Plant Autographs and Their Revelations

Sucessive dots in down or expansive temperature. For obtain- part of curve represent rise of tern- ing a death-record, we be employed with equal success) and subject it to a gradual rise of temperature in a water-bath. The expansive up-movement is represented in this record by a down-curve, while the contractile down- movement is shown by an up-curve. The experiment was commenced at 25° C. The effect of increasing warmth of the bath on the plant, as on our¬ selves, is an expansive relaxation, and the curve is seen to move downwards. This continues until we note a short

pause in the movement at or about 6o° C. Then a sudden spasm — the death-spasm — occurs, the recording lever being jerked up with convulsive violence (fig. 39). Observe how sharp is the point of inversion. The entire record, then, is the curve of life and death. If the plant be cooled before the point of inversion, it survives; but once that point is passed, there can be no recovery. It was at the sharp turn- ing-point that the conflict between life and death was decided.

In young and vigorous specimens this death-throe is very violent ; with advancing age it is less so. In extreme old age the record of the curve of life passes almost imperceptibly into that of death. If the plant has been fatigued by previous over-stimula¬ tion, the death-point is reached at a lower temperature. In a particular case the fatigued plant died at 37° C., the death- point being thus lowered by as much as 23° C. The plant may be regarded as a colony of living units, and it is possible to kill a portion of it without destroying the rest. What would happen if a portion of the plant were killed? If there is an intense excitation at death, then this should be transmitted and cause appropriate response of motile organs at a distance. This was demontrated in the following experiment.

The lower end of a shoot of Mimosa was placed in the bath and the temperature raised. At the critical point, an intense excitatory impulse was generated which, travelling upwards, caused the fall of all the upper leaves in serial succession. That this excitation was due to the local death of the immersed portion was proved by the fact that the upper leaves re-erected themselves in the course of twenty minutes. Cooling the water of the bath and reheating it once more did not give rise to any more impulse, for the portion immersed was already dead. The plant was next lowered about an inch in the bath, and repetition of the experiment caused a new excitatory impulse, due to death of

the fresh portion of the living stem ; the response to trans¬ mitted excitation was exhibited once more by the serial fall of the leaves. This offers conclusive proof of an intense excitation which occurs in the tissue at death. Similarly, a portion of tissue subjected to poison gives out an excitatory impulse at the moment of death. This occurs earlier under strong or virulent poisons. It is delayed under the action of a dilute poison, since this requires pro¬ longed application to bring about the fatal result.

Certain considerations seem to show that the phenomena of life and death are not entirely antithetic, but that there is a continuity which bridges over the chasm. After each shock, the organism becomes irresponsive or dazed for a time, after which it gradually recovers. The duration of insensibility is prolonged with the intensity of the shock. Sensibility and insensibility — tokens of life and death — thus alternate. Our life is, in reality, a succession of incipient deaths ! From a moderate stimulation there is a quick resto¬ ration, but after an excessively intense shock there is no recovery. Death is an extreme case of stimulation.

In records of response of Mimosa taken under moderate, strong, and excessive stimulation, we find certain character¬ istic differences. In the first case, the recovery is completed in the course of fifteen minutes, as seen in the curve of recovery reaching the axis or base-line (see fig. 4). With a still stronger stimulus, the recovery is prolonged for an hour, the recovery-curve meeting the axis at a still greater distance. Up to this time, there is possibility of restoration. But the shock may be so strong as to prove fatal, and the violent spasmodic contraction proves to be the spasm of death. The line of recovery is now parallel to the axis, and never meets it.

Let us examine the suggestive case of a real image of an object formed by a concave mirror. The reflected rays cross and meet at the axis, forming a real image. As the object is brought nearer and nearer, the image, like the line of recovery under stronger stimulation, is projected to a greater distance. A time comes when the reflected rays never meet and no image can be formed on this side of the surface of the mirror. Has it then completely disappeared? Not so, for the phantom is now transferred to the other side of the mirror ! And after the supreme shock of death, though there is no restoration of life on this side of the Great Mirror by which we see Nature reflected, is it possible that there may be a restoration and renewal on the other side that is hidden from us ?

These our mute companions, silently growing beside our door, have now told us the tale of their life-tremulousness and their death-spasm in a script that we can read. May it not be said that their story has a pathos of its own beyond any that we have conceived ? In realising this unity of life, is our final sense of mystery deepened or lessened? Is our sense of wonder diminished when we realise in the infinite expanse of life that is silent and voiceless, the foreshadowing of more wonderful com¬ plexities ? Is it not rather that Science evokes in us a deeper sense of awe? Does not each of her new advances gain for us a step in that stairway of rock which all must climb who desire to look from the mountain-tops of the spirit upon the promised land of truth?

It has been shown that the plant responds to even a feeble stimulus. Each such responsive movement in the plant, as in the animal, can be referred to a definite cause, being due to an antecedent stimulus. There are other movements which are more mysterious than this, inasmuch as they are produced seemingly without any cause. Eor instance, it would seem that the heart beats of its own accord. A sudden contraction of the heart is followed by an expansion, and this rhythm is maintained spontaneously throughout the duration of life. These automatic movements are referred to some unknown and mvsterious inter-

nal cause. What, then, is the solution of the mystery of this automatism? Although such automatic move¬ ments are usually associated only with animal life, yet similar activities are found even in plants. An example of this is found in the Telegraph-plant, Desmodium gyrans, which grows wild on the Gangetic plains. And surely no phenomenon could be more surprising than the ceaseless activity of the leaflets of this plant. Its compound leaf has three leaflets, a large terminal, and two small lateral ones (flg. 40). The small leaflets move up and down like the semaphore formerly employed for telegraphic signalling. On a warm day the leaflets dance continuously up and down, and the Indian peasant’s belief is that they dance to the

The two small lateral leaflets exhibit spon¬ taneous movements. snapping of one’s fingers. In reality, they require no such encouragement, for the minute pulvins at the joint of the leaf¬ let undergoes, of its own accord, a sudden contraction, followed by slow expansion and recovery. for more than twen¬ ty-four hours at a time. The record for four hours is given in fig. 41. The extraordinary uniformity of these pulsa¬ tions is indeed very remarkable.

Automatic Pulsations in Animal and Plant There is an evident similarity between the automatic Fig. 42. Effects of irrigation and drought on pulsations of Desmodium. The first series exhibits normal pulsation ; the second series, arrest under drought ; the third, revival of pulsation after irrigation. pulsation of the leaflet of the Telegraph-plant and that of the animal heart. Is this merely a superficial resemblance, or does it extend deeper? If it be a case of essential simi¬ larity, variation of external conditions should affect both alike.

The reactions of the cardiac tissue, which have been studied with the heart of the frog and of other animals, are such that a certain amount of internal pressure is necessary to start the pulsation. This is well shown in the renewal of pulsation in the quiescent heart of the snail on increasing the intracardiac pressure. Similarly, the pulsations of the Desmodium leaflet become arrested under diminished internal hydrostatic pres¬ sure, consequent on the withdrawal of supply of water to the plant. Irrigation renews the pulsation (fig. 42).

Under anaesthetics, the heart-beat becomes arrested. The above record shows the stoppage of the pulsation of the Telegraph-plant under the action of ether-vapour. The pulsation was seen to come gradually to a stop. The ether- vapour was next quickly blown away and fresh air intro¬ duced into the plant-chamber; the plant revived after nearly a quarter of an hour and resumed its normal pulsation There are numerous other parallelisms between the auto¬ matic pulsations of animal and plant tissues which will be given in a later chapter. I will here describe certain antag¬ onistic reactions under different drugs. Poisonous acids, for example, arrest the pulsation of the heart, but this par-

ticular arrest takes place at diastolic expansion. Alkaline poisons also stop the beating of the heart, but in an oppo¬ site manner, that is to say, at systolic contraction. That the action of these two poisons is antagonistic is further seen in the fact that when the heart-beat is arrested by one, it can be revived by the application of the other. This is a curious instance of one poison acting as the antidote of another. It is wonderful to discover identical reactions in Desmo- dium. Poisonous acids arrest the pulsation at diastole, while alkaline poisons produce an arrest at systole. Finally, the arrest induced by either of these poisons can be counter¬ acted by the antagonistic action of the other.

These experiments conclusively demonstrate the funda¬ mental identity of the pulsatory mechanism in the animal and the plant. But the question still remains, what is the cause of these automatic movements ? We have hitherto been acquainted with a single respon¬ sive movement to a single stimulation ; and now we hnd that there is quite a distinct class of phenomena in which movement takes place apparently without cause. Is there a hiatus between the two, or is there a connecting link, the discovery of which might possibly lead to an explanation of these mysterious automatic movements?

Such connecting links I have been able to discover in the plants Biophytum sensitivum and Averrhoa Carambola. Biophytum is a weed which grows in the neighbourhood of Calculla. Its sensitive leaflets, arranged on the leaf-stalk in two long rows, show excitation by a twitching movement. Averrhoa is a large tree, the leaflets of which are also sensi¬ tive. They are arranged somewhat like Biophytum leaflets, on a long leaf -stalk in two rows. Ordinarily speaking, the leaflets are horizontally outspread. Each leaflet in both Biophytum and Averrhoa, when excited by a stimulus of any kind, responds by a fall, followed by recovery. The impulse caused by strong excitation of one leaflet may radi-

ate to the neighbouring ones, and we are then presented with the most remarkable sight of a progressive rippling movement of all the sensitive leaflets. Biophytum and Averrhoa exhibit characteristics of response which are precisely similar. To avoid repetition, I will describe in detail the reaction of Biophytum under increasing intensity of stimulus. I subjected a particular leaflet to stimula¬ tion by electric shock of increas¬ ing intensity, from 1/20 of a unit to i/io, to 5/10 and then to I and to 2 units. At 1/20 of a unit the plant gave no answer, but when the stimulus was raised to i/io, a pronounced r e - sponse was ob¬ tained. When the intensity was raised to i unit, the response was exactly the same. Hence the Biophytum leaflet exhibits response on the ‘All or None’ principle. It either responds to the utmost or not at all.

It is thus seen that i unit did not produce any larger response than that produced by a stimulus one-tenth of that intensity. What became of the excess of energy that impinged on it in the form of stimulus? It is not necessary to suppose that in every instance the whole energy of the stimulus absorbed is employed in causing the movement ; some of it may be wasted as heat. But on the other hand, it is conceivable that the excess may be stored up for the time being, to And subsequent expression. To take a physi-

cal illustration, the energy stored up in a compressed spring may on release give rise to persistent oscillations. The question arises then, whether there may be an analogous storage of energy in the leaflet of Biophytum after exces¬ sive stimulation. Supposing this to take place, the super- flous energy may be utilised in the form of storage not immediately discerned by an observer; the stored energy may, however, find expression later in the form of rhythmic or repeated movements. That this is actually the case is

Fig, 45. Multiple response in Averrhoa under a single strong electrical shock. seen in the response to a stimulus of intensity of 2 units. Here we find that a single strong stimulus has given rise, not to a single but to multiple responses in recurrent series These multiple responses are given under any form of strong stimulus, such as a mechanical blow, an electric shock, the shock of heat, strong rays of light, or chemical stimulation. The persistence of the multiple activity, more¬ over, depends on the energy of the shock that impinges on the plant, a moderate shock giving rise to a few, and a strong or long continued shock giving rise to many recurring pulsations (fig. 45). The response thus echoes, as it were, or reverberates. There is here an analogy to the vibrations given by a tuning fork ; when gently struck, it vibrates and

emits sound for a short time, but when strongly excited by a stronger blow, it gives a more persistent note. Under natural conditions, the plant is , exposed to the action of various stimuli, supplied by its environment. It is exposed to heat, to the action of light, to the mechanical stimulus of air-currents, and to the action of various chem¬ ical agents present in it or absorbed by it. From the joint action of these external sources of stimulation, the energy stored up by the plant becomes sufficiently great to cause

Fig. 46. Gradual arrest of pulsation of Desmodium after depletion of stored energy. an excitatory ‘overflow. It was our want of sufficiently penetrative analysis of the previous history of the plant that led to the assumption that these movements were self- caused. The internal stimuli for such automatic movements are, in reality, external stimuli which have become trapped. We may, for convenience, divide plants into two classes, ordinary and automatic. Biophytum, under normal condi¬ tions, comes under the first category, since it gives a single response to a single moderate stimulation. But under strong stimulation it gives multiple responses, which are appar¬ ently automatic. There is thus a continuity between ordi¬ narily and automatically responding plants.

Let us now see whether a pronouncedly automatic plant can be brought down to the condition of an ordinary plant. and if it is the store of excess energy that enables Desmo- dium to exhibit spontaneous movement. I took a Des- modium plant and shut it off from the stimulating energies of its environment. Depletion of its store of energy soon brought pulsation to a standstill (fig. 46). A fresh acces¬ sion of energy was now found to renew the pulsation ; a feeble stimulus produced a single response ; a stronger stimu¬ lus gave rise to a series of multiple responses. When restored to the normal stimulation of the environment it regained its automatic activity.

It is the stored energy, then, that bubbles over in apparent spontaneity. This fact casts a flood of light on various activities until now deemed irrational. A healthy baby, on waking after its meal, throws out its limbs over and over again in a rhythmic manner, expressive of overflowing energy. This is also seen in children of a larger growth, when an intensely pleasurable stimulation will cause them to dance for joy, a form par excellence of rhythmic activity. Again, one has often been struck by the long series of zig¬ zag flourishes, characteristic of certain signatures. This multiple rhythmic response is not improbably the expression of an overflowing self-esteem!

This multiple response is not merely characteristic of the grosser mechanical movements, but is equally true in the subtler realm of sensation. Let us think for a moment of the effect on the retina of strong stimulation by light. We stare for a moment at the incandescent filament of a bulb and then close the eyes. Even after the cessation of the stimulus, the after-effect of the light persists as a series of strong visual impressions, the bright filament appearing and disappearing in alterna¬ tion for a considerable length of time. This means that the intense stimulus of light has caused multiple responsive sensations.

The same is true in like manner of all forms of mental stimulation. These, when very intense, are apt to repeat themselves and become persistent. And it is in vain that we seek to escape from their recurrence. We are dogged by our own thoughts, and even in our dreams they return to us. Like other forms of living matter, nervous tissue is tuned or primed to added responsiveness by the very stimu¬ lus that impinges upon it. It is as the cumulative effect of repeated stimulations that the activity of nervous matter ultimately becomes automatic, as it is seen to be in many processes from the birth of thought to inspiration.

The persistence of pulsatory activity on the cessation of external stimulation depends on the capacity for storage. In this respect, I find that plants exhibit two distinct types. In one, the overflow is initiated with very little storage ; and here, the unusual display of multiple activity soon comes to a stop. To maintain such a type in the rhythmic condi¬ tion, constant stimulation from outside is necessary. Plants of this type are extremely dependent on outside influences, and when such sources of stimulation are removed, they speedily come to an inglorious stop. Biophytum is an example of this kind. In the second type of automatic plant- activity, a long continued storage is required before an over¬ flow can begin. But in this case the spontaneous outburst is persistent and of long duration, even when the plant is deprived of any immediately exciting cause. These, there¬ fore, are not so obviously dependent as the others on the sunshine of the world. The Desmodium plant furnishes an example of this type.

Have we not here a suggestive parallel to a certain phe¬ nomenon known among literary and artistic people as inspiration? For the attainment of this exalted condition, it is also necessary to have a previous storage for subse¬ quent effervescent overflow. Inspiration is, after all, an example of fullness and overflow of life without any appar¬ ent effort. If this be so, aspirants to this condition might well decide in whose footsteps they will choose to tread — those of Biophytum, with its immediate dependence on