Plant Autographs and Their Revelations

the different lever-systems were too greatly increased, and the theoretical advantage was wholly nullified in practice. I was, therefore, forced to work out a new solution. Alate- rial contact having proved unworkable, I had to discover a new method of linking which would be immaterial and Fig. 56. Diagrammatic representation of ^Magnetic Crescograph. S-X, magnetic rod supported on fulcrum ; short arm of lever attached to growing plant . Growth-elongation lowers the N end, which causes increasing dedection of the suspended needle with attached mirror M. Deflection magnified by reflected spot of light.

therefore frictionless. This was found in a magnetic con¬ trivance, in which a delicately poised system is upset by the movement of a magnetic lever in its neighbourhood. This will be understood from fig. 56. SN is a light mag¬ netic rod supported on a fulcrum. The short arm of the lever is attached to the tip of a growing plant represented by \V. Growth-elongation lowers the N end of the mag- netised rod, which causes increasing deflection of the sus¬ pended magnetised needle ns with the small attached mirror. The sensitiveness is greatly increased by the employment of a perfectly astatic system of needles for the single ns. A line of light reflected from the mirror is thrown on to a distant scale. The travelling of the reflected light shows the growth-movement magnified from ten to a hundred million times.

It is difficult for the mind to grasp magnification so stu¬ pendous. Some concrete idea of it can, however, be obtained by what the speed of the proverbial snail will become when magnified ten million times by the Magnetic Crescograph. Eor this enhanced speed there is no parallel even in modern gunnery. The 15-inch cannon of the Queen Elisabeth throws out a shell with a muzzle velocity of 2360 feet per second. But the Crescographic snail would move at a speed 24 times faster than the cannon shot. We may next turn to cosmic movements for a closer parallel. A point on the equator whirls round at a rate of 1037 miles per hour. But the Crescographic snail may well look down on the sluggish earth, for by the time the earth had made one revolution, the snail would have gone round it nearly forty times.

No apparatus can be conceived which brings out the invis¬ ible activities of the life of the plant so vividly as this mag¬ netic apparatus. I find many plant-organs come to the end of their growth at maturity. In such cases in which growth had normally ended, I found it sometimes possible to renew it by the action of appropriate stimuli. Here, at any rate, is some support for the ever-recurrent dream of rejuve¬ nescence. Even our own experience tells us that while the stimulus of hope and persistent optimism keep one ever young, pessimism and cynicism bring on premature senility and decadence.

the demonstration of growth before a large audience. No experimental conditions for exhibition of growth could have been more difficult than those prevailing in the depth of an English winter, when the plants were in a state of hibernation. In spite of this, they were made to shake off their lethargy, and the rate of resulting growth was exhib¬ ited by the indicating line of light rushing across a lo-foot scale in the course of some twelve seconds, the actual rate being less than a hundred thousandth part of an inch per second.

Other experiments, even more striking, on the action of drugs in depressing or enhancing the rate of growth can be easily exhibited. The growth of the plant under normal conditions is seen magnified as a line of light rushing across the scale. By a depressing agent the growth becomes para¬ lysed and the line of light is brought to a stop, but a dose of a stimulant instantly removes the depression. The life of the plant thus becomes subservient to the will of the experimenter; he can exalt or depress its activity; he may thus bring it near the point of death by application of poi¬ son, and when the plant is hovering in an unstable poise between life and death, resuscitate it by the timely applica¬ tion of an antidote.

It is true that man is very imperfectly equipped to make daring adventures on the great seas of the Unknown. Of all possible notes of sound, only eleven octaves are audible to him, and a single octave of light circumscribes his vision. Even then the size of the ripple of visible light imposes an impassable barrier. He will never be able to see objects smaller than a fifty-thousandth part of an inch, which is the length of a single light- wave. These limitations have not deterred him, but have, on the contrary, spurred him on to greater efforts in his explorations in the region of the invisible. The mysterious movements of life are not to remain to him inscrutable for all time; but his untiring: effort and single-minded pursuit will reveal to him the secret that lies behind all manifestations of life.

As we step out into the garden there are spread before us multitudinous plants, silent and seemingly impassive. Vari¬ ous shocks impinge on them and they may suffer wounds and death. In what way is the plant affected when the shock has been very excessive, as after a severe wound? I undertook three separate investigations into the effect of wounds. The object of the first was to ascertain the effect of injury on growth; of the second, to demonstrate the reaction of wounds on the throbbing pulsation of the Telegraph-plant; of the third, to analyse the paralysing effect of wounds.

Using the High Magnification Crescograph I subjected a plant to various irritations, from a rough touch to severe wounds, and observed the resulting changes in the rate of growth. In a particular experiment, I first took the record of normal growth, and then irritated the plant by rubbing it with a rough piece of cardboard. Growth was now retarded to two-thirds of its normal rate. The plant was next given a rest of fifteen minutes to recover from the irritation, and the restoration of the rate of growth was found to be only partial. It was a full hour before recovery was complete. Rough touch retards growth, and the rougher the treatment the longer the plant takes to recover from it.

Of this I may give an instance which puzzled me for a long time. I had been attaching numbers of plants to my Crescograph to obtain their records of growth. Though the apparatus was in perfect adjustment, yet no growth could be recorded. A plant happened to be left attached to the apparatus over night, and I found, to my surprise, that the plant which had failed on the previous day to show any growth, now exhibited it vigorously. I then realised that the unavoidable rough handling, in tying the plant to the apparatus, was enough to produce an immediate inhibition of growth, which was renewed only after recovery from the irritation. The procedure now adopted was to mount the plant as gently as possible, and to give it two hours’ rest before taking the record. With this precaution, there was no difficulty in obtaining satisfactory results.

I next set out to find the effect of pin-pricks. These caused a greater irritation than friction or rough handling. Growth was now retarded to nearly a quarter of the normal rate, and the recovery took a proportionately longer time. Even after a full hour, growth recovered to only two-thirds of the normal rate. A more severe wound was inflicted by a longitudinal slit made by a knife; this reduced the growth to about one-fifth of the normal rate. The wound-effect of a transverse cut was far more severe. Such a cut inhibited growth for a very long time. In sensitive specimens it actually produced a convulsive contraction.

Corporal punishment is evidently not conducive to growth, a fact which might be seriously pondered over by schoolmasters! Truth, however, compels me to confess that it may have its uses. For my investigations show that while in actively growing specimens shocks cause a retardation of growth, in others, where the rate of growth is below par, stimulation revives and accelerates the rate. This fact may probably explain the custom among certain Indian peasants of giving a good thrashing to the young crop when it is lagging in growth !

stunted or backward in growth, the branches and leaves presenting an unhealthy look. Lopping off the offending limb is then found to be good for the plant. A severe shock renews growth that had become arrested. The leaflets of the Telegraph-plant, as has been already stated, exhibit automatic pulsation. When the small leaf¬ stalk carrying the leaflets was detached from the parent plant and the cut end of it placed in water, their pulsa¬ tion was arrested by the shock of the operation. The shock-effect of the wound gradually disappeared, and the pulse-throb revived and continued for nearly twenty-four hours. Death, however, found an unguarded spot at the wound, and its march, though slow, was sure. The death- change ultimately reached the throbbing organ, which became permanently stilled. Experiments have been under¬ taken to arrest this march of death. The problem is inti¬ mately connected with the proper understanding of the con¬ ditions which lie behind life, and the other conditions under which the molecular cogwheels become arrested in the rigor of death. The experiments already carried out have been so far successful as to prolong, under proper treatment, the throbbing pulsations of the plant for more than a week.

A severe wound paralyses the motor function of the pul- vinus of Mimosa. On cutting off a short piece of stem bearing a leaf, the shock-effect was transmitted to every part of the parent plant, and all the leaves fell down and remained depressed for a considerable length of time. The detached piece with its cut end placed in a nutrient solution was also depressed, as shown by the fall of its leaf. The subsequent history of the parent plant and of the detached piece exhibited a significant difference.

The parent plant, paralysed by the wound, recovered slowly. A record was taken of the response of one of its leaves. The first record in fig. 57 (i) is the normal response of the leaf before the infliction of the wound. The leaf fell down, after the severe wound, to the position marked in the record by a cross. The paralysing eftect of the wound was proved by means of testing shocks, the response Fig. 57. The effect of wound (i) on parent plant and (2) on detached

(1) First response normal. The x shows contractile fall of the leaf after wound. The subsequent responses exhibit slow but perfect recovery. (2) (a) Vigorous response of leaf of the detached shoot 4 hours after section; (b) depression after 24 hours; and (c) marked depression and final abolition of response at death after 48 hours. being in each case inscribed by the automatic recorder. The excitability remained depressed for nearly two hours, after which the plant gradually recovered its normal excitability in a ‘staircase’ manner. The normal functions of the plant afterwards became fully restored.

The leaf of the detached shoot, fed with the nourishing solution, soon held itself up with an attitude almost of defiance! In its newly found freedom from the parent plant it was unusually energetic in its responses. This vehemence lasted for a whole day, after which a curious change crept in; the vigour of its responses began rapidly to decline. The leaf, hitherto erect, fell over, death having at last asserted its mastery. Similar reactions take place in other plants, including trees.

The wounded plant is thus able to survive the disaster, while the detached shoot with its free leaf, nurtured even in luxury, falls a prey to death. Why should there be this difference? The reason is that the plant or the tree is rooted safely in its own soil. It is the place of birth that provides it with proper nourishment and endows it with strength for its struggle in life. Many waves of change and disaster pass over it. The shocks from outside have not been able to overpower it ; they have only called forth its nascent powers. It has met external change by counter¬ change. The decaying and the effete, such as worn-out leaves, have been cast off as changing times called forth its powers of readjustment.

The tree also derives an additional power from its racial memory. Every particle of the inconspicuous embryo within the seed thus bears the deep impress of the mighty banyan tree; and so the sprouting seedling forces its roots into the yielding earth to anchor more safely; the stem rises high against the sky in search of light, and the branches with their canopy of leaves spread out in all directions. What is the strength, then, that has conferred on the tree its power of endurance and enabled it to emerge victo¬ rious from the struggle of life? It is the strength derived from the place of its birth, its power of perception and quick readjustment to change, and its inherited memories of the past.

The movements induced by external stimulation by con¬ tact and by light have been described in the last chapter. There is little vitality in the outside epidermis or skin of the plant-body upon which the stimulus impinges; hence arises the necessity for special contrivances by which stimu¬ lation at the outside may reach the deeper and more active layers of cells in an intensified form. There are thus vari¬ ous adaptations by which mechanical stimulation or the stimulus of light may induce irritation in the living cells. When we pass the hand gently over our skin, this causes little stimulation; but if there is a thorn in the flesh, the leverage exerted by it will produce an intense irritation. We have tactile hairs or bristles for accentuating the external stimulus of contact. In plants likewise, there are tactile hairs and bristles. In Mimosa, such hairs occur on the under-side of the pulvinus, and by their lever-action cause irritation of the motor tissue. In many of the tendrils, again, there are tactile pits by which the stimulus of con¬ tact is accentuated. As regards the perception of light, Haberlandt has shown that in many leaves the epidermal cells are lens-shaped, so that the incident light becomes focussed on the sensitive protoplasmic layer and thus facili¬ tates the perception of light.

The stimuli enumerated above possess energy and deliver, as it were, a blow on the plant; we can easily make out the direction of stimulation in these cases. But when we come to the effect of the stimulus of gravity, we encounter many difficulties. It should be remembered that gravity only stimulates a plant-member, when that member has been removed from its normal relation to the vertical. The diffi¬ culties which confront us are, firstly, that we cannot see the force of gravity as we do rays of light ; secondly, the force of gravity acts only indirectly through a special sense- organ, as will be subsequently explained.

The problem before us is, first, the determination of the means by which geotropic irritation is effected, and second, the exact localisation of the sense-organ by which the plant is able to perceive when it is out of the vertical, thereby directing its movement so as to become once more erect. Let us begin with consideration of some of the conditions which produce either an acceleration or a retardation of the reaction. High temperature is detrimental to geotropic reaction. I became aware of this from the records obtained under otherwise parallel conditions in Calcutta during summer (temperature 98° F.) and in the hill-station of Darjiling. The geotropic action was far more pronounced in the cooler climate of the hills.

The enhanced geotropic reaction at a moderately low temperature and the depressed reaction at a higher tem¬ perature, offer a satisfactory explanation of the movement of the Praying Palm (see fig. 32). It will be remembered that the inclined stem became increasingly erected by its enhanced geotropic reaction during the fall of temperature, while rise of temperature caused the opposite reaction of declension. The geotropic reaction can, moreover, be extraordinarily exalted by the application of moderate doses of ether vapour. I took two similar leaf-stalks of Tropaeolum majiis (Nasturtium) and subjected them to geotropic stimulation for an hour, N being normal and the other, E, etherised. Figure 58 shows the enhanced reaction under ether as com¬ pared with the normal specimen. While the normal one had

turned up to a moderate extent, the other had completed not only 90° erection, but had gone beyond it. In figure 58A the record shows that the normal rate of geotropic response is greatly enhanced after application of ether as evidenced by the sudden erection of the curve. We now return to the puzzling question as to the nature of the sense-organ by which plants perceive the vertical direction and thus obtain an indication of how to readjust themselves. Masons get their idea of the exact direction of

the force of gravity by the use of the plumb-line, that is to say, by a hanging weight. We ourselves obtain the sense of direction by means of the semi-circular canals in our internal ear. The contained fluid exerts varying pressure in different positions, and thus gives us an idea of the vertical direction. In the lower animals, the lobster, for example, there are ‘otoliths’ and sand-grains mingled with tactile hairs, which by their vertical pressure give these animals their sense of direction in space.

There is a good evidence that the weight of solid par¬ ticles, such as starch-grains, contained in the cells of plants may serve as otoliths, giving the signal and stimulus for vertical adjustment. Observation of the distribution of the starch-grains in the cells and the consideration of the changes in their position within the cells, led to the theory of statoliths ably advocated by Nemec, Haberlandt and the evidence in its sup¬ port is of an indirect na¬ ture. The direct test must lie in ascertaining if the change of position of the starch-grains is accompanied by a physi¬ ological reaction of the plant, giving an unmis¬ takable signal of percep¬ tion of geotropic stimu¬ lation by the plant as it is displaced from its normal vertical position. It is necessary, in fact, to make an exploration inside the still living plant to discover if the particular layer contain¬ ing the statoliths is the one most irritated by

I have been able to solve this problem by the device of the Electric Probe. Suppose G and G' (fig. 59) to be the layers of cells concerned in a stem with the perception of the stimulus of gravity, G and G' being the longitudinal section of a hollow circular cvlinder. The Electric Probe consists of an exceedingly fine platinum wire, enclosed in a capillary glass tube, the Probe being electrically insulated except at the extreme tip. When the Probe, suitably con¬ nected with a galvanometer, is slowly thrust into the stem, the galvanometer, by its deflection, shows the state of irri¬ tation, if any, of every layer of cells through which it passes. WTen the stem is vertical, the exploring Probe, during its transverse passage through the stem, detects no sign of local irritation.

Fig. 59. Localisation of the geo-perceptive laj’er by means of the Electric Probe. Diagram represents the geo- perceptive layer in unexcited vertical and in excited horizontal position. the vertical to a horizontal position. The geotropically sensitive layer G now perceives the stimulus and becomes the focus of excitation as evidenced by the negative electric response registered by the galvanometer. The excitation at the perceptive layer irradiates into the neighbouring cells in radial directions with an intensity diminishing with dis¬ tance. The intensity of responsive electric change is observed to decline in both directions, outwards and inwards, from the focus.

The distribution of the excitatory changes initiated at this perceptive layer, and irradiated in radial directions, is rep¬ resented by the depth of shading, the darkest shadow being on the perceptive layer itself (fig. 55, right fig.)* Had excitation been attended in the plant with a change of light into shade, we should have witnessed the spectacle of a deep shadow spreading from the perceptive layer over the dif¬ ferent layers of cells during displacement of the organ from vertical to horizontal ; the shadow would have been observed to disappear on the restoration of the organ to the vertical position.

After localising the perceptive layer by the electric probe, I made a section of the stem, and found that the cells of cnis layer contained large-sized starch-grains which by their weight were instrumental in causing stimulation. Thus the statolith-theory received independent confirmation. I have been able to devise another crucial experiment for testing of the statolithic theory. The inclination of the stem to the vertical was gradually increased until an abrupt electric response to geotropic stimulation occurred at a critical angle of inclination. This will be understood from the following consideration. In a vertical position, the par¬ ticles are at the base of each cell; excitation can only occur when they fall abruptly on to one side of the cell. Had there been no viscous friction, the particles would have tumbled down at the very beginning of the inclination of the organ; but as there is some friction, the abrupt fall of the particles only occurs at a certain critical angle of inclination.