMENON. I have found various creeping stems, branches and leaves of many trees, exhibit this particular movement of fall with a rise of temperature, and vice versâ. Such movements, I shall, for the sake of convenience, distinguish as belonging to the negative type.

Having found that the temperature is the modifying cause, the next point of inquiry relates to the discovery of the force, whose varying effects under changing temperature induces the periodic movement. I shall, in this connection, first discuss the various tentative theories that may be advanced in explanation of the movement.

TRANSPIRATION AND DIURNAL MOVEMENT.

It may be thought that the fall of the tree during rise of temperature may be due to passive yielding of the tree to its weight, there being increased transpiration and general loss of turgor at high temperature. I shall, however, show that the diurnal movement persists in the absence of transpiration.

Diurnal movement in absence of transpiration: Experiment 5.—In the leaf of Arenga saccharifera, I found that the petiole was the organ of movement. I cut off the transpiring lamina and covered the cut end with collodion flexile. The plant was now placed in a chamber saturated with moisture. The petiole continued to give records of its diurnal movement in every way similar to the record of the intact leaf. In another experiment with the water plant, Ipoemia reptans, immersed in water, the normal diurnal movement was given by the plant, where there could be no question of variation of turgor due to transpiration. (See also Expt. 7.)

In the diurnal movement of the ‘Praying’ Palm the concave curvature of the rigid neck in the morning, became flattened or slightly convex in the afternoon. The force necessary to cause this is enormously great, and could on no account result from the passive yielding to the weight of the upper part of the tree.

From the facts given above it will be seen that the diurnal movement is not brought about by variation in transpiration. I now turn to another phenomenon which appeared at first to have some connection with the movement of the tree. Kraus found that the tissue tensions of a shoot exhibit a daily periodicity. He, however, found that between 10°C. and 30°C., variation of temperature had no effect on the daily period. But as regards the diurnal movement of the tree, it is the temperature which is the principal factor. Kraus also found a daily variation of bulk in different plant-organs; this variation of bulk is connected with transpiration, for the removal of the transpiring leaves arrested this variation. But the periodic movement of the tree, as we have seen, is independent of transpiration.

Millardet observed a daily periodicity of tension in Mimosa pudica. He found that maximum tension occurs before dawn; the petiole becomes erected, the movement being upwards or towards the tip of the stem. Tension decreases during the day, and reaches a minimum early in the evening; in correspondence with this is the fall of the petiole, the movement being away from the tip of the stem.[C] If the plant were placed upside down the periodic movement of the petiole in relation to the stem will evidently remain the same, but become reversed in space. Maximum tension in the morning will make the petiole approach the tip of the stem, i.e., the movement will be downwards instead of upwards as in the normal position. The experiment described below will show that the diurnal movement induced by variation of temperature is not reversed by placing the plant in an inverted position.

Diurnal movement in inverted position: Experiment 6.—I took a vigorous specimen of Arenga saccharifera growing in a pot, and took its normal record, which as explained before exhibited down-movement during rise, and an up-movement during fall of temperature. The plant was now held inverted, the upper side of the petiole now facing the earth. The diurnal curve of movement should now show an inversion, if that movement was solely determined by the anisotropy of the organ. But the record did not exhibit any such inversion. After being placed upside down, the leaf did not, on the first day, show any diurnal movement; there was, on the other hand, a continuous down-movement on account of the fall of the leaf by its own weight. But in the course of 24 hours the leaf readjusted itself to its unaccustomed position, and became somewhat erected under the action of geotropic stimulus. After the attainment of this new state of geotropic equilibrium, the leaf gave a very pronounced record of its diurnal movement which did not show any reversal; the inverted leaf continued to exhibit the same char­ac­ter­is­tic movements as in the normal position, that is to say, a down movement during rise, and an up-movement during fall of temperature. As the plant in the inverted position did not show any reversal of the periodic curve, it is clear that the diurnal movement is determined by the modifying influence of temperature on the physio­logic­al reaction of the plant to some external stimulus which is constant in direction. I shall presently show that it is the constant geotropic stimulus modified by the action of temperature, which determines the diurnal movement of the tree.

This will be better understood if I refer once more to certain characteristics in the movement of the “Praying” Palm. The neck of the tree was seen to be concave in the morning. The physio­logic­al effect of raising temperature is virtually to oppose or neutralise the geotropic curvature as seen in the flattening or slight reversal of curvature in the afternoon. Similarly, various plant organs, growing at an inclination to the vertical, are subjected to geotropic action, and thus assume different char­ac­ter­is­tic angles. This state of equilibrium is not static but may better be described as dynamic; for it will be shown that this state of geotropic balance is upset in a definite way, by variation of temperature.

That geotropism is an important factor in the diurnal movement is supported by the fact that the Sijbaria Palm with an inclination of 20° to the vertical exhibited a daily movement which was only moderate in extent. But the Faridpur Palm growing at an inclination of 60° was subjected more effectively to geotropic action, and exhibited movements which were far more pronounced. I shall now proceed to describe crucial experiments which will demonstrate the effect of change of temperature on geotropic curvature.

EFFECT OF VARIATION OF TEMPERATURE ON GEOTROPIC CURVATURE.

In the instances of diurnal movement already described the trees or their leaves were already at an inclination to the vertical. I now took a radial and erect shoot of Basella cordifolia growing in a pot and laid it horizontally for two weeks. The procumbent stem curved up and attained a state of equilibrium under the action of geotropic stimulus.

Diurnal curve of Basella cordifolia: Experiment 7.—The plant was completely immersed in a vessel of water, and its diurnal curve recorded. This resembled in all essentials the diurnal curve of the Palm; the slight deviation was due to the fact that owing to difference in the season (August) the temperature maximum was attained at 12-25 P.M. and the minimum at 6 A.M. The geotropic curvature was reduced to its minimum at the maximum temperature, and vice versâ. As in the case of the Palm so also in the procumbent stem of Basella there was a physio­logic­al lag, which was 50 minutes in the morning and about the same in the afternoon. The free end of the stem thus exhibited a diurnal movement up and down. The temperature, as stated before, began to rise from 6 A.M. and the down-movement commenced 50 minutes later, i.e., at 6-50 A.M. The temperature, after reaching the maximum, began to fall at 12-25 P.M., and the previous movement of fall of the stem was arrested and reversed into an erectile movement shortly after 1 P.M. There are thus two “turning points,” one at 7 A.M., and the other at about 1 P.M.; at these periods the movement of the plant remains more or less arrested for more than half-an-hour.

Fig. 7. Diurnal curve of movement of procumbent young stem of Mimosa pudica. Successive dots at intervals of 15 minutes.

I obtained records of similar diurnal movements with various procumbent or creeping stems. Figure 7 gives the diurnal record of the procumbent stem of a young specimen of Mimosa pudica.

The experiment that has just been described shows clearly that geotropic curvatures of stems is opposed, or neutralised to a greater or less extent, during rise of temperature, and this antagonistic reaction is removed during the fall of temperature. The diurnal movement of the plant completely immersed under water shows once more that transpiration has little to do with the diurnal movement.

REVERSAL OF NATURAL RHYTHM.

The diurnal rhythm of up and down movement in the particular specimen Basella had become established under the daily variation of temperature. I now attempted to reverse this rhythm by artificial variation of temperature. The plant was placed in water in a rectangular metallic vessel which was placed within a second outer vessel. The plant could thus be subjected, without any mechanical disturbance, to variation of temperature, by circulating warm or cold water in the outer vessel. In order to reverse the natural rhythm I subjected the plant to the action of falling temperature at the “turning” point at 7 A.M., at a time when the plant would have undergone a down-movement under the daily rise of temperature. Conversely the plant was subjected to the action of rising temperature at the second “turning” point at 1 P.M. when the movement under diurnal fall of temperature would have been one of erection.

Effect of fall of temperature: Experiment 8.—As stated before the experiment was carried out in the morning; ice cold water was circulated in the outer chamber, the fall of temperature was in this case sudden, and there was an almost immediate responsive movement. This appeared anomalous, since the latent period of response to slow variation of temperature was found from the diurnal curve to be as long as 50 minutes.

As a result of further in­ves­ti­ga­tions I found that variation of temperature produces two different effects which may be distinguished as transient and persistent. Sudden variation of temperature affects the superficial tissue, and gives rise to a transient reaction, while it takes a long time for temperature variation to react on the geotropically active tissue in the interior. The persistent effect therefore takes place after a latent period from one to three hours according to the thickness of the plant.

Fig. 8. Reversal of normal rhythm: Erectile response Basella to gradual fall of temperature.

Fig. 9. Responsive fall of Basella to gradual rise of temperature. (Dots at intervals of 5 minutes).

The persistent effect of rise of temperature is a movement downwards, that of fall of temperature is a movement upwards. These definite reactions will be seen exhibited in Figs. 8 and 9. The plant was stationary at the turning point in the morning hence the curve at first was horizontal. The temperature was gradually lowered through 5°C., from 29°C., to 24°C. in the course of five minutes and maintained at the lower temperature. There was no immediate effect, but after a latent period of 65 minutes the plant responded by a movement of erection. The natural movement at this period of the day would have been one of fall, but artificial change of temperature in the opposite direction effectively reversed the normal diurnal movement. The latent period for this reverse movement is, as stated before, 65 minutes as against 50 minutes in the normal diurnal movement. The increase in the latent period is probably due to the added physio­logic­al inertia in reversing the normal rhythm.

Effect of rise of temperature: Experiment 9.—The temperature was raised through 5°C at the second turning point, at 1 P.M. After a latent period of 50 minutes the plant began to rise steadily (Fig. 9) thus exhibit­ing once more the reversal of its normal diurnal movement.

From the experiments described above it will be seen that the movement of the Palm, and of other organs growing at an inclination to the vertical, is brought about by the action of temperature in modifying the geotropic curvature. The ever present tendency of geotropic movement is opposed or helped by the physio­logic­al reaction induced by rise and fall of temperature respectively. The state of equilibrium is never permanent, but the dynamic balance is being constantly readjusted under changing conditions of the environment.

The movement of the tree furnishes an example of the negative type of THERMONASTIC MOVEMENT. Parallel phenomena are found in floral organs, where, in the well-known instance of Crocus, the perianth leaves open outwards during rise of temperature and close inwards during the onset of cold. Looked at from above, the opening outwards during rise of temperature is a movement downwards, and therefore belongs to the negative type. In such cases the changed rate of growth by variation of temperature is the most important factor in the movement. It may be asked whether all thermonastic movements must necessarily belong to the negative type, where rise of temperature is attended by a movement downwards. I shall in my Paper on “Thermonastic Phenomena” show that there is also a positive type where rise of temperature induces an up-movement or of closure.

SUMMARY.

The ‘Praying’ Palm of Faridpur, growing at an inclination of about 60° to the vertical, exhibited a diurnal movement by which its head became erected in the morning and depressed towards the afternoon, the outspread leaves pressing against the ground.

The record of the diurnal movement showed that the head was erected to the highest position between 7 and 8 in the morning, after which there was a continuous fall which reached its climax at 3-15 P.M.; after this the movement was reversed and the maximum erection was again reached next morning.

This phenomenon is not unique, but is found exhibited, more or less, by all trees and their branches and leaves.

Diurnal records of temperature, and movement of the tree showed, that the two curves closely resembled each other. Rise of temperature was attended by a fall of the tree, and vice versâ.

The movement is brought about by the physio­logic­al action of temperature; it may be arrested by artificially induced physio­logic­al depression, and is permanently abolished at death.

The movement is primarily determined by the modifying influence of temperature on geotropic curvature. Rise of temperature is found to oppose or neutralise geotropic curvature, the fall of temperature inducing the opposite effect. The ever present tendency of upwards geotropic movement is opposed or helped by the effects of rise and fall of temperature respectively.

The movement of the ‘Praying’ Palm is a thermonastic phenomenon. The tree, apparently so rigid, responds as a gigantic pulvinoid to the changes of its environment.

III.—ACTION OF STIMULUS ON VEGETABLE TISSUES

By

Sir J. C. Bose,

Assisted by

Narendra Nath Sen Gupta.

The leaf of Mimosa pudica undergoes a rapid fall when subjected to any kind of shock. This plant has, therefore, been regarded as “sensitive,” in contra­distinction to ordinary plants which remain apparently immobile under external stimulus. I shall, however, show in course of this Paper that there is no justification in regarding ordinary plants as insensitive.

Let us first take any radial organ of a plant and subject it to an electric shock. It will be found that the organ undergoes a contraction in length in response to the stimulus. On the cessation of excitation the specimen gradually recovers its original length. Different organs of plant may be employed for the experiment, for example, the tendril of Cucurbita, the pistil of Datura, or the flower bud of Crinum. The shortening may be observed by means of a low power microscope. Greater importance is, however, attached to the detailed study of response and its time relations. The pull exerted by a delicate organ during its excitatory contraction is slight; hence arises the necessity of devising a very sensitive apparatus, which would give records magnified from ten to a hundred times.

RESPONSE RECORDERS.

The mag­ni­fi­ca­tion of movement is produced by a light lever, the short arm of which is attached to the plant organ, the long arm tracing the record on a moving smoked plate of glass. The axis of the lever is supported by jewel bearings. The principal difficulty in obtaining accurate record of response of plant lies in the friction of contact of the recording point against the glass surface. This difficulty I have been able to overcome by providing a device of intermittent instead of continuous contact. For this, either the writer is made to vibrate to and fro, or the recording plate is made to oscillate backwards and forwards.

1. The Resonant Recorder.—In this the writing lever is made of a fine steel wire. One end of this wire is supported at the centre of a circular electromagnet; this latter is periodically magnetised by a coercing vibrator, which completes an electric circuit ten hundred, or two hundred times in a second. The writing lever is exactly tuned to the vibrating interrupter and is thus thrown into sympathetic vibration. Successive dots in the record thus measure time from 0.1 to 0.05 second. The employment of the Resonant Recorder enables us to measure extremely short periods of time for the determination of the latent period or the velocity of trans­mission of excitation.[D]

2. The Magnetic Tapper.—Measurement of very short intervals is not necessary in ordinary records of response. In this type of recorders, the circular magnet is therefore excited at longer intervals, from several seconds to several minutes; this is done by completion of the electric circuit at the required intervals, by means of a key operated by a clock.

3. The Mechanical Tapper.—In this, magnetic tapping is discarded in favour of mechanical tapping. The hinged writing lever is periodically pressed against the recording plate by a long arm, actuated by clock-work.

4. The Oscillating Recorder.—Here the plate itself is made to oscillate to-and-fro by eccentric worked by a clock. The frame carrying the plate moves on ball-bearings. The advantage of the Oscillating Recorder lies in the fact that a long lever, made of fine glass fibre, or of aluminium wire, may be employed for giving high mag­ni­fi­ca­tion. A mag­ni­fi­ca­tion of a hundred times may be easily obtained by making the short arm 2.5 mm. and the long arm 25 cm. in length.[E]

RESPONSE OF A RADIAL ORGAN.

Fig. 10. Response of a straight tendril of Passiflora to electric shock. Suc­ces­sive dots at inter­vals of 5 seconds. The ver­ti­cal lines below are at inter­vals of a min­ute. In this and in all fol­low­ing records (unless stated to the contrary) up-curve rep­re­sents con­trac­tion, and down-curve ex­pan­sion or re­cov­ery.

Experiment 10.—As a typical example I shall describe the response of a straight tendril of Passiflora. A cut specimen was mounted with its lower end in water. Suitable electric connections were made for sending a feeble induction shock of short duration through the specimen. In this and all other records, unless contrary be stated, up-curve represents contractile movement. On application of stimulus of electric shock, an excitatory movement of contraction occurred which shortly reached its maximum; the apex-time was one minute and forty seconds, and recovery was completed after a further period of five minutes (Fig. 10). Stronger shocks induce greater contraction with prolongation of the period of recovery. The specimen was afterwards killed by application of poisonous solution of potassium cyanide; this brought about a permanent abolition of response. The experiment just described may be taken as typical of response of radial organs.

In a radial organ contraction takes place equally in all directions; it therefore shortens in length, there being no movement in a lateral plane. But if any agency renders one side less excitable than its opposite, diffuse stimulation will then induce greater contraction on the more excitable side which will therefore become concave.

RESPONSE OF AN ANISOTROPIC ORGAN.

Excessive stimulation is found to reduce the ex­cit­abil­ity of an organ. Under unilateral mechanical stimulation a tendril of Passiflora becomes hooked or coiled, the concave being the excited side. From what has been said, the unexcited convex side will relatively be the more excitable.

Fig. 11. Response of a hooked tendril of Passiflora to electric shock. Successive dots at intervals of 5 seconds.

Experiment 11.—I took a specimen of hooked tendril, and excited it by an electric shock. The response was by the greater contraction of the more excitable convex side, on account of which the curved specimen tended to open out. The record of this response is seen in Fig. 11; the apex-time was nearly two minutes, and the recovery was completed in the further course of 15 minutes.

From the responses of organs rendered anisotropic by the differential action of the environment we pass to others which show certain amount of anatomical and physio­logic­al differentiation between their upper and lower sides. I find that many petioles of leaves show movement in response to stimulus. Many pulvini, generally regarded as insensitive, are also found to exhibit responsive movements.

RESPONSE OF THE PULVINUS OF MIMOSA PUDICA.

Fig. 12. Response of the main pulvinus of Mimosa pudica.

The most striking and familiar example of response is afforded by the main pulvinus of Mimosa pudica of which a record is given in Fig. 12. It is generally assumed that sensibility is confined to the lower half of the organ. It will be shown in a subsequent Paper that this is not the case. The upper half of the pulvinus is also sensitive though in a feeble degree, its ex­cit­abil­ity being about 80 times less than that of the lower half. On diffuse stimulation the predominant contraction of the lower half causes the fall of the leaf, the antagonistic reaction of the upper half being, in practice, negligible. In order to avoid unnecessary repetition, I shall ignore the feeble antagonistic reaction of the less excitable half of the organ, and shall use the word ‘contraction’ for ‘relatively greater contraction.’

It is interesting in this connection to refer to the response of the leaf of Water Mimosa (Neptunia oleracea). Here the reaction is very sluggish in comparison with that of Mimosa pudica. A tabular statement of contractile response of various radial, anisotropic and pulvinated organs will show a continuity in the contractile reaction; the difference exhibited is a question of degree and not of kind.

TABLE 1—PERIODS OF MAXIMUM CONTRACTION AND OF RECOVERY OF DIFFERENT PLANTS.

SpecimenPeriod of maximum contractionPeriod of recovery.

Radial organ: Tendril of Passiflora100 seconds 4 minutes.

Anisotropic organ: Hooked tendril of Passiflora120 "13 "

Pulvinated organ: Pulvinus of Neptunia Oleracea180 "57 "

Pulvinus of Mimosa pudica 3 "16 "

As regards the excitatory fall of the leaf of Mimosa pudica, Pfeffer and Haberlandt are of opinion that this is due to the sudden diminution of turgor in the excited lower half of the pulvinus. The weight of the leaf, no longer supported by the distended lower cells, causes it to fall. This is accentuated by the expansion of the upper half of the pulvinus which is normally in a state of compression. According to this view the excitatory fall of the leaf is a passive, rather than an active, movement. I have, however, found that in determining the rapidity of the fall of Mimosa leaf the factors of expansive force of the upper half of the pulvinus and the weight of the leaf are negligible compared to the active force of contraction exerted by the lower half of the pulvinus (p. 87).

With regard to the fall of turgor, it is not definitely known whether excitation causes a sudden diminution in the osmotic strength of the cell-sap or an increase in the permeability of the ectoplast to the osmotic constituents of the cell. Pfeffer favours the former view, while others support the theory of variation of permeability.[F]

RESPONSE OF PULVINUS OF MIMOSA TO VARIATION OF TURGOR.

Whatever difference of opinion there may be in regard to the theories of osmotic and permeability variations, we have the indubitable fact of diminution of turgor and contractile fall of the pulvinus of Mimosa under excitation. The restoration of the original turgor brings about recovery and erection of the leaf. In connection with this the following experiments on responsive movements of the leaf under artificial variation of turgor will be found of interest:—

Effect of Increased Turgor: Experiment 12.—A young Mimosa plant was carefully transplanted and the root embedded in soil placed in a linen bag. This was held securely by a clamp, and one of the leaves of the plant attached to the recorder. Withholding of water for a day caused a general loss of turgor of the plant. A vessel full of water was now raised from below so that the linen bag containing the roots was now in water. The effect of increased turgor by suction of water by the roots became apparent by the upward movement of the leaf. The distance between the immersed portion of the plant and the leaf was 2 cm. and the up-movement of the leaf was indicated within 10 seconds of application of water (Fig. 13). The velocity with which the effect of increased turgor travelled was thus 2 mm. per second. The leaf exhibited increasing erection with absorption of water.

Fig. 13. Response of Mimosa pulvinus to variation of turgor. Increased turgor by application of water at point marked with vertical arrow induced erectile movement. Diminution of turgor by application of KNO₃ solution at the point marked with the horizontal arrow, brought about the fall of the leaf within 80 seconds. Successive dots at intervals of 5 seconds (The down curve represents up-movement and vice versâ.)

Effect of Diminution of Turgor: Experiment 13.—While the leaf in the above experiment was in process of erection, a quick change was made by substituting KNO₃ solution for the water of the vessel in which the roots were immersed. The plasmolytic withdrawal of water at the roots gave rise to a wave of diminished turgor, the effect of which became perceptible within 40 seconds by the movement of fall of the leaf. (Fig. 13.)

DIFFERENT MODES OF STIMULATION.

In Mimosa excitation is manifested by the contraction of the pulvinus and the consequent movement of the leaf. But in most plants, excitatory movement cannot be realized on account of the rigidity of the plant structure, the thickness of the cell-wall and the want of facility for escape of water from the excited cells. I shall show later how excitation may be detected in the absence of mechanical movement.

As regards stimulation of vegetable tissues, there are various agencies besides electric shock, which induce excitatory contraction; these agencies I shall designate as stimuli. Excitation is detected in Mimosa by the downward movement of the leaf. It will be found that such excitatory movement is caused by a mechanical blow, by a prick or a cut, by the application of certain chemical agents, by the action of electric current and by the action of strong light. The study of the action of these stimuli will be given in greater detail in subsequent Papers.

I shall give below a general classification of different stimuli which cause excitation in vegetable tissues.

Electric Stimulus.—Induction shock, condenser discharge, the make of kathode and the break of anode.

Mechanical Stimulus.—Mechanical blow, friction, prick or cut.

Chemical Stimulus.—Effect of certain acids and of other chemical substances.

Thermal Stimulus.—Sudden variation of temperature; application of heated wire.

Radiation Stimulus.—Luminous radiation of the more refrangible portion of the spectrum; ultra-violet rays; thermal radiation in the infra-red region.

All these different forms of stimulus induce an excitatory contraction, a diminution of turgor, and a negative mechanical response or fall of a motile leaf.

SUMMARY.

A radial organ responds to stimulus by contraction in length; as all its flanks are equally excitable there is no lateral movement under diffuse stimulus.

Physiological anisotropy is induced in an organ, originally radial and isotropic, by the unequal action of the environment on its different sides. Diffuse stimulus induces a greater contraction of the more excitable side.

In a curved tendril the concave side is less excitable than the convex. Diffuse stimulus tends to straighten the curved tendril.

In the pulvinus of Mimosa pudica, the lower half is eighty times more excitable than the upper, and the fall of the leaf is due to the predominant contraction of the more excitable lower half.

A diminution of turgor takes place in the excited cells. Restoration of turgor brings about recovery of the leaf to its normal erect position. Independent experiments show that the fall of the leaf may be brought about by an artificial diminution of turgor, and the erection of the leaf by an increase of turgor.

IV.—DIURNAL VARIATION OF MOTO-EXCITABILITY IN MIMOSA

BY

Sir J. C. Bose.

Several phenomena of daily periodicity are known, but the relations between the recurrent external changes and the resulting periodic variations are more or less obscure. As an example of this may be cited the periodic variation of growth. Here the daily periodicity exhibited by a plant is not only different in varying seasons, but it also differs in diverse species of plants. The complexity of the problem is very great, for not only are the direct effects of the changing environment to be taken into consideration but also their unknown after-effects. Even in the case of direct effect, different factors, such as light, temperature, turgor, and so on, are undergoing independent variations; it may thus happen that their reactions may sometimes be concordant and at other times discordant. The nyctitropic movement of plants affords another example of daily periodicity. The fanciful name of ‘sleep’ is often given to the closure of the leaflets of certain plants at night. The question whether plants sleep or not may be put in the form of the definite inquiry: Is the plant equally excitable throughout day and night? If not, is there any definite period at which it practically loses its ex­cit­abil­ity? Is there, again, another period at which the plant wakes up, as it were, to a condition of maximum ex­cit­abil­ity?

In the course of my in­ves­ti­ga­tions on the irritability of Mimosa pudica, I became aware of the existence of such a daily periodicity; that is to say, the moto-ex­cit­abil­ity of the pulvinus was found to be markedly diminished or even completely abolished at a certain definite period of the day; at another equally definite period, the ex­cit­abil­ity was observed to have attained its climax. The observations on the periodic variation of ex­cit­abil­ity appeared at first to be extremely puzzling. It might be thought, for example, that light would prove to be favourable for moto-ex­cit­abil­ity; in actual experiment the results apparently contradicted such a supposition: for the ex­cit­abil­ity of the plant was found much higher in the evening than in the morning. Favourable temperature, again, might be regarded as an important factor for the enhancement of the moto-ex­cit­abil­ity; it was, nevertheless, found that though the excitatory response was only moderate at that period of night when the temperature was at its minimum, yet the ex­cit­abil­ity was altogether abolished at another period when the temperature was several degrees higher. The obscurities which surrounded the subject were only removed as a result of protracted in­ves­ti­ga­tion and comparison of continuous automatic records made by the plant itself during several months, beginning with winter and ending in summer.

The question whether a plant like Mimosa exhibits diurnal variation of ex­cit­abil­ity can be experimentally investigated by subjecting the plant at every hour of the day and night to a test-stimulus of uniform intensity, and obtaining the corresponding mechanical responses. Under these circumstances the amplitude of response at any time will serve as a measure of the ex­cit­abil­ity of the plant at the particular time. Any periodic fluctuation of response will then demonstrate the periodic character of variation of ex­cit­abil­ity.

The in­ves­ti­ga­tion thus resolves itself into:—

The successful construction of a Response Recorder which will automatically record the response of the plant to uniform periodic stimulation at all hours of day or night;

the study of the effects of various external conditions on ex­cit­abil­ity;

the diurnal variation of ex­cit­abil­ity and its relation to the changes of external conditions.

I will first give a diagrammatic view of the different parts of the apparatus which I devised for this in­ves­ti­ga­tion.[G] The leaf of Mimosa is attached to one arm of a light aluminium lever, L, by means of thread. At right angles to the lever is the writing index W, which traces on a smoked glass plate allowed to fall at a definite rate by clockwork the responsive movement of the leaf. Under a definite stimulus of electric shock the leaf falls down, pulling the lever L, and moving the writer towards the left. (Fig. 14.) The amplitude of the response-curve measures the intensity of excitation. The leaf re-erects itself after a time, the corresponding record exhibit­ing recovery. A second stimulus is applied after a definite interval, say an hour, and the corresponding response shows whether the ex­cit­abil­ity of the plant has remained constant or undergone any variation.

Fig. 14. Diagrammatic representation of the complete apparatus for determination of diurnal variation of ex­cit­abil­ity. Petiole of Mimosa, attached by thread to one arm of lever L; writing index W traces on smoked glass plate G, the responsive fall and recovery of leaf. A, primary, and S, secondary, of induction coil. Exciting shock passes through the plant by electrodes E, E′. A, accumulator. C, clockwork for regulating duration of tetanizing shock. Primary circuit of coil completed by plunging rod, V, dipping into cup of mercury M.

UNIFORM PERIODIC STIMULATION.

Electric mode of excitation.—I find that one of the best methods of stimulating the plant is by means of tetanizing induction shock. The sensitiveness of Mimosa to electric stimulation is very great; the plant often responds to a shock which is quite imperceptible to a human subject. By the employment of a sliding induction coil, the intensity of the shock can be regulated with great accuracy; the secondary if gradually brought nearer the primary till a stimulus is found which is minimally effective. The intensity of stimulus actually employed is slightly higher than this, but within the sub-maximal range. When the testing stimulus is maintained constant and of sub-maximal intensity, then any variation of ex­cit­abil­ity is attended by a corresponding variation in the amplitude of response.

The exciting value of a tetanizing electric shock depends (1) on the intensity, (2) on the duration of shock. The intensity may be rendered uniform by placing the secondary at a fixed distance from the primary, and keeping the current in the primary circuit constant. The constancy of the current in primary circuit is secured by the employment of an accumulator or storage cell of definite electromotive force. It is far more difficult to secure the constant duration of the tetanizing shock in successive stimulations at intervals of, say, one hour during twenty-four hours. The duration of the induction shock given by the secondary coil depends on the length of time during which the primary circuit is completed in successive excitations. I have succeeded in overcoming the difficulty of securing uniformity of duration of shock by the employment of a special clockwork device.

The clockwork plunger.—The alarum clock can be so arranged that a wheel is suddenly released and allowed to complete one rapid revolution at intervals of, say, one hour. There is a fan-governor by which the speed of the revolution can be regulated and maintained constant. This will specially be the case when the alarum spring is long and fully wound. The succession of short releases twenty-four times during the day produce relatively little unwinding of the spring. On account of this and the presence of the fan-governor, the period of a single revolution of the wheel remains constant. By means of an eccentric the circular movement is converted into an up and down movement. The plunging rod R thus dips into a cup of mercury M, for a definite short interval and is then lifted off. The duration of closure can be regulated by raising or lowering the cup of mercury. In practice the duration of tetanizing shock is about 0.2 second.

The same clock performs three functions. The axis which revolves once in twelve hours has attached to it a wheel, and round this is wound a thread which allows the recording glass plate to fall through six inches in the course of twenty-four hours. A spoke attached to the minute hand releases the alarum at regular and pre-determined intervals of time, say once in an hour. The plunging rod R, actuated by the eccentric, causes a tetanizing shock of uniform intensity and duration to be given to the plant at specified times.

Constancy of resistance in the secondary circuit.—In order that the testing electric stimulus shall remain uniform, another condition has to be fulfilled, namely, the maintenance of constancy of resistance in the secondary circuit, including the plant. Electric connections have to be made with the latter by means of cloth moistened with dilute salt solution; drying of the salt solution, however, gives rise to a variation of resistance in the electrolytic contact. This difficulty is overcome by making the electrolytic resistance negligible compared to the resistance offered by the plant. Thin and flexible spirals of silver tinsel attached to the electrodes E, E′ are tied round the petiole and the stem, respectively. In order to secure better electric contact, a small strip of cloth moistened with dilute salt and glycerine is wound round the tinsel. As the resistance of contact is relatively small, and as drying is to a great extent retarded by glycerine, the total resistance of the secondary circuit undergoes practically no variation in the course of twenty-four hours. This will be seen from the following data. An experiment was commenced one day at 1 P.M., when the resistance offered by 8 cm. length of stem and 2 cm. length of petiole was found to be 1.5 million ohms. After twenty-four hours’ record, the resistance was measured the next day and was found unchanged. The fact that the stimulus remains perfectly uniform will be quite apparent when the records given in the course of this paper are examined in detail.

THE RESPONSE RECORDER.

The amplitude of response affords, as we have seen, a measure of the ex­cit­abil­ity of the plant. In actual record friction of the writer against the glass surface becomes a source of error. This difficulty I have been able to overcome by the two independent devices, the Resonant Recorder and the Oscillating Recorder. In the former the writer is maintained by electric means in a state of continuous to and fro vibration, about ten times in a second. There is thus no continuous contact between the writer and the smoked glass surface, friction being thereby practically eliminated. The writer in this case taps a record, the successive dots occurring at intervals of 0.1 second. The responsive fall of the leaf is rapid, hence the successive dots in this part of the record are widely spaced; but the erection of the leaf during recovery takes place slowly, hence the recovery part of the curve appears continuous on account of the superposition of the successive dots. The advantage of the Resonant Recorder is that the curve exhibits both response and recovery. This apparatus is admirably suited for experiments which last for a few hours. There is, however, some drawback to its use in experiments which are continued for days together. This will be understood when we remember that for the maintenance of 10 vibrations of the writer in a second, 10 electric contacts have to be made; in other words, 36,000 intermittent electric currents have to be kept up per hour. This necessitates the employment of an electric accumulator having a very large capacity.

In the Oscillating Recorder the recording plate itself moves to and fro, making intermittent contact with the writer about once in a minute. The recording smoked glass plate is allowed to fall at a definite rate by the unwinding of a clock wheel. By an electro­magnetic arrangement the holder of the smoked glass plate is made to oscillate to and fro, causing periodic contact with the writer.

Fig. 15. The Oscillator. Electromagnet M, M′, periodically magnetized by completion of electric current by clockwork C. Periodic attraction of soft iron armature A moves attached glass plate G to left, making thereby electric contact with writer.

The Oscillator is diagrammatically shown in Fig. 15. M, M′ are the two electro­magnetic coils, the free ends of the horseshoe being pointed. Facing them are the conical holes of the soft iron armature A. This armature carries two rods which slide through hollow tubes. The distal ends of the rods support the holder H, carrying the smoked glass plate. Under normal conditions, the plate-holder is held by suitable springs, somewhat to the right of, and free from contact with, the writer. A clockwork C carries a rotating arm, which makes periodic contact with a pool of mercury contained in the vessel V, once in a minute. On the completion of the electro­magnetic circuit, the armature A is attracted, the recording glass plate being thereby moved to the left making contact with the writer. The successive dots in the record thus take place at intervals of a minute. Only a moderate amount of electric current is thus consumed in maintaining the oscillation of the plate. A 4-volt storage cell of 20 amperes capacity is quite sufficient to work the apparatus for several days.

The responsive fall of the leaf of Mimosa is completed in the course of about two seconds. The leaf remains in the fallen or ‘contracted’ position for nearly fifteen seconds; it then begins to recover slowly. As the successive dots of the Oscillating Recorder are at intervals of a minute, the maximum fall of leaf is accomplished between two successive dots. The dotted response record here obtained exhibits the recovery from maximum fall under stimulation (cf. Fig. 23). The recovery of the leaf in one minute is less than one-tenth the total amplitude of the fall, and is proportionately the same in all the response records. Hence the successive amplitudes of response curves that are recorded at different hours of the day afford us measures of the relative variations of ex­cit­abil­ity of the plant at different times. This enables us to demonstrate the reality of diurnal variation of ex­cit­abil­ity. In my experimental in­ves­ti­ga­tions on the subject I have not been content to take my data from any particular method of obtaining response, but have employed both types of recorders, the Resonant and Oscillating. It will be shown that the results given by the different instruments are in complete agreement with each other.

EFFECTS OF EXTERNAL CONDITIONS ON EXCITABILITY.

Before giving the daily records of periodic variation of ex­cit­abil­ity, I will give my experimental results on the influence of various external conditions in modifying ex­cit­abil­ity. The conditions which are likely to affect ex­cit­abil­ity and induce periodicity are, first, the effects of light and darkness: under natural conditions the plant is subjected in the morning to the changing condition from darkness to light; then to the action of continued light during the day; and in the evening to the changing condition from light to darkness. A second periodic factor is the change in the condition of turgidity, which is at its maximum in the morning, as evidenced by the char­ac­ter­is­tic erect position of the petiole. Finally, the plant in the course of day and night is subjected to a great variation of temperature. I will now describe the effects of these various factors on ex­cit­abil­ity. It should be mentioned here that the experiments were carried out about the middle of the day, when the ex­cit­abil­ity, generally speaking, is found to remain constant.

EFFECTS OF LIGHT AND DARKNESS.

Fig. 16. Effect of cloud. Dotted up-curve indicates responsive fall, and continuous down-line exhibits slow recovery. First four responses normal; next three show depression due to diminution of light brought on by cloud, the duration of which is indicated by horizontal line below. Last three records show restoration of ex­cit­abil­ity brought on by clearing of sky. All records read from left to right.

I have frequently noticed that a depression of ex­cit­abil­ity occurred when the sky was darkened by passing clouds. This is clearly seen in the above records obtained with the Resonant Recorder. Uniform sub-maximal stimuli had been applied to a specimen of Mimosa at intervals of fifteen minutes. The dotted up-line represents the responsive fall, and the continuous down-line, the slow recovery. The first four are the normal uniform responses (Fig. 16). The next three show the depressing effect of relative darkness due to cloudy weather. The sky cleared after forty-five minutes, and we notice the consequent restoration of normal ex­cit­abil­ity.

Effect of sudden darkness and its continuation. Experiment 14.—In the next record (Fig. 17) is shown the immediate and continued action of darkness. The first two are the normal uniform responses in light. By means of screens, the plant was next subjected to sudden darkness; this brought about a marked depression of ex­cit­abil­ity. Subjection to sudden darkness thus acts as a stimulus inducing a marked but transient fall of ex­cit­abil­ity. Under the continuous action of darkness, however, the ex­cit­abil­ity is at first restored and then undergoes a persistent depression.

Fig. 17. Effect of sudden darkness. Plant subjected to sudden darkness beyond horizontal line seen below. First two responses normal. Note sudden depression of ex­cit­abil­ity, revival and final depression under continued darkness.

Effect of transition from darkness to light: Experiment 15.—Here we have to deal first with the immediate effect of sudden transition, and then with the persistent effect of continuous light. In the record given in Fig. 18 the plant had been kept in the dark and the responses taken in the usual manner. It was then subjected to light; the sudden change from darkness to light acted as a stimulus, inducing a transient depression of ex­cit­abil­ity. In this connection it is interesting to note that Godlewski found that in the phenomenon of growth, transition from darkness to light acted as a stimulus, causing a transient decrease in the normal rate. The effect of continued light on Mimosa is an enhancement of ex­cit­abil­ity.

Fig. 18. Effect of change from darkness to light. The first three records are normal under darkness. Horizontal line below indicates exposure to light. Note preliminary depression followed by enhancement of ex­cit­abil­ity.

EFFECT OF EXCESSIVE TURGOR.

I have often found that the moto-ex­cit­abil­ity is depressed under excessive turgor. Thus the “over-turgid” leaf of Biophytum sensitivum does not exhibit any mechanical response on rainy days.

Fig. 19. Effect of enhanced turgor, artificially induced. First two responses normal. Application of water, at arrow, induces depression of moto-ex­cit­abil­ity.

Experiment 16.—The effect of excessive turgor on moto-ex­cit­abil­ity may be demonstrated in the case of Mimosa by allowing its main pulvinus to absorb water. The result is seen in the above record (Fig. 19), where water was applied on the pulvinus after the second response. It is seen how a depression of moto-ex­cit­abil­ity results from excessive turgor brought on by absorption of water. In such cases, however, the plant is found to accommodate itself to the abnormal condition and gradually regain its normal ex­cit­abil­ity in the course of one or two hours.

INFLUENCE OF TEMPERATURE.

The moto-ex­cit­abil­ity of the pulvinus of Mimosa is greatly modified under the influence of temperature. For the purpose of this in­ves­ti­ga­tion I enclosed the plant in a glass chamber, raising the temperature to the desired degree by means of electric heating. Responses to identical stimuli were then taken at different temperatures. It was found that the effect of heightened temperature, up to an optimum, was to enhance the amplitude of response. Thus with a given specimen it was found that while at 22°C. the amplitude of response was 2.5 mm., it became 22 mm. at 27°C., and 52 mm. at 32°C. The ex­cit­abil­ity is enhanced under rising, and depressed under falling temperature. The moto-ex­cit­abil­ity of Mimosa is practically abolished at the minimum temperature of about 19°C.

Fig. 20. Effect of moderate cooling during a period shown by horizontal line below. Moderate depression followed by quick restoration.

Effect of lowering of temperature: Experiment 17.—A simple way of exhibit­ing the effect of lowering of temperature is by artificial cooling of the pulvinus. This cannot very well be done by application of a stream of cooled water, because, as we have seen, absorption of water by the pulvinus is attended by a loss of ex­cit­abil­ity: diluted glycerine has, however, no such drawback. This fluid at ordinary temperature was first applied on the pulvinus, and after an interval of half an hour records were taken in the usual manner. Cooled glycerine was then applied and the record taken once more; the results are seen in Figs. 20 and 21. In the former, the first response was normal at the temperature of the room, which was 32°C.; the next two exhibit depression of ex­cit­abil­ity under moderate cooling; the duration of application of moderately cooled glycerine is there indicated by the horizontal line below. On the cessation of application, the normal temperature was quickly restored, with the restoration of normal ex­cit­abil­ity.

In the next record (Fig. 21) is shown the effect of a more intense cold. It will be noticed that the first effect was a depression, and subsequently, a complete abolition of ex­cit­abil­ity. Thick dots in the record represent applications of stimulus which proved ineffective. It will also be noticed that even on the cessation of cooling, and the return of the tissue to normal temperature the induced abolition of ex­cit­abil­ity persisted as an after-effect for a considerable time. I have likewise found that the after-effect of cold in abolishing the conduction of excitation is also very persistent. These experiments show that owing to physio­logic­al inertia, the variations of ex­cit­abil­ity in the plant often lag considerably behind the external changes which induce them.

Fig. 21. Effect of application of more intense cold. Note sudden depression followed by abolition of ex­cit­abil­ity, also persistent after-effect.

Effect of high temperature: Experiment 18.—It has been shown that the moto-ex­cit­abil­ity is enhanced by rising temperature; there is, however, an optimum temperature above which the ex­cit­abil­ity undergoes a depression. This is seen in the following record (Fig. 22), where the normal response at 32°C. was depressed on raising the temperature to 42°C.; the ex­cit­abil­ity was, however, gradually restored when the plant was allowed to regain the former temperature.

Fig. 22. Effect of temperature above optimum. Note depression of ex­cit­abil­ity induced by high temperature, and gradual restoration on return to normal.

I may now briefly recapitulate some of the important results: darkness depresses and light exalts the moto-ex­cit­abil­ity. Excessive turgor depresses motility. Still more marked is the effect of temperature. Lowering of temperature depresses and finally abolishes the moto-ex­cit­abil­ity: rise of temperature enhances it up to an optimum temperature, but beyond this point the ex­cit­abil­ity undergoes depression. The change in ex­cit­abil­ity induced by the variation of external condition is not immediate; the induced effect, generally speaking, lags behind the inducing cause.

DIURNAL VARIATION OF EXCITABILITY.

I will now give automatic records of responses taken once every hour for twenty-four hours. They prove conclusively the diurnal variation of ex­cit­abil­ity in Mimosa. After studying in detail the variations char­ac­ter­is­tic of particular times of the day, I will endeavour to correlate them with the effects brought on by the periodic changes of the environment.

Experiment 19.—As a typical example I will first give a record obtained in the month of February, that is, say, in spring. From this it will not be difficult to follow the variations which take place earlier in winter or later in summer.

Fig. 23. Record for twenty-four hours, exhibit­ing diurnal variation of ex­cit­abil­ity (spring specimen). The displacement of base-line is due to nyctitropic movement.

The record given in Fig. 23 was commenced at 5 P.M. and continued to the same hour next day. The first thing noticeable is the periodic displacement of the base-line. This is due to the nyctitropic movements of the leaf. It should be remembered that the up-movement of the leaf is represented by down-curve, and vice versâ. After the maximum fall of the leaf, which in this case was attained at 9 P.M., there followed a reverse movement: the highest erection, indicative of maximum turgor, was reached at 6 A.M. The leaf then fell slowly and reached a middle position at noon. The extent of the nyctitropic movement varies in individual cases; in some it is slight, in others very large. The erectile movement began, as stated before, at about 9 P.M.; in some cases, however, it may occur as early as 6 P.M.

In following the char­ac­ter­is­tic variations of response occurring throughout the day, we find that while they are practically uniform between the hours of 5 and 6 P.M., a continuous decline is manifested after setting in of darkness (7 P.M.); the fall of ex­cit­abil­ity continues even after sunrise (6-30 A.M.), response being practically abolished at 8 A.M. The ex­cit­abil­ity is then gradually restored in a staircase manner, the maximum being reached after 12 noon. After attaining this, the ex­cit­abil­ity remains more or less constant till the evening. It will be noticed that the amplitude of response at 5 P.M. on the second day was the same as the corresponding response on the previous day.

The results of this and numerous other records taken in spring may be summarized as:—

1. The maximum ex­cit­abil­ity of Mimosa is attained between 1 and 3 P.M., and remains constant for several hours. In connection with the constancy of response at this period, it should be remembered that when the response is at its maximum a slight increase of ex­cit­abil­ity cannot further enhance the amplitude of response.

2. The ex­cit­abil­ity, generally speaking, undergoes a continuous decline from evening to morning, the response being practically abolished at or about 8 A.M.

3. From 8 A.M. to 12 noon, the ex­cit­abil­ity is gradually enhanced in a staircase manner, till the maximum ex­cit­abil­ity is reached after 1 P.M.

I have obtained numerous records in support of these conclusions, some of which are reproduced in the following figures. In these cases responses to uniform stimuli at intervals of half an hour were taken at different parts of the day, the recorder employed being of the Resonant type.

Mid-day record: Experiment 20.—The record of daily periodicity previously given shows that the ex­cit­abil­ity reaches its maximum after 12 noon, and that it remains constant at the maximum value for several hours. This fact is fully borne out in the following record obtained with a different specimen (Fig. 24). The responses were taken here from noon to 3 P.M., once every half-hour.

Fig. 24. Mid-day record from noon to 3 P.M. exhibit­ing uniform ex­cit­abil­ity. Responses taken once every half-hour.

Evening record: Experiment 21.—The record given in Fig. 23 shows that the amplitude of response falls continuously after 6 P.M. It might be thought that the diminished amplitude in the first part may be due to the natural nyctitropic fall of the leaf. The range of the pulvinar movement being limited, it is clear that the extent of the responsive fall must become smaller on account of the natural fall of the leaf during the first part of the night. That this is not the whole explanation of the decline of response in the evening will be clear from certain facts which I will presently adduce. It was stated that the leaf of Mimosa exhibits nyctitropic fall from 6 to 9 P.M., after which there is a reverse movement of erection. In certain specimens, however, the erectile movement commenced as early as 6 P.M. It is obvious that in these latter cases diminution of amplitude of response cannot be due to the reduction of the range of movement of the leaf. In Fig. 25 is given a series of records from 6 to 10 P.M. obtained with a leaf in which erectile movement had commenced early in the evening. Though the full range of responsive movement was in this case available, yet the amplitude of successive responses is seen to undergo continuous diminution.

Fig. 25. Evening record from 6 to 10 P.M., showing gradual depression of ex­cit­abil­ity.

Record in the morning: Experiment 22.—The ex­cit­abil­ity is, as we have seen, nearly abolished about 8 A.M., after which there is a gradual restoration. This gradual enhancement of ex­cit­abil­ity to a maximum in the course of the forenoon is seen well illustrated in the record below (Fig. 26).

Fig. 26. Morning record from 8 A.M. to 12 noon, exhibit­ing gradual enhancement of ex­cit­abil­ity.

The record of daily periodicity given in Fig. 23 may be regarded as a typical example. Modi­fi­ca­tions may, however, be observed which are traceable to individual peculiarities. As an example of this, I give a record (Fig. 27) obtained with a specimen in which nyctitropic movement was very pronounced. The periodic variation of ex­cit­abil­ity exhibited here is practically the same as shown by other specimens. The interesting variation is in the character of the recovery from stimulus; the leaf was falling from 6 to 9 P.M.; owing to the shifting of the base-line upwards the recovery appears to be incomplete. After 9 P.M. the leaf was erected, at first slowly, then at a very rapid rate. The consequent fall of the base-line late at night is very abrupt; hence there is an apparent over-shooting in the line of recovery.

Fig. 27. Record of diurnal variation of ex­cit­abil­ity; it exhibits marked nyctitropic movement.

EFFECT OF TEMPERATURE ON VARIATION OF EXCITABILITY.

So far I have merely described the observed diurnal variation of ex­cit­abil­ity. We may next inquire whether there is any causal relation between the change of external conditions and the observed variation of ex­cit­abil­ity. It has been shown that the moto-ex­cit­abil­ity is greatly influenced by temperature. In order to find in what manner the diurnal variation of ex­cit­abil­ity was influenced by the daily variation of temperature, I took special care to secure by means of the thermograph a continuous record of temperature variations. The table which follows shows the relation between the hours of the day, temperature, and amplitude of response, in a typical case of diurnal variation of ex­cit­abil­ity.

TABLE II.—SHOWING THE RELATION BETWEEN HOUR OF THE DAY, TEMPERATURE, AND EXCITABILITY. (SPRING SPECIMEN.)

Hours of day.Temperature.Amplitude of Response.Hours of day.Temperature.Amplitude of Response.

5p.m.28°.0C.28.0mm.5a.m.20°.0C.5.0mm.

6"25.5°"28.0"6"20.5°"4.2"

7"24.5°"27.0"7"21°.0"3.5"

8"23°.0"23.5"8"22°.0"2.5"

9"22°.0"21.5"9"24°.0"0.0"

10"21°.0"18.0"10"26°.0"6.0"

11"20.5°"15.0"11"26.5°"15.5"

12"20°.0"13.0"12"28°.0"22.5"

1a.m.20°.0"10.0"1p.m.28°.0"26.0"

2"20°.0"8.0"2"28.5°"28.0"

3"20°.0"7.5"3"28.5°"28.0"

4"19.5°"6.0"4"29°.0"28.0"

From the data given in the table, two curves have been obtained. One of these shows the relation between the hours of the day and temperature; the other exhibits the relation between the hours of the day and the ex­cit­abil­ity as gauged by the amplitude of response (Fig. 28). It will be seen that there is, broadly speaking, a marked resemblance between the two curves, which demonstrate the predominant influence of temperature on diurnal variation of ex­cit­abil­ity.

Fig. 28. The continuous curve shows the relation between the hour of the day and temperature. The dotted curve exhibits relation between the hour of the day and ex­cit­abil­ity.

EFFECT OF PHYSIOLOGICAL INERTIA.

It has been shown (page 59) that owing to physio­logic­al inertia, the change of ex­cit­abil­ity, generally speaking, lags behind the inducing cause. This fact finds striking illustration in the lag exhibited by the curve of ex­cit­abil­ity in reference to the temperature curve. The minimum temperature was attained at about 4 A.M., but the ex­cit­abil­ity was not reduced to a minimum till four hours later and again there is a marked fall of temperature after 5 P.M., but the ex­cit­abil­ity did not become depressed till two hours later.

There is again the factor of variation of light, the effect of which is not so great as that of temperature. The periods of maximum of light and temperature are, however, not coincident.

We may now discuss in greater detail the diurnal variation of ex­cit­abil­ity in Mimosa, taking the typical case, the record of which is given in Fig. 23. The temperature here is seen to remain almost constant, and at an optimum, from 1 to 5 P.M., the condition of light is also favourable. Hence the ex­cit­abil­ity is found to be constant, and at its maximum between these hours. The temperature begins to fall after 6 P.M., and there is, in addition, the depressing action of gathering darkness. Owing to the time-lag, the fall of ex­cit­abil­ity does not commence immediately at 6 P.M., but an hour afterwards, and continues till the next morning. During this period we have the cumulative effect of twelve hours’ darkness and the increasing depression due to cold, the temperature minimum occurring at 4 A.M. On account of the combined effects of these various factors, and phenomenon of lag, the period of minimum ex­cit­abil­ity is in general reached about 8 A.M. In certain other cases this may occur earlier. After the attainment of this minimum, the ex­cit­abil­ity is gradually and continuously increased, under the action of light and of rising temperature, till the maximum is reached in the afternoon.

EFFECT OF SEASON.

It was said that temperature exerted a predominant influence in inducing variation of ex­cit­abil­ity. We may, therefore, expect that the diurnal period would be modified in a certain way according to the season. In winter the night temperature falls very low; hence the depression of ex­cit­abil­ity is correspondingly great, and results in the complete abolition of ex­cit­abil­ity. The after-effect of intense cold is seen in the condition of inexcitability persisting for a very long period in the morning. In summer the prevailing high temperature modifies the diurnal periodicity in a different manner. When the night is warm, the fall of ex­cit­abil­ity is slight. In the day, on the other hand, the temperature may rise above the optimum, bringing about a depression. In such a case the ex­cit­abil­ity in the earlier part of the evening may actually be greater than in the middle of the day. These modi­fi­ca­tions are shown in a very interesting way in the following record (Fig. 29) taken at the end of April. The temperature of Calcutta at this season often rises above 100°F. or 38°C. Table III also exhibits, in the case of the summer specimen, the relation between the hours of the day, temperature, and ex­cit­abil­ity.

Fig. 29. Diurnal variation of ex­cit­abil­ity exhibited by summer specimen.

An inspection of the record given in Fig. 29 shows that the amplitude of response was enhanced after 4 P.M. The temperature up to that time was unusually high (38°C.), and there was in consequence a depression of ex­cit­abil­ity. After that hour there was a mitigation of heat, the temperature returning towards the optimum. Hence we find that the maximum ex­cit­abil­ity was attained between the hours 4 and 6 P.M. The minimum temperature at night was higher in the present case than that of the experiment carried out in February; in the former the minimum was 25.5°C., while in the latter it was 19.5°C. On account of this difference the night record in summer shows a fall of ex­cit­abil­ity which is far more gradual than that obtained in spring. The ex­cit­abil­ity is here not totally abolished in the morning, but reaches a minimum after 8 A.M.; the sensitiveness is then gradually enhanced in a staircase manner.

TABLE III—SHOWING THE RELATION BETWEEN HOURS OF THE DAY, TEMPERATURE, AND EXCITABILITY. (SUMMER SPECIMEN.)

Hours of day.Temperature.Amplitude of Response.Hours of day.Temperature.Amplitude of Response.

1p.m.38°.0C.22.0mm1a.m.26°.0C.21.5mm.

2"38°.0"23.0"2"26°.0"20.0"

3"38°.0"24.5"3"25.5°"18.5"

4"37°.0"28.0"4"25.5°"17.0"

5"35.5°"29.0"5"25.5°"16.0"

6"33°.0"27.0"6"26°.0"15.0"

7"31°.0"26.0"7"27°.0"14.0"

8"30°.0"26.0"8"29°.0"13.0"

9"29°.0"25.0"9"30.5°"11.0"

10"27°.0"24.5"10"33°.0"16.0"

11"27°.0"24.0"11"35°.0"17.0"

12"26.5°"22.5"12"37°.0"21.0"

SUMMARY.

The moto-ex­cit­abil­ity of Mimosa was gauged every hour of the day and night, by the amplitude of the response to a testing stimulus. This is effected by means of automatic devices which excite the plant periodically by an absolutely constant stimulus, and record the corresponding mechanical response.

From the record thus obtained, it was found that the ex­cit­abil­ity of the plant is not the same throughout the day, but undergoes a variation char­ac­ter­is­tic­al­ly different at different times of the day. In a typical case in spring the ex­cit­abil­ity attained its maximum value after 1 P.M. and remained constant for several hours. There was then a continuous fall of ex­cit­abil­ity, the minimum being reached at about eight in the morning. The plant at this time was practically insensitive. The moto-ex­cit­abil­ity was then gradually enhanced in a staircase manner till it again reached a maximum next afternoon.

The effect of sudden darkness was found to induce a transient depression, followed by revival of ex­cit­abil­ity. The effect of persistent darkness was to induce a depression.

Exposure to light from darkness caused a transient depression, followed by an enhancement of ex­cit­abil­ity.

Excessive turgor induced a diminished response.

Lowering of temperature induced a depression of ex­cit­abil­ity, culminating in an abolition of response. The after-effect of excessive cold was a prolonged depression of ex­cit­abil­ity.

Excitability was enhanced by rising temperature up to an optimum; above this point a depression was induced.

Owing to physio­logic­al inertia the change of ex­cit­abil­ity induced by variation of external condition lags behind the inducing cause.

The diurnal variation of ex­cit­abil­ity is primarily due to diurnal variation of temperature. The effect is modified in a minor degree by variation of light.

V.—RESPONSE OF PETIOLE-PULVINUS PREPARATION OF MIMOSA PUDICA

By

Sir J. C. Bose,

Assisted by

Surendra Chandra Das, M.A.

The most suitable plant for researches on irritability of plants is Mimosa pudica, which can be obtained in all parts of the world. An impression unfortunately prevails that the excitatory reaction of the plant can be obtained only in summer and under favourable circumstances; this has militated against its extensive use in physio­logic­al experiments, but the misgiving is without any foundation; for I found no difficulty in demonstrating even the most delicate experiments on Mimosa before the meeting of the American Association for the Advancement of Science held during Christmas of 1914. The prevailing outside temperature at the time was considerably below the freezing point. With foresight and care it should not be at all difficult to maintain in a hot-house a large number of these plants in a sensitive condition all the year round.

In order to remove the drawback connected with the supply of sufficient material, I commenced an in­ves­ti­ga­tion to find whether a detached leaf preparation could be made as effective for the study of irritability as the whole plant. Here we have at the central end of the leaf the pulvinus, which acts as the contractile organ; the conduct­ing strand in the interior of the petiole, on the other hand, is the vehicle for trans­mission of excitation. The problem to be solved is the rendering of an isolated petiole-and-pulvinus of Mimosa as efficient for researches on irritability as the nerve-and-muscle preparation of a frog. On the success of this attempt depended the practical opening out of an extended field of physio­logic­al in­ves­ti­ga­tion which would be unhampered by any scarcity of experimental material.

In connection with this it is well to note the surprising difference in vegetative growth as exhibited by plants grown in soil and in pots. A pot-specimen of Mimosa produces relatively few leaves, but one grown in the open ground is extremely luxuriant. As an instance in point, I may state that for the last five months I have taken from a plant grown in a field about 20 leaves a day for experiment, without making any impression on it. A large box containing soil would be practically as good as the open ground, and the slower rate of growth in a colder climate could be easily made up by planting half a dozen specimens. The protection of the plants from inclemencies of weather can be ensured by means of a glass cover with simple heat-regulation by electric lamps, in place of an expensive green-house.

Returning to the question of the employment of an isolated leaf, which I shall designate as a petiole-pulvinus preparation, instead of the entire plant, the first attempts which I made proved unsuccessful. The cut leaf kept in water would sometimes exhibit very feeble response, at other times all signs of ex­cit­abil­ity appeared to be totally abolished. It was impossible to attempt an in­ves­ti­ga­tion on the effect of changing environment on ex­cit­abil­ity when the normal sensitiveness itself underwent so capricious a change

These difficulties were ultimately overcome from knowledge derived through systematic in­ves­ti­ga­tion on the relative importance of the different parts of the motor apparatus, on the immediate and after-effect of section on the ex­cit­abil­ity of the leaf, and on the rate of decay of this ex­cit­abil­ity on isolation from the plant. The experience thus gained enabled me to secure long-continued and uniform sensibility under normal conditions. It was thus possible to study the physio­logic­al effects of changing external conditions by observing the responsive variation in the isolated petiole-pulvinus preparation. I propose to deal with the different aspects of the in­ves­ti­ga­tion in the following order:—

1. The effect of wound or section in modi­fi­ca­tion of normal ex­cit­abil­ity.

2. The change of ex­cit­abil­ity after immersion in water.

3. Quantitative determination of the rate of decay of ex­cit­abil­ity in an isolated preparation.

4. Effect of amputation of the upper half of pulvinus.

5. Effect of removal of the lower half.

6. Influence of the weight of leaf on rapidity of responsive fall.