Plant Autographs and Their Revelations

A solution of i per cent, of the venom was next applied to the plant. This produced first a great depression, as seen in the down-record, and in a short time the pulse-beat came to a permament stop (fig. 99). Subsequent wilting and decomposition showed that the plant had been killed. Fig. 97. Action of Strychnine. Enhancement of sap-pres¬ sure under a minute, and diminution under a strong, dose. The effect of a minute dose of cobra-venom, one part in a hundred thousand, produced in the plant an accelera¬ tion of activity and an enhanced rate of ascent of sap exhibited by increased pressure (fig. 100). The cut stem

Fig. 98. Effect of Cobra-venom ; Normal cardiac pulsation ; rapid diminution of pulsation and final abolition after appli¬ cation of o.i per cent, dose of venom, as seen in lower record. Note death-spasm. (Fish.) was placed in this solution for several days, and the specimen was found to remain in a vigorous living condition. This led me to investigate the effect of a minute dose of cobra-venom on the cardiac activity. In this connection, I was greatly interested to find that a preparation known as Shiichikavaran, the principal constituent of which is a minute quantity of cobra-poison, has been employed in the Hindu System of Medicine for nearly a thousand years. Its use coincided with the rise of a new school of invest!-

Fig. 99. Effect of cobra-venom in diminishing sap-pressure and abolition of activity. Fig. 1 00, Effect of highly dilute cobra-venom in enhancing sap- pressure. gators who made a systematic study of the medicinal properties of various alkaloids and metallic compounds. Shiichikavaran is still used in desperate emergencies when the patient, in the last stage of illness, is on the point of death from heart-failure. It is said that in such emergencies this preparation of cobra-venom is most effective in reviving and strengthening the cardiac activity.

In order to observe the effect of Shiichikavaran on the animal heart in a depressed condition, I injected a dilute solution of the preparation into the sinus of the fish. This produced a decided improvement in the frequency and amplitude of the pulsation (fig. loi). In other cases irregular pulsations became quite regular after the injection. The incessant activities of life require expenditure of energy that has been previously stored by the organism. Taking, for example, the rise of sap, the ceaseless pumping activity of the propulsive tissue raises enormous quantities of water to a considerable height. The energy of doing this work issues from the breakdown of organic chemical substances in internal combustion or respiration. The loss of energy must be restored by absorption and storage of energy from outside.

The supply of energy may be maintained in either of two ways : it may be absorbed by the organism in the active or kinetic form; or in the inactive, potential or latent form. The former is the method characteristic of plants ; the latter of animals. The plant, in virtue of its green colouring matter, chlorophyll, can absorb the kinetic energ}^ of the sun’s rays and is thereby enabled to build up from the carbonic acid gas of the atmosphere, organic matter, that is, complex carbon compounds in which is stored, in the latent form of chemical combination, the energy of the absorbed light. The organic matter so produced is partly used in the maintenance and growth of the plant-body and consumed in carrying on its various vital activities. The remainder is stored in the tissues of the plant as reserve-material for future growth and, more particularly, in seeds and fruits for reproduction.

The animal, on the contrary, is entirely dependent for its supply of both energy and material upon organic food, which has been, originally at least, produced by green plants. Thus the animal is dependent upon the plant ; and both are dependent upon the sun, the plant directly, the animal indirectly. The sun is, in fact, the prime source of all the energy that is set free in the living organism as heat, electric current, or movement ; and, more generally, in the processes of combustion. To stand before a coal-fire is to bask in the sun which shone millions of years ago in the Carboniferous Period.

Hence the carbon-assimilation of the green plant, Photo¬ synthesis, as it is now termed, is a process of the greatest theoretical and practical interest which deserves thorough investigation with a view, more particularly, to determine the conditions that affect its activity. This can be effected by accurately measuring the gaseous interchange between the plant and the atmosphere during photosynthesis, that is, either the volume of carbon dioxide absorbed, or the equal volume of oxygen evolved. The measurement of the intake of CO2 involves complicated chemical analysis and is too long and laborious a method. The measurement of the output of oxygen is more promising. I have, in fact, been able to devise an apparatus on this principle, which I will now describe.

Water-plants obtain their carbon from the carbonic acid dissolved in the water. When sunlight falls on these plants, carbonic acid gas is broken up, the carbon becomes fixed in the form of organic compounds known as carbohydrates, and an equal volume of oxygen is evolved which rises as a stream of bubbles from the plant. The rate of evolution of oxygen indicates the rate of assimilation. Numerous difficulties were encountered in making this method practical ; they have been overcome by my Automatic Recorder. A piece of a water-plant, e.g. Hydrilla verticillata, is placed in a bottle completely filled with tank-water containing sufficient COo in solution, the open end of which is closed by a special bubbling-apparatus, the Bubbler, for measuring the oxygen evolved. The Bubbler consists of a U-tube, the further end of which is

closed by a drop of mercury acting as a valve. The oxygen evolved by the plant, entering the U-tube, produces an increasing pressure, which eventually lifts the mercury valve and allows the escape of a bubble of the gas. The valve then immediately closes until it is lifted once more for the escape of another equal volume of gas. The movement of the mercury completes an electric circuit, which either rings S, bubbled with stop-cock; E, the electric pencil for completing electric contact through drop of mercury, ]\I ; A, adjusting screw; V, voltaic cell ; C, condenser ; D, revolving drum ; W, electro magnetic writer ; G, governor, shown separately at P with pair of hinged levers, H ; I, ink-recorder. Electric bell not shown.

a bell or makes an electro-magnetic writer inscribe successive dots on a revolving drum (fig. 102). The automatic method eliminates all personal errors of observation ; it is so extremely sensitive that it is possible to measure a deposit of carbohydrate as minute as a millionth of a gram. I will give the following example. The plant with the apparatus is so placed as to face the northern light; the bell rings each time it has evolved a certain amount of oxygen representing an equal volume of absorbed COo. If a person now stands obstructing the light, the assimilation is slowed down and the bell now strikes at longer intervals. When strong sunlight is thrown on the plant, the successive strokes on the bell become greatly quick¬ ened. The plant is such a sensitive detector of light that it may be employed as a photometer for indicating the slightest variations in the intensity of the light of the sky.

I have been successful in devising a contrivance by which the plant actuates an electric switch and turns on a light as soon as the sky-light is dimmed by a passing fog, the switch being turned off as soon as the sky becomes clear. This device may prove useful during winter in London. Still more interesting is the automatic record given by the electro-magnetic writer in the form of successive dots inscribed on the drum. When the rate of assimilation is in any way enhanced, the dots come close together. Depressed assimilation is indicated by widely separated dots.

At what hour of the day does the plant assimilate COo at the quickest rate? For this determination I took suc¬ cessive records from 7.30 a.m. until 5 p.m. for five minutes at a time. When the sun rose at 6.45 a.m. the light was too feeble to be effective. At 7.30 a.m. assimilation began, and the plant evolved four bubbles of oxygen in the course of five minutes ; with the progress of the day it became more and more hungry, until at i p.m. it took four times as much CO2 as it took in the morning. It is curious that the plant should also be most hungry at our lunch-time ! The real reason for increased assimilation at i p.m. is the favourable conditions of light and temperature. The activity declined in the afternoon and became arrested with the onset of darkness (fig. 103).

Fig. 103. Automatic Record of successive Bubblings for five minutes Effect of Stimulation on Assimilation As assimilation of food is essentially a vital process, any irritation by excessive stimulation proves to be highly detrimental. Thus a plant which was assimilating actively and showed its enjoyment by ringing the bell with great rapidity became suddenly depressed after receiving a strong electric shock. It was ‘off its feed’ for a long time, as indicated by the stoppage of the tinkling of the bell. The stronger the stimulation the more prolonged the stoppage of assimilation. The obvious moral is that we should take absolute rest for at least half an hour before meals. Food may indeed turn into poison when we are under an intense irritation.

During the course of my investigations I discovered that the presence of the minutest traces of certain chemical sub¬ stances induced an extraordinarily great enhancement of carbon-assimilation. The dilution employed was one part in a billion (billion in French measure is equal to 1000 millions). With certain substances a dilution of one part in two billions produced an increase of activity of more than a hundred per cent. The activity declined when the strength of the solution was raised above a critical dose. Dilute extract of thyroid gland, in a dilution of one part in a billion, produced a maximum increase of activity of about 70 per cent. The noticeable fact in the action of thyroid extract is that no diminution of activity below normal took place for a considerable range of dilution. The effect of traces of iodine was more or less similar. At first sight it is incon¬ ceivable that infinitesimal traces of certain chemical sub¬ stances should have such a potent influence on life-activity. The immediate and concrete demonstration of the effect of minute traces of chemicals on carbon-assimilation is of special interest, since it enables us to understand the effect of ultra-measurable quantities of vitamins on general assimilation and of hormones on physiological reaction.

The economic life of the present age may be said to be dependent to a great extent on the utilisation of the solar energy that has been stored in past ages by vegetable life. What is the efficiency of the plant-mechanism for the storage? It has hitherto been regarded as extremely low, less than i per cent. : but the methods hitherto employed in this determination have been more or less defective. I therefore undertook a careful re-deter¬ mination by new and highly sensitive methods. The incident solar energy was carefully determined by my Magnetic Radiometer, and the energy stored by the plant was also accurately measured. The efficiency was found to be much higher than had been generally supposed, being as high as 7.4 per cent. It is interesting to compare the efficiency of transformation in an ordinary steam-engine with that in the photosynthetic organ. In the former the potential energy of coal is transformed into the kinetic energy of motion; in the latter the kinetic energy of radia¬ tion is transformed into the potential energy of complex chemical compounds. The efficiency of the photosynthetic organ may be taken as about half that of an ordinary steam- engine. After all, it may not be such an unpractical proposi¬ tion to devise a chlorophyll apparatus for trapping sunlight.

When the tip of one’s finger is gently scratched, an impulse is created which is perceived in the brain as sensation. The message is transmitted along a nerve-thread, which is the definite channel for the conduction of impulse ; when the nerve is injured in any way there is an end of all sensation. The scratch and the resulting sensation appear to be simultaneous, but in reality a short time is required for the impulse to travel from the finger-tip to the brain. The speed of the nervous impulse may be found somewhat as follows : the person on whom the experiment is made gives a signal when he feels the sensation of the scratch on his toe. The interval between the scratch and the signal enables us to calculate the speed of the impulse through the length of the nerve.

If the nerve terminates in a muscle, then the arrival of the impulse is signalled by the twitch of the muscle. Experi¬ ments on nervous impulse are usually carried out with a piece of nerve and muscle of the frog, which may be detached and kept alive for several hours. If now a distant point of the nerve be stimulated by an electric shock, an impulse is transmitted along the nerve to the terminal muscle which is attached to a recording lever. The record is taken on a moving drum, on which time-marks are inscribed by a chronograph. The speed of the impulse is found from the length of the nerve and the time recorded for transmission.

In the nervous circuit of the animal three different parts may be distinguished. The first is the ‘ receptor,’ which receives the shock from outside; the second is the ‘conductor/ the ‘nerve/ by which excitation is carried to a distance, though no visible change occurs in this conducting tissue during the transmission of an impulse. Finally, the impulse impinges on the terminal responding organ, the ‘effector,’ which may be a muscle; the response is then visibly manifested by movement.

The beginnings of such a nerve and muscle system are seen in animals as low as the sea-anemone, where stimulation of its tenacles causes a motile reaction at a distant part, there being no movement in the inter¬ mediate region. The receptor and the effector are thus at a distance from each other, the connecting link being the nerve. This mode of transmission of excitation, where the effect of a stimulus applied at a point is manifested by a movement at a distance, would appear to be not unlike what occurs in the Sensitive Plant Mimosa pudica. Here also the applica¬ tion of stimulus, say of an electric shock, to one of the sub¬ petioles, gives rise to an impulse which, travelling onwards along the leaf-stalk, reaches its motile organ, the pulvinus, the contraction of which produces the sudden fall of the leaf. Though the effects produced in the plant and in the animal are so similar, yet the prevalent opinion has been that impulse travels in the plant in a manner quite different from that in the animal nerve.

Let us consider on what experimental fact this con¬ clusion is based. Pfeffer gave a knife-stab to the plant to stimulate it, and observe how the plant answered under such brutal treatment. Imagine expecting a man to give any rational answer when the stimulus employed is a knife- thrust ; so far from giving any coherent response, he would be thrown into convulsions ! After the knife-thrust into the plant, Pfeffer noticed the escape of sap from the wound. The turgid stem of the plant he imagined to be like an india-rubber tube filled with water, the escape of sap being supposed to produce a sudden diminution of pressure,

causing a traction or pull on the sensitive pulvinus. This hydro-mechanical impulse is considered practically equivalent to the movement of water in a pipe. Ricca is also enamoured of the method of knife-thrust in causing stimulation. He imagines that the wounding of the wood causes it to secrete some stimulating sub¬ stance, a hormone, and that this hypothetical hormone is conveyed by the ascent of sap to the leaf, which it stimu¬ lates to movement. This is a misapplication of the theory of the hormone as enunciated by its authors. Starling and Bayliss, who insisted on the fundamental difference between the two modes of communication between two distant organs, by transfer of matter, and by transmission of motion. The first is exemplified by the slow movement of liquids carrying chemical stimulants in solution, such as occurs in the ascent of sap in the plant, or in the circulation of blood in the animal; the second is the rapid conduction of excitation from point to point associated with the pro¬ pagation of nervous impulse. These two different modes have been aptly likened to communication by post or by telegraph. The difference between the two speeds is so great that it would be an unpardonable mistake to confuse one with the other.

The hydro-mechanical theory and the theory of hormone- transport by ascent of sap are both based on the supposition that a wound is necessary to produce a mechanical dis¬ turbance or the secretion of an irritant causing stimula¬ tion. They stand condemned if it can be shown that an excitatory impulse is generated and conducted in the plant by a feeble stimulus and without any wound. The plant is highly excitable, and a very feeble stimulus is sufficient to start an impulse. The only excuse for using a knife-thrust as a stimulus is the erroneous supposition that plants are very much less sensitive than animals, and must therefore be goaded into activity by violence. This is a gratuitous and totally unfounded assumption, for, as previously stated, I discovered that Mimosa can be excited by an electric shock of one-tenth of the intensity of that

which evokes human sensation. No wound is produced, yet the excitation is transmitted to a considerable distance. This result alone is sufhcient to show the totally unfounded character of both the hydro-mechanical and hormone- ascent theories. I shall describe other crucial experiments which will completely disprove them. a, the Scratch-Stimulator ; b, effect of stimula¬ tion of moderate intensity, s, applied on left gives rise to simultaneous impulses up and down ; c, effect of strong stimulus s' applied on right side causes an impulse which ascends on the right side and then descends on the left.

A hypothetical stimulant is, according to this theory, conveyed to the leaf in the ascending sap. The impulse in this case should always travel upwards in the same direction as the sap, and not downwards against that direction. The speed of the impulse, moreover, should be the same as that of the sap-movement. I applied the stimulus of a superficial scratch on one side of the stem of Mimosa, and found that the excitation travelled simul¬ taneously both upwards and downwards, causing the fall of leaves both above and below. On increasing the inten¬ sity of the stimulus, the excitation was found to ascend along one side of the stem, reach the apex, and then descend down the other side (fig. 104). Rising sap could not possibly have produced such characteristic results. The conduction of excitation up and down the stem can only be attributed to the presence of a special conducting tissue, that is a nervous tissue.

Accurate measurements which I have carried out show, moreover, that the rate of transmission of excitation is several hundred times quicker than that of the ascent of sap. That the movement of sap has nothing whatever to do with conduction of excitation finds a further striking demonstration in the following experiment : A drop of hydrochloric acid was applied to the tip of the uppermost leaf of Mimosa. The impulse generated travelled to a considerable distance downwards against the direction of the normal ascent of sap; subsequent chemical examination proved that the stimulant had not been transported, but had remained localised at the point of application.

Having proved that the transmission of excitation is neither hydro-mechanical nor due to the movement of sap, I shall next adduce evidence to show that the con¬ duction in the plant is a propagation of protoplasmic excitation, just as in the excited nerve of the animal. It is obvious that the mechanical movement of water through a pipe will not be affected by heat or cold within reasonable limits ; the pipe will not lose consciousness and stop the flow of water if it be anaesthetised, nor will its conducting power be abolished by applying round it a bandage soaked in poison. These agents will, on the other hand, profoundly affect the transmission of nervous excitation. The nature of an impulse, whether mechani-

cal or nervous, may be discriminated by several crucial tests. If physiological changes affect the rate of conduction, then the impulse must he of a nervous character; the absence of any such effect, on the other hand, proves the mechanical character of the impulse. There are various physiological means by which it is possible to retard or arrest the nervous impulse, but have no such effect on a mechanical impulse. Some of these are as follows : ( 1 ) When the conducting tissue or nerve is cooled, the speed of the impulse is slowed down, culminating in its arrest.

(2) Poisonous solutions applied on the nerve permanently abolish its conducting power. (3) The conducting power is temporarily arrested by a block produced by the passage of an electric current in a portion of the nerve through which the impulse is being transmitted. This electrotonic block is removed on the stoppage of the current. The tests are carried out by automatically determining the velocity of impulse first under normal conditions, and then under other conditions which are known to modify the transmission of excitation in animal nerve. The normal velocity is found by applying a definite inten¬ sity of electric shock on the leaf-stalk at a specified distance, say 30 mm., from the motile pulvinus, the intensity of stimulus being maintained constant in successive experi¬ ments. The moment of application of the shock is marked in the record by a vertical line. The Resonant Recorder taps successive dots at intervals, say of one-tenth of a second, and thus measures the interval between the appli¬ cation of stimulus and the resulting movement of the leaf indicated by an up-curve. The interval between stimulation and response is seen to be 16.2 spaces, each of the value of o.i second (fig. 105). The total interval is therefore

1.62 second. In order to prove the reliability of the method, two successive records are taken, which show that the time required under normal conditions is practically the same in both experiments. The recorded time includes the latent period of the pulvinus, which represents the time consumed in starting the motor mechanism into action. The length of the latent period, that is, the interval between stimulation and response, is determined by applying stimu¬ lus directly to the pulvinus. In the present case this was

Fig. 105. Determination of velocity of transmission of excitation in the petiole of Mimosa. Two lower records are in response to indirect stimula¬ tion applied at a distance of 30 mm. ; upper record of response to direct stimulation gives the latent period. Recorder 10 v. per second. 0.12 second, and the actual time of transmission of the im¬ pulse through a distance of 30 mm. is therefore 1.62 — 0.12 second, or 1.5 second. The velocity of the impulse is therefore 30/1.5, or 20 mm. per second.