Experiment 58.—High magnification records are taken for successive periods of ten seconds, for selected temperatures, maintained constant during the particular observation. In figure 63 is given records of rate of growth obtained with a specimen of Kysoor at certain selected temperatures. It will be seen that the rate of growth increases with the rise of temperature to an optimum, beyond which the growth-rate undergoes a depression. In the present case the optimum temperature is in the neighbourhood of 35°C.
METHOD OF CONTINUOUS OBSERVATION.
The method of observation that I have described above is not ideally perfect, but the best that could be devised under the circumstances. A very troublesome complication of pulsations in growth, arises at high temperatures, which render further record extremely difficult. Growth is undoubtedly a pulsatory phenomenon; but under favourable circumstances, these merge practically into a continuous average rate of elongation. At a high temperature the effect of certain disturbing factors comes into prominence. This may be due to some slight fluctuation in the temperature of the chamber, or to the effect of thermal radiation from the side of the chamber. This disturbing influence is most noticed at about 45°C, rendering the record of growth above this point a matter of great uncertainty. It will presently be shown that in plants immersed in water-bath growth is often found to persist even up to 57°C.
The only way of removing the complication arising from thermal radiation lies in varying the temperature condition of the plant, by direct contact with water at different temperatures. This procedure will also remove uncertainty regarding the body of the plant assuming the temperature of surrounding non-conducting air. The disturbing effect of sudden variation of temperature is also obviated by a more uniform regulation of rise of temperature. The inner cylinder containing the plant is filled with water; heat from gradually warmed water in the outer cylinder is conducted across the inner cylinder made of thin copper and raises the temperature of the water contained in the inner cylinder with great uniformity. A clock-hand goes round once in a minute; the experimenter, keeping his hand on the stop-cock, adjusts the rate of rise of water in the inner cylinder, so that there is a rise, say, of one-tenth of a degree every 6 seconds or of one degree every minute. The mass of water acts as a governor, and prevents any sudden fluctuations of temperature. The adoption of this particular device eliminated the erratic changes in the rate of growth that had hitherto proved so baffling.
The elongation recorded by the Crescograph will now be made up of (1) physical expansion, (2) expansion brought about by absorption of water, and (3) the pure acceleration of growth. The disentanglement of these different elements presented many difficulties. I was, however, able to find out the relative values of the first two factors in reference to the elongation of growth. This was done by carrying out a preliminary experiment with a specimen of plant in which growth had been completed. It was raised through 20°C in temperature, records being taken both at the beginning and at the end. This was for obtaining a measure of the physical change due to temperature, and also of the change brought about by absorption of water. I should state here that for the method of continuous record of growth which I contemplated, the record had to be taken for about 18 minutes. The magnification had to be lowered to 250 times to keep the record within the plate. With this magnification, the fully grown specimen did not show in the record a change even of 1 mm. in length in 18 minutes, while the growing plant under similar circumstances exhibited an elongation of 100 mm. or more. In records taken with low magnification, the effect of physical change is quite negligible.
DETERMINATION OF THE CARDINAL POINTS OF GROWTH.
The cardinal points of growth are not the same in different plants; they are modified in the same species by the climate to which the plants are habituated; the results obtained in the tropics may thus be different from those obtained in colder climates. At the time of the experiment, the prevailing temperature at Calcutta in day time was about 30°C.
Temperature minimum: Experiment 59.—For the determination of the minimum, I took a specimen of S. Kysoor, and subjected it to a continuous lowering of temperature, by regular flow of ice-cold water in the outer vessel of the plant-chamber. Record was taken on a moving plate for every degree fall of temperature; growth was found to be continuously depressed, till an arrest of growth took place at 22°C. (Fig. 64).
The arrested growth was feebly revived at 23°C., after which with further rise of temperature there was increased acceleration. The optimum point was reached at about 34°C. In some plants the optimum is reached at about 28°C., and the rate remains constant for the next 10 degrees or more.
Temperature maximum: Experiment 60.—For the determination of the maximum, the temperature was raised much higher. At 55°C. growth was found to be greatly retarded with practical arrest at 58°C. At 60°C. there occurred a sudden spasmodic contraction (Fig. 65), which I have shown elsewhere to be the spasm of death. This mechanical spasm at 60°C. is also strikingly shown by various pulvinated organs. An electric spasm of galvanometric negativity, and a sudden diminution of electrical resistance also take place at the critical temperature of 60°C.[U]
I have described the immediate effect at the critical point. Long maintenance at a temperature few degrees below 60°C. will no doubt be attended with the death of the organ. Fatigue is also found to lower the death-point.
THE THERMO-CRESCENT CURVE.
Experiment 66.—I was next desirous of devising a method by which an automatic and continuous record of the plant should enable us to obtain a curve, which would give the rate of growth at any temperature, from the arrested growth at the minimum to a temperature as high as 40°C. In order to eliminate the elements of spontaneous variation, the entire record had to be completed within a reasonable length of time, say about 18 minutes for a rise of as many degrees in temperature. This gives a rate of rise of 1°C. for one minute. Separate experiments showed that at this rate of continuous rise of temperature there is practically no lag in the temperature assumed by thin specimens of plants. For observation during a limited range I use the slower rate of rise at 1°C. per two minutes. But the result obtained by slower rise was found not to differ from that obtained with one degree rise per minute. The curve of growth is taken on a moving plate, which travels 5 mm. per minute. Successive dots are made by the recording lever at intervals of a minute during which the rise of temperature is 1°C. A Thermo-crescent Curve is thus obtained, the ordinate of which represents increment of growth, and the abscissa, the time. As the temperature is made to rise one degree per minute, the abscissa also represents rise of temperature (Fig. 66). The vertical distance between two successive dots thus gives increment of growth in one minute for 1 degree rise of temperature from T to T′. If l represents this length, t the interval of time (here 60 sec.), and m the magnifying power of the recorder, then the rate of growth for the mean temperature (T+T′)2 is found from the formula: rate of growth at (T+T′)2 = lm × t × 60 103µ per sec.
TABLE XI.—RATE OF GROWTH FOR DIFFERENT TEMPERATURES.
| Temperature. | Growth. | Temperature. | Growth. |
| 22°C | 0.00 µ per sec. | 31°C | 0.45 µ per sec. |
| 23°C | 0.02 µ " " | 32°C | 0.60 µ " " |
| 24°C | 0.04 µ " " | 33°C | 0.80 µ " " |
| 25°C | 0.06 µ " " | 34°C | 0.92 µ " " |
| 26°C | 0.08 µ " " | 35°C | 0.84 µ " " |
| 27°C | 0.12 µ " " | 36°C | 0.64 µ " " |
| 28°C | 0.16 µ " " | 37°C | 0.48 µ " " |
| 29°C | 0.22 µ " " | 38°C | 0.30 µ " " |
| 30°C | 0.32 µ " " | 39°C | 0.16 µ " " |
I give in figure 67 a curve showing the relation between temperature and growth.
It will thus be seen that, in the course of an experiment lasting about twenty minutes, data have been obtained which enable us to determine the rates of growth through a wide range of temperature. We have likewise been able by the first method to make very accurate determinations of the temperature maximum and minimum. In short, by adopting the methods described, the cardinal points of growth and the rate of growth at any temperature, may be determined with a precision unattainable by the older methods, of averages or of prolonged observation.
SUMMARY.
Temperature induces variation in the rate of growth. In accurate determination of the growth, the disturbing effect of radiation of heat has not been eliminated.
A continuous record of growth under uniform rise of temperature gives the Thermo-crescent curve, from which the rate of growth at any temperature may be deduced.
Different plant-tissues exhibit characteristic differences in their cardinal points of growth. In Kysoor, growth is arrested at the temperature minimum of 22°C. The optimum temperature is at 34°C., after which growth-rate declines and becomes completely arrested at 58°C. At 60°C. there is a sudden spasmodic contraction of death.
In other plants the cardinal points are different. In some plants the optimum growth is attained at 28°C. and remains constant up to 38°C.
XII.—THE EFFECT OF CHEMICAL AGENTS ON GROWTH
By
Sir J. C. Bose,
Assisted by
Guruprasanna Das.
Chemical agents are found to exert characteristic actions on growth. The method of investigation sketched here opens out an extended field of investigation. The effect of a chemical substance, I find, to be modified by (1) the strength of the solution, (2) the duration of application, and (3) the condition of the tissue. A poisonous substance in minute doses is often found to exert a stimulating action. Too long continued action of a stimulant, on the other hand, exerts a depressing effect. The influence of the tonic condition is shown by the fact that while a given dilution of a poisonous substance kills a weak specimen, the same poisonous solution, applied to a vigorous specimen, actually stimulates and enhances the rate of the growth. I give below descriptions of a few typical reactions.
The reagent, when in a liquid form, is locally applied on the growing organ. The records, taken before and after the application, exhibit the stimulatory or depressing character of the reagent. A different method of application of the reagent is employed for plants with extended region of growth. The specimen is then enclosed in a glass cylinder, with inlet and outlet pipes. The cylinder is first filled with water, and the normal rate of growth recorded. This rate remains constant for several hours; but prevention of access of air for too long a time affects the normal growth. After obtaining normal record, water charged with the giving chemical agent is passed into the cylinder; and the subsequent record shows the characteristic effect of the reagent. The introduction of a gas into the chamber offers no difficulty.
EFFECT OF STIMULANTS.
Hydrogen Peroxide: Experiment 62.—This reagent, as supplied by Messrs. Parke Davis & Co., was diluted to 1 per cent. and applied to the growing plant. Its stimulating action on growth is demonstrated in the right hand record of Fig. 68a, where the rate of growth is seen enhanced two and a half times the normal rate.
Ammonia: Experiment 63.—The immediate effect of dilute vapour of this reagent is an enhancement of growth, seen in the middle record of Fig. 68b, where the rate is seen to be double the normal. Continued action, however, caused a depression; the third record of this series shows this, where the reduction is three-fourths of the normal rate.
EFFECT OF ANÆSTHETICS.
Ether: Experiment 64.—In Fig. 68c, the records exhibit the effect of introduction of ether vapour into the plant chamber, and its recovery after the removal of the vapour. Ether is seen to depress the rate of growth to a little more than a third of the normal rate. The recovery is seen to be nearly complete half an hour after the removal of the vapour.
Carbonic Acid: Experiment 65.—The action of this gas is very remarkable. The plant was immersed in water and normal record taken; the plant chamber was now filled with water, charged with carbonic acid gas. This induced a very marked acceleration of growth (Fig. 69). In a seedling of Onion, the increase was found to be two and a half times. In the flower bud of Crinum, the rate was found enhanced threefold from the normal 0.25 µ to 0.75 µ per second. After this preliminary enhancement, there was a depression of growth within 15 minutes of the application, the rate being now reduced to 0.15 µ per second. These effects were found to take place equally in light or in darkness.
ACTION OF DIFFERENT GASES.
Coal Gas: Experiment 66.—Coal gas induces a depression. It is curious that subjection to the action of this gas does not produce so evil an effect as one would expect. The introduction of the gas had reduced the growth-rate to more than half; but there was a recovery half an hour after the introduction of fresh air.
Sulphuretted Hydrogen: Experiment 67.—This gas not only exerts a depressing effect, but its after-effect is also very persistent. The plant experimented on was very vigorous and its rate of growth was depressed to half by subjection to the action of the gas for a short time. The record taken half an hour after the introduction of fresh air did not exhibit any recovery.
ACTION OF POISONS.
Ammonium Sulphide: Experiment 68.—This reagent in dilute solution retards growth, and in stronger solution acts as a poison. The following results were obtained with a wheat seedling under different strengths of solution:—
| Normal rate | 0.30 µ per sec. |
| 0.5 per cent. solution | 0.15 µ " " |
| 2.0 " " " | 0.08 µ " " |
Copper Sulphate: Experiment 69.—The effect of a solution of this reagent is far more depressing than the last. One per cent. solution acting for a short time depressed the rate from 0.45 µ to 0.13 µ per sec. Long continued action of the poisonous solution kills the plant.
SUMMARY.
The effect of a chemical agent is modified by the strength of the solution, the duration of application and the tonic condition of the tissue.
Dilute solution of hydrogen peroxide induces an acceleration of growth.
The action of dilute vapour of ammonia is a preliminary enhancement followed by depression of growth.
Ether vapour depresses the rate of growth. On the removal of the vapour there is a recovery of the normal rate.
The effect of carbonic acid is a great enhancement of the rate of growth; after this preliminary action, growth undergoes a decline. The effect described takes place equally in light or in darkness.
Coal gas induces a depression of the rate of growth from which there is a recovery after the removal of the gas. The action of sulphuretted hydrogen is far more toxic, the after-effect being very persistent.
Solution of ammonium sulphide induces increasing retardation of growth, with the strength of the solution. Copper sulphate solution acts as a toxic agent, retarding the rate of growth and ultimately killing the plant.
XIII.—EFFECT OF VARIATION OF TURGOR AND OF TENSION ON GROWTH
By
Sir J. C. Bose.
The movements of leaves of sensitive plants are caused by variation of turgor in the pulvinus induced by stimulus. The down movement or negative response of Mimosa is caused by a diminution or negative variation of turgor, while the erection or positive response is brought about by an increase, or positive variation of turgor.
We shall now investigate the change induced in a growing organ in the rate of growth by variation of turgor. Turgor may be increased by enhancing the rate of ascent of sap or by an artificial increase of internal hydrostatic pressure. A diminution of turgor may, on the other hand, be produced by withdrawal of water through plasmolysis. In order to maintain a constant terminology I shall designate an increase, as the positive, and a diminution, the negative variation of turgor.
RESPONSE TO POSITIVE VARIATION OF TURGOR.
In experimenting with Mimosa the plant was subjected to the condition of drought, water being withheld for a day. On supplying water, the leaf, after a short period, exhibited a positive or erectile movement (Expt. 12). The delay was evidently due to the time taken by the water absorbed by root to reach the responding organ.
Method of Irrigation: Experiment 70.—In order to investigate the effect of enhanced turgor on growth, I took a specimen of Kysoor which had been dug up with an attached quantity of soil; this latter was enclosed in a small bag. The plant was then securely clamped and fixed on a stand. This precaution was taken to prevent upward displacement by the swelling of the soil in flower pot of the plant under irrigation. The specimen was then subjected to a condition of drought, water being withheld for a day. The depressed rate of growth is seen in record (Fig. 70). Ordinary cold water was now applied at the root, the effect of which is seen in record C. Finally the record (H) was obtained after irrigation with tepid water. It will be seen that the spaces between successive dots, representing magnified growth at intervals of ten seconds, are very different. While a given elongation took place under drought in 19 × 10 seconds, a similar lengthening took place, after irrigation with cold water, in 13 × 10 seconds, and after irrigation with warm water in 3 × 10 seconds. Irrigation with warm water is thus seen to increase the rate of growth more than six times.
The enhancement of the rate of growth on irrigation with cold water took place after seventy seconds. The interval will obviously depend on the distance between the root by which the water is absorbed and the region of growth. It will further depend on the activity of the process of the ascent of sap. The time interval is greatly reduced when this activity is in any way increased. Thus the responsive growth elongation after application of warm water was very much quicker; in the case described it was less than 20 seconds. With regard to application of warm water, the variation of temperature should not be too sudden; it should commence with tepid, and end with warm water. Sudden application of hot water brings about certain complications due to excitatory effect. As regards the persistence of after-effect of a single application of warm water, it should be remembered that the absorbed water gradually cools down. In an experiment with a peduncle of Zephyranthes the growth under partial drought was found to be 0.04 µ per second; application of warm water increased the growth rate to 0.20 µ per second. After 15 minutes the growth rate fell to 0.13 µ per second; and after an hour to 0.08 µ per second. It will be noted that even then the rate was twice the initial rate before irrigation.
TABLE XII.—EFFECT OF IRRIGATION.
| Specimen. | Condition of Experiment. | Rate of growth. |
| Kysoor | Dry soil | 0.21 µ per second. |
| Irrigation with cold water | 0.30 µ " " | |
| Irrigation with warm water | 1.33 µ " " | |
| Peduncle of Zephyranthes | Dry soil | 0.04 µ " " |
| Irrigation with warm water | 0.20 µ " " |
EFFECT OF ARTIFICIAL INCREASE OF INTERNAL HYDROSTATIC PRESSURE.
Increased turgor was, next, artificially induced by increase of internal hydrostatic pressure.
Experiment 71.—The plant was mounted water-tight in the short limb of an U-tube, and subjected to increased hydrostatic pressure by increasing the height of the water in the longer limb. Table XIII shows how increasing pressure enhances the rate of growth till a critical point is reached, beyond which there is a depression. This critical point varies in different plants.
TABLE XIII.—EFFECT OF INCREASED INTERNAL HYDROSTATIC PRESSURE (Kysoor).
| Specimen. | Hydrostatic pressure. | Rate of growth. |
| Normal | 0.18 µ per second. | |
| No. II | 2 cm. pressure | 0.20 µ " " |
| 4 cm. " | 0.11 µ " " | |
| Normal | 0.13 µ " " | |
| No. II | 1 cm. pressure | 0.20 µ " " |
| 3 cm. " | 0.18 µ " " | |
| 4 cm. " | 0.15 µ " " |
RESPONSE TO NEGATIVE VARIATION OF TURGOR.
I shall now describe the influence of induced diminution of turgor on the rate of growth.
Method of plasmolysis: Experiment 72.—Being desirous of demonstrating the responsive growth variations of opposite signs in an identical specimen under alternate increase and diminution of turgor, I continued the experiment with the same peduncle of Zephyranthes in which the growth acceleration was induced by irrigation with warm water. In that experiment the growth rate of 0.04 µ per second was enhanced to 0.20 µ per second after irrigation. A strong solution of KNO3 was now applied at the root; and the growth-rate fell almost immediately to 0.03 µ per second, or nearly to one-third the previous rate, the depression induced being thus greater than under condition of drought (Fig. 71).
TABLE XIV.—EFFECT OF ALTERNATE VARIATION OF TURGOR ON GROWTH (Zephyranthes).
| Condition of Experiment. | Rate of growth. |
| Dry soil | 0.04 µ per second. |
| Application of warm water | 0.20 µ " " |
| Steady growth after 1 hour | 0.08 µ " " |
| Application of KNO3 solution | 0.03 µ " " |
From the series of results that have been given above, it will be seen that employing very different methods of turgor variation, the rate of growth, within limits, is enhanced by an increase of turgor. A diminution or negative variation of turgor, on the other hand, brings about a retardation or negative variation in the rate of growth. We should, in this connection, bear in mind the fact that, growth is dependent on protoplasmic activity, and the variation of turgor itself is also determined by that activity.
RESPONSE OF MOTILE AND GROWING ORGANS TO VARIATION OF TURGOR.
I have already described (p. 40) the effects of variation of turgor on the motile pulvinus of Mimosa. There is a strict correspondence between the responsive movement of the leaf of Mimosa and the movement due to growth, which is summarized as follows:—
(1) An increase or positive variation of turgor induces an erection or positive response of the leaf of Mimosa, and a positive variation or enhancement of the rate of growth.
(2) A diminution or negative variation of turgor induces a fall or negative response of the leaf of Mimosa, and a negative variation or retardation of the rate of growth.
EFFECT OF EXTERNAL TENSION.
Experiment 73.—The recording levers are at first so balanced that very little tension is exerted on the plant. Record of normal growth is taken of a specimen of Crinum. The tension is gradually increased from one gram to ten grams. The table given below shows how growth-rate increases with the tension till a limit is reached, after which there is a retardation.
TABLE XV.—EFFECT OF TENSION ON GROWTH.
| Tension. | Rate of growth. |
| 0 (Normal) | 0.41 µ per second. |
| 4 grams | 0.44 µ " " |
| 6 " | 0.48 µ " " |
| 8 " | 0.52 µ " " |
| 10 " | 0.40 µ " " |
SUMMARY.
Increase of turgor induced by irrigation enhances the rate of growth. Irrigation with warm water induces a further augmentation of the rate of growth.
The latent period for enhancement of growth depends on the distance of growing region from the root. The latent period is reduced when the plant is irrigated with warm water.
Artificial increase of internal hydrostatic pressure, up to a critical degree, enhances the rate of growth.
A diminution or negative variation of turgor depresses the rate of growth.
There is a strict correspondence between the responsive movement of the leaf of Mimosa, and the movement due to growth. An increase or positive variation of turgor induces an erection of positive response of the leaf of Mimosa, and a positive variation or enhancement of the rate of growth. A diminution or negative variation of turgor induces a fall or negative response of the leaf of Mimosa, and a negative variation or retardation of the rate of growth.
External tension within limits, enhances the rate of growth.
XIV.—EFFECT OF ELECTRIC STIMULUS ON GROWTH
By
Sir J. C. Bose,
Assisted by
Guruprasanna Das.
In plant physiology, the word ‘stimulus’ is often used in a very indefinite manner. This is probably due to the different meanings which have been attached to the word. An agent is said to stimulate growth, when it induces an acceleration. But the normal effect of stimulus is to cause a retardation of growth. It is probably on account of lack of precision in the use of the term that we often find it stated, that a stimulus sometimes accelerates, and at other times, retards growth. In order to avoid any ambiguity, it is very desirable that the term stimulus should always be used in the sense as definite as in animal physiology. An induction shock, a condenser discharge, the make or break of a constant current, a sudden variation of temperature, and a mechanical shock bring about an excitatory contraction in a muscle. These various forms of stimuli cause, as we have seen, a similar excitatory contraction of the motile pulvinus of Mimosa pudica. We shall enquire whether the diverse forms of stimuli enumerated above, exert similar or different reactions on the growing organ.
EFFECT OF ELECTRIC STIMULUS OF VARYING INTENSITY AND DURATION.
The form of stimulus which is extensively used in physiological investigations, is the electric stimulus of induction shock which is easily graduated by the use of the well known sliding induction coil, in which the approach of the secondary to the primary coil, indicated by the higher reading of the scale, gives rise to increasing intensity of stimulus. The retarding effect of electrical stimulus on growth has already been demonstrated in record taken on a moving plate (Fig. 61).
I shall adopt for unit stimulus, that intensity of electric shock which induces a barely perceptible sensation in a human being. It is very interesting to find, as stated before, that growth is often affected by an electric stimulus, which is below the range of human perception.
Effect of Intensity: Experiment 74.—I shall now describe a typical experiment on the effect of intensity of stimulus in retarding the rate of growth. The normal rate of growth of the bud of Crinum was 0.35 µ per second. On the application of electric shock of unit intensity for 5 seconds, the rate became reduced to 0.22 µ per second. When the stimulus was increased to 2 units, the retarded rate of growth was 0.07 µ per second. When the intensity was raised to 4 units, there was a complete arrest of growth. In figure 72 is given records of a different experiment which show the effects of increasing intensity of stimulus in retardation of growth.
Effect of continuous stimulation: Experiment 75.—The effect of continuous stimulation of increasing intensity will be seen in the record (Fig. 73), taken on a moving plate. On application of continuous stimulus of increasing intensity an increased flexure was produced in the curve, which denoted greater retardation in the rate of growth. When the intensity of stimulus was raised to 3 units, there was induced an actual contraction.
CONTINUITY BETWEEN INCIPIENT AND ACTUAL CONTRACTION.
It will thus be seen, that external stimulus of electric shock induces a reaction which is of opposite sign to the normal growth elongation or expansion. We may conveniently describe this effect as ‘incipient’ contraction; for under increasing intensity of stimulus, the contractile reaction, opposing growth elongation, becomes more and more pronounced; at an intermediate stage this results in an arrest of growth; at the further stage, it culminates in an actual shortening of the organ. There is no break of continuity in all these stages. I shall, therefore, use the term ‘contraction’ in a wider sense, including the ‘incipient’ which finds expression in a retardation of growth.
In Table XVI is given the results of certain typical experiments on the effect of stimulus of increasing intensity and duration.
TABLE XVI.—EFFECT OF INTENSITY AND DURATION OF ELECTRIC STIMULUS ON GROWTH.
| Duration of Application. | Intensity. | Rate of growth. |
| Normal | 0.35 µ per second. | |
| 5 seconds | 1 unit | 0.22 µ " " |
| " | 2 units | 0.07 µ " " |
| " | 4 " | Arrest of growth. |
| Continuous stimulation | Normal | 0.30 µ per second. |
| " " | 0.5 unit | 0.20 µ " " |
| " " | 1 " | 0.09 µ " " |
| " " | 3 " | Contraction. |
With regard to the question of immediate and after-effect of stimulus, I find great difficulty in drawing a line of demarcation. Owing to physiological inertia there is a delay between the application of stimulus and the initiation of responsive reaction (latent period); owing to the same inertia, the physiological reaction is continued even on the cessation of stimulus. All responsive reactions are thus after-effects in reality. The latent period is shortened under strong stimulus, but the contractile reaction becomes more persistent. When the stimulus is moderate or feeble, the recovery from incipient contraction takes place within a short time. Stimulus, under certain circumstances, is found to improve the ‘tone’ of the tissue, and as we shall presently see bring about, as the after-effect, an enhancement of the rate of growth.
The effect of electric stimulus is thus an incipient or actual contraction.
SUMMARY.
In normal conditions electric stimulus induces an incipient contraction exhibited by the retardation of the rate of growth. Growth is often affected by an electric stimulus which is below human perception.
Under increasing intensity of stimulus, the contractile reaction opposing growth elongation becomes more and more pronounced. At a critical intensity of stimulus growth becomes arrested. Under stronger intensity of stimulus growing organ undergoes an actual shortening in length.
There is continuity between the incipient contraction seen in retardation, arrest of growth, and contraction of the organ under stronger stimulus.
The latent period of responsive variation of growth is shortened under stronger stimulus, but the period of recovery becomes protracted.
XV.—EFFECT OF MECHANICAL STIMULUS ON GROWTH
By
Sir J. C. Bose.
Amongst the various stimuli which induce excitation in Mimosa may be mentioned the irritation caused by rough contact, by prick, or wound. Friction causes moderate stimulation, from which the excited pulvinus recovers within a short time. But a prick or a cut induces a far more intense and persistent excitation; the recovery becomes protracted, and the wounded pulvinus remains contracted for a long period.
I shall now describe the effect of mechanical irritation on growth. For moderate stimulus, I employ rough contact or friction; more intense stimulation is caused by a prick or a cut.
EFFECT OF MECHANICAL IRRITATION.
Experiment 76.—In this experiment, I took a peduncle of Zephyranthes, which had a normal rate of growth of 0.18 µ per second. I then caused mechanical irritation by rubbing the surface with a piece of card-board. The mechanical stimulation was found to have caused a retardation of growth, the depressed rate being 0.11 µ per second, or three-fifths the normal rate. As this particular mode of stimulation was very moderate, the normal of rate growth was found to be restored after a short period of rest. After 15 minutes the rate became 0.14 µ per second; after an hour the recovery was complete, the rate being now 0.18 µ per second, the normal rate before stimulation (Fig 74a). We shall presently see that not only is the growth rate greatly depressed under intense stimulation, but the period of recovery also becomes very much protracted.
I have often been puzzled by the fact, that specimens apparently vigorous exhibited little or no growth, after attachment to the recorder. After waiting in vain for an hour, I had to discard them for others with equally unsatisfactory results. One of these specimens happened to be left attached to the recorder overnight, and I was surprised to find that the specimen, which had shown no growth the previous evening, was now exhibiting vigorous growth after being left to itself for 12 hours. I then realised that the temporary abolition of growth must have been due to the irritation of somewhat rough handling during the process of mounting and attachment of the specimen to the recorder.
In the matter of mechanical stimulation, some specimens are more irritable than others. The persistence of after-effect of irritation in retardation of growth will be demonstrated in the following experiments, where the stimulus employed was more intense.
EFFECT OF WOUND.
A prick causes an intense excitation in Mimosa. I tried the effect of this form of stimulation on responsive variation in growth.
Experiment 77.—The specimen was the same as had been employed in the last experiment. After moderate stimulation due to friction it had, in the course of an hour, completely recovered its normal rate of growth of 0.18 µ per second. I now applied the stimulus of pin prick; the actual injury to the tissue due to this was relatively slight; but the retardation of growth induced by this more intense mode of stimulation was very great. With moderate mechanical friction the rate had fallen from 0.18 µ to 0.11 µ per second, i.e., to three-fifths the normal rate: in consequence of prick the depression was from 0.18 µ to 0.05 µ per second, i.e., to less than a third of the normal rate. After 15 minutes the rate recovered from 0.05 µ to 0.07 µ per second. After moderate friction the recovery was complete after an hour; but in this case the recovery after an equal interval was only three-fourths of the original, the rate being now 0.12 µ per second (Fig. 74b). I next applied the more intense stimulus caused by a longitudinal cut This caused a depression of growth rate to 0.04 µ per second. A transverse cut, I find, gives rise to a more intense stimulation, than a longitudinal slit.
TABLE XVII.—EFFECT OF MECHANICAL IRRITATION AND OF WOUND ON GROWTH.
(Zephyranthes.)