Researches on Irritability of Plants

A stronger stimulus, generally speaking, requires a longer period for recovery. The influence of season is also a factor to be taken into consideration. Under the physio- logical depression induced by winter, the responsive process is appropriately modified. The excitability of the tissue becomes depressed. An intensity of stimulus which in summer was effective, becomes in winter ineffective. To evoke response much stronger stimulus has to be employed. The latent period is prolonged and the amplitude of response reduced. And lastly, in winter there is, generally speaking, a great prolongation of the period of recovery. In summer, with vigorous specimens, recovery may be_ practically complete in as short a time as 8 minutes. But owing to sluggishness induced in winter, on the other hand, the recovery may be prolonged to 25 minutes or more. In a severe winter response may even be abolished altogether.

I have hitherto dealt in some detail with the responsive movement of Mimosa. In contrast with this may be cited other examples in which the excitatory reaction may be either more rapid or extremely sluggish. Response of Biophytum.—As an instance of relatively quick reaction I give (fig. 15) a record of response of leaflet of Biophytum. The maximum fall was here attained in the course of a second.after the shock. The recovery was completed in the course of only 3 minutes.

Response of Neptunia.—In marked contrast with the quick reaction of Biophytum, I may cite the very slow action Fic. 15.—Record of response of leaflet of Biophytum. Vertical marks below record indicate intervals of ‘5 minute. of the primary leaf of Neptwnia oleracea. In fig. 16 is given a record of its response under an exciting induction-shock of moderate intensity. The clock-interrupter was so adjusted that the successive dots should be at intervals of half a

Fic. 16.—Response of leaf of Neptunia. Successive dots are at intervals of +5 minute in the contractile portion, and 1 minute in the recovery portion of curve. minute during contraction, and at intervals of a minute during recovery. It will be seen that the maximum fall was attained 3 minutes after stimulation, and the recovery was not completed even after 40 minutes, which was the duration of this particular record. The recovery was completed after a further period of 20 minutes, that is to say, the total period of recovery was an hour. The apex time or period of contraction is shortened by the application of a stronger stimulus, but the period of recovery then becomes very much prolonged. The following tabular statement will display the range of variation in the speed of reaction of these sensitive plants :—

Specimen Apex time |_— Period of recovery | Biophytum sensitivum ac I second 3 minutes Mimosa pudica 58 of 3 seconds 16 minutes Neptunia oleracea .. aA 180 seconds . 60 minutes The arbitrary distinction that is generally drawn between the so-called sensitive and ordinary plants may briefly be referred to here. In the case of Mimosa, it is generally supposed that the lower half of the pulvinus is alone sensi- tive ; this however is not an accurate statement. By local application of stimulus it can be shown that the ee half also undergoes a feeble contraction, See an ‘up’ movement of the leaf.

The localised stimulus may be applied by means of the electro-thermic stimulator. Application of stimulus of moderate intensity on the upper half of the pulvinus will be found to give rise to an erectile response. Another prac- tical method of local application of stimulus is by means of sunlight. A narrow beam may thus be thrown on the upper half of the pulvinus; .this will be found to give rise to erectile or ‘up’ response. Two such responses are

shown in fig. 17, where the stimulus of sunlight was applied for 2 minutes followed by a period of recovery for 13 minutes. If the stimulus applied on the upper half be strong or long continued, then the excitatory effect is transmitted across the pulvinus to the more excitable lower half. In these circumstances the ‘up’ is converted to ‘down’ response, on account of the greater contraction of the lower half of the pulvinus. Thus under any form of diffuse stimulation the resultant response in Mzmosa is brought about by the differential excitabilities of the upper and the lower halves of the pulvinus. We should also bear in mind that the slight differential contraction-effect in Mimosa leaf is very much magnified by the long petiolar

Fic. 17.—' Up’ response (represented by down curve) due to local stimulation of upper half of pulvinus of Mimosa. index. There are, again, numerous pulvinar organs whose responsive movements have passed unnoticed. In Desmo- dium gyrans there are two conspicuous pulvini ; the primary pulvinus is at the junction of the petiole with the stem ; there is a secondary pulvinus at the junction of the petiole with the terminal leaflet. The primary pulvinus appears at first sight to be insensitive. But on attaching the primary petiole of Desmodium with the writing-lever, I obtained the series of responses under a very feeble electric shock, as seen in fig. 18. In this particular case the recovery is practi- cally complete in 15 minutes. Other pulvini also exhibit differential contraction under diffuse stimulation. Thus the terminal leaflet of the bean plant (Vicia Fava) exhibits

responsive down movement, though here recovery is very protracted. But we have seen that the recovery of Neptunia is also a very slow process. The responsive movement of Mimosa is due, as has been noted, to the unequal excitabilities of the upper and lower halves of the pulvinus. The excitability of the tissue is again modified by the state of turgor. In Mimosa there is induced a periodic variation in the relative turgescence of the two halves of the pulvinus. On account of this the differential excitability, on which the motile response of

Fic. 18.—Series of responses of leaf of Desmodium gyrans, under electrical stimulation. Mimosa depends, undergoes great variation. The sensitive- ness of this plant is in consequence often found to disappear completely at certain hours of the day. I shall, moreover, show in Chapter VII that the leaf of Mzmosa becomes in- sensitive when its pulvinus absorbs an excess of water. Thus the mechanical movement of the sensitive plants on which depended the assumption that ‘ordinary’ plants were insensitive, rests on a basis which is very unreliable. Responsive movements may, on the other hand, be demonstrated in ordinary plants by the employment of a suitable contrivance. In a radial organ diffuse stimulation induces equal contractions on all sides, which balance each

other. Hence lateral movement, dependent on differential contraction, cannot take place. But if we take a hollow tubular organ of some ordinary plant, say the peduncle of daffodil, it is clear that the protected inner side of the tube must be the more excitable. When this is cut in the form of a spiral strip and excited by means of an electric shock, we observe a responsive movement to take place by curling, due to the greater contraction of the inside of the strip. This mechanical response is at its maximum at that season which is optimum for the plant. When the plant is killed its response disappears.

It will be seen that the division of plants into sensitive and insensitive is without any justification. Moreover, by adopting the electric mode of investigation, I have shown that every plant and every organ of the plant is sensitive and responds to stimulus by a definite electric variation. We have hitherto referred but vaguely to the question of the intensity of the induction-shock employed as stimulus to induce response. We have observed that on making and breaking a current in the primary coil, instantaneous currents are induced in the secondary. The intensity of the induction-current employed for giving a shock depends in the first place on the intensity of the primary current ; secondly, on the suddenness with which the primary current is made or broken; and lastly, on the relative distance separating the secondary from the primary coil. The in- tensity of the current can be maintained uniform if we always employ the same battery, say a 4-volt accumulator or storage-cell. As the break of a current is accomplished more quickly than make, the break-shock, as we have seen, is more intense than the make-shock. The plant may, there- fore, be excited by a single make-shock, or by a single break-shock or a double make-and-break shock, or by a sequence of make-and-break shocks, of definite duration, according to the particular requirements of the experiment.

shown, be increased by sliding the secondary nearer and nearer to the primary coil. At a great distance the intensity of the shock is very feeble, whereas in the nearest position it is most intense. If a scale be placed to mark the relative position of the secondary to the primary, we may be assured of obtaining an identical intensity of shock whenever we place the secondary at the same point on the scale; or we can obtain an increasing intensity of stimulus by progressive movement along the scale towards the primary. There is, however, no simple relation between the distance and the intensity—that is to say, equal decrement of distance does not mean equal increment of intensity. All that we are sure of, is that the sliding in of the secondary coil secures an increasing intensity of stimulation. In order to be certain of obtaining quantitative values of intensity, the scale has to be specially calibrated.

In subjecting the plant to the secondary shock, if we begin with feeble intensity of stimulus, by placing the secondary at a great distance, and gradually increase the intensity by sliding the secondary nearer and nearer, we shall obtain that scale-reading at which the stimulus begins to be effective. This particular intensity, the feeblest that is effective, we designate the minimal stimulus. As we now proceed to increase the stimulation by pushing the secondary nearer to the primary, we find the amplitude of the response is progressively enhanced, and ultimately we reach an intensity beyond which there is no further increment of amplitude. This intensity we designate the maximal stimulus. When the plant is in an exceedingly vigorous condition, the minimal intensity is low and the range between maximal and minimal is narrow. But if the plant be in a less favourable tonic condition, then the minimal stimulus is relatively high and the range between minimal and maximal is wider.

value of the intensity of stimulus as obtained from the scale- reading of a particular coil gives us no idea of the absolute intensity. It appeared desirable, nevertheless, in making quantitative experiments, to adopt some unit of stimulus in terms of which other intensities might be expressed. It would be well, moreover, to select this unit in some way not quite arbitrary, so that it might carry a significance more or less universal. The unit intensity of exciting shock which I have adopted for these reasons is that which barely induces in ourselves a perceptible sensation. The observer dips two fingers, one of each hand, into two troughs of saline solu- tion, which are in series with the experimental Mimosa and the secondary coil. The plant tissue is interposed so as to ensure an identical current to pass through the experimental individual and the plant. The resistance offered by the plant tissue is very great ; in the case of Mimosa under the usual mode of connection, it is about half a million ohms. At the beginning the secondary is placed at a great distance from the primary. The vibrating interrupter of the primary is next started and the secondary gradually pushed in, till at a certain scale-reading the observer, who is kept in ignorance of the position of the secondary, just begins to perceive the shock. This process is repeated several times in the case of the individual observer, and the mean of various consecu- tive readings, which ought not to differ from each other to any extent, is taken as the unit for that particular individual. The same observation is repeated with some ten different individuals, and the mean of these ten readings is finally adopted as that reading of the unit intensity which is to serve as the standard.

Though this reading cannot be regarded as absolute and invariable, yet, in the particular circumstances of the case, it is fairly definite and on the whole satisfactory. It gives us a general idea, moreover, of that intensity which will be effective in stimulating the plant, in terms of the minimal stimulus capable of evoking sensation in man. Having thus obtained the scale-reading corresponding to this unit, we calibrate other positions of the scale in terms of this unit. In this manner the scale is marked so as to indicate intensities of ‘I, ‘5, I, 2,3, 4,5, andsoon. The calibration is carried out by means of a ballistic galvanometer. In subsequent chapters we shall employ these practical units, which will thus have a definite significance.

Having shared the prevailing belief that the sensitive- ness of the plant was very feeble compared with that of the animal, I was considerably surprised to find that the intensity of induction-shock which is barely sufficient to induce sensation in man is quite enough to cause excitatory fall in a Mimosa of moderate sensitiveness. Indeed, I found that in the case of a highly excitable specimen an intensity only one-tenth of this was sufficient to excite it. In other words, under this particular test Mimosa may prove ten times as sensitive as a human subject! Later on I shall give details of measurements which will show that, as far as electric mode of stimulation is concerned, the plant is in no way inferior to the animal in sensitiveness.

The extent of responsive fall in Mzmosa increases with increasing intensity of stimulus. The rate of movement is also greater under stronger stimulus. The rate of responsive movement becomes slower under fatigue. In a given case the normal maximum rate of movement of 50 mm. per second was reduced to 8 mm. under fatigue. Temperature enhances the rate of movement. A rate of 10 mm. per second at a temperature of 22° C. was found enhanced to 105 mm. per second when the temperature was raised to 28° C.

In a typical case of Mimosa, in summer, the latent period was found to be one-tenth of a second. The maxi- mum contraction was attained in 3 seconds and the recovery completed in 15 minutes. The rate of recovery was relatively rapid at the beginning and very slow towards the end. The maximum rate of recovery was ‘og mm. per second in contrast with the maximum rate of contraction of 24 mm. per second. The movement of recovery was about three hundred times slower than the movement of excitatory contraction.

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 is prolonged and amplitude of response reduced. The period of recovery may also become protracted. Different plants exhibit different characteristics of response. Bzophytum sensitivum may be taken as a type of quickly reacting plant, while Neptunza oleracea is very sluggish in its reactions. In Brophytum the apex time is reached in a second and the recovery accomplished in 3 minutes. In Neftunia the apex time is reached in 180 seconds, and recovery completed in 60 minutes.

Mechanical response of Mimosa is due to differential contraction of the upper and lower halves of pulvinus. Erectile responses of Mimosa may be obtained by local stimulation of the upper half of pulvinus. Distinction of plants into sensitive and ordinary is arbitrary. Under suitable conditions, ordinary plants, so- called, may be made to exhibit motile response. By means of electric response it may be shown that every plant, and every organ of the plant, is sensitive and responds to stimulation by a definite electric change.

The sensitiveness of Mimosa to electrical stimulus is high and may even exceed that of a human subject. Greater excitatory efficiency of the break-shock—Additive effect of stimulus—Quantitative relation of additive effect—Effect of load— Measurement of work under different loads—Rate of work—Thermal chamber—Effect of temperature—Effect of increasing intensity In exciting Mimosa by means of induction-currents we may employ either the make- or break-shock. It has already

been stated that the break-shock is more efficient than the make- shock. That is to say, as we gradually push in the secondary nearer the primary, excitation is effected earlier with the break than with the make. I will now proceed to demonstrate this fact by experiments. For obtaining the record I employed a writer which had a vibration-frequency of 20 times per second. The make and break of the primary current was effected by a metronome. In the primary circuit an electrical signal (fig. 19) was also included, which marked at the base of the figure the moments when the

current was made and broken. When the current is made, an up-line is described by the writer attached to the signal. So long as the current is flowing, the writer remains in the up-position and draws a horizontal line (fig. 20). At the time of make it will be noticed that, owing to inertia, the writer was momentarily jerked some- what above the level of this up-position. This jerked line, therefore, always marks the moment of make, and the horizontal line at the higher level the continuation of the current. When the current is broken, the writer falls suddenly to its original level. Thus a jerked up-line indicates the moment of the application of the make-shock,

Fics. 20, 21.—Records showing greater efficiency of break-shock ; frequency of vibrating recorder is 20. Signal below shows by up movement ‘ make’ and by down movement ‘ break.’ and the down-line the application of the break-shock. In the two accompanying figures are given records of the effects of make- and break-shocks. In the record (fig. 20) the secondary coil was placed at the reading of °75 unit. It will be noticed that at ‘ make ’ there was no response. But there was response at ‘ break,’ which took place ‘1 second later, the delay being due to the latent period. In the next experiment, with the same plant, the coil was pushed into the reading of r. It will be seen (fig. 21) that excitation was here effective at “ make,’

a similar delay of ‘I second being again due, as in the previous case, to the latent period. Thus we see that while the stimulus of the feebler intensity of °75 was effec- tive at ‘break,’ it took the stronger stimulus of I to induce response at ‘ make.’ In the responsive tissue of the animal a single stimulus, by itself ineffective, is found to become effective on repeti- tion. In order to test whether this holds good in the case of the plant also, I carried out the experiments which I

Fic. 22.—Stimulus of intensity ‘5 became effective on being repeated four times. shall now describe. With a given specimen I found that a single make-and-break shock of intensity °75 was ineffective in inducing excitation. I then adjusted the secondary for intensity of 5, and made a reed-interrupter interposed in the primary circuit give a series of make-and-break shocks till the leaf responded by a fall. The interrupting reed was adjusted to vibrate five times per second and the number of interruptions is recorded below in the usual manner. It will be seen in the record given in fig. 22 that the make-and- break stimulus, which singly was ineffective, here became effective on being repeated four times.

Desiring next to observe the effect of still further reducing the intensity of stimulus with the same specimen, J adjusted the secondary for an intensity of ‘1. It must be remembered that this is the intensity of tetanisation, which is only one-tenth of what is perceptible to the human subject. Looking at fig. 23 it will be seen that even this very feeble stimulus became effective on being repeated 20 times. In carrying out this experiment I had expected in a general way that a feeble stimulus, to be effective, must be repeated a greater number of times. But I was not prepared for so strictly quantitative a result as came out in these two records. If the summated effect is to prove strictly additive, then effective excitation must be equal to the individual intensity multiplied by the number of repeti- tions. From the record in fig. 22 the effective excitation

Fic. 23.—Stimulus of intensity "1 became effective on being repeated twenty times. was seen to be ‘5 X 4 = 2. From the second record with the same specimen, in fig. 23, it is seen to be "I X 20=2. In other words, for effective excitation the number of additive stimuli varies inversely as the intensity of each. That this is true, within certain limits, is borne out by another set of results obtained from a different specimen, which was found to be somewhat more excitable than the former.

In order to vary the condition of the experiment I ad- justed the reed-interrupter to vibrate twice in a second. There was thus an addition here of the effects of single make-and-break shocks at intervals of half a second, in- stead of one-fifth of a second as in the last case. In fig. 24 is seen the record of the additive effect, the intensity of stimulus being ‘5. We find here that the stimulus became effective on being repeated twice. specimen, but using the reduced stimulus-intensity of °2. The result given in fig. 25 shows that the stimulus had to be repeated five times to become effective. We see once more in this experiment that the additive effect is strictly quanti- tative, and that the effective stimulation is constant under varying intensity of stimulus, being equal to the individual intensity multiplied by the number of repetition. In the