Plant Autographs and Their Revelations

The movement of the floral leaves of the Water-Lily will be shown in a subsequent chapter to be brought about by variation of growth, induced by changes of temperature. In the case of fully grown rigid trees, it is necessary to seek another explanation. I cannot give here an account of a large number of experiments which led to the discovery of a class of phenomena hitherto unsuspected, namely, the influence of temperature on geotropic reaction. When a plant-organ, whether stem, branch, or leaf, grows slantingly, the stimulus of gravity tends to raise it upwards. Lay a potted plant horizontally on the ground, and it will soon be noticed that the stem and leaves raise themselves upwards against the pull of the earth. The geo¬ tropic reaction has, therefore, to overcome the tension of the plant’s tissues. Equilibrium is attained when these counteracting forces balance each other. The experiments carried out show that the geotropic reaction is modified by change of temperature; a rise of temperature diminishes, while a fall accentuates the geotropic up-curvature. Hence the ‘dynamic balance’ is upset in one direction or the other by the rise or fall of temperature, thus causing the daily up and down movements.

Theoretically, all inclined organs sensitive to the stimulus of gravity should exhibit the characteristic movements under variation of temperature. Now the horizontally outspread leaves of plants are also subject to the stimulus of gravity. Do they too exhibit daily movements similar to those of the Praying Palm? This investigation was carried out with the help of an Automatic Recorder specially constructed for the purpose. The apparatus has four recording levers: the first three

record movements of leaves (or of shoots horizontally laid ) of different plants; the fourth lever records the variation of the temperature by means of a metallic thermometer (fig. 34). The smoked glass plate is made to oscillate to and fro by clockwork at intervals, say, of fifteen minutes. Four records are thus made at the same time, the uppermost being the thermograph, and the three others the phytographs of different leaves. The records show that the leaves are in a state of con¬ tinuous movement and that they all exhibit a rise during a fall of temperature, and vice versa. There is, however, an individuality of each species of plant, exhibited in its characteristic autograph (fig. 35). Note the tremulous writing of Papaya; the signature of Croton shows more character, it evidently crosses its ‘t’s’ with greater decision ! It is thus possible to identify a plant from its peculiar hand¬ writing.

The plant-world, as a whole, is thus not passive but fully responsive. The full-grown and apparently rigid tree is sensitive to the slightest external change, even to the passage of a cloud across the sky, and it signals its perception of the change by a definite movement. The arbitrary distinction between ‘ordinary’ and ‘sensitive’ plants thus wholly dis¬ appears : it is a question of degree only. The facts show that not only the ‘Praying Palm’ but every tree and its dif¬ ferent organs perceive changes in the environment, and execute movements in response to them. It is not the cush¬ ioned pulvinus alone that is excitable, but the tree as a whole is instinct with sensibility. Its seemingly rigid trunk is really a gigantic pulvinoid which perceives and responds to the multitudinous stimuli of its environment.

The results given in the previous chapters show that a con¬ tinuity exists between the reactions of animal and vege¬ table organisms, proving that the laws of life are one. In studying this continuity, some are apt to make the mistake of comparing a man with an amceba, and wonder that all the complicated organs in the highest are not found in the lowest. The fact of the matter is that there has been a con¬ tinuous evolution from the lowest to the highest, with corre¬ sponding increase in complexity.

Simple machinery does not easily get out of order, and in the machinery of life we find the simplest mechanism not easily deranged. Hence unicellular organisms are practically immortal. Many of the infusoria may be dried up and apparently killed, but the addition of a drop of water revives these simple organisms, which will now swim about quite vigorously. When the complexity is great, as in higher animals, any local derangement puts the machinery out of gear; the penalty exacted for higher evolution is death.

Can we proceed even lower down in the scale of life, and reaching inorganic matter find in it some sign of irritability which had hitherto been regarded as the special character¬ istic of living matter? Has matter itself then the promise and potency of life? Otherwise how did life make its first appearance on earth ? When the earth was a molten mass, there could have been no life as we know it now. It has been suggested that the seed of life was imported to this earth by the cosmic dust from other worlds ; but this would merely transfer the difficulty backwards. Probably, at some critical period of the earth’s history, the environmental con-

ditions may have favoured the appearance of life out of non-life. I was startled more than thirty years ago by certain quite unexpected characteristics of my detectors for electric waves. I found, for example, that under continuous stimulation by the oncoming message, the sensitiveness of the metallic detector disappeared. But on giving the receiver a sufficient period of rest, it became sensitive once more. On taking records of successive responses, I was surprised to find that they were very similar to those exhibiting fatigue in animal

muscle. And just as animal tissue, after a period of rest, recovers its activity, so did the inorganic receiver re¬ cover after rest. Thinking that a long rest would make my receiver more sensitive, I laid it aside for several days and was astonished to find that it had become inert. In fact, it had become lazy through lack of stimulation. A strong shock Fig. 36. Fatigue of Metals. stirred it up again into readiness for response. Two opposite treatments are thus indicated for fatigue from overwork and for inertness from long passivity!

This was not all, for I further found that the application of certain stimulating drugs rendered my receiver extraor¬ dinarily sensitive so that it could record extremely feeble wireless messages, which it had failed to detect before. Other drugs depressed the sensitiveness, or abolished it altogether. Having thus obtained some indications of reaction wffiich reminded one of the responsiveness of living tissues, experi¬ ments were next carried out upon metallic wires, using the same method of electric response commonly applied to the investigation of animal tissues.

I now describe some of the more important results obtained with inorganic matter, metals for example. Metals, such as tin, copper, and even the chemically inactive platinum, were selected on account of their better conduc¬ tivity, since this ensures larger electric response. As long as the piece of metal was at rest, there was no electric sign of excitation. But when it was excited in any way, a definite response was obtained. A feeble stimulation gave rise to a feeble response and the recovery from the state of excitation was found to take place in a short time. A stronger stimula- t i o n produced a larger response, the period of recov¬ erybeing prolonged.

A feeble stimula¬ tion, individually ineffective, became effective on repeti¬ tion. All these results are strictly parallel to the characteristic responses of nerve and of muscle. Uniform re¬ sponses were obtained under uniform stimulation with suit¬ able periods for complete recovery. But when the metal was subjected to long-continued stimulation, it showed fatigue (see fig. 36). The effect of stimulants in enhancing the reaction will next be described. After taking the normal response of the metal to successive uniform stimulations, it is touched with the stimulating agent, a solution of sodium carbonate, with the result that there is an exaltation of the response

Fig. 37. xA.ction of stimulant in enhancing the electric response of metal. (fig. 37). There are other stimulants which induce a still more astonishing increase of irritability. Thus, as stated in this and in previous chapters, when animal, plant and metal have been subjected to the same questioning shocks they have in all cases given similar replies. They exhibit similar fatigue and show similar exaltation under stimulants. If further confirmation of the unity of reaction be needed, there remains one final test by which physiologists distinguish the characteristic phenomena

A minute dose enhances, and a large dose abolishes, the response. of life. That which is living is capable of dying, and death may be hastened by poison. The pulses of response then wane until they cease altogether. Is it credible that we may in like manner ‘kill’ metals by the administration of poison? Strange as it may appear, the electric response disappears altogether after application of poison. There remains the very curious phenomenon known not only in the investiga¬ tion of physiological response, but also in medical practice, that of the opposite effects produced by the same drug when given in large or in small doses. Here, too, we have the

same phenomenon reproduced in an astonishing manner even in inorganic response. A small dose of poison amplifies the response, while a large dose destroys it (fig. 38). We have thus examined the autographic records of the living and non-living. How similar are the writings ! So similar, indeed, that we cannot tell one from the other. We have watched the responsive pulses wax and wane, in the one as in the other. We have seen response sinking under fatigue, becoming exalted under stimulants, and being abol¬ ished by poisons, in the non-living as in the living.

Among such phenomena, how can we draw a line of demarcation and say, ‘Here the physical process ends, and there the physiological one begins’ ? Such a line can hardly be drawn. Do not the two sets of records of the living and non¬ living tell us of some property of matter common and per¬ sistent? Do they not show us that the same kind of molecu¬ lar upset on stimulation occurs on both the inorganic and the living — that the physiological is closely connected with the physical — that there is no abrupt break, but a uniform march of law?

The dust particle and the earth, the plant and the animal, are all sensitive. Thus, with an enlarged cosmic sense, we may regard the million orbs that thread their path through space, as something akin to organisms, having a definite history of their past and an evolutionary progress for their future. We may then come to realise that they are by no means insensate clods, locked in the rigour of death, but active organisms ‘whose breath, perchance, is luminous iron vapour, whose blood is liquid metal and whose food is a stream of meteorites.’

We shall henceforth turn with renewed courage to the investigation of mysteries which have long eluded us. For every step of science has been made by the inclusion of what seemed contradictory or capricious into a new and harmoni¬ ous simplicity. Her advances have always been towards a clearer perception of underlying unity in apparent diversity. made records and perceived in them one phase of an all- pervading unity that bears within it all things — the mote that quivers in ripples of light, the teeming life upon our earth, and the radiant -suns that shine above us — it was then that I understood for the first time a little of that message pro¬ claimed by my ancestors on the banks of the Ganges thirty centuries ago :

They who see but one, in all the changing manifoldness of this universe, unto them belongs eternal Truth — unto none else, unto none else. What is the difference between life and death? ‘Tremu¬ lous with life,’ we say. What do we mean by this? In con¬ trast rise the words ‘As still as death.’ Life indeed is tremulous, because the living substance is constantly throbbing under the impact of those shocks from within which are vaguely called ‘internal stimuli,’ and shocks from without, or ‘external stimuli.’ But internal and external stimuli are in fact one, seen as it were from differ¬ ent points of view. All that is within the living organism has been acquired from outside. Into the primordial speck of life comes inpouring energy from the universe without. With the capital of a single cell, life begins its bank account, and this grows and waxes great as the accumulation of mat¬ ter and energy increases. Multiplication and enlargement are the visible signs of acquired wealth. At full maturity, income and expenditure are balanced. After this comes loss greater than recuperation; at the end bankruptcy and death.

An organism is actively living as long as it is responsive to the forces of its environment. It gives an answering twitch to the impinging stimulus. It does not necessarily give out all that it receives, for it may store a portion of the energy of the incident stimulus. And when, thus gath¬ ering by driblets, it has accumulated an excess of energy, this overflows or bubbles over in spontaneous movements. This process will be studied in greater detail in a subsequent chapter.

From the physical aspect, an animal or plant may be likened to a heat-engine supplied with fuel. In order to find out how effectively the machine is working, we may attach to it an apparatus like my writing lever, which gives a graphic record of its activity. When the fuel runs short, the beat of the piston and the up and down strokes of the record come to a stop. This stoppage from the exhaustion of the fuel is but temporary, for a fresh supply of fuel renews the movement. But the working of the engine may also be permanently stopped by some breakdown of machin¬ ery and interlocking of cogwheels. In the machinery of the living organism likewise, the activity of life may be tem¬ porarily stopped by the run-down of energy previously stored. The activity may also be permanently stopped by some internal clogging of the living machine.

Under the action of various poisons, the pulse-throb is gradually arrested in death. We may regard every frag¬ ment of the living substance as an aggregate of molecular machines. The change from a living to a dead condition is one of transformation from a state of molecular mobility to that of interlocked rigidity. We see a man instinct with life suddenly falling into a death-like stupor under the action of a narcotic. By timely treatment he may be made to shake off the lethargy and return to life. But if there is delay, then life passes into non-life. The thinnest line thus demarcates life from death. What a crucial moment it is when the living particles are swinging in their unstable poise ; a little tilt this way and the living machine is locked in the grip of death! In this supreme moment is played out the drama of life and its end. If we could, at this crisis, trace out the history of the molecu¬ lar conflict, then and then only would be revealed to us the secret of death. That invisible internal conflict, the struggle between the forces of life and death, is sometimes vividly exhibited by the automatic record of the dying organism.

Discoloration is one of the signs of the death of the plant. A simple method of bringing about death is to place the plant in a bath and gradually raise the temperature, until at the fatal point it is scalded to death. If we take a por¬ tion of a flower, say the milk-white pistil of Datura alba, the normal white turns to brown at the occurrence of death. More striking still is the sudden discoloration of flowers of brilliant hue. The rich mauve petals of the Passion-flower are swiftly transformed, as if by magic, to pallid white. Thus if we immerse one half of the flower in a thermal bath, leaving the other half outside, the immersed half turns white at death, the rest retaining its purple hue. Similarly in the Indian crimson flower Sesbania, the red colour is transformed at death to a pale blue. The fatal temperature in normal specimens is more or less definite, being about 60° C. or 140° F. If the plant be subjected to fatigue by over-stimulation, the death-point becomes lowered, the degree of lowering depending on the extent of fatigue. Like ourselves, the plant when weary cannot struggle effectively against death. It is thus possible to map out, in a coloured petal, invisible zones of fatigue. For this we take a pair of stencil patterns in metal and place the highly coloured petal between the two. The two stencils are then connected with the two electrodes of an electric coil, and shocks are passed through definite areas of the petal, which thus becomes fatigued. On removing the stencil, w'e notice no visible change, though a latent image of fatigue is present at the region subjected to the electric shock.

If we now place the specimen in a bath at, say, 50° C. the fresh portion of the petal shows no change, for the death-discoloration takes place only at a temperature of 60° C. The fatigued area, however, dies at the lower tem¬ perature and exhibts local death by discoloration. Thus the invisible image of the pattern of fatigue is now seen visibly developed as the discolored pattern of death. This is the thermograph, the heat-record and map of death.

Is there anything resembling this mapping of incipient death in plants under natural conditions? Let us consider the variegations of certain leaves ; the leaves of the Elephant Creeper of India show white patches in the midst of bright green. These patches represent, in reality, the pallor of approaching death, and if we keep these leaves under observa¬ tion we find that it is at these particular spots that the tissue becomes disintegrated by decay which spreads outwards. The brilliant tint of leaves in autumn is but the hectic flush preceding death.

I have already explained how changes which would have remained beyond our scrutiny can be detected by the elec¬ trical variations by which they are accompanied. Thus a sudden excitation gives rise to a negative electric change. I will presently show that an intense excitation is produced at the moment of death, with the result that there is an elec¬ tric discharge at the moment of death of the tissue. A simple method of demonstration is to take one half of a green pea, and connect its inner and outer surfaces with a galvanometer. The half pea is slowly raised in tempera¬ ture in a heating bath. At the death-point of 6o° C., an intense electric discharge passes through the organism. The electric change at death is very considerable, being often as high as 0.5 volt. If 500 pairs of half peas are suitably arranged in series, the terminal electric pressure will be 500 volts, more than sufficient to cause electrocution of unsus¬ pecting human victims. It is well that the cook does not know the danger she runs in preparing this particular dish, and it is fortunate for her that the peas are not arranged in series !

We have next to consider a very interesting secondary effect in connection with the electrical spasm which we found immediately to precede death. I refer to that sudden revival of memory of which we have all heard. Here I can briefly refer to the enquiry which I carried out into the funda¬ mental background of memory. After each shock of stimu¬ lus the responsive surface undergoes a molecular distortion, from which recovery takes place very gradually; this recov¬ ery is, however, never quite complete. Traces are left of the impression made by the stimulus and these remain as

latent images, invisible even under microscopic examina¬ tion. Under suitable conditions, however, this invisible script becomes conspicuous. I have thus been able to form impressions on a metallic plate of which no sign whatever can be detected, and yet when the place is subjected to diffuse stimulation, the invisible images are readily seen. Similarly all the impressions made on our sensory surface remain dormant as latent memory images, which become revived under the impact of the internal stimulus of will. The revival of memory, then, is the result of a strong stimu¬ lation being thrown on the impressioned surface, so as to wake up the dormant images. Now we have seen that dur¬ ing the struggle of death an electrical spasm sweeps through every part of the organism and this strong and diffuse stimulation — now involuntary — may be expected to crowd into one brief flash a panoramic succession of all the memory images latent in the organism.

The various symptoms of death in ordinary plants, such as drooping, withering, and discoloration, do not manifest themselves exactly at the moment of death, but at a later period. Even after a plant has been subjected to a tem¬ perature in excess of the fatal degree it may continue for a time to appear fresh and living. How, then, can it be possible to distinguish a living from a dead plant, and how to And the exact moment of transition? Such discrimination is possible by watching for the dis¬ appearance of some reaction characteristic of the living con¬ dition. The ideally perfect method, however, would be the discovery of a reaction which, at the moment of death, underwent a sudden reversal to its opposite. There would then be not even that minor degree of uncertainty which is inseparable from the determination of the vanishing point of a waning effect. Such a perfect method I have been a*ble to render practicable by the discovery of the death-spasm in plants analogous to the death-throe of the animal.

In securing the record of the onset of death in plants, two different methods have been found suitable. The first is to subject the plant to a continuous rise of temperature, until the fatal degree is reached. The second method, not so per¬ fect as the first, is to apply a dose of dilute poison which proves fatal after a shorter or longer period, depending on the dose and the virulence of the poison. I have succeeded in de¬ vising a Death-Recorder by which the dying organ¬ ism, by means of a con¬ tinuous script, records its exact death-point. The recording apparatus is of an oscillating type ; the smoked glass plate is made to oscillate to and fro by an electro-magnetic contrivance, thus produc¬ ing a series of dots at