Plant Autographs and Their Revelations

And lastly, when, in any of the manifold ways, life is finally extinguished, will it be possible to detect the critical moment ? Does the dying plant give any unmistakable sign at the moment, after which its activities cease for ever? With all the preceding questions before us, we have to face the difficulty that plants are dumb and apparently passive creatures ; yet may they not give us some signals from which we can read their inner history ? Dumb people make themselves understood by movements of their fingers ; let us see how we can learn something of their vital condition. We tap a dumb person on the finger; that will be the ques¬ tioning shock or stimulus, and the finger twitches in answer. But to what extent? That will depend on how much alive he is. If he be very much alive, the twitch of his finger will be strong; if depressed, the same blow will cause a feeble twitch; if he is dead, there will be no answering twitch at all.

Suppose we had tied a pencil to his finger and had a piece of paper moving past the pencil. We should, in the experiments described, get three kinds of record. If the dumb person were fully alive, the line recorded by the twitch would be long; if in a depressed condition, the answering record would be short, and in death there would be no recorded movement. Thus from the length of the lines we should be able to judge of the condition of this dumb person.

Somewhat in the same way we shall be able to discover the vitality of the plant, by the amount of movement made in answer to a testing shock. While all plants have a limited power of movement, none have such active movements, comparable to those of animals, as the so-called ‘sensitive’ plants in general and Mimosa pudica in particular. A rep¬ resentation of a pair of leaves of Mimosa is given in fig. i : the left leaf is in the normal outspread position; the right leaf has fallen down under a shock.

It is a very fascinating sight to watch the movements of the Mimosa. In Bengal it is called the ‘coy maiden,’ because of its sensitive shrinking away from all touch. Chil¬ dren love to play with it by the hour, attracted by the impulsive movement in answer to every kind of irritation. The entire leaf of the Mimosa may be likened to our own horizontally outstretched arm with the fingers outspread. In order to avoid the use of long descriptive phrases, it is advisable to use short names for different parts of the leaf. The main leaf-stalk or petiole is like the arm, and the four

Fig. I. Leaves of Mimosa in expanded condition (left) and con¬ tracted condition after stimulation (right). sub-petioles are like the outspread fingers. Strike the arm, and it bends in contraction and the fingers close ; so also a Mimosa leaf, when struck, falls down, and the sub-petioles are drawn close together. The sub-petioles bear numerous pairs of leaflets which are also sensitive and close upwards. The movement is brought about by contraction of the sensi¬ tive cushion-like mass of tissue, the pulvinus, at the differ- the joints, which contracts like the muscle of the animal. There is a large pulvinus at the junction of the leaf with the stem; there are four smaller pulvini at the junctions of the four sub-petioles with the main petiole, and there are numerous minute pulvinules at the junction of the leaflets with the sub-petioles.

It is a wonderful leaf -mechanism, that of the Mimosa ; three distinct and differing motions in one single leaf. The main leaf-stalk drops down; the four sub-petioles are moved laterally and drawn together; and the small pairs of leaflets fold upwards. Is it any wonder that we should be fas¬ cinated by such a lively display of movement in a plant? If any one of the terminal leaflets be pinched or cut with a pair of scissors or touched with a hot wire, an impulse is found to travel inwards, that is to say, towards the stem. The different pairs of leaflets fold themselves upward one after another in regular succession; the sub-petioles are drawn together, and the main petiole drops down. If the irritation has been very great, the excitation passes along the stem, causing the fall of other distant leaves.

Now let us conflne our attention to the movement of the leaf-stalk and enquire why it should fall. Observe the cushion of tissue, the leaf- joint or pulvinus, which is the motile organ and also serves as a hinge for the movement of the leaf. The lower half of the pulvinus is highly sensi¬ tive, and the upper half less so: by careful measurement, I find that the lower half of the pulvinus is about 8o times more sensitive than the upper half. Not only so, but the lower half is more actively motile than the upper. Hence when we stimulate the leaf, both halves contract, but the greater contraction of the lower half causes the leaf to fall. The actual shortening of the lower half of the pulvinus is very small, but as the long leaf-stalk acts as a magnifying index, the movement of response becomes very conspicuous.

I shall now describe the method of recording the respon¬ sive movement under stimulation. A leaf of a Mimosa is attached by means of a fine thread to one end of the lever (fig. 2), which is pivoted on jewel bearings. The other end of the lever carries a small weight by which the string is kept taut. From the middle of the lever and at right angles to it, there extends a thin wire with a curved end serving as the writer. The tip of this wire just touches a

smoked glass plate on which the record is made, which is allowed to slide down under the action of gravity, at a rate regulated by clockwork. We may stimulate the plant by employing agents that excite human beings — a pinch, the application of a hot wire or of a drop of acid. These, however, may readily cause serious injury to the plant, and are, therefore, unsuitable, especially for repetition. A better way is to employ an electrical shock from an induction coil, which has the following advan¬ tages : first, it does not injure the plant, provided the shock is not too in¬ tense ; second, successive shocks can be kept absolutely constant; and third, the intensity of the shock can, when necessary, be gradually varied. In the induction apparatus there are two coils, a primary and a secondary. The in¬ tensity of the shock can be gradually increased by bringing the secondary nearer to the primary coil (fig. 3).

When the leaf falls on excitation it pulls down the right end of the lever, so that the writer moves to the left, marking an up-line. The leaf slowly recovers from the effect of the shock and again becomes erect, producing the down-curve of recovery. The up- and down-curves together constitute a single pulse of response. The up-curve, representing contraction, is com¬ pleted in a little over a second, whereas the recovery takes as long as twelve minutes.

In fig. 4 is shown a record of the response of Mimosa; it also represents vividly the course of our own sensation after getting a shock. We feel the pain of a blow almost immediately, but it takes some time for the painful sensa- tion to disappear. After the Mimosa has completed its recovery, we may give it a second shock of the same inten- Fig, 3. Arrangement for applying induction shock. K, key in the primary circuit. sity. If the surrounding conditions are the same as before, the second response is the same, and so are the succeeding

Fig. 4. Response-curve of primary leaf of Mimosa. The vertical lines below the record indicate intervals of one minute each. ones (fig. 5). Should the plant be depressed in any way, the fact is at once revealed by depression in its responses. I have not yet referred to a serious difficulty in obtaining accurate plant-records. In the case of muscle, the force of contraction is considerable, so the friction offered by the recording surface presents no serious difficulty. In the case of plants, however, the pull exerted by the motile organ is feeble. If the lever were allowed to remain in continuous contact with the smoked glass, the pressure would be enough to modify or arrest the movement. This difficulty is over¬ come by making an intermittent in¬ stead of a perma¬ nent contact, thus giving rise to a series of dots in the record instead of a continuous line. This has been accomplished by my Resonant Re¬ corder (fig. 6), the principle of which is based on sympathetic vibration.

If the strings of two violins are exactly tuned, then a note sounded on one will cause the other to vibrate in sympathy. Suppose we tune the writer V to vibrate a hundred times in a second; if now we sound a note which causes an air- vibration of one hundred times per second, the writer will vibrate in sympathy. It will no longer remain in continuous contact with the recording plate, but will deliver a succes¬ sion of taps a hundred times a second. The record will, therefore, consist of a series of dots, the distance between one dot and the next representing an interval of one hun¬ dredth part of a second.

With finer recorders it is possible to measure intervals of time as short as a thousandth part of a second. It will now be understood how, by the device of the Resonant Recorder, not only is the difficulty due to friction removed. but the record itself is made to measure the time, however short the intervals required may be. In this way I have succeeded in making the plant itself write an answering script to a definite testing stimulus ; and in order that the results obtained should not be influenced by any personal bias, arrangements were made that the plant

Fig. 6. Upper part of Resonant Recorder. (From a photograph.) attached to the recording apparatus should be automatically excited by an absolutely constant stimulus, and should make its own responsive record, going through its own period of recovery and repeating the same cycle without assistance or interference at any point on the part of the observer. Fig. 7 illustrates the complete apparatus. We have been accustomed to regard the plant-world as insensitive, Mimosa and a few other ‘sensitive plants’ being

considered to be exceptions. Our justification for dividing plants into these two classes is that while Mimosa makes an active movement when struck, ordinary plants are, to all seeming, passive and immobile. Yet may not this absence of motile indications be due not to want of sensibility, but to some opposing force which prevents the movement from being realised? Imagine two persons of equal size and strength tied back to back and both struck at the same time. Each would try to bend forward and thus balance and counteract the other’s movement. Yet should we be justified in regarding the two persons as insensitive to blows?

As regards the movement of Mimosa, let us for a moment think what would have happened if the upper half of the pulvinus were as sensitive as the lower. The two halves would thus have acted like opposing muscles, preventing all movement. Paradoxical as it may appear, this ‘sensitive¬ ness’ of Mimosa depends on the fact that one half of its motile organ is comparatively inactive. It is as if in our human illustration we had tied a strong man to a weakling ; the response of the former would then have been mani¬ fested to all eyes.

In the young stem of ordinary plants the tissue is almost equally sensitive all round, so that a contractile movement of one side is counteracted by that of the diametrically opposite side. Consequently, when symmetrically stimu¬ lated, the stem does not respond by bending to one side or the other. The whole length, however, undergoes a con¬ traction and shortening, but to so slight an extent as to be imperceptible. This contraction of ordinary plants may, however, be recorded by means of a Magnifying Recorder. The response of the ordinary stem is then found to be essen¬ tially similar to that of the sensitive Mimosa, though less pronounced.

The following is a striking experiment demonstrating the sensitiveness of ordinary plants. A tendril which has twined round a support is cut off and freed from the sup¬ port. It appears to be altogether insensitive; but on send¬ ing a strong electric shock through it, the spiral tendril begins at once to uncurl and exhibit violent contortions, like the writhings of a worm under torture. The explanation of this extraordinary phenomenon is as follows : the tendril, when it touches a twig, coils, as every one knows, around the support. The inner surface of the spiral becomes toughened by constant friction, while the outer side of the spiral remains relatively more sensitive. The outer or convex side of the tendril is, in fact, like the lower half of the pulvinus, and exerts a predominant contraction under excitation. The greater contraction of the convex side on electrical stimulation causes the uncurling of the tendril. Experiments such as these prove that the distinc¬ tion hitherto made between ‘sensitive’ and ‘ordinary’ plants is quite unjustified; the difference between them is only one of degree.

We shall see in the next chapter that plants are pro¬ foundly affected by changes in external conditions, and that it is possible to make the plants themselves reveal those subtle internal changes, by means of responses to question¬ ing shocks applied to them. The diverse movements of animals are carried out by a muscular mechanism which, in all of them, serves a definite purpose. When rapid movement is needed, the reaction is quick, while in sluggish animals the response is slow. This fact is corroborated by the study of the reactions of the muscles of dififerent animals. Thus, when we pass an elec¬ tric shock through the leg-muscle of the frog, it takes a certain time to start the muscular machinery into action, there being an appreciable loss of time between the appli¬ cation of shock and the resulting movement. The lost time is known as the latent period; in the frog, it is about i/ioo of a second. The latent period is very much longer in the sluggish tortoise. On what does this difference in the rapidity of the motor mechanism depend? An attempt will be made in a later chapter to answer this question.

In the human subject also, the reaction to a shock is not instantaneous, and a short time elapses between the stimulus and the response. The duration of this latent period, more¬ over, is not constant at all times of the day, even with the same individual. We are more or less sluggish when get¬ ting up in the morning, become fully alert during the middle of the day, and grow fatigued and slow to respond towards the end of the working day. Does the motile machinery of the plant exhibit the same peculiarities as that of the animal? Does the plant instan¬ taneously respond to the shock, or is there a definite latent period? For making these very accurate measurements, our sense is not keen enough; it is, therefore, necessary to make the plant itself record these minute fractions of a

second. It has been explained how the Resonant Recorder traces the response on the smoked glass plate as it slides down under the action of gravity. At a certain definite point during its descent, the moving plate establishes an electrical contact by which a shock is given to the plant. It must be remembered that, in the meanwhile, the vibrating writer is continuously tapping the time-marks. The suc¬ ceeding taps in the present experiment were at intervals of 1/200 part of a second. Though the plant had received the shock at the vertical mark (fig. 8), there was no imme-

Fig. 8. Record of Latent Period of Mimosa, with 200 Vibration diate answer. Ten dots had already been marked, still no response. Only after the fifteenth dot did movement of the plant begin to take place. The latent period of the speci¬ men was, therefore, 0.075 of a second. We shall next find out the effect of fatigue on the latent period. The latent period of a particular specimen, when fresh, was found to be o.i second. The second record was taken before the plant had recovered from its previous shock and was, therefore, in a state of fatigue. The latent period was now prolonged from o.io to 0.14 second. In the fresh condition, the answer is vigorous and the curve more erect.

Under fatigue, the reaction is sluggish and the curve more slanting (fig. 9). The plant thus becomes slug¬ gish and inactive after being fatigued ; when excessively fa¬ tigued, it altogether refuses to give any answer. In this dazed condition, it requires at least half an hour of complete rest to regain its equanimity. way in which fatigue is exhlb- prolonging the Latent Period, ited by the plant, namely, in the Upper record, normal; lower First three uniform responses obtained at intervals of 15 minutes. The second three, with the interval of rest shortened to 10 minutes, exhibit fatigue. On re¬ turning to an interval of 15 minutes, the last record shows restoration of nor¬ mal response.

sponse. In this experiment, three normal responses are first obtained, allowing an interval of fifteen minutes for complete rest. The resting period is then reduced to ten minutes for the next three responses, and a de¬ pression is noticed due to fa¬ tigue. We once more allow the plant a full period of recovery, and find that its normal vigour has been regained (fig. 10). In all this, we see how human¬ like is plant behaviour. In cer¬ tain respects, the plant may even be found to be superhuman! I was awakened to this one day when taking a record of Mimosa in my laboratory. The response I was getting was uniform; but all of a sudden, there was a de¬ pression, for which I could at first discover no cause, since all

the surrounding conditions appeared to be unchanged. On looking out of the window, however, I noticed a wisp of cloud passing across the sun. The plant had perceived the slight darkening which I had not noticed. As the cloud passed by, the plant recovered its normal exuberance, as the record shows (fig. II ). Not only is the plant affected by changes in light, but also by changes in temperature. There is a more or less definite temperature at which Mimosa is most active. This optimum temperature is found to be 33° C. or 91° F. When the temperature is lowered to 11° C. the plant gets numbed and more or less paralysed by cold; a few degrees of further lowering of temperature renders it quite insensitive. This is seen in fig. 12, which shows that the plant, when subjected to excessive cold, had completely lost its sensitiveness. The line below indicates the duration of the exposure to cold, after which the plant was allowed to regain the normal and favour¬ able temperature. But the paralysing effect persisted for a considerable length of time, as seen by the lack of response to shock given at the points marked with thick dots. When

the temperature is raised too high, the plant becomes oppressed with heat and its power of response undergoes a decline. At too high a temperature it gets something like a heat-stroke, and dies. Man’s pretensions as a highly sensitive being receive a rude shock when certain plants are found to be a great deal more sensitive than the lord of creation. It must, however, be admitted that the plant is practically deaf, for it is little affected by sound. The case is quite different in regard to its perception of different octaves of visible and invisible light-rays. Of the multitudinous ether waves, the human

Note sudden depression followed by abolition of excitability, also retina perceives only a single octave, lying between the red and the violet. The plant not only perceives visible light, but also invisible ultra-violet and wireless infra-red waves at the two opposite ends of the spectrum. In human beings the most sensitive organ is the unruly tongue! Before the invention of delicate electric instru¬ ments very feeble electric currents were detected by the tongue. This can easily be verified by placing two coins, one copper and one silver, above and underneath the tip of the tongue. As soon as the two coins touch, a feeble electric current is generated by the contact of the two different metals. The irritation caused by the passage of the current gives rise to a peculiar taste. An average European can per¬ ceive by his tongue a current as feeble as six micro-amperes — a micro-ampere being one millionth part of the unit of electric current. This value appears to be subject to certain variations, depending on racial characteristics. One might expect that the tongue of the Celt would be more excitable than that of the stolid Anglo-Saxon. In any case, my Hindu students exhibit a sensitiveness twice as great as that of an average European. But the plant Biophytum is found to be eight times more sensitive than a European and four times more so than a Hindu. These revelations are as unexpected as they are startling. They show that the pretensions of man and animal to greater sensitiveness than their despised ‘vege¬ table brethren’ do not bear the test of close scrutiny.

In this general description, I have been speaking of the perception of shock by the plant, the effect of fatigue in dull¬ ing the alertness with which it gives an answering twitch, and its awareness of the slightest fluctuation of light — an awareness surpassing that of human beings. The question naturally arises here whether consciousness should be imputed to plants. The difflculty lies in defining consciousness and in drawing a line below which conscious¬ ness does not exist, and above which it enters the domain of life.

There are two ways in which the boundary of knowledge in regard to life may be extended. We may start with the theory that the different types of living organisms have been specially created, with specific characteristics, physiological and psychological. Or, finding similarities between related forms, we may believe in gradual evolution; and thanks to Darwin’s life-work, this theory has been accepted as regards the origin of different forms of life. The evolutionary process has been active not only in the development of new forms, but also in the development of special mechanisms for the performance of the various vital functions. There still exists a long prevalent idea that the physiological mechanisms of animals and plants, because they have developed along divergent lines, are fundamentally different ; but the evidence adduced in this work will suffice to show that this idea is totally unfounded.