Life Movements in Plants

The leaf of Mimosa pudicci undergoes a rapid fall when subjected io any Mud of shock. This plant has* therefore, been regarded as “sensitive,'” in contradistinction to*ordinary plants which remain apparently immobile under extern a! stimulus, I shall, however, show in course of this Paper f hat there is no justification in regarding ordinary plants as insensitive. Lei ns 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 ^sample, the tendril, of Cucurbita, the pistil . of Datura or the Sower bud of Crinum. The shoitening may he observed by. means of a low power microscope. Greater importance |g 9 however, .attached to the detailed study of response and its tlpae : relations. The pull exerted by a delicate organ during its excitatory contraction is slight; hence arises* the neees-

sity of devising a very sensitive apparatus, which would wive records magnified from ten to a hundred times. 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 com¬ pletes an electric circuit ten hundred, or two hundred timek 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 OT 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.*

2 . The Magnetic Tapper .—Measurement of very short intervals is not necessary in ordinary records of res- ' *'For detailed description cj, Bose.—“An Automatic Method for Investigation of Velocity of 'Transmission of Excitation in Mimosa. 11 —Phil. Trans., B. vol. 2.04» ponse. 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 titfe plate itself is made to oscillate io-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, #

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 ail other records, unless, contrary be stated, up-curve re¬ presents, contractile movement. On application of stimulus of electric shock, an excitatory movement of contrac¬ tion occurred which shortly reached its m^xiihum; the apex¬ time was one minute and forty seconds, ' and recovery was completed after' a further period oi five minutes

Fig. 10. Response of a straight tendril of Passiflora to electric shock. Suc¬ cessive doi s at intervals of o 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. with prolongation of the period of recovery. The sped-' men was afterwards killed by application of poisonous solution of potassium cyanide ; this . brought about a per¬ manent abolition' of response. The- experiment just- described may be taken as. typical of response of Tadia! 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 §ide less excitable than its opposite, diffuse stimulation will then induce greater contraction on the more excitable side which will therefore become concave. Excessive stimulation is found to reduce the excitability of an organ. Under unilateral mechanical stimulation a

tendril of Paesiflora becomes. ■ booked or coiled, the con¬ cave being the excited side. From what has been' said, the unexcited convex side will relatively be the more excitable. 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 carved specimen tended to -open out. The record of this response is seen in Fig. 11;

Fig. 11. Response of a hooked tendril of Passifiora lo electric shock. Suc¬ cessive dots an intervals of 5 seconds. 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 physiologi¬ cal . differentiation between, ■ their : ■ upper , and, lower sides. 1 find that. many petioles of leaves show movement in ref sponse to stimulus. Many pulvini, generally regarded 'as

insensitive, are also found to exhibit responsive move¬ ments. The most striking and familiar example of response is afforded by the main puivinus of Mimosa /ntdica of which a record is given in Fig. 12. It is generally assumed Fig. 12. '.Response of the main puivinus of Jfiniufiu ptu/ica. 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 puivinus is also sensitive though in a feeble degree, its excitability being about 80 t ; mes less than that of the lower half. On dill'use. stimu¬ lation 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 un¬ necessary 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 con¬ traction.’

It is interesting, in. this connection to refer'to \ttie. re¬ sponse of the leaf of Water Mimosa {Neptania olsraeeQ^). Here the reaction is very sluggish, in comparison with that of Mimma pudica. A tabular statement of contractile re¬ sponse of various radial, anisotropic and pulvinated organs will shown continuity in the contractile reaction ; the'differ¬ ence exhibited is a question of degree; and not of .kind. TABLE ?.— PERIODS OF MAXIMUM CONTRACTION AND OF RECOVERY OF'

As regards .the excitatory fall of the leaf . of Mimosa pudica, .Pfeifer and Haberlandt are' of"-opinion that' this,-is- due to- the sudden diminution of turgor in tjie excited .lower half of the pul vinos. 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 pal'viiius. which is ■ normally. ih‘ 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. Pfeft'er favours the former view, while /Others support the theory of variation of permeability.*

Whatever difference of opinion there may be in regard to the theories of osmotic and permeability variations, we have the indubitaole fact of diminution of turgor and contractile fall of the pulvinus of Mimosa under excita¬ tion. 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: -

* IVith reference to the fall of Mimosa leaf Jost says: “When the pressure of the cell decreases we naturally assume this to be due to a decreasin'- osmotic pressure due to alterations in the permeability of the plasma, and an excretion of materials from the cell. It is a remarkable fact that plasmolytic research (Hilburg 1881) affords no evidence of any decrease in osmotic pressure. No complete insight into the mechanism of the stimulus movement in Mimosa has yet been obtain-d, although one thing is certain, that there is a decrease in the expansive power on the under side of the articulation."-Jost, “ Plant ysiology ” English Translation, p. 515. Clarendon Press (1907). Blackman and Paine thin : that the loss of turgor on excitation “is probably due to the dis- appearance or inactivation of a considerable portion of the osmotic substances of the cells. -Annals of Botany, Vol. XXXII, No. CXXXV, Jan 1918

Effect of Increased Turgot ; Experiment 12 .—A young Mimosa plant was carefully transplanted and 4|ie 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

Fig. 13. Response of Mimosa pulvinas to variation of turgot. Increased turgor . by application of water at point marked with vertical arrow induced erectile i$ove- me^fc. Diminution of turgor by application of KN0 3 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 *?tce versd,) increased turgor travelled was thus 2 mm. per second. The leaf exhibited increasing erection with absorption of

Effect of Diminution of Turgor : Experiment IS . While the leaf in the above experiment was in process of erection, a quick change was made by substituting- KNO.^ solution for the water of the vessel in which the roots were immersed. The plasmoiytie 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. (lug. Id.) •In Mimosa excitation is manifested by tfce contraction of the pulvinus and the consequent movement of the leaf. But in most plants, -excitatory movement cannot' be , realiz¬ ed 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 mechani¬ cal movement.

As regards stimulation of vegetable tissues, there are various agencies besides electric shock, which induce excita¬ tory contraction ; these agencies I shall, designate as stimuli. Excitation is detected in Mimosa by the downward move¬ ment of the leal 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. 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 regiqn. - All these different forms of stimulus induce an excita¬ tory contraction, a diminution of turgor, and » negative mechanical, response or fall of a motile leaf.

A radial organ responds to stimulus by contraction in length; as ail its flanks are equally excitable there is no lateral movement under diffusa stimulus. Physiological anisotrophy is "induced in an organ, origin-, ally radial and isotropic,.. by the unequal action of the en¬ vironment on its different sides.. Diffuse stimulus induces . a greater contraction of the more excitable side. In a curved tendril the concave side is less excit¬ able than the convex. Diffuse stimulus tends to straighten the.''.curved 'tendril.'

In the pul vines of Mimosa pudiea, 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 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 exper.ments 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.

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, temperatn-e, turgor, and so on, are undergoing independent vaviatior' ; 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 period¬ icity. 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 t>f 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 excit¬ ability was observed to have attained its climax. The observations on the periodic variation of excitability ap¬ peared at first to be extremely puzzling. It might be thought, for example, that light would prove to be favour¬ able for inoto-excitability ; in actual experiment the results apparently contradicted such a supposition : for the excit¬ ability of the plant was found much higher in the even¬ ing than in the morning. Favourable temperature, again, might -be regarded as .an- important factor for the en¬ hancement 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 re¬ cords made by the plant itself during several months, beginning with winter and ending in summer.

The question whether a plant like Mimosa exhibits diurhal ^ variation of excitability can be experimentally in¬ vestigated 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.

ThS- 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 -diurnal variation excitability and its relation to the changes of external conditions, 1 will first-give a diagrammatic view of the different parts of the apparatus - which I devised for this investiga¬ tion. . 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

,Fr©. ■ 14, --Diagrammatic representation ■ of the complete apparatus far deter-, ruination of diurnal variation of excitability. Petiole of Mimosa, attached by, thread to ■ one arm of lever X; writing index W traces on sfnoked glass plate Gb the responsive fall and recovery of leaf. A,-primary, and S. secoAaslry,.-of in¬ duction roil. Exciting “shock passes ■ through the plant by electrodes E,-K A, accumulator. C, clockwork for regulating duration of thanking shock. Prim¬ ary circuit' of coil completed by plunging-rod,. T, dipping into cup of mercury M.

* See also Bose.-—The Diurnal Yariation of Koto-excitability in Mnosa ^Annals of Botany,Oct, 1913. ’ measures the intensity of excitation. The , leaf re-ereets 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. 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 ex citability is attended by a corresponding variation in the amplitude ot response.

The exciting value of a tetanizing electric shock depends (1). on the intensity, (g). on. the duration of shock. The intensity may be rendered uniform by placing the second¬ ary -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 employ¬ ment of an accumulator or storage ceil of definite electro* motive forest It is far more difficult to secure the constant duration of? the tetanizing shock in successive stimulations