The Motor Mechanism of Plants

required for more or less complete restoration of excitability. I give a record of the variation of excitability as shown by the leaf of another cut shoot (fig. 49) : (a) is the record of response to the testing stimulus 4 hours after section. 1 he excitability, under constant external conditions, remained fairly constant for about 12 hours. A steady but slow decline of excitability (h) then occurred for tne next 12 hours. On the second day the fall of excitability was very rapid (c), reaching zero in about 5° hours after isolation. 1 he whole cycle of change may be described as follows : the isolated preparation is rendered insensitive by the shock of section for nearly an hour ; the excitability is then gradually restored

almost to its normal value before operation. Under constant external conditions the excitability of the isolated specimen remains fairly constant for about 12 hours, after which depres¬ sion slowly sets in. The rate of fall of excitability becomes rapid 40 hours after the operation, being finally abolished after the fiftieth hour. It is probable that the rate of fall of excitability in a colder climate would be much slower. (a) Vigorous response of leaf of the isolated shoot 4 hours after section ; ( b ) depression after 24 hours, and ( c ) marked dep.-ession after 48 hours, culminating in total abolition of response and death (Mimosa).

It may be said in anticipation that the effects of variation of external conditions on the isolated pet i ole-pul vinus pre¬ paration are identical with those manifested in the intact plant. I describe additional experiments which are greatly facilitated by the more easily managed cut specimens. For instance, the condition of subtonicity is more easily induced in isolated specimens, and greater facility is offered for studying the effect of external stimulation in raising the tonic level oi the specimen. In regard to the action of various chemical solutions, the particular dnig can be applied at the cut end of the stem. «

The marked change in excitability in an intact plant under variation of light has already been described ( see fig. 38, p. 69). It was shown that a slight darkening due to the passage of a cloud caused immediate depression, while t he disappearance of the cloud was followed by an equally quick restoration of excita¬ bility. In order to ascertain whether or not this effect of light is connected with photosynthetic action, I took a petiole-pulvinus preparation from which the sub-petioles bearing the photosynthetic leaflets had been cut off ; the specimen was placed in a room illu¬ minated by diffuse daylight.

Experiment 47. — The normal responses were taken under diffuse light, the temperature of the room being 30° C. The room was then darkened by pulling down the blinds, and the record was continued in darkness, the temperature of the room remaining unchanged at 30 °. The record (fig. 50) Fig. 50. Stimulating action of light, a. A depressing action of darkness. shows that a great depression of excitability had occurred in darkness. The blinds were then pulled up, and the restora¬ tion of the normal amplitude of response demonstrates the

stimulating action of light. The results establish the fact that light has a direct tonic action quite apart from photo¬ synthesis. This finds independent support from the fact, which will be demonstrated later, that the enhancement of excitability is induced in a subtonic tissue not merely by photic stimulation but also by electric stimulation. Experiment 48. — The action of barium chloride on the contractile response of muscle is a prolongation of the period of recovery, and a characteristic double contraction. The sluggishness in recovery and the double contraction dis¬ appear in a short time after t repeated stimulation by tetanising electric shocks. The effect of the reagent on the response of the pulvinus is very similar. After the application of 1 per cent, solution BaCl2, the response exhibited a double contraction and incomplete re¬ covery (fig. 51). The sluggish¬ ness induced was so great that the next stimulation, repre¬ sented by a thick dot, was ineffective. The pulvinus was now subjected for a short time to tetanising electric shocks, the response to which was not recorded. The after-effect of tetanisation was the removal of sluggishness, as shown in the next three records, which were taken under the original testing stimulus. The response is now a single contraction followed by complete recovery.

Fig. 51. Incomplete recovery under the action of FaC]2 and transient restoration under tetanisation a t r (Mimosa) Acid and alkali are known to exert antagonistic actions on the spontaneous beat of the animal heart. agents also produce their antagonistic effects on the contrac¬ tile response of Mimosa. This is illustrated in fig. 52, where the application of 1 per cent, solution of lactic acid arrested the response in contraction ; the antagonistic action of dilute NaOli is shown in the restoration of excitability ; its con¬ tinuous application, however, caused a second arrest, but this time in a state of relaxation.

Arrest of response in contraction under lactic acid ; restoration and final arrest in expansion under NaOH (Mimosa). The most important outcome of the results described is that all the investigations on the irritability of the into.ct Mimosa plant can be successfully repeated wTith the petiole- pulvinus preparation. The difficulty arising from the supply of material has thus been entirely removed. Several experiments, practically impossible with the intact plant, can be successfully carried out with the isolated preparation, which allows far greater facility for manipulation.

For investigations on irritability, the isolated petiole- pul vinus preparation of Mimosa can be substituted for the intact plant. It is shown that in Mimosa conductivity is independent of moto-excitability. The pulvinus may have been rendered imrnotile, yet the excitatory impulse is transmitted along the enclosed nervous tissue in both centripetal and centrifugal directions, causing the fall of other leaves on the stem and the closure of leaflets on the sub-petioles.

I he age of the leaf has influence on its conductivity and moto-excitability which attain their maxima in the fully grown young leaf. The conducting power of a fullv grown young leaf is eight times that of a very young leaf, and sixteen times that of a moderately old leaf. 1 he effect of a severe wound is to cause intense excitation, the after-effect of which is a depression of excitability. Excitability is, however, gradually restored in a staircase manner.

After recovery from the effects of section of the stem, the excitability of the isolated preparation remains uniform for nearly 12 hours, during which experimental investiga¬ tions can be carried on with greater facility than with an intact plant. There is a gradual fall of excitability from the twelfth to the twenty- fourth hour. The depression becomes rapid 40 hours after isolation, excitability being abolished after the fiftieth hour. Several instances of daily periodicity are known, but the relations between the recurrent external changes and the re¬ sulting periodic variations are more or less obscure. The complexity of the problem is great, for not only have the effects of changing environment to be taken into account, but also their unknown after-effects. Different factors again, such as light, temperature,^ turgor, etc., are under¬ going independent variation, and the physiological changes induced by them are either concordant or antagonistic. Among the instances of diurnal periodicity may be men¬ tioned the nyctitropic movements of plants. The fanciful name of f sleep * is commonly given to the closure of the leaflets of certain plants at night. Whether plants really sleep or not can only be ascertained by a definite inquiry as to whether or not the plant is equally excit¬ able throughout the day and night. If it is not, then is there any definite period during which it practically loses its sensibility ? Is there, again, another period at which the plant wakes up, as it were, to a condition of maximum

The diurnal variation of the excitability of Mimosa was experimentally investigated by subjecting the plant at every hour of the day and night to a test-stimulus of uniform intensity, and obtaining the record of the resulting mechani¬ cal response at different periods during 24 hours. Under these circumstances the amplitude of response at any hour serves as a measure of the excitability of the plant at that particular moment. A periodic fluctuation of response would demonstrate the periodic character of variation of excitability.

automatically record the response of the plant to uniform periodic stimulation at all hours of the day and night. I give a diagrammatic view (fig. 53) of the different parts of the apparatus which was devised for this investigation. The leaf of Mimosa is attached to one arm of a light aluminium lever L by means of a thread. At right angles to the lever is the writing -index W, which traces the responsive move¬ ment of the leaf on a smoked-glass plate, allowed to fail at a definite rate by clockwork. Under a definite stimulus 01 electric shock, the leaf falls down, and the response is recorded on the plate by the writing -lever.

Successive stimulations of constant intensity are applied at intervals of, say, an hour ; comparison of the responses shows whether the excitability of the plant had remained constant or undergone any variation. The leaf was periodically stimulated by tetanising electric shocks of constant and sub-maximal intensity. The exciting value of the tetanising shock depends (1) on trie intensity, and (2) on the duration, of the shock. The intensity can be rendered uniform by hxing the secondary at a definite

distance from the primary ; the current in the primary circuit is kept constant by the employment of a storage-cell of definite electromotive force. The duration of the induc¬ tion-shock given by the secondary coil depends on the length of time during which the primary circuit is completed in each, stimulation. The completion of the primary circuit Fig. 53. Diagrammatic representation of the complete apparatus for determination of the diurnal variation of excitability.

Petiole of Mimosa, attached by thread to one arm of leyer ; the writing-index w traces on the smoked-glass plate g me responsive fall and recovery of leaf, p primary, and s secondary, of induction-coil. Exciting shock pr sses through plant by electrodes e, b'. c, clockwork for regulating duration of shock. Primary circuit of coil completed by plunging rod R, dipping into cup of mercury m. is made by a plunging rod R, which dips into a cup of mercury M. The duration of closure of the circuit is adjusted by clock-mechanism (fig. 53) '• in t^le following

The clock employed performs three functions. 1 he axis, which revolves once in 12 hours, has attached to it a wheel, round which is wound a thread which allows the recording-glass plate to fall through 6 inches in the course of 24 hours. A spcke attached .to the minute hand releases the alarm at intervals of an hour and actuates the plunging rod R, which then completes the primary circuit of the induction-coil. The plant is thus subjected at specified times to tetanising induction-shocks of uniform intensity and duration. The record of response was taken on an Oscillating Recorder.

Before giving the daily records of the hourly variation of excitability, I will give some experimental Results which illustrate the effect of external variations in modifying excitability. Undpr natural conditions the plant is subjected (1) to periodic variation of light and darkness ; (2) to hourly variation of temperature. I will explain how each of these factors modifies the moto-excitability of the plant. J he plant is subjected to light from morning to evening and to continuous darkness from evening to early morning. The excitability of the plant has been shown to increase under light and to decrease under darkness ( cf . figs. 38, 50b

The experiments on periodic variation of moto-excita¬ bility were carried out in early spring (February) and in summer (May). The highest temperature was attained at different seasons between 12 noon and 3 P.M. The minimum temperature was reached between 5 and 6 a.m. The maximum and minimum temperatures in spring were 29 ( . and 200 C. respectively. The summer maximum was ] have shown that the excitability of the plant increases with moderate rise of temperature, the maximum excita¬ bility being attained at or about 350 C., which is the optimum.

In order to determine the changes of excitability induced bv changes of temperature below and above the optimum, I carried out the following experiments. Experiment 50. Effect of moderate lowering of tempera¬ ture - -A simple way of exhibiting the effect of lowering of Fig. 54. Effect of moderate cooling in depressing excitability. Duration of application of cold shown by the horizon „ai line temperature is by the artificial cooling of the pulvinus. This cannot very well be done by the application oi a stream of cooled water, since absorption of excess of water by the pulvinus is attended by a loss of excitability ; dilute solution of glycerine has, however, no such drawback, this liquid at ordinary temperature was first applied on the pulvinus, and record was taken in the usual manner. ( ooled liquid was then applied and record taken once more (fig. 54'* The first or normal response was taken at the temperature of the room, which was 32° C. * the next two exhibit de¬ pression of excitability under moderate cooling, the duration

of which is indicated hr7 the horizontal line below. After this the plant regained the normal temperature of the room, with the restoration of normal excitability. Experiment 51. Effect of excessive cooling* — The record (%• 55) shows the effect of cooling by ice-cold glycerine solution. The immediate effect was depression, followed by complete abolition of excitability. Thick dots in the record represent applications of stimulus which proved Note sadden depression followed by abolition of excitability; also persistent after-effect, (Mimosa).

ineffective. It will aiso be noted that even after the cessa¬ tion of cooling and the return to normal temperature the induced abolition of excitability persisted as an after-effect for a considerable length of time. The results prove that, on account of physiological inertia, the variation of excitability persists beyond jthe external change which induced it. , • Expei ime^f 52. Effect of high temperature. — The moto- excitability undergoes depression at any temperature above the optimum, [his is seen in the following record (fig. 56), where the normal response, at 32 0 C. was depressed^ by gradual rise of the temperature of the plant-chamber to

420 C. ; the excitability was slowly restored when the plant was allowed to regain the former temperature. I briefly recapitulate some of the important results : Light enhances the moto-excitability, while prolonged darkness depresses it. Far more marked is the eifect of ,■ temperature : lowering of temperature depresses and Note depression of excitability induced by high temperature, and gradual restoration on return to normal (Mimosa). finally abolishes the moto-excitability ; rise of temperature enhances it up to an optimum temperature, above which the excitability undergoes a decline. The change of excita¬ bility induced by variation of external conditions is not immediate, but lags behind the inducing cause ; the after¬ effect alsu persists for a time.

The automatic record of the responses under uniform testing stimulus every hour exhibits the characteristics of the diurnal variation of moto-excitability. 1 Experiment 53. — As a typical example I reproduce a record (fig. 57) obtained in spring in the month of February. The record was commenced at 5 p*m., and continued to the! same hour next day, the plant being stimulated once every A hour. Examination of the characteristic variations of response throughout the 24 hours shows that a continuous decline occurred after the setting in of darkness at 7 p.m. The

fall of excitability continued even after sunrise, at about 6 A.M., and the response was practically abolished between 8 and 9 a.m. The excitability then gradually increased in a staircase manner, the maximum being reached after 12 noon; the excitability after this hour remained more or less constant till late in the afternoon. It will be noted that the amplitude of the response at 5 p.m. on the second day was the same as that of the corresponding response on the previous day.

Fig. 58. Midday record from noon tu 3 r.M., exhibiting uniform The results of numerous records taken in spihig may be summarised as follows : (1) The maximum excitability of Mimosa is attained at or about noon, and remains constant for several hours. I give a series of records taken every half an hour on a fast moving plate (fig. 58) in which the | uniformity of responses demonstrates constancy of excitability during the period. In studying the influence of external variations on excitability, it is best to carry out the experiments during the midday period, for the observed change will then be due to the external agent and not to variation in the plant.

(2) Excitability undergoes a continuous decline from evening to morning, the response being practically abolished between 8 and 9 a.m. (3) After o a.m. the excitability is gradually increased in a staircase manner till the maximum is reached a 1 ’noon ; records taken once every half an hour on a faster moving plate show this in a very clear manner (fig. 59). It will be observed in the record (fig. 57) that the level of the base-line varies. 1 bis is due to rising and sinking of the leaf itself, which, in turn, is caused by changes in the turgor of the pulvinus ; hence the variations in level ol the base-line represent and record variations of turgor. The changes in the external conditions affected not only the rnoto- excit ability of the leaf, but the turgor of its pulvinus as well , but inversely, maximum moto-excitability coinciding

I0O CHAP. VIII. DIURNAL VARIATION OF MOTO-E XC IT AB I LI T Y with minimum turgor, and vice ve*sa. This is in accord¬ ance with a previous observation (cf. p. 70) that the motility of the leaf is diminished when turgor becomes too high. The highest erection of the leaf, indicative of maximum turgor, was reached at 6 a.m. : the leaf then fell slowly and reached the middle position at noon, when the turgor-condi¬ tion was neither too high nor too low. On account of the sudden change of light to darkness, an excitatory fall of the leaf (with diminution of turgor) occurred after 6 p.m., as shown by the displacement of base-line upwards. A continuous increase of turgor was recorded after 9 p.m., with displacement of the base-line downwards, the maximum turgor being attained, as already stated, at 6 a.m.

The causal relation between change in the external condi¬ tions and the variation of excitability may now be discussed. 1 1 has been shown that moto-excitability is greatly influenced bv temperature. In order to find in what manner the diurnal variation of excitability is influenced b}^ the hourly variation of temperature. I took special care to secure, by means of the thermograph, a continuous record cf the temperature- variations. The following table shows the relation between the hours of the day, the temperature, and the excitability measured by the amplitude of response.

Table V. — Showing the Relation between Hours of the DAy, Temperature, and the Amplitude of Response (Spring-Specimen). In fig. 60 are shewn two curves, (i) of variation of temperature and (2) of variation of excitability, daring the 24 hours. The data for these were obtained from records taken on a day later in spring when the maximum temperature was 33 0 C. and the minimum 22 0 C. The striking resemblance between the two curves demonstrates I he upper curve gives the variation of temperature and the lower the corresponding variation of excitability.