Life Movements in Plants

in the afternoon, when the Palm, was at its lowest posi¬ tion. After this hour excitability fell continuously till 7 or 8 next morning. Corresponding to this is the con¬ tinuous 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 tem¬ perature on the diurnal curve of excitability in Mimosa , and the diurnal curve of movement of the Palm. This will.fee-. quite evident from the inspection of the tempera¬

I have shown elsewhere* that the variation of moto- excitabiliiy of* the pulvinus of Mimosa is a physiological function of temperature. The remarkable similarity be-, tween the diurnal variation of moto-excitability of Mimesa and diarnal movement of the Palm is due to the fact that both are determined by the physiological action of tem¬ perature. 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 Ms the diurnal variation of temperature, and mot of light that is effective in inducing the periodic movement of the tree. Further experiments will be.given in..support of this" con¬ clusion. 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 negli¬ gible 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 'down¬ wards with rise of temperature and vice versa.

There are many practical advantages in working ^ ith -a small specimen. It can easily be placed under glass cover or taken to a glass house, thus completely eliminat¬ ing 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 pho-o-tonic condition of the plant. But such unhealthy condition did not make its appearance in the first few days.

There may be a misgiving that the movement of the tree might be due to physical effect of temperature. If the upper strip of % differential thermometer be made of the wore 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 physi¬ cal movement would have persisted, while the physio¬ logical action would have disappeared. As this test was not practicable, I tried the effect of physiological depres¬ sion on the periodic movement of the leaf of Arenga saccharifera.

Effect of Drought; Experiment 3 .—In Fig. 6 is given a series of records of movement of the leaf-stalk of Arenga, first under norm.|l 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, (6) 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 poison¬ ous solution of potassium cyanide. The diurnal movement Fig. 6. Effect of physiological depression on diurnal movement of the petiole of Arenga aceUrifera. The uppermost curve exhi temperature, (a), normal diurnal ourve, (6), modification a ter ys aftir 7 days’ withholding of water.

These experiments conclusively prove that the periodic njovement of the leaf-stalk induced by variation of tem¬ perature is a physiologic? 1 phenomenon, and from analogy we are justified in drawing the inference that the move¬ ment of the Faridpur tree is also physiological. The question, however, was finally settled by the unfortunate death o£ the tree which occurred the other day, nearly a year after I commenced my investigations. While presid¬ ing at my lecture on the subject, His Excellency Lord- Ronaldshay, the Governor of Bengal, announced that a tele¬ gram 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 versa. Such movements,

1 shall, for the sake of convenience, distinguish as belong¬ ing 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- connec¬ tion, first discuss the various tentative theories that may be advanced in explanation of the movement. It may be thought that the fall of the tree during rise of temperature may be due to passive fielding of the

tree to its weight, there being increased transpiration and general loss o£ turgor at high temperature. I shall, how¬ ever, show that the diurnal movement persists in the absence of transpiration. Diurnal movement in absence of transpiration : Experi¬ ment 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 replans, immersed in water, the normal diurnal movement was given by the plant, where there could be no question of variation of turgor due to trans¬ piration. (See alsQ Expt. 7.)

In the diurnal movement of the 4 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 move¬ ment 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 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* Miliardet 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.* 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, ?*.#., 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 posi-. tion.

Diurnal movement in inverted position : Experiment d.—I took a vigorous specimen of Arenga saccharlfera grow¬ ing 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 she?w 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 tha first day, show any diufnal movement; there was, on the other hand, a continuous down-movement

oi 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 attain¬ ment 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 con¬ tinued 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 dmrna movement is determined by the modifying influence ot temperature on the physiological reaction of the plant to some external stimulus which is constant m direction, 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 tempera¬ ture is virtually to oppose or neutralise the geotropic curva¬ ture 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 de¬ scribed as dynamic ; for it will be shown that this state of geotropic balance is upset, in a definite way, by variation of temperature. *

movement is supported by the fact that the Sijjbaria 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 demon¬ strate the effect of change 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 Baaella cordifolia growing in a pot and laid it horizontal¬ ly for two weeks. The procumbent stem curved up and attained a state of equilibrium under the action of geotropic stimulus. Diurnal curve of Basella cormfolia: 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 attain¬ ed at 12-25 P. M., and the minimum at 6 A. M. The geotro¬ pic curvature was reduced to its minimum at the maximum temperature, and vice versa. As in the case of the Palm 30 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

Fig. 7. Diurnal curve of movement of procumbent young stem of Mimosa pudica. Successive dots at intervals of 15 minutes. the maximum, began to fall at 12-25 p. m., and the pre¬ vious -movement of fall of the stem was arrested .and reversed _ into ■ an erectile movement shortly; after 1 p. i. There are thus two “ turning points,” one at I a, I,; and the other at about 1 P.M. ; at these periods the movement of the plant remains more or less arrested; for more than haif-an-hour.:

I obtained records of similar diurnal movements with various procumbent or creeping stems. Figure 7 gives the diurnal record of the procumoent 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 tem¬ perature, 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 move- ment. The diurnal rhythm of up and down movement in the particular specimen Basella had become established under the daily variation of temperature. I now attempt¬ ed to reverse this rhythm by artificial variation of tem¬ perature. The plant was plaeed 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. Jn- 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 under¬ gone a down-movement under the daily rise of temnerature. 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 fair 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, andl 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 ass 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

tissue in the interior. The persistent effect therefore takes place after a lateiit period from one to three hours accord¬ ing to the thickness of the plant. The persistent effect of rise of temperature is a move¬ ment downwards, that of fall of temperature is a move¬ ment 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 v C, to 24 C. in the course of five minutes and maintained at the lower temperature. ' There v r as 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

Fig. 8. Re\ ersal of normal rhythm : Erectile response Basella to gradual fall of temperature. Fig.^._ esponsive fall of Basella to gradual rise of temperature. movement is, as stated before, 65 minutes as against 50 minutes • in the normal diurnal movement. The increase in the latent period is probably tine to the added, physio¬ logical inertia in reversing the normal rhythm. 'Effect of rise of temperature : Experiment 9 .—The tem¬ perature 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) thns 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 lespectivelv. The state of equili¬ brium 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 out¬ wards during rise of temperature and close inwards during the onset of cold. Looked at from above, the opening out¬ wards 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- neces¬ sarily belong to the' negative type, where*rise of tempera-- ture 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. The 'Praying 5 Palm of Faridpur, growing at an inclination of about 60 5 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 move¬ ment was reversed and the maximum erection was again reached next morning.

.This phenomenon is not unique, but is found exhibited, more or fess, by all trees and their branches and leaves. Diurnal records of temperature, and movement of the tree ’ showed, that the two carves closely resembled each Other. Rise of temperature was attended by a fall of the tree, .and vice versa . ■ "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 modify¬ ing influence of temperature on geotropic curvature. Rise of. temperature is found to- oppose or neutralise geotropic purvatura, 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 u Praying n Palm is a thermonastic phenomenon. The tree, apparently so rigid, responds as a gigantic puldnoid to the changes of its environment.