We shall in the succeeding series of papers deal with the subject of tropism in general. Different plant organs undergo curvature or bending, sometimes towards and at other times away from the stimulus which induces it. The problem is very intricate; the possibility of its solution will depend greatly on the accurate determination of the immediate and after-effects of various stimuli on the responding organ. The curvature induced in the growing organ is brought about by variation, often extremely slight, of the rate of growth; the result, moreover, is liable to be modified by the duration and point of application of stimulus. The difficulties connected with the problem can only be removed by the detection and measurement of the minutest variation in growth, and by securing a continuous and automatic record of the entire history of the change.
In the chapter on High Magnification Crescograph an account is given of the apparatus which I have devised by which the rate of growth may be magnified from ten thousand to ten millions times. It is thus possible to measure the imperceptible growth of plants for a period shorter than a single second. The variation of normal rate of growth is also found by measuring successive growth records on a stationary plate at regular intervals, say of ten seconds, or from the flexure in the growth-curve taken on a moving plate (p. 163).
I was next desirous of exalting the sensitiveness to a still higher degree by an independent method, which wouldnot only reveal very slight variation induced in the rate of growth, but also the latent period and time-relations of the change. For this purpose I at first devised the Optical Method of Balance[1]which was considered at the time to be extremely sensitive. The spot of light from the Optical Lever (which magnified the rate of growth) was made to fall upon a mirror to which a compensating movement was imparted so that the light-spot after double reflection remained stationary. Any change of rate of growth—acceleration or retardation—was at once detected by the movement of the hitherto stationary spot of light in one direction or the other.
A very careful manipulation was required for the adjustment of the Optical Balance; the record moreover was not automatic. For these reasons I have been engaged for several years past in perfecting a new apparatus by which, (1) the balance could be directly obtained with the utmost exactitude, (2) where an attached scale would indicate the exact rate of growth, and (3) in which the upsetting of the balance by external stimulus would be automatically recorded, the curve giving the time relations of the change.
I shall take a concrete example in explanation of the method of balance. Taking the rate of growth per second of a plant to be1⁄50,000inch or 0·5 µ, per second (equal to the wave length of sodium light), the tip of the plant will be maintained at the same point in space if we succeeded in making the plant-holder subside exactly at the same rate. The growth-elongation of the plant will then be exactly balanced by a compensating movement downwards. The state of exact balance is indicated when therecording lever of the Crescograph traces a horizontal line on the moving plate. Overbalance or underbalance will deflect the record below or above the horizontal line.
Fig. 93.Fig. 93.—Arrangement for compensation of growth-movement by equal subsidence of plant-holder; S, adjusting screw for regulation of speed of rotation; G, governor; W, heavy weight; P, plant-holder.
Fig. 93.—Arrangement for compensation of growth-movement by equal subsidence of plant-holder; S, adjusting screw for regulation of speed of rotation; G, governor; W, heavy weight; P, plant-holder.
For securing exact balance the holder of the plant P, in the given example, will have to subside at a rate of1⁄50,000inch per second. This is accomplished by a system of reducing worm and pinion, also of clock wheels (Fig. 93). The clock at first used for this purpose was worked by the usual balance wheel. Though this secured anaveragebalance yet as each tick of the clock consisted of sudden movement and stoppage, it caused minute variation in the rate of subsidence; this became magnified by the Crescograph and appeared as a series of oscillations about a mean position of equilibrium. This particular defect was obviated by the substitution of a fan governor for the balance wheel. But the speed of rotation slows down with the unwinding of the main spring, and the balance obtained at the beginning was found to be insufficient later on. The difficulty was finally overcome by the use of a heavy weight W, in the place of coiled spring. The complete apparatus is seen in figure 94.
Fig. 94.Fig. 94.—Photographic reproduction of the Balanced Crescograph. L, L', magnifying compound lever. R, recording plate. P, plant. C, clock work for oscillation of the plate and lateral movement. G, governor. M, circular growth-scale. V, plant-chamber.
Fig. 94.—Photographic reproduction of the Balanced Crescograph. L, L', magnifying compound lever. R, recording plate. P, plant. C, clock work for oscillation of the plate and lateral movement. G, governor. M, circular growth-scale. V, plant-chamber.
For purpose of simplicity of explanation, I assumedthe growth rate to have a definite value of1⁄50,000inch per second. But the rate varies widely in different plants and even in the same plant at different days and seasons. In practice the rate of growth for which compensation has to be made varies from1⁄150,000to1⁄25,000inch, or from 0·17 µ to 1·0 µ per second. We have thus to secure some means ofcontinuousadjustment for growth, the rate of which could be continuously varied from one to six times. This range of adjustment I have been able to secure by the compound method of frictional resistance and of centrifugal governor. As regards frictional resistance the two pointed ends of a hinged fork rub against a horizontal circular plate not shown in the figure. By means of the screw head S, the free ends of the fork spread out and the circumference of the frictional circle continuously increased. The centrifugal governor is also spread out by the action of the adjusting screw. By the joint actions of the frictional control and the centrifugal governor, the speed of rotation can be continuously adjusted from 1 to 6 times. When the adjusting screw is set in a particular position, the speed of rotation, and therefore the rate of subsidence of plant-holder, remains absolutely constant for several hours. The attainment of this constancy is a matter of fundamental importance, and it was only by the employment or the compound system of regulation that I was able to secure it.
The method of obtaining balance now becomes extremely simple. Before starting the balancing movement by clock regulation, the plant is made to record its magnified growth by the Crescograph. The compensation is effected as follows: the speed of the clockwork is at the beginning adjusted at its lowest value, and the pressure of a buttonstarts the balancing movement of the plant downwards. On account of partial balance the record will be found to be less steep than before; the speed of the clock is gradually increased till the record becomes perfectly horizontal under exact balance. Overbalance makes the record slope downwards. In figure 95 is seen records of underbalance (a) and overbalance (b), to the extent of about 3 per cent.
Fig. 95.Fig. 95.—Balanced Crescographic record: (a) showing effect of underbalance and (b) overbalance of about 3 per cent. (Magnification 2,000 times.)
Fig. 95.—Balanced Crescographic record: (a) showing effect of underbalance and (b) overbalance of about 3 per cent. (Magnification 2,000 times.)
It will thus be seen that the effect of an external agent may be detected by the upsetting of the balance; an up-movement indicates (unless stated to the contrary) an enhancement of the rate of growth above the normal; and a down-movement, on the other hand, a depression of the normal rate.
Calibration.—The calibration of the instrument is obtained in two different ways. The rate of subsidence of the plant-holder, by which the balance is obtained, is strictly proportional to the rate of rotation of the vertical spindle and the attached train of clock-wheels. A striker is attached to one of the wheels, and a bell is struck at each complete revolution. The clockwork is adjustedat a medium speed, the bell striking 35 times in a minute. A microscope micrometer is focussed on a mark made on the plant-holder, and the amount of subsidence of the mark determined after one minute; this was found to be 0·0525 mm. As this fall occurred after 35 strokes of the bell the subsidence per stroke was 0·0015 mm.
Determination of the absolute rate of growth.—If growth be found balanced at N strokes of bell per minute, the rate of subsidence per second
= N ×·0015⁄60mm. per second= N × ·000025 mm. per second= N × ·025 µ per second= N × 10-5inch per second.
Example.—The growth of a specimen ofZea Mayswas found balanced when the number of strokes of the bell was 20 times in a minute.
Absolute rate of growth= 20 × ·025 µ = 0·5 µ per secondor= 20 × 10-5inch "or=1⁄50,000" "
If we take the wave length of sodium light λ as our standard, the growth in length per second is equal to λ. This will give us some idea of the sensitiveness of the Crescograph employed in recording the movement of growth.
The Balanced Crescograph enables us not merely to determine the absolute rate of growth, but the slightest fluctuation in that rate.
Indicator Scale.—All necessity of calculation is obviated by the scale provided with the apparatus. The speed of clockwork which brings about the balance of growth is determined by the position of the adjusting screw S, thegradual lowering of which produces a continuous diminution of speed. A particular position of the screw therefore indicates a definite rate of subsidence for balancing growth. By a simple mechanism the up or down movement of the screw causes rotation of an index pivoted at the centre of a circular scale. Each division of the scale is calibrated by counting the corresponding number of strokes of the bell per minute at different positions of the adjusting screw. The scale is calibrated in this manner to indicate different rates of growth from 0·2 µ to 1·2 µ per second.
The determination of the rate of growth now becomes extremely simple. Few turns of the screw bring about the balance of growth and the resulting position of the index against the circular scale automatically indicates the absolute rate. The procedure is even simpler and more expeditious than the determination of the weight of a substance by means of a balance.
Perhaps the most delicate method of measuring lengths is that afforded indirectly by the spectrum of a light. A good spectroscope resolves differences of wave lengths of D1(= 0·5896 µ) and D2(= 0·5890)i.e.of 1 part in a thousand. The average rate of growth ofZea Maysis of this order; being about 0·5 µ per second. Let us consider the question of the possibility of detecting a fractional variation of the ultra-microscopic length by means of the Balanced Crescograph. In reality the problem before us is more intricate than simple measurement of change of length; for we have to determine therate of variationof length.
The sensitiveness of the balance will, it is obvious, depend on the magnifying power of the Crescograph. By the Method of Magnetic Amplification referred to in page 170, I have succeeded in obtaining a magnification of tenmillion times. In this method a very delicate astatic system of magnets undergoes deflection by the movement of a magnetised lever in its neighbourhood. A spot of light reflected from a small mirror attached to the astatic system, thus gives the highly magnified movement of the rate of growth, which may easily be raised to ten million times. I shall in the following describe the results obtained with this easily managed magnification of ten million times.
Determination of sensitiveness: Experiment 99.—A seedling ofZea Mayswas placed on the Crescographic Balance; and the magnetic amplification, as stated above, was ten million times. With 18 strokes of the bell per minute the spot of light had a drift of + 266 cm. per minute to the right; this is because the growth was underbalanced. With faster rate of clock movement,i.e., 21 strokes in 68 seconds or 18·53 strokes per minute, the drift of the spot of light, owing to overbalance, was to the left at the rate of - 530 cm. per minute. Thus
(1) 18 strokes per minute caused a drift of + 266 cm. per minute.(2) 18·53 strokes per minute caused a drift of - 530 cm. per minute.
(1) 18 strokes per minute caused a drift of + 266 cm. per minute.
(2) 18·53 strokes per minute caused a drift of - 530 cm. per minute.
Hence by interpolation the exact balance is found to correspond to 18·177 strokes per minute.
Therefore the absolute rate of growth
= 18·177 × 0·025 µ per second.= 0·45 µ per second.= 0·000018 inch per second.
We learn further from (1) and (2) that a variation of(18·53 - 18)⁄18·177produces a change of drift of the spot of light from + 266 to - 530 cm.,i.e., of 796 cm. per minute.As it is easy to detect a drift of 1 cm. per minute a variation of0·53⁄(18·177 × 796), or 1 part in 27,000 may thus be detected by the Method of Balance. The spectroscopic method enabled us, as we saw, to detect change of wave length 1 part in a thousand. The sensibility of the Balanced Crescograph is thus seen to rival, if not surpass that of the spectroscope.
For obtaining a general idea of the sensitiveness, the absolute of growth in the instance given above was 0·00018 inch per second, and the Balanced Crescograph was shown capable of discriminating a variation of 1 part in 27,000; hence it is possible to detect by this means a variation of1⁄1,500millionth of an inch per second.
This method of unprecedented delicacy opens out a new field of investigation on the effect of changes of environment in modification of growth; instances of this will be found in subsequent chapters. I give below accounts of certain demonstrations which will no doubt appear as very striking.
After obtaining the exact balance a match was struck in the neighbourhood of the plant. This produced a marked movement of the hitherto quiescent spot of light, thus indicating the perception of such an extremely feeble stimulus by the plant.
Breathing on the plant causes an enhancement of growth due to the joint effects of warmth and carbonic acid gas. A more striking experiment is to fill a small jar with carbonic acid and empty it over the plant. A violent movement of the spot of light to the right demonstrates the stimulating effect of this gas on growth.
The method described above is excessively sensitive; for general purposes and for the method of direct record, a lesssensitive arrangement is sufficient. I give below accounts of several typical experiments in which the recording form of Crescograph was employed, the magnification being only 2,000 times.
Fig. 96.Fig. 96.—Record showing the effect of CO2. Horizontal line at beginning indicates balanced growth. Application of CO2at arrow induces enhancement of growth shown by the up-curve followed by depression, shown by the down-curve. Successive dots at intervals of 10 seconds. (Seedling of wheat.)
Fig. 96.—Record showing the effect of CO2. Horizontal line at beginning indicates balanced growth. Application of CO2at arrow induces enhancement of growth shown by the up-curve followed by depression, shown by the down-curve. Successive dots at intervals of 10 seconds. (Seedling of wheat.)
Effect of carbonic acid on Balanced growth: Experiment 100.—I have already shown that carbonic acid diluted with air induces an enhancement of the rate of growth, but its long continued action induces a depression (p. 185). I shall now employ the Method of Balance in studying the effect of CO2on growth. It should be remembered in this connection that the horizontal record indicates the balance of normal rate of growth. An up-curve exhibits the induced enhancement and a down-curve, a depression of growth. In the present experiment after obtaining the exact balance, pure carbonic acid gas was made to fill up theplant-chamber at the point marked with an arrow (Fig. 96). It will be seen that this induced an almost immediate acceleration of the rate, the latent period being less than five seconds. The acceleration continued for two and half minutes; the accelerated rate then slowed down, became enfeebled, and the growth returned for a short time to the normal as indicated by the horizontal portion at the top of the record; this proved to be the turning point of inversion from acceleration into retardation of growth. The stronger is the concentration of the gas the earlier is the point of inversion. With diluted carbonic acid the acceleration may persist for an hour or more.
Effect of Ether: Experiment 101.—Dilute vapour of ether is found to induce an acceleration of rate of growth which persist for a considerable length of time. This is seen in the upsetting of the balance upwards on the introduction of the vapour (Fig. 97a.).
Fig. 97.Fig. 97.—(a) Effect of ether, acceleration of growth, (b) effect of chloroform preliminary acceleration followed by depression.
Fig. 97.—(a) Effect of ether, acceleration of growth, (b) effect of chloroform preliminary acceleration followed by depression.
Effect of Chloroform: Experiment 102.—The effect of chloroform vapour is relatively more depressing than ether. Application of chloroform is seen to induce at first an acceleration which persisted for 50 seconds, but after this depression set in (Fig. 97b). Prolonged application of the anæsthetic is followed by the death of the plant.
In the Method of Balance the movement of growth upwards is compensated by an equal movement of the plant downwards, with the result that the record remains horizontal.
The effect of an external agent is immediately detected by the upsetting of the balance, up-record representing acceleration above normal, a down-record the opposite effect of depression below the normal rate.
The latent period and the after-effect of stimulus may thus be obtained with the highest accuracy.
The sensitiveness of the Method of Balance may be raised so as to indicate a variation of rate of growth smaller than1⁄1000millionth of an inch per second.
[1]"Plant Response"—p. 413.
[1]"Plant Response"—p. 413.
The diverse movements induced by external stimuli in different organs of plants are extremely varied and complicated. The forces in operation are manifold—the influence of changing temperature, the stimulus of contact, of electric current, of gravity, and of light visible and invisible. They act on organs which exhibit all degrees of physiological differentiation, from the radial to the dorsiventral. An identical stimulus may sometimes induce one effect, and at other times, the precisely opposite. Thus under unilateral stimulation of light of increasing intensity, a radial organ exhibits a positive, a dia-phototropic, and finally a negative response. Strong sunlight brings about para-heliotropic or 'midday sleep' movement, by which the apices of leaves or leaflets turn towards or away from the source of illumination. The teleological argument advanced, that in this position the plant is protected from excessive transpiration, does not hold good universally; for under the same reaction, the leaflets ofCassia montanaassume positions by which the plant risks fatal loss of water. InAverrhoa carambolathe movement is downwards, whichever side is illuminated with strong light; inMimosaleaflet the movement, under similar circumstances is precisely in the opposite direction. The photonastic movement, apparently independent of the directive action of light, has come to be regarded as a phenomenon unrelated to phototropic reaction, and due to a differentkind of irritability, and a different mode of response. So very anomalous are these various effects that Pfeffer, after showing the inadequacy of different theories that have been advanced, came to the conclusion that "the precise character of the stimulatory action of light has yet to be determined. When we say that an organ curves towards a source of illumination because of its heliotropic irritability, we are simply expressing an ascertained fact in a conveniently abbreviated form, without explaining why such curvature is possible or how it is produced."[2]
The contradictory nature of the various responses is however not real; the apparent anomaly had lain in the fact that two definite fundamental reactions of opposite signs induced by stimulus had not hitherto been recognised and distinguished from each other. The innumerable variations in the resultant response are due to the summation of the effects of two fluctuating factors, with further complications arising from: (1) difference in the point of application of stimulus, (2) the differential excitability of the different sides of the responding organ, and (3) the effect of temperature in modifying tropic curvature. It is therefore most important to have the means for automatic record ofcontinuouschange in the response brought about by various factors, which act sometimes in accord, and at other times in conflict. The autograph of the plant itself, giving a history of the change in response and its time-relations, is alone decisive in explanation of various difficulties in connection with plant movements, as against the various tentative theories that have been put forward. The analysis of the resulting effect, thus rendered possible, casts new light on the phenomena of response, proving that the anomalies which had so long perplexed us, are more apparent than real.
One of the causes of uncertainty lay with the question, whether response changed with the mode of stimulation. I have, however, been able to show thatall forms of stimuliinduce a definite excitatory reaction of contraction (p. 218).
Tropic movements induced by unilateral action of stimulus may, broadly speaking, be divided into two classes depending on the point of application of stimulus:
In the first, the point of application of unilateral stimulus is not on the responding organ itself, but at some distance from it. The question therefore relates toLongitudinal Transmissionof effect of stimulus.
In the second, unilateral stimulus acts directly on the responding organ. For the determination of the resultant movement, it is necessary to take account of effects induced on the two sides of the organ. The side adjacent to the stimulus I shall designate as theproximal, and the diametrically opposite as thedistalside. The question to be investigated in this case relates toTransverse Transmissionof effect of stimulus. It will be shown that the resulting movement depends on:—
(a) whether the tissue is a conductor or a non-conductor of excitation in a transverse direction, and(b) whether it is the proximal, or the distal side of the organ that is the more excitable.
(a) whether the tissue is a conductor or a non-conductor of excitation in a transverse direction, and
(b) whether it is the proximal, or the distal side of the organ that is the more excitable.
In connection with the response to environmental changes, a source of uncertainty is traceable to the absence of sufficient knowledge of the physiological effect of heat, which has been regarded as a form of stimulus: it will be shown that heat induces two distinct effects dependent on conduction and radiation. We shall in the succeeding chapters, take up the study of the physiological effects induced by changes in the environment.
[2]Pfeffer—Ibid—Vol. II, p. 74.
[2]Pfeffer—Ibid—Vol. II, p. 74.
I have in previous chapters explained that the direct application of stimulus gives rise in different organs to contraction, diminution of turgor, fall of motile leaf, electro-motive change of galvanometric negativity, and retardation of the rate of growth. I have also shown that indirect stimulation (i.e.application of stimulus at some distance from the responding organ) gives rise to a positive or erectile response of the responding leaf or leaflet (indicative of an increase of turgor), often followed by normal negative response. The positive impulse travels quickly. The interval of time that elapses, between the application of stimulus and the erectile response of the responding leaf, depends on the distance of the point of application, and the character of the transmitting tissue: it varies in different cases from 0·6 second to about 40 seconds. The positive is followed by a slower wave of protoplasmic excitation, which causes the excitatory fall. The velocity of this excitatory impulse is about 30 mm. per second in the petiole ofMimosa, and about 3 mm. per second inBiophytum. The positive followed by the negative thus gives rise to a diphasic response. The excitatory impulse is much enfeebled during transit: the negative impulse may thus fail to reach the respondingorgan, if the stimulus be feeble or if the intervening distance be long or semi-conducting. Hence moderate stimulus applied at a distance gives rise only to positive response; direct application of strong stimulus gives rise, on the other hand, to the normal negative. By employing the electric method of investigation, I have obtained with ordinary tissues the positive, the diphasic, and the negative electric response, in correspondence with the responses given by a motile organ (p. 214). The mechanics of propagation of the positive and the negative impulse are different. It is therefore necessary to distinguish the quicktransmissionof the positive impulse from the slowconductionof the negative impulse due to the propagation of excitatory protoplasmic change.
It should be borne in mind in this connection that all responsive movements are ultimately due to protoplasmic changes which are beyond our scrutiny. We can infer the nature of the change by the concomitant outward manifestations, which are of two kinds: thepositive, associated with increase of turgor, expansion, and galvanometric positivity, and thenegativewith concomitant decrease of turgor, contraction, and galvanometric negativity. Thus positive and negative reactions indicate the fundamental protoplasmic changes of opposite characters.
The movement and curvature induced by stimulus have, for convenience, been distinguished aspositive curvature, (movement towards stimulus), andnegative curvature(movement away from stimulus). Though these curvatures result from protoplasmic reactions, yet thepositive curvatureis not necessarily associated withpositive protoplasmic reaction. It will be shown that the curvature of an organ is determined by the algebraical summation of effects induced at the proximal and distal sides of the responding organ.
Physiologists have not been aware of the dual character of the impulse generated by stimulus, and the term "transmission of stimulus" is thus misleading since its effect may be an expansion, or its very opposite, contraction. It is therefore necessary to discriminate the effect of one from the other: the impulse which induces an increase of turgor, expansion, and galvanometric positivity will be distinguished as positive, in the sense that it causes an enhancement of turgor. The other, which induces diminution of turgor and contraction, will be termed as the excitatory impulse. Transmission of the latter is dependent on conducting power of the tissue; the positive impulse is practically independent of the conducting power.
In animal physiology again, there is no essential difference between the effect of the direct and indirect stimulation. In a nerve-and-muscle preparation, for example, indirect stimulation at the nerve induces the same contraction as the direct stimulation of the muscle. The only difference lies in the latent period, which is found to be longer under indirect stimulation by the time interval necessary for the excitation to travel along the conducting nerve. It is probable that stimulus gives rise to dual impulses in the animal tissue as in the plant. But the detection of the positive impulse in the animal nerve is rendered exceedingly difficult on account of the high velocity of conduction of excitation. I have explained that the separate effects of the two impulses can only be detected if there is a sufficient lag of the excitatory negative behind the positive, so that the relatively sluggish responding organ may exhibit the two impulses one after the other. In a highly conducting tissue the lag is very slight, and the negative will therefore mask the positive by its predominant effect. In spite of the difficulty involved in the problem, I have recently been successful in demonstrating the dual impulses in the animal nerve.
In any case it is important to remember the following characteristic effects of indirect stimulation.
TABLE XXII.—SHOWING THE EFFECT OF INDIRECT STIMULATION.
Intensity of Stimulus.Character of intervening tissue.Responsive effect.ModerateHighly ConductingContraction."Non-conductingExpansion."Semi-conductingExpansion followed by contraction.Feeble" "Expansion.
These effects of indirect stimulation have been fully demonstrated in the case of pulvinated organs (p. 136) and growing organs (p. 215).
Having demonstrated the fundamental reactions of direct and indirect stimulation, we shall next study the tropic effects induced in growing organs by the effect of unilateral application of indirect stimulus.
Fig. 98.Fig. 98.—Diagrammatic representation of effects of indirect and direct stimulation. Continuous arrow represents the indirect stimulation, and the curved continuous arrow above, the induced negative curvature: dotted arrow indicates the application of direct stimulus, and the dotted curve above, the induced positive curvature.
Fig. 98.—Diagrammatic representation of effects of indirect and direct stimulation. Continuous arrow represents the indirect stimulation, and the curved continuous arrow above, the induced negative curvature: dotted arrow indicates the application of direct stimulus, and the dotted curve above, the induced positive curvature.
Experiment 103.—I have already explained, how thermalradiationis almost as effective in inducing contraction and retardation of growth as the more refrangible rays of the spectrum. The thermal radiation was produced by the heating of a platinum spiral, short of incandescence, by the passage of an electric current. The intensity of radiation is easily varied by adjustment of the current by means of a rheostat. The experimental specimen was a flower bud ofCrinum. It was held by a clamp, a little below the region of growth. Stimulus was applied below the clamp so that the transmitted effect had to pass through S, the securely held tissue (Fig. 98). A feeble stimulus was applied on one side, at the indifferent point about 3 cm. below the region of growth. The positive effect of indirect stimulus reached the region of growth on the same side, bringing about an acceleration of growth with expansion and convexity, the resulting movement beingnegativeor away from the stimulus. The latent period was ten seconds, and maximum negative movement was completed in the further course of ten seconds, after which there was a recovery in the course of 75 seconds. A stronger stimulus S' gave a larger response; but when the intensity was raised still higher to S", the excitatory negative impulse overtook the positive within 15 seconds of its commencement; the convex wasthus succeeded by the concave curvature (Fig. 99). Direct application of stimulus at the growing region gave rise to a positive curvature.
Fig. 99.Fig. 99.—Tropic curvature ofCrinumto unilateral indirect stimulation of increasing intensities: S, S' of moderate intensity induced negative tropic effect (movement away from the stimulated side); stronger stimulus S" gave rise to negative followed by positive. Successive dots at intervals of 5 seconds Magnification 100 times.
Fig. 99.—Tropic curvature ofCrinumto unilateral indirect stimulation of increasing intensities: S, S' of moderate intensity induced negative tropic effect (movement away from the stimulated side); stronger stimulus S" gave rise to negative followed by positive. Successive dots at intervals of 5 seconds Magnification 100 times.
The effect of feeble stimulus transmitted longitudinally is thus found always to induce convexity, anegative curvatureand movement away from stimulus. I have obtained similar responsive movement of negative sign with various plant organs, and under various forms of stimuli. Thus in the stem ofDregea volubilisthe longitudinally transmitted effect of light of moderateintensity was a negative curvature; direct application of light on the growing region gave, on the other hand, a positive curvature and movement towards light.
Thus while the effect of direct unilateral stimulation is a positive curvature, the effect of indirect stimulation is a negative curvature. The following table gives a summary of results of tropic effects under unilateral application of indirect stimulus.
TABLE XXIII.—SHOWING TROPIC EFFECT OF UNILATERAL APPLICATION OF INDIRECT STIMULUS.
Stimulus.Character of intervening tissue.Sign of tropic response.ModerateConductingPositive curvature."Semi-conductingNegative followed by positive.FeebleSemi-conductingNegative Curvature.ModerateNon-conducting" "Direct application of unilateral stimulus induces a positive curvature.
In sensitive plants stimulus applied at a distance induces in the responding region an expansion indicative of increase of turgor.
The effect of indirect stimulation is also exhibited by an electric change of galvanometric positivity, indicative of enhancement of turgor and expansion.
Indirect stimulus induces in growing organs an enhancement of rate of growth.
Unilateral application of stimulus causes an expansion higher up on the same side to which the stimulus is applied; the result is an induced convexity, a movement away from the stimulus,i.e., a negative curvature. Direct stimulus applied unilaterally at the responding region induces, on the other hand, a positive curvature.
We have next to consider a very large class of phenomena arising out of the direct stimulation of one side and its transversely transmitted effect on the opposite side. The unilateral stimuli to which the plant is naturally exposed are those of contact, of light, of thermal radiation, and of gravity. There is besides the stimulation by electric current. I shall presently show that these tropic curvatures are determined by the definite effects of direct and indirect stimulations.
Under unilateral stimulus, the proximal side is found to become concave and the distal side convex; the organ thus moves towards stimulus, exhibiting a positive curvature. This movement may be due: (1) to the diminution of turgor, contraction or retardation of rate of growth of the proximal side, (2) to the increase of turgor, expansion or acceleration of rate of growth on the distal side, or (3) to the joint effects of contraction of the proximal and expansion of the distal side.
As regards the reaction of the proximal side, it has been shown that direct stimulation induces local contraction in a pulvinated organ, and retardation of growth in a growing organ. The effect induced on the distal sidehad hitherto remained a matter of uncertainty. In regard to this we must bear in mind that it is the effect of indirect stimulus that reaches the distal sided, inducing an enhancement of turgor and expansion of that side.
For obtaining a complete explanation of tropic curvatures in general, it is important that the induction of enhanced turgor at the distal side (by the action of stimulus at the proximal side) should be corroborated by independent methods of enquiry. One of the methods I employed for this purpose was electrical. Two electric connections were made, one with the distal point (diametrically opposite to the stimulated area), and the other, with an indifferent point at a distance. On application of stimulus of various kinds, the distal point was found to exhibit galvanometric positivity, indicative of enhancement of turgor.[3]
I have since been able to devise a new experiment by which the enhancement of turgor on the distal side is demonstrated in a very striking manner.
I have shown (p. 39) that the movement of the motile leaf ofMimosais a reliable indicator of the state of turgor, increase of turgor inducing erection, and diminution of turgor bringing about the fall of the leaf. I shall employ the mechanical response of the leaf to demonstrate the enhancement of turgor induced by transverse transmission of effect of stimulus.
Fig. 100.Fig. 100.—Increased turgor due to indirect stimulation, inducing erection of Mimosa leaf: (a) diagram of the experiment, point of application of stimulus indicated by arrow. (b) erectile response (shown by down-curve) followed by rapid fall (up-curve) due to transverse conduction of true excitation. (Successive dots at intervals of 10 seconds.)
Fig. 100.—Increased turgor due to indirect stimulation, inducing erection of Mimosa leaf: (a) diagram of the experiment, point of application of stimulus indicated by arrow. (b) erectile response (shown by down-curve) followed by rapid fall (up-curve) due to transverse conduction of true excitation. (Successive dots at intervals of 10 seconds.)
Unilateral photic stimulation: Experiment 104.—AMimosaplant was taken, and its stem was held vertical by means of a clamp. We apply a stimulus at a point on one side of the stem, and observe the effect of this on the state of turgor at the diametrically opposite side. In my first experiment on the subject of detection of induced change of turgor I employed the stimulus of light. A narrow beam from a small arc lamp was made to fall on the stem, at a point diametrically opposite to the motile leaf, which was to serve as a indicator for induced variation of turgor at the distal side. The leaf was attached to the recording lever, the successive dots in the record being at intervals of ten seconds. Stimulation by light caused a positive or erectile movement within 20 seconds of application. The positive response afforded a conclusive proof of the induction of an increase of turgor at the distal point. When the stimulus is moderate or of short duration, the response remains positive. But with strong or prolonged stimulation, the slower excitatory negative impulse is conducted to the distal point and brings about the sudden fall of the leaf (Fig. 100). In the present case the excitatoryimpulse reached the motile organ 200 seconds after the initiation of the positive response. The stem was thin, only 2 mm. in diameter. The velocity of excitatory impulse in a transverse direction is thus 0·01 mm. per second; transverse transmission is, for obvious reasons, a much slower process than longitudinal transmission of excitation; in theMimosastem this is about 4 mm. per second.
Fig. 101.Fig. 101.—Response of leaf ofMimosaunder transverse transmission of electric stimulus. (Compare this with fig. 100.)
Fig. 101.—Response of leaf ofMimosaunder transverse transmission of electric stimulus. (Compare this with fig. 100.)
Unilateral electric stimulation: Experiment 105.—In order to show that the effects described above are not due to any particular mode of stimulation but to stimuli in general, I carried out an additional experiment, the stimulus employed being electrical. Two fine pin-electrodes were pricked into the stem, opposite to the responding leaf ofMimosa; these electrodes were placed vertically one above the other, 5 mm. apart. After a suitable period, allowed for recovery from mechanical irritation, feeble tetanising electric shock was passed through the electrodes. The responsive effects at the distal side of the stem is precisely similar to those induced under unilateral photic stimulation; that is to say, the first effect was an erectile movement of the leaf, indicative of an inducedenhancement of turgor; the excitatory negative impulse then reached the distal point and caused a sudden fall of the leaf (Fig. 101).
The experiments that have just been described are of much significance. An organ like the stem ofMimosa, since it exhibits no contraction, may appear insensitive to stimulation; but its perception of stimulus is shown by its power of transmitting two characteristic impulses, one of which is the positive, giving rise to an enhancement of turgor, and the other, the true excitatory negative, inducing the opposite reaction or diminution of turgor. Unilateral stimulation gives rise to both these effects in all organs: pulvinated, growing, and non-growing. It was the fortunate circumstance of the insertion of the motile leaf on one side of theMimosastem that enabled us to demonstrate the important facts given above.
The underlying reactions, which give rise to tropic curvature, could have been foretold from the Laws of effects of Direct and Indirect stimulation, established in previous chapters (pp. 136, 216). The resulting curvature is thus brought about by the joint effects of direct stimulation of the proximal, and indirect stimulation of the distal side. We may now recapitulate some of the important facts relating to tropic curvatures:
Indirect stimulation gives rise to dual impulses, positive and negative; of these the positive impulse is practically independent of the conducting power of the tissue; but the transmission of the excitatory negative impulse is dependent on the conducting power. No tissue is a perfect conductor, nor is any a perfect non-conductor of excitation, the difference is a question of degree. In a petiole or a stem the conducting power along the direction of length is considerable, but very feeble in a transverse direction. In a semi-conducting tissue, a feeble stimulus will transmitonly the positive impulse; strong or long continued stimulation will transmit both positive and negative impulses, the positive preceding the negative. The transmitted positive gives rise to increase of turgor, expansion, and acceleration of rate of growth; the negative induces the opposite reaction of diminution of turgor, of contraction, and of retardation of rate of growth. Transverse transmission is only a particular instance of transmission in general; the only difference is that the conducting power forexcitationis very much less in the transverse than in the longitudinal direction. Owing to feeble transverse conductivity, the transmitted impulse to the distal side often remains positive; it is only under strong or continued stimulation that the excitatory negative reaches the distal side and neutralises or reverses the previous positive reaction. If the distal is the more excitable side, the reversed response will appear as pronounced negative. I give a table which will clearly exhibit the effects of stimulus on the proximal and distal sides of the responding organ.
TABLE XXIV.—SHOWING RESPONSIVE EFFECTS COMMON TO PULVINI AND GROWING ORGANS UNDER UNILATERAL STIMULATION.