Researches on Irritability of Plants

dilute NaHO solution (fig. 190), on the other hand, induces exactly the opposite effect—namely, an arrest at systolic contraction. The effect of chemical reagent on the pulsating activity of Desmodium is to a certain extent modified by the tonic condition of the specimen. The effect is also dependent on the strength of the reagent. Very dilute application often induces an effect opposite to strong. Alcohol induces a transient exaltation followed by depression. The period of pulsation is lengthened.

Carbonic acid when dilute induces an exaltation of amplitude, with prolongation of period of each pulsation. Long application of undiluted gas induces an arrest : revival takes place on substituting fresh air. Vapour of ether induces a temporary arrest of pulsation ; the pulsation may be revived on blowing off the vapour. The effect of chloroform is similar to that of ether, but its toxic effect is greater, any excess causing permanent arrest. Diluted vapour causes a preliminary exaltation followed by arrest. Revival may take place after quick substitution of fresh air.

Vapour of carbon disulphide applied for a short time also induces a transient arrest. Copper sulphate solution when applied directly on the pulvinule causes an almost immediate arrest. But when applied at the cut end of petiole, the arrest does not take place till the lapse of a period, required for the solution to ascend to the motile organ. The poisonous reaction of potassium cyanide solution is more powerful than that of copper sulphate. The effect of acids and alkalis on the rhythmic move- ments of Desmodium are, as on the animal heart, antagonistic. In both, acids induce a standstill in diastole, while alkalis induce arrest in systole.

For the study of phenomenon of irritability, the so-called ‘sensitive’ plants have been selected for the purpose of experimental investigation. From this it must not be inferred that the fundamental reactions that have , been demonstrated are different in ordinary plants. By the employment of the electrical mode of investigation, I have shown elsewhere that not sensitive plants alone, but every plant, and also every organ of every plant, is excitable. Even in the matter of mechanical response, longitudinal contraction under excitation may be demonstrated in the case of various organs of ordinary plants; but the extent of movement in such cases is not very great. A ‘sensitive plant’ differs from others only in the possession of mobile mechanism by which excitatory change of form is mani- fested by a conspicuous movement.

In the case of Mimosa, the responsive down move- ment or the fall of the leaf is brought about by the differential excitabilities of the upper and lower halves of the pulvinus, the movement being magnified by the long petiolar index. It is these advantageous circum- stances that render the motile apparatus of Mimosa a good indicator of excitation. The leaf may thus by its responsive movement indicate the passage of an excitatory impulse originated at a distance just as the excitation transmitted through a nerve is detected by the mechanical response of the attached muscle. In connection with this it should be remembered that the plant indicator may become inefficient under unfavourable circumstances, Absorption of excess of water, for example, will annul the

motility of the pulvinus of Mimosa. Under these circum- stances, we may have an excitatory impulse without any external manifestation of its passage through the plant. It will thus be understood how an excitatory impulse in an ordinary plant may pass unnoticed on account of the absence of an efficient motile indicator. The exist- ence of such an impulse can, however, be detected by means of electric response. In the present work the various excitatory phenomena of the plant have been investigated by means of mechanical response under the action of a testing stimulus. The different responses of plants may be included under three classifications : (1) Simple response, where a single stimulus evokes a single response ; (2) Multiple response, where a single strong stimulus gives rise to multiple series of responses ; and (3) Automatic response so called, where the pulsations appear to be spontaneous. In reviewing these in their proper sequence we shall be struck by the extraordinary similarities which are revealed between the response of the plant and the animal.

In the course of this work it has been shown that physio- logical changes induced in the plant owing to the action of the environment may be revealed by the characteristic variation of the amplitude and time relations of the normal curve of response. In obtaining records of response errors are, however, introduced on account of friction of the writing point against the recording surface. In recording the pulsations of Desmodium leaflet it is found that a weight as small as ‘03 gram is enough to arrest the pulsatory movement. The difficulty of friction has been overcome by the method of intermittent instead of con- tinuous contact for record. In the Resonant Recorder the writer is made to vibrate to and fro at a known and definite rate. The record consists of series of dots giving

definite time intervals. In this manner time interval shorter than a hundredth part of a second can be measured. Owing to the extreme lightness of the recorder the error due to inertia is reduced to a minimum (p. 14). The various stimuli which evoke motile response in the animal are also found effective in giving rise to excitatory response in the plant. Thus the plant may be excited by mechanical, chemical, thermal, and electrical modes of stimulation. Electric stimulation may be caused by the polar action of a constant current, by the discharge of a condenser, or by the induction current. The electrical method allows the intensity of stimulus to be maintained constant, or varied in a quantitative manner. As in the skeletal muscle of animal, so also in the pulvinus of Mimosa, the break induction shock is more effective than the make shock. The sensitiveness of Mimosa to an electric shock is very great. It often reacts to an intensity of shock which is only one-tenth of the minimum perceived by a human subject (p. 23).

Different plants exhibit different characteristics of response. The reactions are relatively quick in some and slow in others. In a typical case of Mimosa in summer the latent period was one-tenth of asecond. The maximum fall of the leaf was attained in three seconds, and the recovery completed in 15 minutes. The rate of recovery was rapid at the beginning and very slow towards the end. The maximum rate of recovery was ‘og mm. per second in contrast to the maximum rate of fall of 24 mm. per second. The movement of recovery was about three hundred times slower than the movement of the excitatory fall. The extent of responsive fall in Mimosa increases with the

increasing intensity of stimulus. The rate of movement is also increased under the action of stronger stimulus and higher temperature; it is decreased under fatigue. A stronger stimulus, generally speaking, requires a longer period for recovery. Under the physiological depression induced by winter the responsive reactions are modified, the latent period prolonged, and the amplitude reduced. Biophytum sensitivum may be taken as typical of a quickly reacting plant. The leaflets undergo closure within a second after receiving the excitatory shock ; recovery is accomplished in the course of three minutes. In marked contrast with this is the extremely sluggish reaction of the leaf of Neptunia oleracea, where the apex time is reached only after an interval of 180 seconds, and recovery com- _ pleted in 60 minutes (p. 44).

In the response of animal tissue it is found that a single stimulus by itself ineffective becomes effective on repetition. The same is found to be the case with plant tissues. Thus, with a given specimen of Mimosa, it was found that, while an electrical stimulus of intensity ‘I was singly ineffective, it became effective after being repeated twenty times. It is found, moreover, that this additive effect is, within limits, strictly quantitative. Thus with the identical specimen of Mimosa, when the intensity of individual stimulus was increased from ‘1 to ‘5, the number of repetitions necessary to cause effective excitation was reduced from 20 to 4. Here ‘I X 20 ='5 X 4. The effective stimulation is thus found constant, being equal to individual intensity multiplied by the number of repetitions (p. 54).

The response of Mimosa is abolished at a low tempera- ture. With the rising temperature the amplitude of re- sponse is increased and the period of recovery shortened. As in the response of animal muscle, so also in Mimosa there is a range between minimal and maximal stimulations where increasing amplitude of response occurs under increasing intensity of stimulus. The range within which the increasing effect is observed to take place is relatively extended in the case of plants in a somewhat sub-tonic condition. The range of variation is, however, restricted in specimens which are highly excitable (p. 61).

During the responsive movement the pulvinus of Mimosa can do the work of lifting a weight. The effect of load on the response of Mimosa is similar to that on the contractile response of muscle. In both, under increasing load, the height of response undergoes a _ progressive diminution with shortening of the period of recovery. Within limits, the amount of work performed by the muscle increases with the load. The same is true of the work performed by the pulvinus of Mimosa. Thus it is found that, while under a load of 100 mg. the work performed was 1340 mm. mg., it became enhanced to 8666 mm. mg. under a load of 2000 mg.

The response of the pulvinus of Mimosa exhibits charac- teristics which are similar to those of the response of muscle. Under normal conditions, and with sufficient intervening periods of rest, the successive responses are found to be of uniform height. Under conditions of incomplete re- covery, however, the responses exhibit signs of fatigue. The excitability of the plant in a sub-tonic condition is enhanced by the action of stimulus itself. Under such conditions the response exhibits a staircase increase.

Under extreme sub-tonicity the normal negative response may even be converted into abnormal positive. The anomalous erection, after a preliminary fall of the leaf of Mimosa under continuous stimulation, is explicable on the common characteristics of response in plant and animal tissue. In both contraction is reversed to relaxa- tion under fatigue (p. 84). The motile excitability of Mzmosa undergoes abolition under sudden change from light to darkness. After a certain time the plant accommodates itself to the changed condition, with the restoration of normal excitability.

There is again a variation of motile excitability depending on the time of the day. The motility is at its maximum at a certain hour, and minimum at a different hour. This peculiarity is probably connected with the question of variation of turgor. The excess of turgor in the pulvinus caused by absorption of water is attended by a reduction or abolition of motile response. The lost motility may,’ however, be restored by the application of glycerin.

Very characteristic are the effects exerted by the different gases. Some induce a stimulating action, others give rise to depressing or toxic effects. Ozone enhances the excita- bility. Carbonic-acid gas and vapour of alcohol induce a moderate depression of excitability from which the plant recovers on readmission of fresh air. Coal gas and vapour of carbon disulphide also induce a depressing effect. The vapour of ether exerts a moderate narcotic action. The effect of chloroform is far more pronounced, the loss of excitability under its action being more complete and persistent. Ammonia has marked effect in the abolition of excitability. Sulphuretted hydrogen, nitrogen dioxide, and sulphur dioxide are very toxic in their action; their

application is attended by quick abolition of excitability followed by the death of the plant (p. 94). One test by which a dead plant may be distinguished from a living one is that of electric response. The response of galvanometric negativity characteristic of living condi- tion is abolished at death. When the plant is subjected for a time to a temperature of 60°C. its electric response, generally speaking, disappears. This temperature is, there- fore, fatal for most plants.

When the leaf of Mimosa is continuously raised in tem- perature there is produced a progressive erectile movement ; but at a critical temperature the erectile movement is suddenly reversed into a spasmodic contraction. This inversion takes place under standard conditions at or about 60° C., after this the response of the plant is permanently abolished. The death record is a V-shaped curve, the point of inversion being the death-point. After death a repetition of experiment shows no further inversion. Various other plants, sensitive and ordinary, exhibit this characteristic death-spasm at or about 60°C. In radial organ this movement consists of an abrupt longitudinal contraction. In taking an electrical record it is found that an electric-spasm also takes place at the critical tem- perature which is very near 60° C.

The death contraction in plants is similar to that seen in the animal. The death-point is found lowered under physiological depression. Thus, in a certain case, fatigue lowered the death-point of the plant from the normal 60° C. to 37°C. In another case dilute solution of poison lowered the death-point by 18° C. (p. 106). The fundamental unity of excitatory phenomena in the animal and plant finds a striking illustration in the characteristic effects induced at the kathode and anode.

In animal tissues, with feeble current it is found that excitation takes place only at the kathode at make. On moderately increasing the current excitation is found to take place at the make of kathode and break of anode. These effects are included in Pfluger’s law of polar excita- tion in animal tissues. I have shown that effects precisely similar to these take place in the vegetal tissues. That is to say, the laws of polar excitation in plant under feeble and moderate currents are :

I. With feeble current the kathode excites at make and not at break. The anode excites at neither make nor break. II. With moderately strong current, the kathode excites at make and not at break. The anode excites at break and not at make. The polar reactions in the undifferentiated protoplasm of the plant body are thus identical with those of highly differentiated animal tissues (p. 233). The effect of feeble ascending and descending currents in the petiole of Mimosa are parallel to those in nerve-and- muscle preparations. In both cases excitation takes place earlier when the kathode is nearer to the responding organ. As in animal, so also in Mimosa, single induction shock of moderate intensity is, as regards polar actions, effective at the commencement and not termination of the current (p. 206).

The sensitiveness of Biophytum to an electrical current is remarkably high ; compared to the very sensitive human tongue, the sensitiveness of Biophytum is about ten times as great (p. 251). Pfluger’s law cannot be taken as a complete statement of the polar action of currents (p. 265). For strong currents there are induced two additional types of reaction: III. Under the action of strong current, excitation takes place at the make of kathode and make and break of anode.

Physiological changes are also found to modify the polar effects of current. Under the separate or joint action of an increasing current and physiological modification, types of reaction are observed in plants which are similar to the so-called anomalous response in Protozoa (p. 276). In the animal heart the contrasted effects of anode and kathode are exhibited by characteristic modification of its pulsating activity. Effects precisely similar are shown in the rhythmic tissue of Desmodium gyrans. The effect of the make of anode is to induce an expansion ; this is shown in the record of pulsation of Desmodium by a reduction of normal limit of systolic contraction. The opposite effect of contraction due to the make of kathode is seen in the reduction of normal limit of diastolic expansion (p. 236).

In nerve-and-muscle preparation the effects of ascending and descending currents are found to be modified by the intensity of the current. Effects in every way parallel are observed in experimenting with petiole-pulvinus of Biophy- tum. These characteristic modifications are easily trace- able in Biophytum to the contrasted effects of anode and kathode and of make and break. Excitability is enhanced by the make of kathode and break of anode. It is depressed by the make of anode and break of kathode (p. 239).

A very important problem in plant physiology relates to the question as to whether in plants there is any trans- mission of true excitation. The prevailing opinion has been that in plants like Mimosa pudica there is merely a passage of hydro-mechanical disturbance, unlike the trans- mission of excitatory protoplasmic change, which takes place in an animal nerve. This view has been based on the experiments of Pfeffer and Haberlandt, who found transmission of stimulus to take place in spite of narcotisa- tion or scalding of the intervening tissue. It is shown that those experiments are not conclusive, inasmuch as super- ficial narcotisation or scalding is not effective in abolish- ing the conducting power in the interior of the tissue.

For the settling of the question whether the transmission is physical or physiological it was necessary to have quanti- tative measurements of the highest accuracy in order to determine whether physiological changes affect in a definite manner the velocity of transmission. For the accurate determination of velocity it is essential to allow for the latent period of the responding pulvinus and its variations under different conditions. By the employment of the Resonant Recorder the value of latent period can be accurately determined within a hundredth part of a second. Successive determinations of the latent period under constant external conditions are found to give identical results. The shortest value of the latent period in vigorous Mimosa is ‘06 second, the average value in summer being ‘r second. The latent period is in general shorter under stronger intensity of stimulus; but the value becomes constant above a maximal stimulus. In the optimum condition of the plant the latent period is the same for feeble or strong stimulus. Fatigue prolongs the latent period ; a rise of temperature, on the other hand, shortens the latent period (p. 130). The latent period of the pulvinus of Neptunia oleracea is ‘6 second.

Successive values of velocity of transmission are found constant when the applied stimulus is constant, and when the intervening period of rest allows complete protoplasmic recovery. Consistent results have been obtained by the employment of the Direct and the Differential Methods. The automatic records afford measurement of time as short as ‘05 second. The highest velocity of transmission of excitation that has been found in the petiole of Mimosa is 30 mm. per second. In a sub-tonic tissue the velocity of transmission of excitation is enhanced under increased intensity of stimulus. The tissue became a better conductor of excitation in consequence of previous stimulation. Fatigue depresses the rate of conduction of excitation. Velocity of transmission becomes markedly enhanced at a higher temperature (p. 140).

The transmission of excitation takes place in both directions ; but the velocity is not necessarily the same in the two directions. In Buophytum the velocity in the centrifugal direction is quicker than in the centripetal. The enhancement of velocity under favourable condition of rising temperature proves the physiological character of the transmitted effect. In support of this there are various confirmatory proofs, some of which may be regarded as crucial.

Transmission of excitatory electric impulse.—The excita- tory change in the plant is accompanied by a concomitant electric change of galvanometric negativity. Electrical investigation shows that this excitatory electric impulse is transmitted to a distance through certain plant organs. Excitatory impulse in absence of mechanical disturbance.— The hydro-mechanical theory presupposes the occurrence of a strong mechanical disturbance to give rise to the transmitted impulse. But initiation of excitatory impulse is found to take place under the polar action of an electrical current in the absence of any mechanical disturbance. This is realised when we find that an excitatory impulse is initiated and transmitted by the action of a current which is so feeble as not to be perceived by the very sensitive