Researches on Irritability of Plants

The after-effect of moderate stimulation converts the polar reaction of a given type to one of higher type. Under the same exciting current the resulting type of reaction depends on the age of the specimen. A moderately young specimen gives higher type of reaction than one which is very young or one which is old. On account of modification of the tissue, its excitability to anode-break undergoes progressive diminution. In . consequence of this, modified types of reaction Km Am and Km Kb Am are sometimes found to occur in Mimosa.

The polar reactions in Protozoa are not exceptional. Similar effects are observed in Mimosa under specific conditions. The Oscillating Recorder—Latent period of Biophytum—Refractory period—Response on ‘all-or-none’ principle—Multiple electrical response to a single strong stimulus—Multiple mechanical response to strong stimulus in Biophytum and Averrhoa—Continuity of multiple and automatic response—Ordinarily responding Biophytum con- verted into automatically responding condition by excess of stored energy—Automatically responding Desmodium converted to ordinarily responding condition by depletion of stored energy.

WE have studied in detail the responsive characteristic of the leaf of Mimosa. We next take up the study of the responses of the leaflet of Bzophytum. In doing this, we shall observe a certain new class of phenomena of great theoretical importance come into play. The difficulties encountered here, however, in the taking of automatic records are extremely great. The leaflets are very slender, and the pull exerted in the course of the excitatory fall is very slight. I overcame the difficulties in the taking of the record by the use of the Optical Lever, in which a moving spot of light either traced the response- curve on a travelling photographic-plate, or was itself followed by the pen of the observer on a moving drum covered with paper. These devices have their disad- vantages, and I was desirous of contriving means to secure records at once simple, effective, and perfectly automatic.

The Resonant Recorder was not found specially suit- able for the tracing of Biophytum movements, inasmuch as the lightest steel wire was still too heavy for the slight pull exerted by the leaflet ; the only lever that was sufficiently light to give a slightly magnified record of these movements was some special kind of dry grass haulm, which combined rigidity with excessive lightness. I have tried the finest feathers from small birds, but these were not so efficient

Fic. 131.—Photograph of the Oscillating Recorder, reduced to one- fourth the natural size. as the selected specimen of grass which I was so fortunate as to obtain later. Though in this I was successful in obtaining a recording-lever, there yet followed the difficulty that owing to its being non-magnetic it could not be thrown into resonant vibration by the electro-magnet. I had therefore to devise some arrangement by which the recording-plate could be maintained in a state of oscillation. The recording- plate was consequently made as light as possible, using the glass plate employed for covering magic-lantern slides ; the carrying-frame was made of aluminium. By means of an electric motor and an eccentric device, the plate-carrier was made to oscillate to and fro, the frequency of oscillation being regulated by an adjustment of the electrical current (fig. 131).

The plate-carrier has small wheels which run between horizontal rails, above and below. The recording-plate thus travels in a horizontal instead of vertical direction. There is also a knock-over key or trigger arrangement, not shown in the figure. During a particular part of the travel of the plate the trigger is released, causing a single break-induction-shock to pass through the plant. The moment of stimulation is marked in the usual manner on the travelling-plate, and the number of dots intervening between this mark and the beginning of response enables us to deter- mine the latent period when the stimulation is direct, or the velocity of transmission when the stimulation is indirect. — In the latter case it is of course necessary to know the distance between the point of application of stimulus on the midrib and the responding leaflet.

For the determination of the latent period, and of the velocity of transmission, the oscillation-frequency of the plate should be about Io times in a second. But for the mere obtaining of response-records the frequency of oscilla- tion need not be high. Taking a record of Biophytum on a fast-moving plate, with a recorder having a frequency of oscillation of 10 times a second, it is seen (fig. 132) that there are four spaces between the incidence of stimulus and the initiation of response; it would thus appear that the latent period of the leaflet is *4 of

a second. This seems very high compared with the latent period of Mimosa, whichis ‘1 second. Perhaps this difference may be due to certain characteristics that mark the response of Biophytum. In animal tissues it is found that while the singly responding skeletal tissue of the frog has a latent period of about ‘or second, its multiple responding cardiac tissue has a latent period of about ‘I second, or ten times as long. We shall presently see that the leaflet of Biophytum exhibits multiple response.

Ifasecond stimulus be applied a short time after the first, it is found that it is ineffective unless a certain minimum Fic. 132.—Record giving the latent period of the leaflet of Biophytum. Frequency of vibration of recorder 10 times per second. interval of time elapses between the two. In these circum- stances the leaflet takes no account of the second stimulus, becoming apparently refractory to it. The minimum interval that must elapse before the second stimulus can be effective—the refractory period—varies somewhat in different specimens. In Biophytum it is usually Io seconds.

In a previous chapter I have given the record of a response in Biophytwm under a single stimulus (fig. 15). We will now study the effect of increasing intensities of stimulus on the amplitude of response. We have seen that in the case of Mimosa increasing intensities of stimulus induce, generally speaking, increasing amplitudes of response, which however reach a limit. The same is true of the responses of a skeletal muscle of frog. In carrying out experiments on Biophytum, I first determined the minimal intensity of induction-shock that was effective in inducing response. This happened when the intensity of stimulus was ‘r unit. The record of this response under minimal stimulation was then taken. After this, a second response to stimulus which was ten times as strong, was recorded. It will be seen that both minimal and maximal stimuli induced practically the same effect (fig. 133). In other words, we have here an example of what is

Fic. 133.—Record of responses of Biophytum to stimuli ‘1 and I unit, respectively. known as response on the ‘all-or-none ’ principle. The leaflet either responds to its fullest or not at all. In the various characteristics which have just been described, the responses of the vegetal organism bear a curious resemblance to those of the cardiac tissue of the animal. In the response of the animal heart the latent period is relatively long, and it exhibits a similar prolonged refractory period. Its responses are also on the ‘ all-or-none’ principle—it either responds to the utmost or not at all.

Returning to Biophytum, we have seen that in order to induce any response a certain minimal intensity of stimulus was necessary. But when the intensity of this stimulus was further increased, the outward expression, or response of the leaflet, remained apparently as before. What then became of the excess of energy that impinged upon it in the form of stimulus ? It is not necessary to suppose that in every instance the whole energy imparted by stimulus gives rise to useful work. Some of it may be wasted as heat. But, on the other hand, it is conceivable that the excess of this energy may be stored up, for the time being, to find subsequent expression. To take a physical illustra- tion, the energy stored up in a compressed spring may, on release, give rise to long-continued and rhythmic oscilla- tions. The question arises, then, whether in the leaflet of Biophytum, impressed by an excess of stimulation, there may be any analogous storage of energy. Supposing this to take place, the superfluous energy might be utilised to do some internal work not discernible by the observer ; or it might, in favourable circumstances and in the presence of suitable motile indicators, find expression in rhythmic movements.

I have referred elsewhere to another mode of recording excitatory. response in plants—namely, the electrical. Using this, I have frequently observed that although under a moderate stimulus a single stimulus gives rise to a single electric response, yet under a strong stimulus there would arise a series of responses. Thus, while a feeble stimulus induced a single response, a strong stimulus gave rise to multiple responses. Having this in mind and the peculiar characteristics of Biophytum response, I expected to demonstrate the occur- rence of multiple responses by means of mechanical move- ments. It had already been noticed that Biophytum when strongly excited closed its leaflets, not by one but by two successive twitches. It appeared to me that this curious phenomenon was parallel to the multiple response of rhythmic animal tissues ; and I expected, if this were so, that instead of two it would be possible to obtain from it a long series of rhythmic responses comparable with the multiple rhythmic responses in animal tissues. In order to obtain this in the case of Biophytum it was necessary to prevent the complete closure of the leaflet, in consequence of which further exhibition of mechanical response is rendered impossible. This was secured by applying a light counterpoise in the

second arm of the lever. The effect of this was by exerting a tension to hasten recovery, and thus oppose the complete closure of the leaflet. Fic. 134.—Response of Biophytum leaflet to stimuli *1, *5, I, and 2. Response is seen to be multiple with the last. Fic. 135.—Multiple response in Biophytum under a single strong electric shock. titative stimuli of increasing intensity, the stimuli being applied at intervals of 3 minutes (fig. 134). The successive stimuli were of intensities ‘I, ‘5, I, and 2. Owing to incom- plete recovery during the intervening resting-intervals,

the base-line is seen to be shifted upwards. The amplitude of successive responses is about the same, though the stimuli are increasing. At the application of the fourth Fic. 136.—Multiple response in Avervrhoa under a single strong electrical stimulus. Vertical marks below indicate time- interval of 1 minute in this and in the following records. stimulus, of intensity 2, we find that the response becomes multiple. Thus we see that while a single stimulus of moderate intensity gives rise to a single response, a strong

Fic. 137-—Multiple excitation in Biophytum under the action of constant light. stimulus gives rise to a multiple series of responses. In fig. 135 are depicted multiple responses in Biophytum to a very strong electrical stimulus, there being four multiple responses in the course of 6 minutes. In fig. 136 are shown the multiple responses obtained in Averrhoa carambola under a single stimulation caused by strong induction-shock. Here there are six responses in the course of 15 minutes, the average period of a single pulsation being 2°5 minutes.

We have also seen multiple responses induced in Bio- phytum under the action of constant current (cf. fig. 124). Multiple responses also take place in Biophytum and in Averrhoa under the action of constant light. I give a Fic. 138.—Multiple response under the action of single strong thermal shock. record (fig. 137) in which is seen the occurrence of five multiple-responses in the course of 8 minutes under the continued action of light from an arc-lamp.

I have also obtained multiple responses under other forms of strong stimulation. In fig. 138 is given a record of multiple responses induced by the action of a strong thermal stimulus. In certain other cases I obtained with Biophytum as many as sixteen recurrent responses under a single thermal shock. In fig. 139 is seen a series of multiple responses induced by strong chemical stimulation. This was caused by the application of a drop of hydrochloric acid on the petiole.

From these experiments it is clear that a rhythmic series of effects need not have a periodic antecedent cause. We see on the other hand that under strong stimulation there is not only an immediate response but that the surplus of energy remains over and is held latent by the tissue to be given out later in the form of recurrent responses. It is the excess of latent or Internal Energy that gives rise to phenomenon of multiple response. Sometimes we may not have noticed the antecedent external stimuli the

Fic. 139.—Multiple response induced by strong chemical stimulation. absorption of which had contributed to that storage of internal energy which gave rise to the rhythmic activity. In these circumstances the pulsations appear to us as spontaneous or automatic. Under natural conditions, the plant is exposed to the action of various stimuli supplied by its environment. It is exposed to warmth, to the action of light, to internal hydrostatic pressure, to the action of various chemical agents—present in it or absorbed by it. We have seen that each of these factors exerts its stimulating action independently. From the joint action of these external sources of stimulation, the energy stored up by the plant may become sufficiently great to cause an excitatory overflow. It will thus be seen how, by the cumulative effects of these various stimuli, the excitability of the

plant may become so excessive as to manifest itself by outward response, apparently automatic. Thus I have obtained from Biophytum seemingly auto- matic pulsation by subjecting a vigorous plant to the favourable conditions of light and warmth. In a particular case the favourable temperature was found to be 35° C. When this was lowered to 29° C. the pulsations came to a stop. It is thus clearly seen that there is no strict line of demar- cation between multiple and automatic responses so called. An ordinarily responding plant like Biophytum, which gives a single response to a single moderate stimulus, and multiple responses to a strong stimulus, will in very favourable circum- stances, that is to say, when it has absorbed an excess of energy from without, become automatically responding.

Conversely, an automatically responding plant in un- favourable circumstances is found to be converted to the condition of an ordinarily responding plant. The leaflet of Desmodium gyrans under favourable conditions is found to execute pulsatory movements which appear to be spon- taneous. But when this plant is subjected to unfavourable conditions, then its spontaneous rhythmic activity comes toastop. In this condition of standstill, the reaction of the leaflet is like that of Biophylum. It then gives rise to a single response to a single moderate stimulus, and multiple responses to a strong stimulus.

There is thus seen a continuity in the multiple and auto- matic responses. Biophytum is equivalent to Desmodium when brought to a state of standstill by depletion of storage of energy. Pulsating leaflets of Desmodium may, on the other hand, be regarded as equivalent to Biophytum with an overflow of energy. In plant a single moderate stimulus is found to give rise to a single electric response. A strong stimulus, on the other hand, often gives rise to a multiple series of electric responses.

In Biophytum, similarly, while a moderate stimulus gives rise to a single mechanical response, a strong stimulus gives rise to a multiple series of responses. These multiple responses are induced by various modes of strong stimulation such as induction-shock, constant current, strong light, thermal shock, and chemical excita- tion. Certain plant tissues have the power of holding the excess of stimulus latent, to be given out later in the form of recurrent responses.

The characteristics of the response of Biophytum are like those of cardiac tissue of the animal. Both are characterised by long refractory period and response on ‘ all-or-none ’ principle. A single moderate stimulus gives rise to a single response in both, and a strong stimulus gives rise to a multiple series of responses. There is no strict line of demarcation between the phenomena of multiple and of automatic response. In very favourable circumstances of absorption of excess of energy from without, an ordinarily responding plant like Biophytum will become converted into an apparently automatically responding plant like Desmodium.

Conversely, under unfavourable conditions, that is to say, when the sum-total of its energy is below par, an automatically responding plant like Desmodium will become converted into an ordinarily responding plant like Biophytum. Its leaflets then come to a state of standstill. Activity of detached leaflet of Desmodium—Pulsation maintained uniform under constant internal hydrostatic pressure—The plant chamber— Time-relations of pulsating movement derived from dotted record— Significance of down and up movements—Systole and diastole— Table showing rates of movement of Desmodium leaflet at different phases.

WE have hitherto studied the responsive movement in sensitive plants, where such movement was initiated by a directly exciting stimulus. We shall now take up the ‘consideration of another class of move- ments, which are apparently auto- matic or without any immediately preceding cause. Incertain plants we observe what are known as spontane- ous movements, of which Desmodium gyvrans or the telegraph-plant fur- nishes an example. This telegraph-plant grows wild in the Gangetic plain; its Indian EE aoa name is Bon Charal, or ; forest churl,’ iwo small lateral tue popular, belief being that it leafletsexhibitspon- dances to the clapping of the hands. taneousmovements. There is, however, no foundation for this belief. It is a papilionaceous plant with trifoliate leaves, of which the terminal leaflet is large, and the two lateral, very small (fig. 140). Each of these is inserted on the petiole by means of pulvinule. The lateral leaflets are seen to execute pulsating

unlike the rhythmic movement of the heart. The extra- ordinary similarity under various conditions of the rhythmic reactions in the plant and the animal will be seen in the experiments which will be presently described. It will, moreover, be shown that, strictly speaking, there is no such thing as spontaneous movement. The energy which expresses itself in pulsating activity is derived by the plant either directly from immediate external sources or from the excess of such energy already accumulated and held latent in the tissue. When the storage is exhausted, the spontaneous movement, so called, is found to cease. In this condition of standstill the rhythmic activity can be renewed by an accession of fresh stimulus.

In the intact plant, under favourable conditions, these spontaneous movements are observed to take place more or less continuously ; but there are times when they come to a standstill. For this reason and because of the fact that a large plant cannot easily be manipulated as a whole and subjected to the various changing conditions which the purposes of investigation demand, it is desirable if possible to experiment with the detached petiole carrying the pul- sating leaflet. The required amputation, however, may be followed by arrest of the pulsating movements. But, as in the case of the isolated heart in a state of standstill, I find that the movement of the leaflet can be renewed in the detached specimen by the application of internal hydrostatic pressure. Under these conditions, the rhythmic pulsations are easily maintained uniform for many hours. This is a great advantage, inasmuch as in the undetached specimens the pulsations are not usually found to be so regular as they now become. So small a specimen, again, can easily be subjected to changing experimental conditions, such as variations of internal hydrostatic pressure and temperature, application of different drugs, vapours, and gases.

The petiole after detachment should be put in water immediately, to prevent complications arising from drying of the cut end. It is then mounted water-tight, in the shorter open end of a narrow u-tube filled with water. For this purpose an indiarubber cork is taken, and a slit made from circumference to centre. The petiole is then slipped into the centre of the cork, which is gently forced on to the short open end of the u-tube. The longer end of the u-tube partly consists of indiarubber tubing. By raising or lower-