Life Movements in Plants

at" intervals of, say, one hour during twenty-four hours. The duration of the induction shock given by the second¬ ary .. coil depends on the length of time during which the primary circuit is completed in successive excitations, 1 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, wiji 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 revo¬ lution of the wheel remains constant. By means of an eccentric the circular movement is converted into an up and down movement The plunging roc! R thus dips into a cup of mercury M, for a definite short .interval and is then lifted off. The duration of closure can . hv regu¬ lated by s 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 Tail through six inches in the course of twenty-four. hours. A spoke attached to the minute, hand,.releases the alarum at regular and ' pre-deter- jEnined' intervalsof time, • say .once in 'an hour* The plunging • rod,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 uni¬ form, another condition lias 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 elec¬ trolytic resistance negligible compared to the resistance offered by the plant. Thin and flexible spirals of. 1 silver tinsel attached to the electrodes E, E 1 are tied round the petiole and the stem, respectively. In order to secure better, electric contact, a. small strip of cloth moistened witty, "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 tbtal resist¬ ance 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 uit. changed. The fact that the stimulus remains perfectly uniform will he quite apparent when the records given in the course of this paper are examined' in detail.

The amplitude of response affords,, as to. 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 con¬ tinuous to and fro vibration, about ten times m 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 GT 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. Tnis wili 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. B.U. lo. The Oscillator. Electromagnet II, M', periodically magnetized bv 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.

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. Undey 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 «£ mercury contained in the vessel V, once in a minute. Ou the completion of the electromagnetic circuit, -the armature- A is attracted, the recording glass plate being thereby moved to the left making contact with

tli© writer. The successive dots in the record thus take place at internals of a minute. Only a moderate amount of elec¬ tric 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 4 contracted * position for nearly fifteen seconds ; it then begins to recover slowly. As the succes¬ sive dots of the Oscillating Recorder are at intervals of a minute, the maximum fall of leaf is accomplished be¬ tween two • successive dots. The dotted response record here obtained exhibits the recovery from maximum fail under stimulation (c/. 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 res- • ponse curves, that are recorded at different hours .-of the day afford us measures of the relative variations of ex¬ citability of the plant at different times. This enables us to, demonstrate the reality of diurnal variation, of excit¬ ability.. 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 ■•Ins¬ truments' are ' in complete agreement with each other.

Before giving ■ the daily records of .periodic variation of excitability, I will give my experimental results on the' iiifluence of various' external conditions in -modifying ex¬ citability The conditions which are likely to affect ex¬ citability and induce periodicity are, first, the effects of light ' and darkness :' under natural conditions tEe 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, os mentioned here that the experiments were carried out about the middle of the day, when the excitability, generally speaking, is found to remain constant.

I have frequently noticed that a depression of excit¬ ability 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 Fig. 16. Effect of cloud. Dotted up-curve indicates responsive fall, and •continuous down-lire 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 'Qt excitability brought on by clearing $f e&y. Aft ^cords read from left to right

stimuli ' had been applied to a specimen of Mimosa at intervals of fifteen minutes. The dotted up-line repre¬ sents the responsive fall, and the continuous down-line, the slow recovery. The first four are the normal uni¬ form responses (Fig. 16). The next three show the de¬ pressing effect of relative darkness due to cloudy weather. The sky cleared after forty-five minutes, and we notice the consequent restoration of normal excitability. Wm. 17- Effect of sudden darkness. Plant subjected to sudden darkness beyond horizontal line - : een below. First, two responses normal. Hofce sudden depression of excitability, revival and final depression under, continued darkness.

immediate and continued action of darkness. The first two are the normal uniform responses in light. By means of screens, the plant was nest subjected to sudden dark - ness; this brought about a marked depression of excit¬ ability. Subjection to sadden -darkness thus acts as- a stimulus inducing a marked but transient, fall of., excit¬ ability. Under the 'Continuous action of darkness, however, the excitability is at first restored and then-.undergoes a persi$te»t depression.

Effect of transition from darkness to light : Experiment 7 5.—Here we have to deal first with the immediate effect of sadden transition, and then with the presistent 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 Godiewski found that in the phenomenon of growth, transition from dark- ness to light acted as a stimulus, ■ causing a transient

Fia. 18 . Effect oi change from darkness to light. , The first three records aie normal under darkness. Horizontal line below indicates exposure to light. Note preliminary depression followed by enhancement of excitability. decrease in the normal rate. The effect of continued light on Mimosa is an enhancement of excitability. I have often found that the moto-excitability is depress¬ ed under excessive turgor. Thus the “over-turgid'leaf of Biophytum sensitivum does not exhibit any mechanical response ou rainy davs.

Experiment 16 .—The effect of excessive tnrgoj; on moto- excitabiiity may be demonstrated in the case of Mimosa Fig. 19. Effect of enhanced turgor, ariificailly induced. .First two responses normal. Application of water, at arrow, induces depression of moto-excitability. 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-excitabilifcy results from excessive turgor, brought on by absorption of water. In such cases, how r ever, 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.

■The,. . mofco-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 Identi¬ cal .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 practical¬ ly abolished at the minimum temperature of about 19°C. Effect of lowering of temperature : Experiment 17.—A simple way of exhibiting the effect of lowering of temper¬ ature is by artificial cooling of the pulvinus. This cannot

Fig-. 20. Effect of moderate cooling during a period shown by horizontal line below. Moderate depression followed by quick restoration. •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 w6re taken in the usual manner. Cooled glycerine was then applied and the fecord 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 moderate¬ ly cooled glycerine is there indicated by the horizontal line .below. On the cessation of application, the normal tem¬ perature 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 reedrd represent appli-

Fig. 21 . Effect of application of more intense cold. Note madden depression followed by abolition of excitability, also persistent after-effect. cations, 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 m the plant often lag considerably behind the external changes which induce them, . Effect of high temperature ; Experiment 18 ,—It has been shown that the moto-excitabiiity 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 1 excitability was, however, gradually restored when the plant was allowed ( to regain the former temperature.

Fig. 22. Effect of tempera tare 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- e^citability. 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. I will now give automatic records of responses taken once every hour for twenty-four hours. They prove con¬ clusively the diurnal, variation of excitability in Mimosa. After studying in detail the variations characteristic of particular times of the day, I will endeavour to cirrelate them with the effects brought on by the periodic changes of the environment.

Experiment 19. —As a typical example 1 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. 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 ths nyctitropic movements of the- leaf. It should be remembered that the up-movement of the leaf is represented by down-curve, and vice versa. After the maximum fall of the leaf, which in this case was attained at 9 P.M., there followed a revise 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.

are* practically uniform between the hours of 5 and 6"' p.h., 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 Miftiosct is attained- between" 1 and 3 P.M., . and remains constant for several hours. In connection with the con-, stancy of response at this period, it should be remembered that when the response Is at its maximum a slight increase of excitability can¬ not 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 gradual¬ ly 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 1 in the •follow¬ ing 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 shous 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.

Ftg. 21. Mid-day record from noon to 3 i\m. exhibiting uniform excitability ^Responses taken once every haif^hour. Everting record: Experiment 21 .—The record given in Fig. 23 shows that the amplitude of response falls contin¬ uously after 6 f.m. -It might be thought that the diminished amplitude in the first part may be due to the natural n^yctitropic fall of. the leal 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 tie 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 fal

from 6 to 9 P.M., after which there is a reverse move¬ ment 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 Fig. 25. Eyenirtg record from 6 to 10 p.ji., showing gradual depression 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.

Record in. the morning : Experiment 22,—The excita¬ bility 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 above record (Fig. 26). Fig. 26. Horning record from 8 a.m. to 12 noon, exhibiting gradual enhancement 1’he 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-lire upwards the recovery appears to be incom¬ plete* 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

So far^I have merely described the observed' diurnal, variation of excitability. We may next inquire whether fcernal 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 man¬ ner 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 excitaoility.