Chapter V: Part III: Tropism in Plants (2)

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It is generally supposed that the upper side of the tendril of _Passiflora_ is devoid of contractility. This is however not the case, for my experiments show that stimulation of the upper side also induces contraction and concavity of that side, though the actual movement is relatively feeble.

_Experiment 109._--In order to subject the question to quantitative test I applied feeble stimulus of the same intensity on upper and lower side alternately. Successive stimuli were kept more or less uniform by employing the following device. I took a flat strip of wood 1 cm. in breadth, and coated 2 cm. of its length with shellac varnish mixed with fine emery powder. On drying the surface became rough, the flat surface was gently pressed against the area of the tendril to be stimulated, and quickly drawn so that the rough surface 2 cm.×1 cm. was rubbed against the tendril in each experiment. Stimulation, thus produced, induced a responsive movement of each side of the organ. The extent of the maximum movement was measured by the microscope micrometer. The following results were obtained with four different specimens.

TABLE XXVII.--SHOWING THE RELATIVE INTENSITIES OF RESPONSES OF THE UPPER AND UNDER SIDE OF TENDRIL (_Passiflora_).

+------------------------------------------------------------+
|Movement induced by | Movement induced by | B |
| stimulation of under | stimulation of upper | Ratio ---. |
| side, A. | side, B. | A |
+----------------------+-------------------------------------+
|(1) 85 divisions | 14 divisions | 1/6 |
|(2) 106 " | 15 " | 1/7 |
|(3) 60 " | 8 " | 1/7 |
|(4) 80 " | 10 " | 1/8 |
+------------------------------------------------------------+

It will thus be seen that the upper side of the tendril is not totally inexcitable, its power of contraction being about one-seventh that of the under side.

NEGATIVE CURVATURE OF THE TENDRIL.

I shall now describe certain remarkable results which show that under certain definite conditions the tendril moves away from the stimulated side. I have explained, how in growing organs the effect of unilateral stimulus longitudinally transmitted, induces an expansion higher up on the same side to which the stimulus is applied, resulting in convexity and movement away from the stimulus (cf. Laws of Tropic Curvatures, p. 286). As the reaction of tendril is in no way different from that of growing organs in general, it occurred to me that it would be possible to induce in it a negative curvature by application of indirect unilateral stimulus.

_Experiment 110._--A tendril of _Passiflora_ was held in a clamp, as in the diagram (Fig. 106) in which the left is the more excitable side of the organ. The responsive movement of the tendril is observed by focussing a reading microscope on a mark on the upper part of the tendril. Direct mechanical stimulation at the dotted arrow makes the tendril move in the same direction, the response being _positive_. But if stimulus be applied on the same side below the clamp the tendril is found to move away from stimulus, the response being now _negative_. This reversal of response, as previously stated, is due to the fact that the transmitted effect of indirect stimulus induces an acceleration of growth higher up on the same side, which now becomes convex. The result though unexpected, is in every way parallel to the response of the flower bud of _Crinum_, in which the normal positive response was converted into negative by changing the point of application of stimulus, so that it became indirect (p. 216).

SUMMARY.

The response of tendril is in no way different from that of growing organs in general.

Direct stimulus, electrical or mechanical, induces an incipient contraction; the after-effect of a feeble stimulus is an acceleration of growth above the normal. Indirect stimulus induces an enhancement of the rate of growth.

Under unilateral mechanical stimulus of short duration the directly excited proximal side undergoes contraction, the indirectly stimulated distal side exhibits the opposite effect of expansion. The induced curvature is thus due to the joint effects of the contraction of one side, and the expansion of the opposite side.

As the after-effect of direct stimulus is an acceleration of growth above the normal, the stimulated side undergoes an expansion by which the recovery is hastened.

Unilateral application of direct stimulus induces a _positive_ curvature, but the same stimulus applied at a distance from the responding region induces a _negative_ curvature.

The tendril of _Passiflora_ is excitable both on the upper and under sides: the excitability of the under side is about seven times greater than that of the upper side.

Stimulation of one side of the tendril induces an expansion of the opposite side, even in cases where the contractility of the stimulated side is feeble.

The response to stimulation of the more excitable side of the tendril is thus inhibited by the stimulation of the opposite side. This is because of the neutralisation of the effect of direct by that of indirect stimulation.

XXVII.--ON GALVANOTROPISM

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.

Before describing the effect of unilateral application of an electrical current in inducing tropic curvature, I shall give an account of the polar effect of anode and cathode on the pulvinated and growing organs. In my previous work[6] on the action of electrical current on sensitive pulvini I have shown that:--

(1) at the 'make' of a current of moderate intensity a
contraction takes place at the cathode; the anode induces no
such contractile effect;

(2) at the 'make' of a stronger current both the anode and
cathode induce contraction.

[6] "Irritability of Plants," p. 212.

I have also carried out further investigations on the polar effect of current on the autonomous activity of the leaflet of _Desmodium gyrans_. These rhythmic pulsations can be recorded by my Oscillating Recorder. Each pulsation consists of a sudden contractile movement downwards, corresponding to the systole of a beating heart, and a slower up movement of diastolic expansion. Application of cathode at the pulvinule was found to exert a _contractile_ reaction, exhibited either by the reduction of normal limit of diastolic expansion, or by an arrest of movement at systole. The effect of anode was precisely the opposite; the induced _expansion_ was exhibited either by reduction of normal limit of systolic contraction, or by arrest of pulsation at diastole.

From the above results it is seen that with a feeble current:

(1) contraction is induced at the cathode, and

(2) expansion is brought about at the anode.

These effects take place under the action of a feeble current. Under strong currents, contraction takes place both at the anode and the cathode.

POLAR EFFECT OF ELECTRICAL CURRENT ON GROWTH.

The object of this investigation was to determine whether anode and cathode exerted similar discriminative and opposite effects on growth. For this experiment I took a specimen of _Kysoor_ and determined the region where growth was maximum. A piece of moist cloth was wrapped round this region to serve as one of the two electrodes. The second electrode was placed in the neighbouring indifferent region where there had been a cessation of growth.

_Effect of Cathode: Experiment 111._--The particular specimen of _Kysoor_ had a normal rate of growth of 0·48 µ per second. On application of the cathode the rate was reduced to 0·14 µ per second, or to less than a third. This will be seen in record (Fig. 107), where N is the normal rate of growth and K, retarded rate under the action of the cathode.

_Effect of anode: Experiment 112._--If the cathode induced a retardation, the anode might be expected to induce an acceleration of growth. But in my first experiment on the action of anode, I could detect no perceptible variation of rate of growth. In trying to account for this failure, I found that the specimen employed for the experiment had normally a very rapid rate of growth. It appeared that an induced acceleration would be brought out more conspicuously by choosing a specimen in which the growth-rate was low, rather than in one in which it was near its maximum. Acting on this idea, I took another specimen of _Kysoor_ in which the normal rate was as slow as 0·10 µ per second. On applying the anode to the growing region, there was an enhancement to one and half times the normal rate (Fig. 108).

TABLE XXVIII.--EFFECT OF ANODE AND CATHODE ON GROWTH (_Kysoor_).

Specimen A Normal rate 0·10 µ per sec.
Acceleration under anode 0·155 µ per sec.

Specimen B Normal rate 0·48 µ per sec.
Retardation under cathode 0·14 µ per sec.

The effects given above take place under the action of a feeble current. Strong current on the other hand induces a retardation or an arrest of growth.

I have in the above experiments demonstrated the normal effect of anode in inducing expansion and acceleration of rate of growth; the cathode was shown to induce contraction and retardation of growth. Unilateral application of anode and cathode thus induces appropriate curvatures in pulvinated and in growing organs.

SUMMARY.

The effects of an electric current on growth is modified by the direction of current. A feeble anodic current enhances the rate of growth; a cathodic current on the other hand induces a retardation of the rate. Strong current, both anodic and cathodic, induces a retardation.

XXVIII.--ON THERMONASTIC PHENOMENA

_By_

SIR J. C. BOSE,

_Assisted by_

SURENDRA CHANDRA DAS.

In describing thermonastic curvatures Pfeffer says that "a special power of thermonastic response has been developed by various flowers, in which low temperatures produce closing movements, and high temperatures, opening ones. The flowers of _Crocus vernus_ and _Crocus luteus_ are specially responsive, as also those of _Tulipa Gesneriana_ for these flowers perceptibly respond to a change of temperature of half a degree centigrade."[7]

[7] Pfeffer--_Ibid_, Vol. III, p. 112.

We have hitherto studied the response of various organs to _stimulus_; we have now to deal with the effect of thermal variation. Does rise of temperature act like other forms of stimuli or is its action different? We have therefore to find:

(1) The physiological effect of variation of temperature.

(2) Whether thermonastic irritability is confined only to
certain classes of organs, or is it a phenomenon of very wide
occurrence?

(3) Whether variation of temperature induces in anisotropic
organs only one type of response, or two types, positive and
negative.

(4) The law which determines the direction of responsive
movement.

EFFECT OF RISE OF TEMPERATURE.

As regards the effect of rise of temperature we have seen that, within normal limits, it induces expansion and acceleration of the rate of growth. Stimulus, on the other hand, induces precisely the opposite effect. Hence the physiological reaction of steady rise of temperature is, generally speaking, antagonistic to that of stimulus. This conclusion is supported by numerous experiments which I have carried out with various plant organs. Example of this will be found in the present and subsequent chapters.

DIFFERENT THERMONASTIC ORGANS.

The only condition requisite for the exhibition of response is the differential excitability of an anisotropic organ. It is therefore likely to be exhibited by a large variety of plant organs, such as pulvini, petioles, leaves, and flowers, and my results show that this is actually the case. This particular sensibility, moreover, is not confined to delicate structures, but is extended to rigid trees and their branches.

Before proceeding further, it is necessary to draw attention to the confusion which arises from the use of the common prefix '_thermo_' in thermonasty and thermo-tropism. With regard to this Pfeffer says "It is not known whether radiated and conducted heat exercise a similar thermotropic reaction."[8] I shall show that the reactions to radiant heat, and to conducted heat (rise of temperature) are of opposite character, radiation inducing contraction, and rise of temperature, expansion. It is therefore advisable to distinguish the thermal, or temperature effect, from the radio-thermal effect of infra-red radiation.

[8] Pfeffer--_Ibid_, Vol. III, p. 177.

TWO TYPES OF RESPONSE.

As regards the effect of variation of temperature I shall proceed to show that there are two distinct types, which I shall, for convenience, distinguish as the _Positive_ and _Negative_.

Positive thermonastic reaction is exhibited by organs in which the upper half is the more excitable. Response to rise of temperature is by _downward_ or _outward_ movement. In floral organs this finds expression by a movement of opening. In illustration of this may be cited the examples of the well known Crocus and also of _Zephyranthes_.

Negative thermonastic movement is shown by organs in which the lower half is the more excitable. Here the response to rise of temperature is by an _upward_ or _inward_ movement. I shall show that an example of this is furnished by the flower of _Nymphæa_ which closes under rise, and opens during fall of temperature.

POSITIVE THERMONASTIC RESPONSE.

_Response of Zephyranthes: Experiment 113._--Viewed from the top, the inner side of the petal of a flower is the upper side. The _Crocus_ flower under rise of temperature opens outwards by expansion of the inner side, which must be the more excitable. As _Crocus_ was not available in Calcutta, I found the flower of _Zephyranthes_ (sometimes called the Indian _Crocus_) reacting to variation of temperature in a manner similar to that of _Crocus_, that is to say, the flower opens under rise and closes with a fall of temperature. For obtaining record all the perianth segments but one was removed. This segment was attached to the recording lever. On lowering of temperature through 5°C. there was an up-movement, or a movement of closure. Rise of temperature induced, on the other hand, a movement of opening.

_Effect of thermal radiation: Experiment 114._--I stated that the effect of thermal radiation acts as a stimulus, inducing a reaction which is antagonistic to that of rise of temperature. In verification of this, I subjected the specimen to the action of infra-red radiation acting from all sides. The result is seen in the responsive movement of closure (Fig. 109 R). These experiments demonstrate clearly that the responses to rise of temperature and thermal radiation are of opposite signs.

As a movement of closure was induced by the diffuse stimulus of thermal radiation, it is evident that this must have been brought about by the greater contraction of the inner half of the perianth; hence the inner half of the organ is relatively the more excitable.

NEGATIVE THERMONASTIC RESPONSE.

_Response of_ Nymphæa: _Experiment 115._--Many of the Indian _Nymphæaceæ_ have their sepals and petals closed during the day, and open at night. I find that the perianth leaves of this flower are markedly sensitive to variation of temperature. The Thermonastic Recorder employed in this investigation is shown in figure 110. The record given in figure 111 shows that the perianth segment, subjected to a few degrees' rise of temperature, responded by an up-movement of closure, due to greater expansion of the outer half. The latent period was 6 seconds, and the maximum effect was attained in the further course of 21 seconds. This experiment shows that the thermonastic response of this flower is of the negative type.

_Effect of stimulus: Experiment 116._--In the positive type of thermonastic organs, where rise of temperature induced a movement of opening, stimulus induced the opposite movement of closure (Expt. 114). We shall now study the effect of stimulus on the movement of _Nymphæa_, which undergoes closure during rise of temperature, as seen in the first part of the record in figure 111. Stimulus of electric shock was applied at the point marked with an arrow; the response is seen to be by a movement of opening. Here also we find the effects of rise of temperature and of stimulus to be antagonistic to each other. This will be clearly seen in the following tabular statement.

TABLE XXIX.--SHOWING THE EFFECT OF RISE OF TEMPERATURE AND OF STIMULUS ON THERMONASTIC ORGANS.

In _Nymphæa_ it is the outer side of the perianth that is relatively the more excitable since diffuse electric stimulus induces a movement of opening due to the greater contraction of the outer side. It is by the greater expansion of this more excitable side that the movement of closure is effected during rise of temperature.

From the results of experiments given above we arrive at the following:--

LAW OF THERMONASTIC REACTION

RISE OF TEMPERATURE INDUCES A GREATER EXPANSION OF THE MORE EXCITABLE HALF OF AN ANISOTROPIC ORGAN.

SUMMARY.

Thermonastic movements are induced by the differential physiological effect of variation of temperature on the two halves of an anisotropic organ.

Rise of temperature induces greater expansion, and enhancement of rate of growth of the more excitable half of the organ; lowering of temperature induces the opposite effect.

Two types of thermonastic movements are met with, the _positive_ exhibiting a movement of opening during rise of temperature; in these the inner half of the organ is relatively the more excitable. Example of this is seen in the _Crocus_ and in _Zephyranthes_.

In the _negative_ type, rise of temperature induces a movement of closure. Here the outer half of the organ is the more excitable. The flower of _Nymphæa_ belongs to this type.

The effect of stimulus is antagonistic to that of rise of temperature. In positive thermonastic organs stimulus induces a movement of closure; in the negative type it induces a movement of opening.

XXIX.--ON PHOTOTROPISM

_By_

SIR J. C. BOSE.

In different organs of plants the stimulus of light induces movements of an extremely varied character. Radial organs exhibit tropic movements in which the position of equilibrium is definitely related to the direction of incident stimulus. Nastic movements under the action of light are, on the other hand, regarded as curvatures of the organ which show "no relation to the stimulus but is determined by the activity of the plant itself".[9] There are thus two classes of response to light which seem to be unrelated to each other. Returning to the directive action of light, radial stems often bend towards the light, while certain roots bend away from it. It may be thought that this difference is due to specific difference of irritability between shoot and root, the irritability of the former being of a positive, and of the latter, of a negative character. But there are numerous exceptions to this generalisation. Certain roots bend towards the light, while a stem, under different circumstances, moves towards light or away from it. Again an identical organ may exhibit a positive or a negative curvature. Thus the leaflets of _Mimosa pudica_ acted on by light from above fold upwards, the phototropic effect being _positive_. But the same leaflets acted on by light from below exhibit a folding upwards, the phototropic effect being now _negative_. Effects precisely the opposite are found with the leaflets of _Biophytum_ and _Averrhoa_. They fold downwards whether light acts from above or below. Finally, a radial organ in found to exhibit under light of increasing intensity or duration, a positive, a dia-phototropic, or a negative phototropic curvature.

[9] Jost--_Ibid_, p. 428.

In these circumstances the theory of specific positive and negative irritabilities is untenable; in any case, it throws no light on the phenomenon of movement. The difficulties of the problem are thus clearly stated by Pfeffer: "When we say that an organ curves towards a source of illumination, because of its heliotropic irritability and we are simply expressing an ascertained fact in a conveniently abbreviated form, without explaining why such curvature is possible or how it is produced.... Many observers have unfortunately devoted their attention to artificially classifying the phenomenon observed, and have entirely neglected the explanation of causes underlying them."[10]

[10] Pfeffer--_Ibid_, Vol. II, p. 74.

COMPLEXITY OF PROBLEM OF PHOTOTROPIC REACTION.

The complexity of phototropic reaction arises from the summated effects of numerous factors; for explanation of the resultant response it is therefore necessary to take full account of the individual effect of each of them.

Among these operative factors in phototropic reaction may be mentioned:--

(1) The difference of effects induced by light at the proximal
and distal sides of the organ.

(2) The modification of the latent period with the intensity of
stimulus.

(3) The after-effect of stimulus.

(4) The modifying influence of tonic condition on response.

(5) The effect of direction of light.

(6) The effect of intensity of light.

(7) The effect of duration of stimulation.

(8) The transmitted effect of light.

(9) The effect of unequal excitability in different zones of
the organ.

(10) The effect of transverse conduction in modification of
the sign of response.

(11) The effect of temperature on phototropic action.

(12) The modification of response due to differential
excitability of the organ.

(13) Nastic and tropic reactions.

(14) The torsional effect of light.

The sketch given above will give us some idea of the complexity of the problem. In this and in the following papers I shall describe the investigations I have carried out on the subjects detailed above.

ACTION OF LIGHT.

I have shown that there is no essential difference between the responses of pulvinated and growing organs, that diminution of turgor induced by stimulus brings about contraction in the one, and retardation of the rate of growth in the other. Indirect stimulation, on the other hand, induces an expansion and acceleration of the rate of growth. The experimental investigation on the tropic effect of light may therefore be carried out both with pulvinated and growing organs.

As regards the effect of direct stimulus of light on growing organs we found (p. 208) that it induces an incipient contraction, seen in diminution of the rate of growth; this incipient contraction culminates in an actual contraction under increasing intensity of light. The contraction under direct stimulation is also observed in pulvinated organs. When light acts from above the upper half of the pulvinus undergoes contraction, resulting in erection of the motile leaf or leaflets. As regards the effect of indirect unilateral stimulus of light on the distal side of the organ, we found that its effect is an enhancement of turgor (p. 281). Hence the positive tropic curvature under light is brought about, as in the case of other forms of stimuli, by the contraction of the proximal, and expansion of the distal sides of the organ.

Various analogies have been noticed between phototropic and geotropic reactions, and it has been supposed that the two phenomena are closely related to each other. This has even led to assumption that there are phototropic particles which function like statoliths in geotropic organs. There is, however, certain outstanding difference between the two classes of phenomena. In the case of light, the incident energy is entirely derived from the outside. But in geotropism, the force of gravity by itself is ineffective without the intervention of the weight of cell-contents to exert pressure on the sensitive ectoplasm, and thus induce stimulation. This aspect of the subject will be treated in greater detail in a subsequent chapter.

POSITIVE PHOTOTROPIC CURVATURE.

I shall now describe the phototropic effect of unilateral light in pulvinated, and in growing organs. From the explanation that has already been given, it will be understood that the side of the organ directly acted on by light undergoes contraction and concavity.

_Tropic curvature of pulvinated organs: Experiment 117._--For this experiment I employed the terminal leaflet of the bean plant. The source of illumination was 32 c.p. electric lamp, enclosed in a metallic tube with circular aperture for passage of light. The leaflet was attached to an Oscillating Recorder. Light was applied on the upper half of the pulvinus for 20 seconds; this induced an up-movement of the leaflet, due to the contraction of the upper half of the organ. Recovery took place in course of 8 minutes (Fig. 112).

POSITIVE PHOTOTROPIC CURVATURE OF GROWING ORGANS.

_Effect of moderate stimulation: Experiment 118._--I shall presently show that the intensity of phototropic reaction depends on the intensity and duration of the incident light. A moderate and effective stimulation may thus be produced by short exposure to strong light. For my present experiment I took a stem of _Dregea volubilis_, and applied light from a small arc lamp to one side of the organ for 1 minute; this induced a positive curvature followed by complete recovery on the cessation of light (Fig. 113).

_Effect of strong stimulation: Experiment 119._--After recovery of the stem of the last experiment, the same light was applied for 5 minutes. It is seen that the curvature is greatly increased (Fig. 114). Thus the phototropic curvature increases, within limits, with the duration of stimulation. The curvature induced under stronger stimulation remained more or less persistent. In certain instances there was a partial recovery after a considerable length of time; in others curvature was fixed by growth.

PHENOMENON OF RECOVERY.

On the cessation of stimulus of moderate intensity the heliotropically curved organ straightens itself; similar effects are also found in other tropic curvatures. Thus a tendril straightens itself after curvature induced by contact of short duration. The theory of rectipitality has been proposed to account for the recovery, which assumes the action of an unknown regulating power by which the organ is brought back to a straight line; but beyond the assumption of an unknown specific power, the theory affords no explanation of the mechanism by which this is brought about.

The problem before us is to find out the means by which the organ straightens itself after brief stimulation. It will also be necessary to find out why there is no recovery after prolonged stimulation. We have thus to investigate the after-effect of stimulus of various intensities on growth, and the Balanced Method of recording Growth offers us an unique opportunity of studying the characteristic after-effects.

IMMEDIATE AND AFTER-EFFECT OF LIGHT ON GROWTH.

As regards the effect of light I have already shown:

(1) that a sub-minimal stimulus induces an acceleration of
growth, but under long continued action the acceleration is
converted into normal retardation (p. 225),

(2) that a stimulus of moderate intensity induces the normal
retardation of the rate of growth.

It is evident that there is a _critical intensity_ of stimulus, above which there is a retardation, and below which there is the opposite reaction of acceleration. This critical intensity, I have found to be low in vigorous specimens, and high in sub-tonic specimens. Thus the same intensity of stimulus may induce a retardation of growth in specimens the tonic condition of which is _above par_, and an acceleration in others, in which it is _below par_. The following experiments will demonstrate the immediate and after-effect of light of increasing intensity and duration.

_Effect of light of moderate intensity: Experiment 120._--The source of light was a small arc lamp placed at a distance of 50 cm., the intensity of incident light was increased or decreased by bringing the source of light nearer or further away from the plant. Two inclined mirrors were placed behind the plant so that the specimen was acted on by light from all sides. A seedling of wheat was mounted on the Balanced Crescograph, and record was first taken under exact balance; this gives a horizontal record. The up-curve represents retardation, and down-curve acceleration of rate of growth. The source of light was at first placed at a distance of 50 cm. from the plant, and exposure was given for 4 minutes at the point marked with an arrow (Fig. 115a). We shall find in the next chapter that the _intensity of phototropic effect is proportional to the quantity of incident light_. This quantity at the beginning proved to be sub-minimal, and hence there was an acceleration at the beginning. Continued action induced the normal effect of retardation, as seen in the subsequent resulting up-curve. On the cessation of light, the balance was upset in an opposite direction, the resulting down-curve showing an acceleration of the rate of growth above the normal. This acceleration persisted for a time, after which the normal rate of growth was restored, as seen in the curve becoming once more horizontal. _The after-effect of light of moderate intensity is thus a temporary acceleration of rate of growth above the normal._

_Effect of strong light: Experiment 121._--The same specimen was used as in the last experiment. By bringing the source of light to a distance of 25 cm. the intensity of light was increased fourfold; the duration of exposure was kept the same as before. The record (Fig. 115b) shows that a retardation of rate of growth occurred from the very beginning without the preliminary acceleration. This is for two reasons: (1) the increased intensity was now above the critical minimum, and (2) the tone of the organ had become improved by previous stimulation. On the cessation of light, the after-effect showed no enhancement of rate of growth, the recovery from retardation to the normal rate being gradual. In the next experiment (the result of which is not given in the record) the intensity of light was increased still further; the retardation now became very marked, and it persisted for a long time even on the cessation of light.

We thus find that:

(1) The immediate effect of light of moderate intensity is a
preliminary acceleration, followed by normal retardation. The
acceleration is the effect of sub-minimal stimulation. The
immediate after-effect is an acceleration above the normal.

(2) The immediate effect of strong light is a retardation
from the beginning; the immediate after-effect shows no
acceleration, the growth rate being gradually restored to the
normal.

(3) Under very strong light the induced retardation is very
great, and this persists for a long time even on the removal
of light.

The experiments described explains the reasons of complete recovery after moderate stimulation, and also the absence of recovery after strong stimulation. The immediate after-effect of moderate stimulation is shown to be an acceleration of rate above the normal. Returning to tropic curvature, the contraction at the proximal side induced by unilateral light is thus compensated by the accelerated rate of growth on the cessation of light. There is no such compensation in the case of strong and long continued action of light; for the after-effect of strong light shows no such acceleration as the immediate after-effect.

We may perhaps go a step further in explaining this difference. Stimulus was found to induce at the same time two physico-chemical reactions of opposite signs (p. 144). One is the 'up' or A-change, associated with increase of potential energy of the system, and the other is associated with 'down' or D-change, by which there is a run-down or depletion of energy. With moderate stimulation the A-and-D effects are more or less comparable to each other. But under strong stimulation the down-change is relatively greater. Hence on cessation of moderate stimulation the increase of potential energy, associated with A-change, finds expression in enhancement of the rate of growth. The depletion of energy under strong stimulation is, however, too great to be compensated by the A-change.

LATENT PERIOD OF PHOTOTROPIC REACTION.

With reference to the latent period Jost thus summarizes the known results:[11] "The latent period of the heliotropic stimulus has already been determined. According to Czapek it amounts to 7 minutes in the cotyledons of _Avena_ and in _Phycomyces_; 10 minutes in hypocotyls of _Sinapis alba_ and _Beta vulgaris_, 20 minutes in the hypocotyl of _Helianthus_, and 50 minutes in the epicotyl of _Phaseolus_. If one of these organs be unilaterally illuminated for the specified time, heliotropic curvature ensues afterwards in the dark, that is to say, we meet with an after-effect in this case as in geotropism. We are quite ignorant, however, as to whether and how the latent period is dependent on the intensity of light."

[11] Jost--_Ibid_, p. 473.

With regard to the question of relation of the latent period to the intensity of stimulus I have shown (p. 166) that the latent period is shortened under increasing intensity of stimulus. In the case of tropic curvature induced by light, I find that the latent period is reduced under increasing intensity of light. The shortest latent period found by Czapek, as stated before, was 7 minutes. But by employing high magnification for record, I find that the latent period of phototropic action under strong light to be a question of seconds.

_Determination of the latent period: Experiment 122._--I give a record of response (Fig. 116) of the terminal leaflet of _Erythrina inidca_ to light acting from above. The recording plate was made to move at a fast rate, the successive dots being at intervals of 2 seconds. The latent period in this case is seen to be 35 seconds. By the employment of stronger light I have obtained latent period which is very much shorter.

The term latent period is used in two different sense. It may mean the interval between the application of stimulus and the initiation of response. In the experiment described above, the latent period is to be understood in this sense. But in the extract given above, Jost uses the term latent period as the shortest period of exposure necessary to induce phototropic reaction as an after-effect. What then is the shortest exposure that will induce a retardation of growth? For this investigation I employed the very sensitive method of the Balanced Crescograph.

GROWTH-VARIATION BY FLASH OF LIGHT FROM A SINGLE SPARK.

_Experiment 123._--I stated that the more intense is the light, the shorter is the latent period. The duration of a single spark discharge from a Leyden jar is almost instantaneous, the duration of discharge being of the order of 1/100,000th of a second. The single discharge was made to take place between two small steel spheres, the light given out by the spark being rich in effective ultra-violet rays. The plant used for the experiment was a seedling of wheat. It was mounted on the Balanced Crescograph, and its normal growth was exactly compensated as seen in the first part of the record. The spark gap was placed at a distance of 10 cm. from the plant; there was the usual arrangement of inclined mirrors for illumination of the plant. The flash of light from a single spark is seen to induce a sudden retardation of rate of growth which lasted for one and half minutes. The record (Fig. 117) shows another interesting peculiarity of acceleration as an after-effect of moderate stimulation. After the retardation which lasted for 90 seconds, there is an acceleration of growth above the normal, which persisted for 6 minutes, after which the rate of growth returned to the normal.

In order to show that the induced variation is due to the action of light and not to any other disturbance, I interposed a sheet of ebonite between the spark-gap and the plant. The production of spark produced no effect, but the removal of the ebonite screen was at once followed by the characteristic response.

MAXIMUM POSITIVE CURVATURE UNDER CONTINUED ACTION OF LIGHT.

The positive curvature is, as we have seen, due to the contraction of the proximal side and expansion of the distal side. The curvature will increase with growing contraction of the proximal side; a maximum curvature is however reached since:

(1) the contraction of the cells must have a limit,

(2) the bending organ offers increasing resistance to
curvature, and

(3) the induced curvature tends to place the organ parallel
to the direction of light when the tropic effect is reduced
to a minimum.

The pulvinus of _Erythrina_ exemplifies the type of reaction in which the positive curvature reaches a maximum, (see below Fig. 132) beyond which there is no further change. This is due to absence of transverse conductivity in the organ. The modifying effect of transverse conductivity on response will be dealt with in the next chapter.

SUMMARY.

The positive phototropic curvature is brought about by the joint effects of the directly stimulated proximal, and indirectly stimulated distal side.

The phototropically curved organ undergoes recovery after brief stimulation.

The recovery after moderate stimulation is hastened by the previously stimulated side exhibiting an acceleration of the rate of growth above the normal. The after-effects of photic and mechanical stimulation are similar.

The latent period of photic reaction is shortened with the increasing intensity of light. The seedling of wheat responds to a flash of light from an electric spark, the duration of which is about a hundred thousandth part of a second.

Tissues in which the power of transverse conduction is negligible, the positive phototropic curvature under continued action of light attains a maximum without subsequent neutralisation or reversal.

XXX.--DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.

I have explained how under the action of unilateral light the positive curvature attains a maximum. There are, however, cases where under the continued action of strong light the tropic movement undergoes a reversal. Thus to quote Jost: "Each organism may be found in one of the three different conditions determined by the light intensity, _viz._ (1) a condition of positive heliotropism, (2) a condition of indifference, (3) a condition of negative heliotropism"[12]. No explanation has however been offered as to why the same organ should exhibit at different times, a positive, a neutral, and a negative irritability. These changing effects exhibited by an identical organ is thus incompatible with the theory of specific sensibility, assumed in explanation of characteristic differences in phototropic response.

[12] Jost--_Ibid_--p. 462.

In regard to this I would draw attention to an important factor which modifies the tropic response, namely, the effect of transverse conduction of excitation. I shall presently describe in detail a typical experiment of the effect of unilateral stimulus of light on the responsive movement of main pulvinus of _Mimosa pudica_. The results will be found of much theoretical interest, since a single experiment will give an insight to all possible types of phototropic response. Before describing the experiment I shall demonstrate the tropic reactions of the two halves of the pulvinus of _Mimosa_.

UNEQUAL EXCITABILITY OF UPPER AND LOWER HALVES OF PULVINUS TO PHOTIC STIMULATION.

I have by method of selective amputation shown that as regards electric stimulation the excitability of the upper half of the pulvinus is very much less than that of the lower half (p. 85). I have obtained similar results with photic stimulation.

_Tropic effect of light acting from above: Experiment 124._--Light of moderate intensity from an incandescent electric lamp was applied on the upper half of the pulvinus of _Mimosa_ for 4 minutes; this induced a contraction of the stimulated upper half and gave rise to an up or erectile response. On the stoppage of light recovery took place in the course of ten minutes. The phototropic curvature is thus seen to be positive. A series of such positive responses of the upper half of the pulvinus is given in figure 118.

_Effect of light acting from below: Experiment 125._--Light was now applied from below; this also induced a contraction of the lower half of the pulvinus, causing a down-movement (Fig. 119). As the responsive movement is towards light, the phototropic effect must be regarded as positive. The greater excitability of the lower half of the pulvinus is shown by the fact that the response of the lower half of the pulvinus to ten seconds' exposure is even larger than that given by the upper half under the prolonged exposure of 240 seconds.

TRANSFORMATION OF POSITIVE TO NEGATIVE PHOTOTROPIC CURVATURE.

_Experiment 126._--A beam of light from a small arc lamp was thrown on the upper half of the pulvinus. After a latent period of 5 seconds, a positive curvature was initiated, by the contraction of the upper and expansion of the lower side of the organ. But under continued action of light, the excitatory impulse reached the lower half of the organ, causing a rapid fall of the leaf, and a _negative_ curvature. The arrival of transmitted excitation at the more excitable distal half of the organ is clearly demonstrated by the very rapid down-movement, seen as the up-curve in the record (Fig. 120). In sensitive specimens this movement is so abrupt and rapid, that the writing lever is jerked off above the recording plate before making a dot on it. The thickness of the pulvinus was 1·5 mm., the distance which the excitatory impulse has to traverse to reach the lower half would thus be about 0·75 mm. The period for transverse transmission of excitation under strong light was found to vary in different cases from 50 to 80 seconds. The velocity of transmission of excitation in a transverse direction through the pulvinus is about 0·011 mm. per second, which is not very different from 0·010 mm. per second in the stem (p. 282).

Returning to the main experiment we find that:

(1) As a result of unilateral action of light, there was
positive phototropic curvature which lasted for 50 seconds.

(2) Owing to the internal conduction of excitation the
positive effect underwent neutralisation by the excitatory
contraction of the distal side. This neutralisation depends
on four factors: (_a_) on the intensity of the stimulus,
(_b_) on the conductivity of the organ in a transverse
direction, (_c_) on the thickness of the intervening tissue,
and (_d_) on the relative excitability of the distal as
compared to the proximal side. The extent of positive
curvature also depends on the pliability of the organ.

(3) In anisotropic organs where the distal side is
physiologically the more excitable than the proximal, the
internally diffused excitation brings about a greater
contraction of the distal, and the _positive_ phototropic
curvature becomes reversed to a very pronounced _negative_.
The effect of the internally diffused stimulus is thus the
same as that of external diffuse stimulus.

(4) When the stimulus is applied on the more excitable half
of the organ, the result is a predominant contraction of that
half, which cannot be neutralised by the excitation conducted
to the less excitable half of the organ. As the curvature is
towards the stimulus, the phototropic curvature thus remains
positive, even under continued stimulation.

The positive curvature is due to the differential action of unilateral stimulus on the proximal and distal sides. But when a strong light is made to act continuously on one side of an organ, the excitation becomes internally diffused, and the differential effect on the two sides is reduced in amount or vanishes altogether. Owing to the weak transverse conductivity of the tissue, while the effect of a feeble stimulus remains localised, that of a stronger stimulus is conducted across it.

Oltmanns found that the seedling of _Lepidium sativum_ assumed a transverse or dia-phototropic position under intense and long continued action of light of 600,000 Hefner lamps. He regards this as the indifferent position. But the neutralisation of curvature is not, as explained before, due to a condition of indifference, but to the antagonistic effects of the two opposite sides of the organ, the proximal being stimulated by the direct, and the distal by the transversely conducted excitation. I obtained such neutralisation with _Dregea volubilis_ under the prolonged unilateral action of arc-light. The first effect was positive; this was gradually and continuously neutralised under exposure for two hours; even then the neutralisation was not complete. I shall presently adduce instances where the neutralisation was not merely complete, but the final effect was an actual reversal into negative response.

SUPPOSED PHOTOTROPIC INEFFECTIVENESS OF SUNLIGHT.

I may here consider the remarkable fact that has been observed, but for which no explanation has been forthcoming, that "direct sunlight is too bright to bring about heliotropic curvature, only diffuse, not direct sunlight has the power of inducing heliotropic movements."[13] But we cannot conceive of light suddenly losing its phototropic effect by an increase of intensity. The experiment just described will offer full explanation for this apparent anomaly. Feeble or moderate stimulus remains, as we have seen, localised, hence the contraction of the proximal side gives rise to positive curvature. But the intense excitation caused by sunlight would be transmitted to the distal side and thus bring about neutralisation. It is the observation of the final result that has misled observers as to the inefficiency of direct sunlight. A continuous record of the response of the organ shows, on the other hand, that the first effect of strong light is a positive curvature, and that under its continuous action the positive effect becomes neutralised (cf. Fig. 121). In the study of phototropic action, the employment of strong light has many advantages, since the period of experiment is, by this means, materially shortened. The continuous record then gives an epitome of the various phases of reaction.

[13] Jost--_Ibid_--p. 464.

NEGATIVE PHOTOTROPISM.

I shall next show the continuity of responsive phototropic effects, from the positive curvature to the negative, through the intermediate phase of neutralisation. I have in the preceding paragraph described an experiment where under a given intensity and duration of exposure the excitations of the proximal and distal sides bring about neutralisation, the organ assuming a dia-phototropic position. If the intensity or duration of the stimulating light be further increased, it is easy to see that while excitation transmitted to the distal side is being increased, the excitatory contraction on the proximal side may, at the same time, be decreased owing to fatigue brought on by over-stimulation.

In connection with this it should be borne in mind that the pulvinus of _Mimosa_ exhibits under continuous stimulation, a fatigue relaxation instead of normal contraction. Similar effects are known to take place in animal muscles. The effect of relatively greater excitation will thus give rise to negative phototropic curvature. The transverse conductivity of organs of diverse plants will necessarily be different. The neutralisation and reversal into negative will thus depend on three factors: the transverse conductivity of the organ, the intensity, and duration of stimulus.

_Neutralisation and reversal under increased intensity of light: Experiment 127._--It is advisable to employ thin specimens (in which the transverse distance is small) for the exhibition of reversal effect. I took a hypocotyl of _Sinapis nigra_ and subjected it to unilateral action of light from a 16 candle-power incandescent electric lamp placed at a distance of 10 cm. A maximum positive curvature was induced in the course of 50 minutes. The intensity of light was afterwards increased by bringing the lamp nearer to a distance of 6 cm. This resulted in a process of neutralisation of the preceding response; after an exposure of 70 minutes the specimen assumed a dia-phototropic position in which it remained in equilibrium. Sunlight was next applied, and in the further course of 30 minutes there was a pronounced reversal into negative phototropic curvature.

_Neutralisation and reversal under continuous stimulation: Experiment 128._--In the last experiment the different changes in the response were brought about by successive increase in the intensity of light. In the present experiment, very strong light was applied from the beginning, and continuous record was taken of the change in the response. In order to reduce the period of experiment I employed a mercury vapour lamp which emits the most effective violet and ultra-violet rays. The specimen used was a seedling of the rice plant (_Oryza sativa_). The first effect of light was a positive curvature which attained its maximum; after this there was a neutralisation in less than six minutes after the application of light. The further continuation of light induced a pronounced negative curvature (Fig. 121).

I shall in the next chapter give other instances which will show that all organs (pulvinated and growing) possessed of power of transverse conduction, exhibit a transformation of response from positive to negative under continued action of strong light.

Thus an identical organ, under different conditions of intensity and duration of stimulus, exhibits _positive_ phototropic, _dia_-phototropic, and _negative_ phototropic curvatures, proving conclusively that the three effects are not due to three distinct irritabilities. The responsive movements are, on the other hand, traced to a fundamental excitatory reaction, remaining either localised or increasingly transmitted to the distal side.

NEGATIVE PHOTOTROPISM OF ROOTS.

From the analogy of opposite responses of shoot and root to stimulus of gravity, it was surmised that the root would respond to light by a negative curvature. This was apparently confirmed by the negative phototropic curvature of the root of _Sinapis_. The supposed analogy is however false; for while the stimulus of gravity acts, in the case of root, only on a restricted area of the tip, the stimulus of light is not necessarily restricted in the area of its action. That there is no true analogy between the action of light and gravitation is seen from the fact that while gravitation induces in the root a movement opposite to that in the stem, in the case of light, this is not always so; for though a few roots turn away from light, others move towards the light.

As regards negative phototropic response of the root of _Sinapis_, it will be shown (p. 376) to be brought about by algebraical summation of the effects of direct and indirect photic stimulus.

SUMMARY.

The normal positive phototropic curvature is modified by transverse conduction of true excitation to the distal side of the organ.

The extent of neutralisation or reversal due to internal conduction of excitation from the proximal to the distal side of the organ depends: (_a_) on the intensity of the incident stimulus, (_b_) on the conductivity of the organ in a transverse direction, (_c_) on the thickness of the intervening tissue, and (_d_) on the relative excitability of the distal as compared to the proximal side.

The dia-phototropic position is not one of indifference, but of balanced antagonistic reactions of two opposite sides of the organ.

The supposition that direct sunlight is phototropically ineffective is unfounded. The response is fully vigorous, but the first positive curvature may in certain cases be neutralised by the transmission of excitation to the distal side.

Under light of strong intensity and long duration, the transmitted excitation to the distal side neutralises, and finally reverses the positive into negative curvature.

The _positive_-phototropic, the _dia_-phototropic, and the _negative_ phototropic curvatures are not due to three distinct irritabilities but are brought about by a fundamental excitatory reaction remaining localised or increasingly transmitted to the distal side.

XXXI.--THE RELATION BETWEEN THE QUANTITY OF LIGHT AND THE INDUCED PHOTOTROPIC CURVATURE

_By_

SIR J. C. BOSE,

_Assisted by_

SURENDRA CHANDRA DAS, M.A.

I shall in this chapter describe experiments in support of the important proposition that _the intensity of phototropic action is dependent on the quantity of incident light_. The proportionality of the tropic effect to the quantity of light will be found to hold good for the median range of stimulation; the deviation from this proportionality at the two ends of the range of stimulation--the sub-minimal and supramaximal--is, as we shall find, capable of explanation, and will be fully dealt with in the next chapter.

The quantity of light incident on the responding organ depends: (1) on the intensity of light, (2) on the angle of inclination or _the directive angle_,[14] and (3) on the duration of exposure. I shall give a detailed account of the investigation relating to the individual effects of each of these factors on the tropic reactions not merely in pulvinated but also in growing organs.

[14] The directive angle [Greek: th] is the angle of inclination of
the rays of light to the responding surface. The angle
[Greek: th] is complementary to the angle of incidence _i_ in
optics. Sin [Greek: th] = Cos i.

EFFECT OF INCREASING INTENSITY OF LIGHT ON TROPIC CURVATURE.

The intensity of light was increased in successive experiments, in arithmetical progression 1:2:3 by suitably diminishing the distance between the plant and the source of light, and the resulting tropic curvatures recorded.

_Effect of increasing intensity of light on the pulvinus of_ Desmodium gyrans: _Experiment 129._--The source of light was a 50 candle-power incandescent lamp, and the duration of exposure was 1 minute. The specimen employed was a terminal leaflet of _Desmodium gyrans_ (Fig. 122) the pulvinus of which is very sensitive to light. It is more convenient to manipulate a cut specimen of the leaf, instead of the whole plant. The petiole is placed in water contained in a U-tube; the depressing effect of wound passes off in the course of an hour or so. Light of increasing intensity is applied from above; this induces a contraction of the upper half of the pulvinus, and the resulting response is recorded by means of the Oscillating Recorder (Fig. 123).

The first record was obtained under a given intensity, and the second, under an intensity twice as great. The tropic effects are seen to increase with the intensity (Fig. 124). If the tropic curvature increased proportionately to the intensity, the two responses should have been in the ratio of 1:2; the actual ratio was however slightly greater, _viz._ 1:2·6. In this connection it will be shown in the next chapter, that strict proportionality holds good only in the median range, and that the susceptibility for excitation undergoes an increase at the beginning of the phototropic curve.

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Life Movements in Plants, Volume IIChapter V: Part III: Tropism in Plants (2)

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