Researches on Irritability of Plants

Two opposite kinds of responses, negative and positive—Excitatory contraction, negative turgidity variation, fall of leaf, and concomitant negative electric variation—Positive electric response—Positive or erectile mechanical response—Dual impulses under different forms of stimuli—Exhibition of positive and negative impulses by different plants—Conditions for obtaining positive response—Characteristics of positive impulse—Masking and unmasking of positive effect—Laws of Direct and Indirect effects of stimulus.

WHEN the leaf of Mimosa is excited, the lower half of the pulvinus undergoes relatively greater contraction ; in conse- quence of this differential action the leaf falls down. There is a concomitant expulsion of water from the excited cells, with diminution of turgor. I have shown elsewhere ! that the excitatory reaction of plant tissue may also be detected by a method altogether different—namely, by means of electric variation. As regards the sign of electrical change, the excited point is found to become galvanometrically negative ; a similar electrical change is also known to take place in the excited animal tissue. The excitatory electro- motive change of galvanometric negativity is therefore the same in the plant and the animal.

Excitation in a plant tissue is thus characterised by concomitant effects of contraction, diminution of turgor or negative turgidity variation, mechanical fall of the leaf, and by the electrical response of galvanometric negativity. For the sake of clearness we shall designate this normal excitatory effect as the negative response. ' Bose: Comparative Electro-physiology (Longmans, Green & Co.); stimulation I was often surprised to find the occurrence— which appeared at first as an abnormal response—of gal- vanometric positivity preceding the true excitatory reaction of galvanometric negativity. Thus it appeared as if, in consequence of stimulation, there originated two distinct impulses, positive and negative, which travelled with different velocities. The former travelled faster and, reaching the responding point earlier, induced there the response of galvanometric positivity. The negative wave with its slower velocity of transmission reached the responding- point later and induced the true excitatory effect of galvanometric negativity.

It was also found that the negative effect was much the stronger of the two; in consequence of this, if the two impulses reached the responding-point about the same time, the positive was completely masked by the predominant negative. Hence in order to bring out the positive it was necessary to apply the stimulus at a distance. The slow- moving negative then lagged behind the positive to such an extent as not to mask it. The following conditions are found to favour the exhi- bition of the positive effect :—

(t1) The stimulus should be applied at a distance from the responding point; the response is then found to be diphasic, positive followed by negative. A distance may again be found where, owing to the enfeeblement of trans- mitted excitatory effect, the negative impulse fails to reach the responding-point ; in such a case there is an exhibition of only the positive effect. Conversely, if the stimulus be applied too near the responding-point, the negative effect alone is exhibited, the positive being masked by the predominant negative.

(2) From what has been said it is easy to understand that, with a very highly conducting tissue, the negative or true excitatory effect will be transmitted with great rapidity ; it will therefore mask the positive effect. Hence the which the velocity of transmission of excitation is relatively slow. (3) As the velocity of transmission of true excitation is greater under stronger stimulus, a feeble stimulation should be employed for the exhibition of positive effect.

The above are the results obtained from the electric mode of investigation. We have seen that the electric variation of galvanometric negativity corresponds to the excitatory reaction of negative turgidity variation, con- traction, and concomitant motile-response by the fall of the leaf. The positive electric response should connote effects the very reverse of these—positive turgidity variation, expansion, and concomitant motile-response of erection of the leaf. Hence if in consequence of stimulation two distinct impulses are sent. out with different velocities, we should be able to demonstrate their existence in an alto- gether different manner—namely, by means of two distinct mechanical responses of erection and of fall of the leaf.

The difficulty that confronts us in this demonstration lies in the relatively small amplitude of the positive response, which is liable to pass unnoticed unless a high magnification be employed. It is easier to demonstrate the existence of the positive impulse by the employment of a magnifying optical lever. A very light mirror is fixed to the fulcrum rod of a lever, one arm of which is attached to the motile leaf or leaflet. Under excitatory fall there is produced a rotation of the fulcrum rod with its attached mirror ; a spot of light reflected from the mirror thus exhibits a responsive down-movement, indicative of normal negative response by contraction. An erectile positive response, on the other hand, is recognised by the movement of the spot of light upwards. Responsive movement may in this manner be magnified from a hundred to a thousand times. The optical method is simple and efficient. It is also well suited for purposes of demonstration before a large audience.

a vecord of the positive response by means of a writing-lever. The weight of the lever stands in the way of obtaining any high magnification, especially in the case of leaflets, where the force of responsive expansion or contraction is very slight. By the employment of an extremely light recording- lever made of aluminium, I was able to secure a magnifi- cation of twenty times, which was the highest that could be obtained in the circumstances. With very good specimens this magnification is often sufficient to exhibit the positive effect. In order to reduce friction and obtain time-records, the recording-plate was made to oscillate to and fro once in a second or once in 2 seconds. The successive dots in the record thus indicate intervals of I or 2 seconds. It has sometimes been possible to employ the Resonant Recorder with long writing-index for recording the positive response of Mimosa. The vibration-frequency of the writer in such a case was five times in a second.

Having thus secured two different methods of observa- tion by means of optical and recording levers, I proceed to demonstrate :— (t1) That whatever may be the form of stimulation em- ployed, two impulses are transmitted, of which the positive travels with a higher velocity than the negative. (2) That such double impulses are exhibited not by any particular plant but by various species of plants. The phenomenon may therefore be regarded as universal.

The positive effect, as previously stated, may be separated from the negative by taking advantage of the different rates of transmission of the two impulses. The lag of one impulse behind the other may be increased (1) by taking a specimen in which the velocity of transmission of excitation is low, and (2) by the application of the stimulus at a distance from the responding organ. It should be remembered different species of plants, but varies to a certain extent in different individuals of the same species. Again, the transmission under feeble stimulus is slower than under strong stimulus. This accounts for the result frequently obtained—that the propagation-time is much quicker under the strong stimulus of thermal shock than under the moderate stimulation by constant current.

Another interesting fact which has attracted my attention is that the velocity of transmission is, generally Fic. 96.—Positive followed by negative impulse in Bizophytum, caused by single indirect thermal stimulus. In this and in the following records, ‘down’ curve represents positive, and ‘ up’ curve negative, response. Frequency of oscillation once in a second, speaking, slower in the stem than in the petiole. There also seems to be a loss of time when excitation has to pass from the stem to the leaf. Taking advantage of these facts we may, when desired, obtain long periods of transmission by applying stimulus of moderate intensity on the stem.

I will now describe experiments giving quantitative results, which demonstrate the occurrence of two distinct impulses under different forms of stimuli, the experimental specimen being Biophytum sensitivum. Thermal stimulus.—The electro-thermic stimulator was employed for the application of thermal shocks. The record was taken by means of the Oscillating Recorder, the fre- quency of oscillation being once in a second. Successive dots are thus at intervals of a second. In the experiment, the record of which is given in fig. 96, stimulus was applied on the petiole at a distance of 50 mm. from the responding leaflet. It is seen from the record that in answer to the stimulus two distinct responses of opposite signs occurred in succession. The positive or erectile response (repre- sented by down curve) is seen to have occurred 1°5 second after the application of the thermal shock ; the excitatory or negative response took place much later, that is to say, 22 seconds after the shock. The velocity of the positive impulse is here 33 mm. per second, that of the negative being 2°3 mm. per second.

From the experiment just described it is clear that a single stimulus gives rise to two impulses, positive and negative. The positive travels at a faster rate and gives rise at the responding-point to the erection of the leaflet indi- cative of positive turgidity variation. The negative or the excitatory impulse travels at a slower rate, inducing at the responding organ negative turgidity variation, contraction, and the fall of leaflet. It was stated that the transmission-time is relatively long when stimulus is applied on the stem instead of on the petiole. This will be seen clearly from the results of experi- ments which I shall now describe. Thermal stimulus was first applied on the stem, the distance of the responding leaflet being 10 mm. of stem and 20 mm. of petiole. The positive response took place 3 seconds and the negative 2I seconds after the application of the thermal shock. Stimulus was next applied on the petiole at a distance of 20 mm, Had the velocity of transmission in the stem and the petiole been the same, then the negative impulse would have reached the leaflet after an interval of 3° x 21 seconds or 14 seconds. Instead of this, the transmission-period in the petiole was found to be much shorter—namely,

TABLE I.—SHOWING DIFFERENCE OF TIMES OF TRANSMISSION THROUGH Chemical stimulus.—The next record was taken under the stimulus caused by the application of a drop of hydro- chloric acid on the petiole, at a distance of 30 mm. from the Fic. 97.—Positive response followed by negative in Biophytum by the application of chemical stimulus. Successive dots at intervals of I second. responding leaflet. The record (fig. 97) shows that the positive response occurred zr second and the negative 14 seconds after the application of the stimulus.

Induction shock.—Stimulus was applied on the petiole at a distance of 30 mm. The positive response took place 2 seconds and the negative 14 seconds after the application of the stimulus. Constant current.—Stimulation was here induced by the “make ’ of kathode. The intervening distance was 20 mm. : the. positive impulse traversed this length in 3 seconds, the negative requiring a longer period—namely, 9 seconds. Condenser discharge-—The distance of the responding leaflet from the point of application of stimulus was 25 mm. : the positive impulse reached the leaflet 4 seconds after the discharge-shock, but the transmission time of the negative was much longer—namely, 15 seconds.

TABLE II.—SHOWING TRANSMISSION OF POSITIVE AND NEGATIVE IMPULSES IN Biophytum UNDER DIFFERENT FORMS OF STIMULI on different specimens of Biophytum. In some cases the stimulus applied was on the stem, the distance mentioned being the sum of the length of stem and petiole through which the two impulses were transmitted. It has thus been shown that under the action of various modes of stimulation two distinct impulses are transmitted, of which the positive travels faster than the negative. The specimen employed for these demonstrations was Biophytum sensitivum. I next proceed to show that these results are not confined to any particular plant but are universally present.

Averrhoa cavambola.—The velocity of transmission of excitation in the petiole of this plant is low, being I mm. per second or even less. In the first experiment of the series I applied the stimulus of induction-shock at the moderate distance of 10 mm. from the responding leaflet, and obtained automatic re- cord by means of the Oscillating Recorder. The successive dots here Fics. 98, 99.—Positive followed by nega- are at intervals. of noticed that a respon- sive movement of erection took place after an interval of three dots or 6 seconds; the negative response occurred after the much longer interval of 20 seconds (fig. 98).

In the next experiment with a different specimen stimulus was applied at a distance twice as great as in the previous case, that is to say, 20 mm. The positive response took place 14 seconds, and the true excitatory effect 48 seconds, after the application of the stimulus (fig. 99). In an experiment where the chemical mode of stimu- lation was employed, the distance to be traversed was 40 mm. The positive impulse reached the responding leaflet Ig seconds, and the negative impulse 50 seconds, after the application of the stimulus.

Thermal stimulus was applied in another experiment at a distance of 70 mm. The transmission periods for the positive and negative impulses were 22 seconds and 65 seconds respectively. The negative impulse thus lagged behind the positive by as much as 43 seconds. Mimosa pudica.—It was stated that in speci- mens like the petiole of Mimosa, where the velo- city of transmission of excitation was high, the positive response was _ Fic. 100.—Records showing positive liable to be masked by response in Mimosa followed by the pre Beeainant nega- negative. Stimulus was applied on

the petiole 30 mm. from pulvinus. tive. It is therefore only Vibration-frequency 5 per second. on rare occasions that I obtained a positive response by stimulating the petiole of Mimosa. This is seen in fig. 100, where stimulus of induction-shock was applied at a point on the petiole 30 mm. from the responding pulvinus. The Resonant Recorder having a long writing-index was employed for obtaining the record. The vibration-frequency of the writer was five times in a second, hence successive dots represent time-intervals of -2 second. It will be seen that the positive response took place ‘6 second and the negative 3°2 seconds after the application of the stimulus.

distance on the stem, the transmission period could be made as long as desired. This will be seen in fig. ror, where thermal stimulus was applied on the stem of Mimosa at Fic. 101.—Positive response, preceding the negative in Mimosa. Stimulus was applied on the stem. Frequency of oscilla- tion 1 per second. some distance from the pulvinus. The record was taken on a slowly moving plate, by means of the Oscillating Recorder, the successive dots being made at intervals of a

TABLE III.—PrErR1Iops oF TRANSMISSION OF PosITIVE AND NEGATIVE IMPULSES IN THE PETIOLE OF Averyhoa AND STEM OF Mimosa second. The positive response is here seen to take place 4 seconds, and the negative 41 seconds, after the application of stimulus. The negative impulse in this case lagged as much as 37 seconds behind the positive. In Table III will be seen the transmission-periods of positive and negative impulses in different specimens of Averrhoa and Mimosa.

I will now describe experiments which bring out the conditions which are favourable for the manifestation of either the positive or the negative response :— 1. Positive response is more easily obtained under feeble stimulus.—This is demonstrated by the following experiment Fic. 102.—Effect of intensity of stimulus in the induction of positive or negative response. Lowest record under stimulus-intensity of 1, middle record under 5, and the uppermost record under 8 units. Vibration frequency 5 times per second.

on Mimosa, where successive stimuli were applied at the same distance on the stem, the intensity being gradually increased in a known manner. The distance of application was 10 mm. The vibration-frequency of the writer was 5 times per second. The first and the lowest record of the series (fig. 102) was taken under the stimulus intensity of I. It will be seen that under this relatively feeble stimulus a positive or erectile response, indicative of positive turgidity

variation, was alone induced °7 second after the application of stimulus; there was no indication whatsoever of the later occurrence of the negative response. After the usual interval of 20 minutes the next or middle record was taken under the enhanced intensity of stimulus of 5 units. We now observe the appearance of negative following the positive. The positive took place ‘6 second and the negative g'4 seconds after the application of stimulus. Finally, when the stimulus-intensity was raised to 8 units, the positive response took place at the same interval as before, namely ‘6 second, but the negative or excitatory response took place earlier than in the last case, that is to say, after an interval of 4°6 seconds, instead of 9°4 seconds.

In another experiment with a different specimen, the positive response took place *5 second after the application of stimulus of intensity 2; there was no negative response. The point of application of stimulus was kept always TABLE IV.—EFFECT OF INTENSITY OF STIMULUS ON THE POSITIVE AND ; eT. . Transmission . Stimulus Transmission period F Specimen. : : : period for intensity. for + impulse. 3 : — impulse. second Negative ( ii | absent I j 5 6 9°4 seconds | 8 6 4°6 seconds { 2 5 Negative II absent | 4 “5 12 seconds

the same—namely, 10 mm. When the intensity of stimulus was increased to 4, the negative response was found to occur after the positive. The former took place ‘5 second, and the latter 12 seconds, after the application of the stimulus. From these experiments it is seen that while under feeble stimulus we obtain only the positive response, on increasing the intensity of stimulus the negative or excitatory response makes its appearance in succession to the positive. Another noteworthy fact is that while an increasing intensity of stimulus enhances in a marked manner the velocity of transmission of the negative or excitatory impulse, it has

Fic. 103.—Effect of diminishing the distance of point of application of stimulus. Stimulus applied at a distance of 20 mm. gives rise only to positive response (lower record). Reduction of distance to half gives rise to positive followed by negative response (upper record). Vibra- tion frequency five times in a second. little or no effect on the velocity of the positive impulse. Thus in the first experiment of the series, while the trans- mission-period of the negative impulse was_ shortened from 9°4 seconds to 4°6 seconds by the stimulus increasing from 5 to 8 units, the transmission period of the positive impulse remained unchanged at ‘6 second.

2. Positive response 1s more easily obtained under a feeble stimulus applied at a distance-—For the demonstration of this, a feeble stimulus is applied on the stem of Mimosa at a distance of 20 mm. from the responding pulvinus. The response is only positive, occurring I second after the application of the stimulus. When the distance of application is reduced to half, we find that the negative or excitatory response makes its appearance in succession to the positive. The positive is seen to take place ‘8 second and the negative ro seconds after the application of the stimulus. It will be noticed that the reduction of the distance to half causes only a slight diminution in the transmission-period of the positive impulse—from 1 second to ‘8 second. The transmission-period of the negative impulse, on the other hand, undergoes as we have seen a rapid diminution with diminishing distance.