Plant Autographs and Their Revelations

It is seen in fig. 25 how the anaesthetic gradually abolished excitability, as indicated by the increasing depression of the response. Long-continued action of the narcotic ultimately proved fatal and produced a permanent abolition of electric response. These facts suffice to show that the modification of vital activity can be accurately gauged by means of the electric response, which gives the clearest indication of the depres¬ sion or abolition of vitality. This is strikingly shown in the following record of a case in which the plant was gradu¬ ally scalded to death by the introduction of steam into the plant-chamber. The first two records were taken at the temperature of the room, which was 20° C. Steam was gradually introduced ; this, at first, caused a transient excita¬ tion, after which there was a rapid decline, and the response was completely abolished at death by scalding (fig. 26).

Electro-motility, that is to say the power of exhibiting electro-motive change under stimulation, is the test for the vital condition of a tissue. Similar reactions have long been known in animal tissues, and now we have plants reacting in the same way. By the employment of this test, we are able to prove that every plant and every organ of the plant is sensitive; they all answer to stimulation by a definite electric response. It has also been shown that in

the modification of the electric response by fatigue, by high and low temperature, by the action of narcotics and poisons, the phenomena strictly correspond to similar reactions of animal tissues. The first two responses (left) took place under a tem¬ perature of 17° C. Steam was then admitted. At the end of 5 minutes the Carrot was dead and response ceased. The vertical line (right) represents o.i volt. plant to stimulation have been discussed. The conclusions to be drawn are that all plants give a response of negative electric variation, and that certain plants possessing a spe¬ cial motile mechanism also give a mechanical response of movement; the two forms of response are completely con¬ current in all respects.

The leaflets of many plants are sensitive to light; they become completely closed at the approach of darkness, and open again when light returns. Misled by the similarity to the closure of our eyes in sleep, the closing of the leaflets was taken to signify sleep in the plant. The resemblance is merely fanciful, for a similar closure is often found to take place at midday under the stimulation of strong light. Many involuntary activities of life show no sign of cessa¬ tion, but persist both day and night; in the animal, the heart continues to beat, the processes of digestion and of building up of the tissues continue without intermission. Similarly in the plant, circulation of food-material continues for the supply of the local wants of organs overworked by day and the accumulation of material for to-morrow’s needs; growth also persists, increasing the bulk of the organism and making good its injuries. Life is sleepless activity.

Yet we are periodically switched off, and then become unconscious of all external happenings, and pass through a daily cycle of what we know as sleeping and waking. The question as to whether plants sleep or not can be put into the form of a definite enquiry: is the plant equally aware of external happenings throughout day and night? If not, is there any period when it loses its perceptive power? Is there, again, another period when it wakes up as it were to a condition of maximum alertness.

How are we to find whether the plant really sleeps or not? When we wish to know if our friend is asleep, we shake him and ask, 'Are you awake?’ If he is, he shouts, ‘Yes/ If he is half asleep, he gives some indistinct answer. But when he is fast asleep, he gives no answer. It is only when his sleep is over that he awakes and gives his normal and vigorous answer. A somewhat similar procedure has to be adopted in the case of the plant. We take the sensitive Mimosa and ques-

Fig. 27. Diagrammatic representation of the complete apparatus for determination of the diurnal variation of excitability. Petiole of Mimosa, attached by thread to one arm of lever L ; the writing index W traces on the smoked glass plate G the respon¬ sive fall and recovery of leaf. P primary, and S secondary, of induction coil. Exciting shock passes through the plant by elec¬ trodes E, E'. C, clockwork for regulating duration of shock. Primary circuit of coil completed by plunging rod, R, dipping into cup of mercury, M.

tion it hour after hour : ‘Are you awake ?’ ‘Are you awake V ‘Are you awake?’ and the changing vigour of the plant’s answer at different times indicates if and how much it is asleep or awake. The difficulty of the problem lies in delivering the same questioning shock every hour of the day and night, and recording the plant’s answer in the form of a responsive movement. It was solved by the special Sleep-Recorder devised for the purpose, represented diagrammatically in fig. 27.

The leaf of Mimosa is attached to one arm of an alu¬ minium lever L by means of a thread ; at right angles to the lever is the writer W, which traces the responsive move¬ ment of the leaf on a smoked glass plate, which is allowed to fall at a definite rate. Under a definite stimulus the leaf Fig. 28. Effect of moderate cooling during the period shown by the horizontal line below. falls down, the amplitude of response measuring the excita¬ bility of the plant at the given moment. A second stimulus is delivered after a definite interval, say one hour, and the corresponding response shows whether the excitability or awareness of the plant has remained constant or has under¬ gone any variation.

The questioning shock is electrical, the efifective intensity of which has to be maintained absolutely uniform. The sec¬ ondary coil is placed at a suitable distance from the primary, and the duration of the shock is kept constant by completing the primary electric circuit for a definite length of time. The completion of the primary circuit is made by the plung¬ ing-rod R, which dips into a cup of mercury for only 0.2 second. The record of the response is obtained on the smoked plate, which is kept oscillating to and fro by clock¬ work.

It will presently be shown that the plant goes through a daily cycle of sensibility and insensibility which may be aptly described as waking and sleeping. The cause of this Fig. 29. Midday record from noon to 3 p.m., exhibiting uni¬ form and maximum excitability. has been fully analysed elsewhere.^ I may briefly enumerate some of the factors which cause sleep or insensibility of the plant. The plant becomes insensitive under prolonged dark¬ ness; lowering of temperature has even a more pronounced efifect in depressing the excitability (fig. 28). It should be remembered that the plant is subjected, during a complete day, to considerable variations of temperature, the mini¬ mum being early in the morning and the maximum at about noon.

^ Bose — “The Diurnal Variation of Moto-Excitability in Mimosa” — Annuls of Botany, Oct. 1913. The ‘awareness’ of the plant is at its maximum through¬ out the middle of the day. This will be seen in the record reproduced (fig. 29), which shows how vigorous and uni¬ form are its responses. This period may be regarded as the ‘office hour.’ Its wakefulness shows a striking change in the morning and evening. Fig. 30. The gradual waking up of the plant from 8 to 12 a.m.

the complete record for twenty-four hours. I reproduce the record (fig. 30) taken every half an hour, illustrating the process of slowly awakening at about 9 a.m. Note the human way in which it is trying to rouse itself gradually. Record for Day and Night This record was commenced at 5 p.m. and taken every hour of day and night for twenty-four hours. The plant was fully awake for the first two hours. It became some¬ what sleepy between 9 p.m. and 2 a.m., but was still awake at 6 A.M., after which it fell sound asleep, and remained so until past 8 a.m. (fig. 31).

Rousseau told us that modern life is decadent and that the only hope of salvation was a return to primitive life. Surely we could not go back to anything more primitive Fig. 31. Diurnal record showing variation of sensibility of Mimosa and unspoilt than unsophisticated plant-life. And yet what are the facts ? Apparently Mimosa has forestalled the dissi¬ pated life of the modern Babylons of London, Paris, and New York, keeping up all night and going to bed only after the rising of the sun!

As if in protest against the imputation of leading a fast life, some plants assume an appearance of a highly sanctimonious character. At least, that was the impression made on the credulous public by the remarkable performance of the Praying Palm. While the temple bells rang, calling the people to prayer in the evening, this tree bowed down its head in adoration. By the morning its head was again erect, and this process was repeated every day of the year. This extraordinary phenomenon was regarded as miracu¬ lous, and pilgrims were attracted in large numbers. It was alleged that offerings made to the tree had been the means of effecting marvellous cures. It is not necessary to pro¬ nounce any opinion on this subject; these cures may have been effective in the same way as other faith cures now prevalent in the West.

This particular Date-Palm {Phoenix sylvestris) was a full-grown rigid tree, its trunk being about lo inches in diameter. It must have been displaced by a storm, so that the trunk had an average inclination of about 6o° to the vertical. In the course of its daily movement the trunk, throughout its entire length, was elevated in the morning and depressed in the afternoon : the upper end of the trunk thus moved up and down through a distance of 3 feet. The ‘neck,’ or upper end of the trunk, bearing the leaves, was concave to the sky in the morning; in the afternoon the curvature was reversed. The large and long leaves which pointed high up against the sky in the morning were thus swung round in the afternoon through a vertical dis¬ tance of about 15 feet.

The upper photograph represents the morning, and the lower photograph the afternoon position. To the popular imagination the tree was a living giant, rearing its head in the morning to more than twice the height of a human being, leaning forward in the evening from this towering height, and bending its neck till the head of the leaves pressed against the ground in an attitude of devotion. I took two photographs of the tree, one in the morning and the other late in the afternoon (fig. 32). I was com¬ pletely puzzled by the peculiar behaviour of the tree, the like of which had never been noticed before. Natural science does not believe in the occult, for to it nothing is extra¬ physical, but only mysterious owing to some hitherto unas¬ certained cause.

What could be the cause or causes behind the mystery? The phenomenon repeated itself day after day, and the only changing factors were light and temperature. Could it be light ? This was not likely, for in order that it could act, light would have to come into contact with the living tis¬ sues. But the thick bases of dead leaves completely screened the deep-lying living tissues from light. The only other factor was the changing temperature, which, by affecting growth, causes movement, as illustrated by the movement of the Water-Lily described in a later chapter. But this Palm-tree was old and not in a state of active growth.

The only means of solving the problem was the construc¬ tion of a special apparatus which would record the move¬ ment of the tree continuously throughout the day and night : the up-movement was recorded as an up-curve and the down-movement as a descending one. The changing tem¬ perature was also recorded simultaneously on the same recording plate by means of a metallic thermometer, the rise of temperature being indicated by the fall of the thermal curve, and vice versa. The principle of the recording appa¬ ratus will be understood from the illustration given later (see fig-^34)-

A difficulty arose at the beginning in obtaining the sanc¬ tion of the proprietor to attach the recorder to the tree. He was apprehensive that the miraculous power of the tree might disappear under the profane contact of foreign-look¬ ing instruments. His misgivings were removed on the assurance that the instrument was made in my laboratory in India, and that it would be attached to the tree by my assistant who was the son of a priest. The results obtained were most striking. A casual observation had led people to believe that the tree lifted itself at sunrise and prostrated itself at sunset, but the con¬ tinuous record obtained showed that the tree was never at rest, but in a state of continuous movement, which under¬ went periodic reversal (fig. 33). The movement was not passive, but was efifected with an active force sufficient to lift a man ofif the ground.

The tree attained its maximum erection at seven in the morning, after which there was a rapid fall. The downward movement reached its maximum at 3.15 p.m., after which it began to lift itself very slowly, then more rapidly, until, as stated before, it lifted itself to the highest position at seven next morning. The accurate record made by the tree itself showed that the maximum fall did not take place exactly at prayer time ; but people who are out to impute miracles would not be dis¬ turbed in their faith by such a slight discrepancy.

The possible explanation of the daily movement was some periodically changing condition, such as light or tem¬ perature. Light, as we saw, could not be the effective cause. Closer inspection of the curve of daily movement also sup¬ ported this contention. For if the movement were due to light, then there would be a climax at noon and the oppo¬ site at midnight. But instead of this we found that the highest point was reached not at noon but at seven in the morning. The lowest point, on the other hand, was attained not at midnight but in the afternoon.

Turning next to the variation of temperature, we were at once struck by the fact that the curve of the movement of the tree was practically a replica of the curve of the varia- Thermographic curve for 24 hours, commencing at 9 in the evening, is given in the upper record, in which the fall of temperature is represented by an up-curve. The corresponding diurnal curve of movement of the tree is given in the lower. Succes¬ sive dots mark intervals of 15 minutes; thick dots intervals of an hour.

tion of temperature. This rise of the tree was seen to follow the fall of temperature, and vice versa. The movement of the tree lagged behind the change of temperature, and the delay was due to two causes : it took some time for the thick trunk of the tree to attain the temperature of its surround¬ ings ; in addition to this physiological inertia delayed the reaction. Having proved that the movement was caused by varia¬ tion of temperature, it now remained to be seen whether this was due to mere physical expansion and contraction, or to some specific reaction of life. The conclusive test would be to observe what happened after the death of the tree. In such a case, the physical movement would persist, while the physiological action would disappear. In order to decide this question it was. of course, impossible to take liberties with the life of a tree which was so unique and held in such reverence.

But a year afterwards, while presiding at my lecture on this subject. Lord Ronaldshay, the Governor of Bengal, announced that a telegram had just reached him from his officer at Faridpore stating that 'the palm-tree is dead and its movements have ceased.’ This much lamented demise of the tree offered the means of solving the problem. It was unfortunate that the tree had to die in order to prove that its movements had been due to its vital activity.

The superstitious might connect the death of the tree with the libertv I took of recording its movements. Need- less to say, the two had no causal relation ; the tree was old and died a natural death more than a year after my experi¬ ments. Nevertheless I felt uncomfortable about it and wished the matter to be buried in oblivion. But this was not to be for a considerable time, since the Press took up the matter and made comments, of which the following is a specimen :

'Tragedy in high life! The praying palm of Faridpore, as already reported, expired suddenly on the very eve of Dr. Bose’s lecture. It must have heard that the distin¬ guished scientist was about to expose a humbug. That pious attitude, we are told, was entirely due to the effort to keep itself warm! At the same time, we cannot help thinking that it may have been a sign of disease, for how is it that other palms never “pray” ?’ For once the omniscience of the Press was at fault. For not only the sacred tree of Faridpore but other Palms and even common trees have this habit of Spraying.’ I have received information of another Palm growing by the side of a pool, the trunk of the tree being inclined towards it. The uplifted leaves of this tree are swung round in the afternoon and dipped into the water of the pool.

The phenomena described above are not marvels of the mystical East ; a similar thing happened in the prosaic neighbourhood of Liverpool. An English friend sent me the following extract from the Liverpool Mercury, dated December 13, 1811: 'Remarkable Phenomenon: There is at present a willow tree of considerable height and about three yards in cir¬ cumference, growing on the banks of a rivulet on the farm called Tubsill, the property of the Rev. Mr. Wasney, near Shipton, which actually appears animated; it will at times prostrate itself at full length on the ground, and then rise to its original perpendicular position. Incredible as this may appear, it is a fact and has been the astonishment of hun¬ dreds who have seen it!’

Another incident has recently been related to me by a coco-nut planter in South Africa. Owing to the strong prevailing winds from the sea, the rows of Palm-trees on his plantation had grown slantingly. It had been a perpetual wonder to him that while in the morning all the Palm-trees were fully erect, so that the coco-nuts were beyond reach, in the afternoon the trees were so accommodating as to bend down to make plucking easy! Trees growing at even a slight inclination show char¬ acteristic up and down movements ; these are so slow that a casual observer fails to notice them. The movements

traced by the automatic recorders, however, are very marked and definite. The movements of trees like the Faridpore Palm become conspicuous for two reasons. First, the geotropic action, on the variation of which the movements depend, is most intense when the tree is nearly horizontal, and less so when more erect. Moreover, the movement of an inclined tree is more noticeable, since the ground affords a fixed object for reference. With a more erect tree, the movement would escape notice, since it would be executed with empty space as a background.