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.

[Illustration: FIG. 116.--Latent period for photic stimulation at

vertical line. Successive dots at intervals of 2 seconds. (_Erythrina

indica_).]

_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.

[Illustration: FIG. 117.--Effect of a single electric spark on variation

of growth. Record taken by Balanced Crescograph. Up-curve shows induced

retardation of growth; the after-effect is an acceleration (down-curve)

followed by restoration to normal.]

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.

[Illustration: FIG. 118.--Series of up-responses of _Mimosa_ leaf to

light applied on upper half of pulvinus.]

[Illustration: FIG. 119.--Down-responses given by the same plant on

application of light from below.]

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).

[Illustration: FIG. 120.--Record of effect of continuous application of

light on upper half of pulvinus of _Mimosa_ leaf. Note erectile response

(positive curvature) followed by neutralisation and pronounced reversal

into negative due to transverse conduction of excitation. Up-movement

shown by down curve, and _vice versâ_.]

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.

[Illustration: FIG. 121.--Positive and negative phototropic responses of

_Oryza_ under continued unilateral stimulus of intense light from arc

lamp.]

_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.

[Illustration: FIG. 122.--Leaf of _Desmodium gyrans_, with the terminal

large, and two lateral small leaflets. These latter exhibit automatic

pulsations.]

_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).

[Illustration: FIG. 123.--The Oscillating Recorder (From a photograph).]

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.

[Illustration: FIG. 124.--Tropic effect of increasing intensity of

light 1:2; on the response of terminal leaflet of _Desmodium gyrans_.]

[Illustration: FIG. 125.--Tropic effect of increasing intensity of

light 1:2:3 on growing organ (_Crinum_).]

_Effect of increasing intensity of light on the tropic curvature of

growing organs._--As the tropic curvature is primarily due to the

retardation of growth induced by light at the proximal side of the

organ, it will be of interest to recapitulate the results I obtained (p.

208) on the effects of increasing intensity of light on growth itself.

The normal rate of growth of the specimen in the dark was 0·47 µ per

second; this was reduced to 0·29 µ under an intensity of one unit, to

0·17 µ under two, and to 0·10 µ under three units. Growth became

arrested when the intensity was raised to four units. Thus increasing

intensity of light induces an increasing retardation of growth at the

proximal side of the organ. This aided by the effect of indirect

stimulus at the distal side brings about an increasing positive

curvature.

_Experiment 130._--The flower bud of _Crinum_ was used for the

experiment, the source of light being a small arc lamp. The duration of

exposure was one minute. Increasing intensity of light gave rise to

increasing positive curvatures (Fig. 125) in the ratio of 1:2·5:5 under

increasing intensities which varied as 1:2:3.

THE EFFECT OF INCREASING ANGLE.

The quantity of light which falls on an unit area of the responding

organ varies as sin [Greek: th] where [Greek: th] is the directive angle

_i.e._ the angle made by the rays with the surface. Some allowance has

to be made for the amount of light reflected from the surface, this

being greater at 45° than at 90°.

[Illustration: FIG. 126.--The Collimator.]

_Tropic response of pulvinus of_ Desmodium gyrans: _Experiment

131._--For application of light at various angles an incandescent

electric lamp was mounted at one end of a brass tube, a collimating

lens being placed at the other (Fig. 126). The parallel beam of light

from the collimator could be sent at various angles by rotating the

collimator tube round an axis at right angles to the tube. The specimen

employed was the terminal leaflet of _Desmodium gyrans_; light was

applied for a minute in the two successive experiments for the two

angles of 45° and 90°. The record (Fig. 127) shows that the phototropic

effect increases with the directive angle. In the present case the ratio

of the two effects is 1·6:1, which is not very different from the ratio

sin 90°/sin 45° = 1·4.

[Illustration: FIG. 127.--Effect of angle of inclination of light on the

tropic curvature of pulvinus. The first response is to light at 45° and

the second, to 90°. (_Desmodium gyrans_).]

[Illustration: FIG. 128.--Series of tropic curvatures of growing bud of

_Crinum_ to alternate stimulation by light at 45° and 90°.]

_Tropic response of growing organs: Experiment 132._--Similar experiment

was carried out with the flower bud of _Crinum_, held vertical. Light

was applied alternately at 45° and 90°, in two successive series. The

object of this was to make due allowance of possible variation of

excitability of the organ during the course of the experiment. I have

explained (p. 147), how the excitability of a tissue in a condition

slightly below par, is increased by the action of previous stimulation.

Series of responses obtained under alternate stimulations at 45° and

90° enable us to find out, whether any variation of excitability

occurred during the course of the experiment and make allowance for it.

The records show that stimulation did enhance the excitability of the

organ to a small extent. Thus the first stimulation at 45° induced an

amplitude of response of 5 mm.; the second stimulation at 45° _i.e._ the

third response of the series, induced a slightly larger response 7 mm.

in amplitude. Similarly the two responses at 90° gave an amplitude of 9

mm. and 11 mm. respectively (Fig. 128). Taking the mean value of each

pair, the ratio of tropic effects for 90° and 45° is = 10/6 = 1·7

nearly.

EFFECT OF DURATION OF EXPOSURE.

_Experiment 133._--The specimen employed for the experiment was a flower

bud of _Crinum_ in a slightly sub-tonic condition. Successive responses

exhibited on this account, a preliminary negative[15] before the normal

positive curvature. The successive durations of exposure were for 1, 2,

and 3 minutes. The amplitudes of responses (Fig. 129) are in the ratio

of 1:2·5:5.

[15] An explanation of this preliminary effect will be found in the

next chapter.

[Illustration: FIG. 129.--Effect of increasing duration of exposure

1:2:3 minutes, on phototropic curvature. Note preliminary negative

response. (_Crinum_).]

We may now recapitulate the tropic effects of light of increasing

intensity, directive angle, and duration of exposure. It has been shown

that the tropic effect is enhanced under increasing intensity of light;

it is also increased with the angle increasing from grazing to

perpendicular incidence. And finally, the tropic effect is enhanced with

the duration of exposure. Taking into consideration the effects of these

different factors we arrive at the conclusion that _phototropic effect

increases with the quantity of incident light_. It will be shown in the

next chapter that strict proportionality of cause and effect holds good

in the median range of stimulation, and the slight deviation from this,

above and below the median range, is due to the fact that susceptibility

for excitation is low at these two regions.

SUMMARY.

Increasing intensity of light induces increasing tropic curvature.

Tropic curvature increases with the directive angle, the effect being

approximately proportional to sin [Greek: th], where [Greek: th] is the

angle made by the rays with the responding surface.

Tropic curvature also increases with the duration of exposure.

The intensity of induced tropic effect is determined by the quantity of

incident light.

XXXII.--THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS

_By_

SIR J. C. BOSE.

When a plant organ is subjected to the continued action of unilateral

stimulus of light, it exhibits increasing tropic curvature, which in

certain cases reaches a limit; in other instances a reversal takes

place, seen in neutralisation, or in the conversion of the positive into

negative curvature. I shall in this chapter enter into a detailed study

of the phototropic curve, and determine its characteristics.

As the vague terminology at present in use has been the source of much

confusion, it is necessary here to define clearly the various terms

which will be employed in this investigation. It is first of all

necessary to distinguish between cause and effect, between external

stimulus and the excitation induced by it. As regards stimulus itself I

have shown elsewhere[16] that its effective intensity becomes summated

by repetition. This was demonstrated by the two following typical

experiments carried out with the pulvinus of _Mimosa_.

[16] "Irritability of Plants"--p. 54.

(1) The intensity of a single electric shock of intensity of 0·5 unit

was found to be ineffective in inducing excitation; but it became

effective on being repeated four times in rapid succession.

(2) The same specimen was next subjected to a feebler stimulus of

intensity of 0·1 unit, and it required a repetition of 20 times for the

stimulus to become effective.

The total stimulus in the first case was 0·5 × 4 = 2, and this was found

to be the same as 0·1 × 20 = 2 in the second case. Thus the intensity of

stimulus is increased by repetition; in the limiting case where the

interval between successive stimulus is zero, the stimulus becomes

continuous. Bearing in mind the additive effects of stimulus we see that

its effective intensity increases with the _duration_ of application.

This important conclusion found independent support from the results of

Experiment 133 given in the last chapter.

We shall now take up the general question of the characteristics of the

phototropic curve, which gives the relation between increasing stimulus

and the resulting excitation. As regards stimulus we found that its

effectiveness increases with the duration of application. The induced

excitation in growing organs may be measured by concomitant retardation

of growth caused by stimulus. In the excitation curves which will be

presently given, the abscissae represent increasing stimulus and

ordinates the resulting excitation. This excitation curve may be

obtained by making the plant record on a moving plate its retardation of

growth by means of the High Magnification Crescograph. I reproduce below

two records of the effects of continuous photic and electric

stimulation. The ordinate of the 'excitation curve' (Fig. 130) exhibits

increasing incipient contraction (retardation of growth) culminating in

an arrest of growth; the abscissa represents increasing stimulus

consequent on increased duration of application. The record shows that

the incipient contraction is slight at the first stage; it increases

rapidly in the second stage; finally, it declines and reaches a limit.

The excitatory reaction is thus not constant throughout the entire curve

of excitation, but undergoes very definite and characteristic changes.

We shall find similar characteristics in the phototropic curves under

unilateral stimulus which will be given presently. The explanation of

the similarity is found in the fact that the tropic curvature is also

due to incipient contraction or retardation of the rate of growth, which

remains confined to the directly stimulated proximal side of the organ.

[Illustration: FIG. 130.--Effects of continuous (_a_) electric, and

(_b_) photic stimulation on rate of growth. Abscissa represents duration

of application of stimulus. Note induced retardation, and arrest of

growth.]

For facility of explanation of what follows, I shall have to use a new

and necessary term, _susceptibility_, to indicate the relation of cause

and effect, of stimulus and resulting excitation. _Susceptibility_ is

thus = Excitation/Stimulus. Different organs of plants exhibit unequal

susceptibilities; some undergo excitation under feeble stimulus, while

others require more intense stimulus to induce excitation. But even in

an identical organ the susceptibility undergoes, as we have seen, a

characteristic variation, being feeble at the beginning of the

excitation curve, considerable in the middle, and becoming feeble once

more towards the end of the curve. The most difficult problem that faces

us is an explanation of this characteristic difference in different

parts of the tropic curve.

GENERAL CONSIDERATIONS.

Before entering into the fuller consideration of the subject, it will be

helpful to form some mental picture of the phenomena of excitation,

however inadequate it may be. The excitation is admitted to be due to

the molecular upset induced by the shock of stimulus[17]; the increased

excitation results from increasing molecular upset brought on by

enhanced stimulus. The condition of molecular upset or excitation may be

detected from the record of any one of the several concomitant changes,

such as the change of form, (contraction or expansion) or change of

electric condition (galvanometric negativity or positivity). These means

of investigation are not in principle different from those we employ in

the detection of molecular distortion in inorganic matter under

increasing intensities of an external force.

[17] I shall use the term _stimulus_ in preference to _stimulation_,

for the latter is often taken in the sense of the resulting

excitation.

THE CHARACTERISTIC CURVE.

Thus the molecular upset and rearrangement, in a magnetic substance

under increasing magnetising force are inferred from the curve obtained

by means of appropriate magnetometric or galvanometric methods. I

reproduce the characteristic curve of iron (Fig. 131) in which the

abscissa represents increasing magnetising force, and the ordinate, the

induced magnetisation. This characteristic curve, giving the relation of

cause and effect, will be found to be highly suggestive as regards the

similar characteristic curve which gives the relation between increasing

stimulus and the resulting enhanced tropic effect in vegetable tissues.

The parallelism will be found to be very striking.

Inspection of figure 131 shows that, broadly speaking, the curve of

magnetisation may be divided into four parts. In the first part, under

feeble magnetic force, the slope of the curve is very small; later, in

the second part, as the force increases, the curve becomes very steep;

in the third part the slope of the curve remains fairly constant; and

finally in the fourth part, the curve rounds off and the rate of ascent

again becomes very small. The _susceptibility_ for induced magnetisation

is thus very feeble at the beginning; under increasing force, in the

second stage, the susceptibility becomes greatly enhanced; in the third

stage, the susceptibility remains approximately constant; and in the

fourth stage it becomes diminished. We shall presently find that the

susceptibility for excitation also undergoes a similar variation at the

four different stages of stimulation.

CHARACTERISTICS OF SIMPLE PHOTOTROPIC CURVE.

I have shown (Fig. 130) the relation between the stimulus and the

resulting excitation, the latter being determined from the diminution of

the rate of growth. Under unilateral action of light, the excitatory

contraction gives rise to tropic curvature. We may thus obtain the

characteristic excitation curve, by making the plant organ record its

tropic movement under continuous action of light applied on one side of

the organ.

[Illustration: FIG. 131.--Characteristic curve of iron under increasing

magnetising force. (After Ewing).]

[Illustration: FIG. 132.--Simple characteristic curve of phototropic

reaction. Excitation increases slowly in the first part and rapidly in

the second; it is uniform in the third, and undergoes decline in the

fourth part (_Erythrina indica_).]

_Experiment 134._--I give below the characteristic curve of excitation

(Fig. 132) of the pulvinus of _Erythrina indica_, traced by the plant

itself, and exactly reproduced by photomechanical process. A parallel

beam of light from a Nernst lamp was thrown on the upper leaf of the

pulvinus, and the increasing positive curvature was recorded on a smoked

glass plate which was moved at an uniform rate. The successive dots are

at intervals of 20 seconds; the horizontal distances between successive

dots are equal, and represent equal increments of stimulus; the vertical

distances between successive dots represent the corresponding

increments of excitation. The gradient at any point of the

curve--increment of excitation divided by increment of stimulus--gives

the susceptibility for excitation at that point. The following table

will show how the susceptibility for excitation undergoes variation

through the entire range of stimulus. The average susceptibility for

each point has been calculated from the data furnished by the curve.

TABLE XXX.--SHOWING THE VARIATION OF SUSCEPTIBILITY FOR EXCITATION AT

DIFFERENT POINTS OF THE TROPIC CURVE.

+--------------------------------------+

| Successive points | Susceptibility |

| in the curve. | for excitation. |

+-------------------+------------------+

| 1 ... ... | 0 |

| 2 ... ... | 0·187 |

| 3 ... ... | 0·44 |

| 4 ... ... | 0·625 |

| 5 ... ... | 0·875 |

| 6 ... ... | 1·25 |

| 7 ... ... | 1·87 |

| 8 ... ... | 3·12 |

| 9 ... ... | 5·0 |

| 10 ... ... | 6·25 |

| 11 ... ... | 8·75 |

| 12 ... ... | 8·87 |

| 13 ... ... | 8·12 |

| 14 ... ... | 6·6 |

| 15 ... ... | 4·4 |

| 16 ... ... | 2·5 |

| 17 ... ... | 1·87 |

| 18 ... ... | 1·5 |

| 19 ... ... | 1·12 |

| 20 ... ... | 0·937 |

| 21 ... ... | 0·75 |

| 22 ... ... | 0·562 |

| 23 ... ... | 0·375 |

| 24 ... ... | 0·25 |

| 25 ... ... | 0·187 |

| 26 ... ... | 0·062 |

+--------------------------------------+

The induced excitation is seen to be increased very gradually from the

zero point of susceptibility, known as the latent period at which no

excitation takes place. In the second part of the excitation curve, the

rate of increase is vary rapid; the maximum rate is nearly reached at

point 11 of the curve and remains fairly constant for a time. This is

the median range where equal increment of stimulus induces equal

increment of excitation. The susceptibility for excitation then falls

rapidly, and increase of stimulus induces no further increase of tropic

curvature. The maximum tropic curvature was, in the present case,

reached in the course of nine minutes. The attainment of this maximum

depends on the excitability of the tissue, and the intensity of incident

stimulus. The characteristics that have been described are not confined

to the phototropic curve but exhibited by tropic curves in general.

Similar characteristics have been found in the curve for electric

stimulus (Fig. 130a), and will also be met with in the curve for

geotropic stimulus (Fig. 161).

I may here refer incidentally to the three types of responses exhibited

by an organ to successive stimuli of uniform intensity; these appear to

correspond to the three different regions of tropic curve; in the first

stage, the plant exhibits a tendency to exhibit a 'staircase' increase

of response; in the intermediate stage, the response is uniform; and in

the last stage, the responses show a 'fatigue' decline.

For purpose of simplicity, I first selected the simple type of

phototropic curve, where the specimen employed was in a favourable tonic

condition, and the stimulus was, from the beginning, above the minimal.

Transverse conduction, which induces neutralisation or reversal into

negative, was moreover absent in the specimen. I shall now take up the

more complex cases: (1) where the condition of the specimen is slightly

sub-tonic, (2) where the stimulus is gradually increased from the

_sub-minimal_, and (3) where the specimen possesses the power of

transverse conduction.

EFFECT OF SUB-MINIMAL STIMULUS.

It is unfortunate that the terms in general use for description of

effective stimulus should be so very indefinite. A stimulus which is

just sufficient to evoke excitatory _contraction_ is termed the minimal,

stimulus below the threshold being tacitly regarded as ineffective. The

employment of sensitive recorders has, however, enabled me to discover

the important fact that stimulus below the minimal, though ineffective

in inducing excitatory _contraction_, is not below the threshold of

perception. The plant not merely perceives such stimulus, but also

responds to it in a definite way, by _expansion_ instead of

_contraction_. I shall designate the stimulus below the minimal, as the

_sub-minimal_. There is a critical point, which demarcates the

sub-minimal stimulus with its expansive reaction from the minimal with

its responsive contraction.

The _critical_ stimulus varies in different species of plants. Thus the

same intensity of light which induces a retardation of growth in one

species of plants will enhance the rate of growth in another. Again, the

critical point will vary with the _tonic_ level of the same organ; in an

optimum condition of the tissue, a relatively feeble stimulus will be

sufficient to evoke excitatory contraction; the critical point is

therefore low for tissues in tonic condition which may be described as

_above par_. In a sub-tonic condition, on the other hand, strong and

long continued stimulation will be necessary to induce the excitatory

reaction. The critical point is therefore high, for tissues in a

condition _below par_. Stimulus below the critical point will here

induce the opposite physiological reaction, _i.e._, expansion. The

physico-chemical reactions underlying these opposite physiological

responses have, for convenience, been distinguished as the "A" and "D"

change (pp. 143, 223). The assimilatory 'building up', A change, is

associated with an increase of potential energy of the system; the

dissimilatory 'break down', D change, on the other hand, is attended by

a run-down of energy.

Stimulus was shown (p. 225) to give rise to _both_ these reactions,

though the A effect is, generally speaking, masked by the predominant D

effect. The "A" change is quicker in initiation, while the "D" effect

developes later; again the "A" effect under moderate stimulation may

persist longer. Thus owing to the difference in their time-relations the

A effect is capable of being unmasked at the onset of stimulus or on its

sudden cessation. For the detection of the relatively feeble expansive A

effect, a special recorder is required which combines lightness with

high power of magnification. The earlier expansive reaction and

acceleration of rate of growth, followed by normal retardation, are

often found in the response of growing organs. The corresponding effect

of unilateral stimulation, even when direct, is a transient expansion at

the proximal side, inducing a convexity of that side and movement away

from stimulus (negative curvature); this is followed by contraction and

concavity with normal positive curvature. The interval between the A and

D effects is increased with increasing sub-tonicity of the specimen. But

it nearly vanishes when the excitability of the specimen is high, and

the two opposite reactions succeed each other too quickly for the

preliminary A reaction to become evident. It is probable that in such a

case the conflict between the two opposite reactions prolongs the latent

period. But in other instances a preliminary expansive response is found

to herald the more pronounced contractile response. Example of this is

seen in figure 129 given in page 344.

The A effect was detected in the records referred to above by its

earlier appearance. Its longer persistence, after moderate stimulation,

is also to be found on the cessation of moderate stimulation. This was

seen in the _acceleration_ of growth which was the after-effect of

stimulation (Figs. 104, 115). The presence of two conflicting

physiological reactions is also made evident on sudden cessation of long

continued stimulation. This particular phenomenon of "overshooting" will

be more fully dealt with in a subsequent chapter.

Owing to the difference in the time relations of the two opposing

activities, A and D, a phase difference often arises in their respective

maxima. It is probably on this account that rhythmic tissues originally

at standstill, exhibit under continued stimulation a periodic up and

down-movement, which persists even on the cessation of the stimulus. The

persistence of after-oscillation depends, moreover, on the intensity and

duration of previous stimulation.[18]

[18] "Plant Response"--p. 293, etc.

The facts given above cannot be explained by the prevalent theory that

stimulus acts merely as a releasing agent, to set free energy which had

been previously stored up by the organism, like the pull of a trigger

causing explosion of a charged cartridge. It is true that in a highly

excitable tissue, the external work performed and the run down of energy

are disproportionately greater than the energy of stimulus that induces

it. But in a sub-tonic tissue, stimulus induces an effect which is

precisely the opposite; instead of a depletion, there is an enhancement

of potential energy of the system. Thus the responding leaf instead of

undergoing a fall becomes erected; growing organs similarly exhibit a

'building up' and an acceleration of rate of growth, in contrast with

the usual 'break down' and depression of the rate. It is obvious that

these new facts relating to the action of stimulus necessitate a theory

more comprehensive and satisfactory than the one which has been in

vogue.

THE COMPLETE PHOTOTROPIC CURVE.

I have explained the characteristics of the simple phototropic curve in

which the tropic curvature, on account of the favourable tonic condition

and strong intensity of incident light, was positive from the

beginning, and in which the curvature reached a maximum beyond which

there was no subsequent reversal. If the intensity of the stimulus be

feeble or moderate, the quantity of light incident on the responding

organ at the beginning may fall below the critical value, and thus act

as a sub-minimal stimulus. This induces as we have seen (p. 344) a

negative tropic curvature; continued action of stimulus, however,

converts the preliminary negative into the usual positive. The

preliminary negative curvature may be detected by the use of a

moderately sensitive recorder with a magnification of about 30 times. It

is comparatively easy to obtain the preliminary negative response in

specimens which are in a slightly sub-tonic condition.

Semi-conducting tissues exhibit under continued stimulation, a

neutralisation and reversal into negative (p. 331). Since this reversal

into negative usually takes place under prolonged exposure to

exceedingly strong light, it is difficult to obtain in a single curve

all the different phases of transformation. I have, however, been

fortunate in obtaining a complete phototropic curve which exhibits in a

single specimen all the characteristic changes from a preliminary

negative to positive and subsequent reversal to negative. I shall

describe two such typical curves obtained with the terminal leaflet of

_Desmodium gyrans_ and the growing seedling of _Zea Mays_.

_Complete phototropic curve of a pulvinated organ: Experiment 135._--A

continuous record was taken of the action of light of a 50 c.p.

incandescent lamp, applied on the upper half of the pulvinus of the

terminal leaflet of _Desmodium gyrans_. This gave rise: (1) to a

negative curvature (due to sub-minimal stimulus) which lasted for 3

minutes. The curve then proceeded upwards, at first slowly, then

rapidly; it then rounded off, and reached a maximum positive value in

the course of 18 minutes; after this the curve underwent a reversal, and

complete neutralisation occurred after a further period of 24 minutes

(Fig. 133). Beyond this the induced curvature is negative.

[Illustration: FIG. 133.--Complete phototropic curve given by pulvinated

Eq. organ. Positive curvature above, and negative curvature below the

horizontal zero line. Preliminary negative phase of response due to

sub-minimal stimulus. The positive increases, attains a maximum, and

undergoes a reversal. Successive dots at intervals of 30 seconds.

Abscissa represents duration of exposure and quantity of incident light.

(Terminal leaflet of _Desmodium gyrans_.)]

_Complete phototropic curve of growing organs: Experiment 136._--I

obtained very similar effects by subjecting the seedling of _Zea Mays_

to unilateral light from an arc lamp for two hours. The characteristic

of this curve is similar to that given by the terminal leaflet of

_Desmodium gyrans_. At the first stage, the sub-minimal stimulation is

seen to induce a negative curvature, transformed into positive after an

interval of 10 minutes. The maximum positive curvature is reached after

50 minutes, and neutralisation completed in a further period of 43

minutes (Fig. 134). After this the response became transformed into

negative.

[Illustration: FIG. 134.--Complete phototropic curve of a growing organ

(_Zea Mays_).]

In a complete phototropic curve we may thus distinguish 4 distinct

stages:--

(1) The stage of sub-minimal stimulation.

(2) The stage of increasing positive curvature culminating in

a maximum.

(3) The stage of neutralisation.

(4) The stage of complete reversal into negative.

The curve thus crosses the zero line of the abscissa twice; the first

crossing takes places _upwards_ at the critical point of stimulation

which demarcates the sub-minimal from the minimal. The second crossing

downwards occurs beyond the point of complete neutralisation.

In a tissue in which transverse conductivity is absent, and the stimulus

applied from the beginning is above the minimal, the simple tropic

curve is confined to the second stage (see Fig. 132).

WEBER'S LAW.

If we neglect the preliminary negative portion under sub-minimal

stimulus, the curve of excitation under increasing photic stimulation

obeys what is known as Weber's law. This is equally true of modes of

stimulation other than that of light as is seen in figure 130 of the

contractile effect of continued electric stimulus on growth; the

excitatory effect is also seen to reach a limit.

Weber's law is applicable for a limited range of stimulation. For the

quantitative relation fails in the region of sub-minimal stimulus, where

the physiological reaction is _qualitatively_ different, namely

expansion instead of contraction. This holds good even in the case of

animal tissues, for here also my recent experiments show that two

opposite reactions--expansion and contraction--take place under

stimulus, and that a very feeble stimulus tends to induce expansion

instead of contraction. The responsive reaction of a kitten under gentle

caressing strokes must be _qualitatively_ different from that of a blow.

The psychological effects under the two treatments evidently differ

qualitatively rather than quantitatively.[19]

[19] "It has been argued by James that the feeling does not cause,

but is caused by the bodily expression.... Münsterberg

concludes that the feeling of agreeableness is the mental

accompaniment and outcome of reflexly produced movements of

extension, and disagreeableness of the movement of flexion."

Schäfer--Text Book of Physiology, Vol. II, p. 975 (1900).

SUMMARY.

The excitation curve exhibits a slow ascent in the first part; in the

second part the gradient is steep, indicating rapid rise in excitation;

in the third part it is uniform; and in the last part the curve rounds

off and the rate of ascent becomes very small.

The susceptibility for excitation is feeble at the beginning; it

increases very rapidly with increasing stimulus; finally it undergoes a

fall, increase of stimulus inducing no further enhancement of

excitation.

In a complete phototropic curve the first part is negative; this is due

to the physiological expansion induced by sub-minimal stimulus. The

curve then crosses the abscissa upwards, and the positive curvature

reaches a maximum. This is followed by neutralisation and reversal into

negative; the curve crosses the zero line and proceeds in the negative

direction.

Weber's law is not applicable for the entire range of stimulation. The

quantitative relation fails in the region of sub-minimal stimulus, where

the physiological reaction is _qualitatively_ different.

XXXIII.--THE TRANSMITTED EFFECT OF PHOTIC STIMULATION

_By_

SIR J. C. BOSE,

_Assisted by_

JYOTIPRAKASH SIRCAR, M.B.

Plant organs exhibit, as we have already seen, a heliotropic curvature

under direct stimulation. Still more interesting is the transmitted

effect of light giving rise to a curvature. Thus if the tip of the

seedling of wheat be exposed to light, the excitation is transmitted

lower down into the region which acts as the responding organ. Growth is

very active in this particular zone, and the change of growth, induced

by the transmitted effect of stimulus, brings about a curvature by which

the tip of the seedling bends towards light. The seedling thus appears

to be differentiated into three physiological zones subserving three

different functions. The tip is the perceptive zone, the intervening

distance between the tip and the growing region is the zone of

conduction, and the growing region is the responsive zone. These

differentiations are shown in a striking manner by certain Paniceae,

_Setaria_ for example. In this seedling the tapering sheathing leaf or

cotyledon is about 5 mm. in length, and it is the upper part of the

cotyledon that is most sensitive to light. Below the sheathing leaf is a

narrow length which will be distinguished as the hypocotyl, and where

growth is very active. The apex of the leaf perceives the stimulus, and

the effect is transmitted to the hypocotyl, which responds by becoming

curved so that the seedling bends towards light.

It is necessary here to make special reference to the confusion that

arises from want of precision in the use of the term stimulus, used

indifferently to denote both the cause and the resulting effect. An

external agent, say light, causes certain excitatory change in the

tissue, and we refer to the agent which induces it, as the _stimulus_.

Thus in the instance cited above, light is the _stimulus_, and it is the

stimulus-effect that is transmitted to a distance. But in physiological

literature no distinction is made between the stimulus and its effect,

hence arises frequent use of the phrase 'transmission of stimulus'. It

is obvious that it is not light but its effect that is transmitted.

Such want of precision in the use of the term stimulus would not have

seriously affected the truth about the description of facts, had the

transmitted effect been only of one kind. In a nerve-and-muscle

preparation, the velocity of transmission of excitation is so great,

that it completely masks the positive impulse (assuming the existence of

such an impulse). The effect of indirect stimulation is, therefore, the

same as that of direct stimulation. Any indefiniteness in the use of the

term stimulus for its transmitted effect does not, in animal physiology,

seriously militate against the observed facts. But lack of precision in

the employment of the term in plant physiology leads to hopeless

confusion. For owing to the semi-conducting nature of vegetable tissue,

the transmitted effect is not of a definite sign, but may be positive or

negative; in the first case, the response is by expansion, in the

latter, by contraction. Thus the transmitted effect will be very

different in the two cases, according as the intervening tissue is a

good or a bad conductor. These facts accentuate the urgent necessity of

revision of our existing terminology.

I have shown that the effects of other forms of stimuli are also

transmitted from the perceptive to the responding region along the

intervening path of conduction. Thus the petiole of _Mimosa_ perceive

any form of stimulus applied to it, and the induced excitation is

conducted to the distant pulvinus to evoke the familiar responsive fall

of the leaf. The pulvinus, moreover, perceives and responds to direct

stimulation. In a nerve-and-muscle preparation the responding muscle is

alike perceptive and responsive.

But in _Setaria_ we meet with certain characteristics of reaction which

are quite inexplicable. Thus if

"the seedling be illuminated on one side, a sharp heliotropic curving

takes place at the apex of hypocotyl. The curvature makes itself

apparent only if the cotyledon be illuminated from one side whether the

hypocotyl be exposed to light or not. If the cotyledon be shaded and the

light be permitted to fall on one side of the hypocotyl, no heliotropic

curving takes place. Hence we may conclude that it is only the cotyledon

that is sensitive to the light stimulus, and it is only the hypocotyl

which can carry out the movement. The excitation which the light effects

in the cotyledon must be transmitted to the hypocotyl and curvature

takes place only from such a transmitted excitation. We have thus in

this case a definite organ for the perception of the stimulus of light,

viz., the cotyledon, and as Rothert has shown, it is more specially the

apex of that organ that is the sensitive part: on the other hand, the

motile organ, the hypocotyl, is some distance away from the sensitive

organ, and in it the power of perception is entirely absent. From the

behaviour of these organs we may draw the further conclusion that

perception and heliotropic excitation are two distinct phenomena, which

depend on different properties of the protoplasm and which are

independent of each other.... We may, therefore, conclude from this

experiment that these two types of excitation are fundamentally distinct

processes, for it is only after indirect or transmitted and not after

direct excitation that a reaction occurs in the case of the seedlings of

the Paniceae".[20]

[20] Jost--_Ibid_--p. 468.

The noteworthy deductions on the above facts are:--

(1) That the motile organ in _Setaria_ is totally devoid of

perception, since direct action of light induces no effect.

(2) That perception and heliotropic excitation are two

distinct phenomena, which depend on different properties of

the protoplasm, and which are independent of each other.

Though the conclusions thus arrived at appear to follow from the facts

that have been observed, yet it is difficult to accept the inference,

that a responding organ should be totally devoid of the power of

perception, and that excitation and perception are to be regarded as

dependent on different properties of protoplasm. It therefore appeared

necessary to re-investigate the subject of the perceptive power of the

cotyledon, and the responding characteristics of the hypocotyl.

The criterion employed for test of perception is the movement induced in

response to stimulus. The responsive _mechanical_ movement is rendered

possible only by the contractility of the organ, and mechanical and

anatomical facilities offered by it for unhampered movement. The

petiole of _Mimosa_ when locally stimulated does not itself exhibit any

movement. The fortunate circumstance of the presence of a motile

pulvinus in the neighbourhood enables us to recognise the perceptive

power of the petiole, since it transmits an impulse which causes the

fall of the leaf. There is no motile pulvinus in ordinary leaves, and

stimulation of the petiole gives rise to no direct or transmitted motile

reaction; from this we are apt to draw the inference that the petiole of

ordinary leaves are devoid of perception. This conclusion is, however,

erroneous, since under stimulus the petiole exhibits the electric

response characteristic of excitation. Moreover my electric

investigations have shown that every living tissue not only perceives

but also responds to stimulation.[21] Hence considerable doubt may be

entertained as regards the supposed absence of perception in the

hypocotyl of _Setaria_.

[21] "Response in the Living and Non-Living"--p. 17.

I shall in the present paper describe my investigations on the

mechanical response of _Setaria_ under direct and indirect stimulation

which will be given in the following order:--

(1) The response to unilateral stimulation of the tip of the

seedling.

(2) The response of growing hypocotyl to direct stimulation.

(3) Summated effects of direct and indirect stimulation.

EXPERIMENTAL ARRANGEMENTS.

_The Recorder._--The pull exerted by the tropic curvature of the

seedling is very feeble; it was therefore necessary to construct a very

light and nearly balanced recording lever. A long glass fibre is

supported by lateral pivots on jewel bearings. The seedling is attached

to the short arm of the lever by means of a cocoon thread. The recording

plate oscillates to and fro once in a minute; the successive dots give

therefore the time relations of the responsive movement. The positive

curvature towards light is recorded as an up-curve, the negative

curvature being represented by a down-curve.

[Illustration: FIG. 135.--Arrangement for local application of light to

the tip and the growing region. O, O', apertures on a metallic screen.

Light is focussed by a lens on the tip, and on the growing region at o,

o'. Figure to the right shows front view of the shutter resting on a

pivot and worked by string, T.]

_Arrangement for local stimulation by light._--The device of placing tin

foil caps on the tip employed by some observers labours under the

disadvantage, that it causes mechanical irritation of the sensitive tip.

The appliance seen in figure 135 is free from this objection and offers

many advantages. A metallic screen has two holes O and O'; these

apertures are illuminated by a parallel beam of light from an arc lamp.

A lens focusses the light from O, on the hypocotyl, and that from O',

on the tip of the cotyledon. A rectangular pivoted shutter S, lies

between the apertures O and O'. In the intermediate position of the

shutter, light acts on both the tip and the growing region. The shutter

is tilted up by a pull on the thread T, thus cutting off light from the

growing region; release of the thread cuts off light from the tip. Thus

by proper manipulation of the shutter, the tip or the growing hypocotyl,

or both of them, may be subjected to the stimulus of light. The

experiment was carried out in a dark room, special precaution being

taken that light was screened off from the plant except at points of

localised stimulation.

[Illustration: FIG. 136.--Response of seedling of _Setaria_ to

unilateral stimulation of the tip applied at dotted arrow.

Note preliminary negative curvature reversed later into positive.]

EFFECT OF LIGHT AT THE TIP OF THE ORGAN.

_Experiment 137._--If the tip of the seedling of _Setaria_ be

illuminated on one side, it is found that a _positive_ curvature

(_i.e._, towards light) is induced in the course of an hour or more. But

in obtaining record of the seedling by unilateral stimulation of the

tip, I found that the immediate response was not towards, but away from

light (negative curvature). The latent period was about 30 seconds and

the negative movement continued to increase for 25 minutes (Fig. 136).

This result, hitherto unsuspected, is not so anomalous as would appear

at first sight. Indirect stimulus, unilaterally applied, has been shown

to give rise to two impulses: a quicker positive and a slower excitatory

negative. The former induces a convexity on the same side, and a

movement away from stimulus (negative curvature); the excitatory

negative, on the other hand, is conducted slowly and induces contraction

and concavity, and a movement towards the stimulus (positive curvature).

In semi-conducting or non-conducting tissues, the excitatory negative is

weakened to extinction during transit, and the positive reaction with

negative curvature persists as the initial and final effect.

But in _Setaria_ the excitatory negative impulse is transmitted along

the parenchyma which is moderately conducting; the speed of transmission

of heliotropic excitation is, according to Pfeffer, one or two mm. in

five minutes or about 0·4 mm. per minute. Thus under the continued

action of light, the excitatory impulse will reach the growing region,

and by its predominant reaction neutralise and reverse the previous

negative curvature.

Inspection of figure 136 shows that this is what actually took place;

the intervening distance between the tip of the cotyledon and the

growing region in hypocotyl was about 20 mm., and the beginning of

reversal from negative to positive curvature occurred 29 minutes after

application of light. The velocity of transmission of excitatory impulse

under strong light is thus 0·7 mm. per minute. The positive curvature

continued to increase for a very long time and became comparatively

large. This is for two reasons: (1) because the sensibility of the tip

of the cotyledon is very great, and (2) because the positive curvature

induced by longitudinally transmitted excitation is not neutralised by

transverse conduction (see below).

[Illustration: FIG. 137.--Effect of application of light to the growing

hypocotyl at arrow induced positive phototropic curvature followed by

neutralisation. Application of indirect stimulus at dotted arrow on the

tip gave rise at first to negative, subsequently to positive curvature.

(Seedling of _Setaria_).]

RESPONSE TO UNILATERAL STIMULUS IN THE GROWING REGION.

_Experiment 138._--The growing region of the hypocotyl of _Setaria_ is

supposed to be totally devoid of the power of perception. In order to

subject the question to experimental test, I applied unilateral light on

the growing region of the same specimen, after it had recovered from the

effect of previous stimulation. The response now obtained was vigorous

and was _ab-initio_ positive. Direct stimulus has thus induced the

normal effect of contraction and concavity of the excited side. The

belief that the hypocotyl of _Setaria_ is incapable of perceiving

stimulus is thus without any foundation. The further experiment which I

shall presently describe will, however, offer an explanation of the

prevailing error. On continuing the action of unilateral light, the

positive curvature after attaining a maximum in the course of 15

minutes, underwent a diminution and final neutralisation (Fig. 137). On

account of this neutralisation the seedling became erect after an

exposure of 30 minutes; in contrast with this is the increasing positive

curvature under unilateral illumination of the tip (Fig. 136) which

continues for several hours. The explanation of this neutralisation

under direct stimulation of the growing region is found in the fact that

transverse conduction of excitation induces contraction at the distal

side of the organ and thus nullifies the positive curvature. The seeming

absence of tropic effect under direct stimulation is thus not due to

want of perception, but to balanced antagonistic reactions on opposite

sides of the organ.

EFFECT OF SIMULTANEOUS STIMULATION OF THE TIP AND THE HYPOCOTYL.

Though stimulation of the hypocotyl results in neutralisation, yet the

illumination of one side of the organ including the tip and hypocotyl is

found to give rise to positive curvature. This will be understood from

the following experiment.

After the neutralisation in the last experiment light was also applied

to the tip from the right side at the dotted arrow (Fig. 137). The

record shows that this gave rise at first to a negative curvature (away

from light); under the continued action of light, however, the negative

was subsequently reversed to a positive curvature, towards light.

Inspection of the curve shows another interesting fact. The positive

curvature induced by direct stimulation is very much less than that

brought out by indirect stimulation. This is due to two reasons: (1) the

sensitiveness of the tip of the organ is, as is well known, greater than

that of the hypocotyl, (2) the positive curvature under direct

stimulation cannot proceed very far, since it is neutralised by

transverse conduction of excitation.

It will be seen from the above that the illumination of the tip

practically inhibits the neutralisation and thus restores the normal

positive curvature. The question now arises as to how this particular

inhibition is brought about.

ALGEBRAICAL SUMMATION OF THE EFFECTS OF DIRECT AND INDIRECT

STIMULATIONS.

An instance of inhibition, though of a different kind, was noticed in

the response of the tendril of _Passiflora_ (p. 296); the under side of

the organ is highly sensitive, while the upper side is almost

insensitive. Stimulation of the under side of the tendril induces a

marked curvature, but simultaneous stimulation of the diametrically

opposite side inhibits the response. This neutralisation could not be

due to the antagonistic contraction of the upper side since the

irritability of that side is very slight. I have shown that the

inhibition results from the two antagonistic reactions, contraction at

the proximal side due to direct stimulation and expansion caused by the

positive impulse from the indirectly stimulated distal side.

We have in the above an algebraical summation of the effects of direct

and indirect stimulations. The longitudinally transmitted effect of

indirect stimulus in _Setaria_ may, likewise, be summated with the

effect of direct stimulus. The phenomenon of algebraical summation is

demonstrated in a very striking and convincing manner in the following

experiment, which I have been successful in devising.

_Experiment 139._--I have explained, (Expt. 126) that unilateral

application of stimulus of light on the upper half of the responding

pulvinus of _Mimosa_ induces an up or positive curvature, followed by a

neutralisation and even a reversal into negative, the last two effects

being brought about by transverse conduction of excitation to the distal

side. When the incident light is of moderate intensity, the transmitted

excitation only suffices to induce neutralisation without further

reversal into negative; while in this state of balanced neutralisation

let us apply indirect stimulus by throwing light on the stem at a point

directly opposite to the leaf (Fig. 138).

[Illustration: FIG. 138.--(_a_) Diagrammatic representation of direct

application of light (v) on the pulvinus and the indirect application on

the stem (-->) (_b_) Record of effect of direct stimulus, positive

curvature followed by neutralisation. Superposition of the positive

reaction of indirect stimulus induces erectile up-response followed by

down movement due to transmitted excitatory impulse (_Mimosa_).]

Two different impulses are thus initiated from the effect of indirect

stimulus. In the present case the positive reached the responding

pulvinus after 30 seconds and induced an erectile movement of the leaf;

the excitatory negative impulse reached the organ 4 minutes later and

caused a rapid fall of the leaf. The record (Fig. 138) shows further

that the previous action of direct stimulus which brought about

neutralisation, does not interfere with the effects of indirect

stimulus. The individual effects of direct and indirect stimulus are

practically independent of each other; hence their joint effects exhibit

algebraical summation.

We are now in a position to have a complete understanding of the

characteristic response of Paniceae to transmitted phototropic

excitation.

(1) Local stimulation of the tip gives rise to two impulses, positive

and negative. The former induces a transient negative movement (away

from light); the latter causes a permanent and increasing positive

curvature towards light.

(2) Local stimulation of the growing hypocotyl gives rise to positive

curvature, subsequently neutralised by the transverse conduction of

excitation to the distal side. The absence of tropic effect in the

growing region is thus due not to lack of power of perception, but to

balanced antagonistic reactions of two opposite sides of the organ.

(3) The effects of direct and indirect stimulations are independent of

each other; hence, on simultaneous stimulations of the tip and the

growing hypocotyl, the effects of indirect stimulus are algebraically

summated with the effect of direct stimulus (neutralisation). The

indirect stimulation of the tip on the right side gives rise to two

impulses, of which the expansive positive reached the right side of the

responding region earlier, inducing convexity and movement away from

stimulus (negative curvature). This is diagrammatically shown in Fig.

139. Had the intervening tissue been non-conducting, the slow excitatory

negative impulse would have failed to reach the responding region, and

the negative curvature induced by the positive impulse would prove to be

the initial as well as the final effect. In the case of _Setaria_,

however, the excitatory impulse reaches the right side of the organ

after the positive impulse; the final effect is therefore an induced

concavity and positive curvature (movement towards stimulus).

[Illustration: FIG. 139.--Diagrammatic representation of the effects of

direct and indirect stimulus on the response of _Setaria_. Direct

stimulation, represented by thick arrow gives rise to antagonistic

concavities of opposite sides of responding hypocotyl, resulting in

neutralisation.

Indirect stimulus represented by dotted arrow gives rise to two

impulses, the quick positive impulse represented by a circle, and the

slower negative impulse represented by crescent (concave).]

The results given above enable us to draw the following

generalisations:--

1. In an organ, the tip of which is highly excitable, the balanced state

of neutralisation, induced by direct stimulation of the responding

region, is upset in two different ways by two impulses generated in

consequence of indirect stimulation at the tip. Hence arises two types

of resultant response:--

Type A.--If the intervening tissue be semi-conducting, the positive

impulse alone will reach the growing region and induce convexity of the

same side of the organ giving rise to a _negative_ curvature.

Type B.--If the intervening tissue be conducting the transmission of the

excitatory impulse will finally give rise to a _positive_ curvature.

Type B is exemplified by the seedling of _Setaria_ where the

transmission of excitatory impulse from the tip upsets the neutral

balance and induces the final positive curvature.

Example of type A is found in the negative phototropism of the root of

_Sinapis_.

_Negative phototropism of root of_ Sinapis: _Experiment 140._--For

investigation of the negative phototropism of the root of _Sinapis

nigra_ I took record of its movement under unilateral action of light by

means of a Recording Microscope, devised for the purpose.[22] When the

root-tip alone was stimulated by unilateral light, the root moved away

from the source of light. This was due to the longitudinal transmission

of positive impulse to the growing region at some distance from the tip.

The intervening distance between the tip and the growing region is

practically non-conducting, hence the excitatory impulse could not be

conducted from the tip. After a period of rest in darkness, I next took

record of its movement under direct unilateral illumination of the

growing region; the result was at first a positive movement; but this,

on account of transverse conduction of excitation under continued

stimulation, underwent a neutralisation and slight reversal. In taking a

third record, in which both the tip and growing region were

simultaneously subjected to unilateral stimulation of light, I found

that a resultant responsive movement was induced which was away from

light.

[22] "Plant Response"--p. 604.

Thus in the root of _Sinapis_, the expansive effect of indirect

stimulation of the tip is superposed on that of direct stimulation of

the growing region (neutral or slightly negative). The final result is

thus a movement away from light or a _negative_ phototropic curvature.

SUMMARY.

The effect induced by stimulus of light is transmitted to a distance, in

a manner precisely the same as in other modes of stimulation.

In the Paniceae, the local unilateral stimulation of the tip of the

cotyledon induces positive curvature in the growing hypocotyl, at some

distance from the tip. This is due to transmitted excitatory effect of

indirect stimulation; the earlier positive impulse induces a preliminary

negative curvature, which is reversed later by the excitatory negative

impulse into positive curvature.

Contrary to generally accepted view the hypocotyl not only perceives but

responds to light. The positive curvature induced by direct stimulation

is, however, neutralised by transverse conduction of excitation.

The effects of direct and indirect stimulus are independent of each

other; the final effect is determined by their algebraical summation.

XXXIV.--ON PHOTONASTIC CURVATURES

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.

Phototropic response, positive or negative, is determined by the

directive action of light. But photonastic reaction is supposed to

belong to a different class of phenomenon, where the movement is

independent of the directive action of light. I shall, however, be able

to establish a continuity between the tropic response of a radial and

the nastic movement of a dorsiventral organ. The intermediate link is

supplied by organs originally radial, but subsequently rendered

anisotropic by the unilateral action of stimulus of the environment. In

a dorsiventral organ, owing to anatomico-physiological differentiation,

the responsive movement is constrained to take place in a direction

perpendicular to the plane of separation of the two unequally excitable

halves of the organ. Even in such a case, it will be shown, that light

does exert a directive action; the direction of movement will further

be shown to be distorted by the lateral action of light.

PHOTOTROPIC RESPONSE OF ANISOTROPIC ORGANS.

The different sides of a radial organ, such as the young stem of

_Mimosa_, are equally excitable. The response to unilateral light of

moderate intensity is therefore positive; owing to equal excitabilities

of the two sides the response of the opposite sides are alike. Diffuse

stimulation therefore induces no resultant curvature. If, however, the

plant is allowed to form a creeping habit, the excitabilities of the

dorsal and ventral sides will no longer remain the same. Thus in the

creeping stem of _Mimosa_ the lower or the shaded side is, generally

speaking, found to be the more excitable. In fact such anisotropic stem

of _Mimosa_ acts somewhat like the pulvinus of the same plant. Diffuse

stimulation induces, in both, a concavity of the more excitable lower

half with the down movement of the leaf or the stem.

_Experiment 141._--I took four creeping stems of _Mimosa_ in vigorous

condition and tied them in such a manner that their free ends should be

vertical. The shaded sides of the four specimens were so turned that

each faced a different point of the compass--east, west, north and

south. Subjected thus to diffuse stimulus of light from the sky, they

all executed curvatures. The specimen whose under side faced the east,

became bent towards the east; the same happened to those which faced

north, south, and west, that is to say they curved towards the north,

south, and west respectively (Fig. 140). The fundamental action by which

all these were determined was the induced concavity of the under or

normally shaded side, which was the more excitable. I obtained similar

results with various other creeping stems.

[Illustration: FIG. 140.--Photonastic curvature of creeping stem of

_Mimosa pudica_: in the central figure the stem is seen to be vertical:

action of diffuse light induced appropriate curvatures by greater

contraction and concavity of the more excitable lower or shaded side, as

seen in figures to the right (_b_) and left (_c_).]

It has been shown that under prolonged unilateral stimulation,

excitation becomes internally diffused; this gives rise to an effect

similar to that of external diffuse stimulus. Under strong light the

shaded side becomes concave, and thus press against the ground or the

support; this will be the characteristic response of creeping stems in

which the shaded side is the more excitable. The facts given above will

probably explain the response of midribs of leaves, of the creeping stem

of _Lysimachia_, all of which, in response to the action of strong light

acting from above, exhibit concavity of the shaded and more excitable

side.

PARA-HELIOTROPISM.

Under strong sunlight, the leaflets of various plants move sometimes

upwards, at other times downwards, so as to place the blades of leaflets

parallel to incident light. This 'midday sleep' has been termed

_para-heliotropism_ by Darwin. It has been thought that para-heliotropic

action has nothing to do with the directive action of light, since many

leaflets either fold upwards or downwards, irrespective of the direction

of incident light. I shall for convenience distinguish the leaflets

which fold upwards under light as _positively_ para-heliotropic, and

those which fold downwards as _negatively_ para-heliotropic. This is

merely for convenience of description. There is no specific irritability

which distinguishes one from the other.

POSITIVE PARA-HELIOTROPISM.

_Para-heliotropic response of_ Erythrina indica _and of_ Clitoria

ternatea: _Experiment 142._--For the purpose of simplicity I have

described the type of movement of these leaflets as upwards; but the

actual direction in which the leaflets point their apices is towards the

sun. Both the plants mentioned here are so remarkably sensitive that the

leaflets follow the course of the sun, in such a way that the axis of

the cup, formed by the folding leaflets at the end and the sides of the

petiole, is coincident with the rays of light. The pulvinus makes a

sharp curvature which is concave to light, the blade of the leaflet

being parallel to light. I have taken record of continuous action of

strong light acting on the responding pulvinus of the leaflets from

above. The result is an increasing positive curvature which reached a

limit (Fig. 141). There was no neutralisation or reversal,

demonstrating the absence of transverse conduction (_cf._ Fig. 132).

[Illustration: FIG. 141.--Positive para-heliotropic response of leaflets

of _Erythrina indica_.]

_Para-heliotropic movement of leaflets of_ Mimosa pudica: _Experiment

143._--These leaflets, as previously stated, fold themselves upwards,

when strongly illuminated either from above or below. Diffuse electric

stimulation also induce a closing movement upwards; hence the upper half

of the pulvinule of these leaflets are the more excitable. In order to

obtain a continuous record of the leaflet under the action of unilateral

light, I constructed a very delicate recording lever magnifying about

150 times. Light of moderate intensity from a 100 candle-power

incandescent lamp was applied on the less excitable lower side of the

pulvinule. The record (Fig. 142) shows that the immediate response is

positive, or a movement towards the light. But owing to transverse

conduction, through the thin and highly conducting pulvinule, the

response was quickly reversed into a very pronounced negative, or

movement away from light. Had a delicate means of obtaining magnified

record not been available, the slight positive twitch, and the gradual

transition from positive to negative phototropic curvature would have

passed unnoticed. Application of light from above gave, on account of

the greater excitability of the upper half of the pulvinule, a

pronounced positive response or movement towards light. The anomaly of

an identical organ appearing as positively heliotropic when acted by

light from above, and negatively heliotropic when acted from below, is

now fully removed. The response of the leaflets is also seen to be

determined by the directive action of light, though the short-lived

response of the less excitable lower side is quickly masked by the

predominant reaction of the more excitable upper side of the organ.

[Illustration: FIG. 142.--Response of leaflet of _Mimosa_ to light

applied below: transient positive followed by pronounced negative

curvature.]

[Illustration: FIG. 143.--Response of leaflet of _Averrhoa_, to light

applied above: transient positive followed by pronounced negative

curvature.

Up-curve represents up-movement, and down-curve, down-movement.]

NEGATIVE PARA-HELIOTROPISM.

_Response of leaflet of_ Averrhoa carambola: _Experiment 144._--The

leaflets of this plant, and also those of _Biophytum sensitivum_ fold

downwards under action of strong light, applied above or below. In these

leaflets diffuse electric stimulation induce a fall of the leaflets

demonstrating the greater excitability of the lower half of the

pulvinule. The analysis of reaction under light is rendered possible

from the record of response of leaflet of _Averrhoa_, given in Fig.

143. Light of moderate intensity from an incandescent electric lamp

acted from above: the result was a feeble and short-lived positive

response, quickly reversed to strong negative by transmission of

excitation to the more excitable lower side. Illumination from below

gave rise only to strong positive response. Thus in _Averrhoa_ the

effect of continuous light applied above or below is a downward

movement; in _Mimosa_ the movement is upwards. The explanation of this

difference lies in the fact, that in _Mimosa_ leaflet it is the upper

half of the pulvinule that is more excitable; while in _Averrhoa_ and in

_Biophytum_ the lower is the more excitable half of the organ.

[Illustration: FIG. 144.--Diagrammatic representation of different types

of phototropic response. (See text.)]

As a summary of the tropic action of light I shall give diagrammatic

representations of various types of phototropic response, including the

photonastic (Fig. 144). The direction of the arrow indicates the

direction of incident light. Dotted specimens are those which possess

transverse conductivity. Thick lines represent the more excitable side

of an anisotropic or dorsiventral organ. The size of the circles, with

positive and negative signs, represents the amplitude and sign of

curvature.

_a._ Radial thick organ, in which transverse conduction is

absent. Curvature is _positive_, _i.e._, movement towards

light. The result will be similar when light strikes in an

opposite direction, _i.e._, from right to left.

_b._ Radial thin organ. There is here a possibility of

transverse conduction. Sequence of curvature: _positive_,

_neutral_, and _negative_. Reversal of direction of light

gives rise to similar sequence of responses as before

(_e.g._, seedling of _Sinapis_).

_c._ Anisotropic thick organ; transverse conduction possible.

Thick line represents the more excitable distal side.

Sequence of curvature: positive, neutral and pronounced

negative. When light strikes from opposite direction on the

more excitable side the curvature will remain positive, since

the pronounced reaction of the more excitable side cannot be

neutralised or reversed by transmitted excitation to the less

excitable distal side (_e.g._, leaf of _Mimosa_).

_In the absence of transverse conduction_, the curvature

remains positive (_e.g._, leaflet of _Erythrina_).

_d._ Anisotropic thin organ with high transverse

conductivity. Sequence of curvature: transient positive,

quickly masked by predominant negative. Light striking on the

more excitable side will give rise only to _positive_. The

response in relation to the plant, will apparently be in the

same direction whether light strikes the organ on one side or

the opposite (_e.g._, leaflets of _Mimosa_, _Averrhoa_ and

_Biophytum_).

I have shown that tissues in sub-tonic condition exhibit an acceleration

of the rate of growth under stimulus (p. 224) the corresponding tropic

reaction would therefore be away from stimulus or _negative_ curvature.

The tonic condition is, however, raised to the normal by the action of

stimulus itself, and the tropic curvature becomes positive.

I give below a table which will show at a glance all possible variations

of phototropic reaction.

TABLE XXXI.--MECHANICAL RESPONSE OF PULVINATED AND GROWING ORGANS UNDER

LIGHT.

+--------------------------------------------------------------------+

|Description of | Action. | Effect observed. |

| tissue. | | |

+---------------+------------------------+---------------------------+

|I Tissue |Stimulus causes increase|Expansion or enhanced rate |

| sub-tonic. | of internal energy. | of growth, _e.g._, |

| | | _Pileus_ of _Coprinus_ |

| | | drooping in darkness, |

| | | made re-turgid by light. |

| | | Renewed growth of dark |

| | | rigored plant exposed to |

| | | light. |

|II Normally |A 1. Moderate light, |1. Curvature towards light,|

| excitable | causing excitatory | _e.g._, flower bud of |

| organ under| contraction of proximal| _Crinum_. |

| unilateral | and positive expansion | |

| light. | of distal. | |

| A. Organ |A 2. Strong light. |2. Neutralisations, _e.g._,|

| radial. | Excitatory effect | seedling of _Setaria_. |

| | transmitted to distal, | |

| | neutralising first. | |

| |A 3. Intense and |3. Reversed or negative |

| | long-continued light. | response, _e.g._, seedling|

| | Fatigue of proximal and| of _Zea Mays_. |

| | excitatory contraction | |

| | of distal. | |

| B. Dorsi- |B 1. Excitatory |1. Positive response, |

| ventral | contraction of proximal| _e.g._, upward folding of |

| organ. | predominant, owing | leaflets in so-called |

| | either to greater | "diurnal sleep" of |

| | excitability of | _Erythrina indica_ and |

| | proximal or feeble | _Clitoria ternatea_. |

| | transverse conductivity| |

| | of tissue. | |

| |B 2. Transmission of |2. Negative response, |

| | excitation through | _e.g._, downward folding |

| | highly conducting | of leaflets in so-called |

| | tissue to more | "diurnal sleep" of |

| | excitable lower or | _Biophytum_ and |

| | distal. Greater | _Averrhoa_. |

| | contraction of distal. | |

|III Rhythmic |Considerable absorption |Initiation of multiple |

| tissue. | of energy, immediate | response in _Desmodium |

| | or prior. | gyrans_ previously at |

| | | standstill; multiple |

| | | response under continuous |

| | | action of light in |

| | | _Biophytum_. |

+--------------------------------------------------------------------+

SUMMARY.

There is no line of demarcation between tropic and nastic movements.

In a differentially excitable organ the effect of strong unilateral

stimulus becomes internally diffused, and causes greater contraction of

the more excitable side of the organ.

In the absence of transverse conduction, the positive curvature reaches

a maximum without neutralisation or reversal. The leaflets of _Erythrina

indica_ and of _Clitoria ternatea_ thus fold upwards, the apices of the

leaflets pointing towards the sun.

Internally diffused excitation under strong light induces greater

contraction of the more excitable half of the pulvinule, causing upward

folding of _Mimosa_ leaflet, and downward folding of the leaflets of

_Biophytum_ and _Averrhoa_.

XXXV.--EFFECT OF TEMPERATURE ON PHOTOTROPIC CURVATURE

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.

I shall in this chapter deal with certain anomalies in phototropic

curvature, brought about by variation of temperature and by seasonal

change; certain organs again are apparently erratic in their phototropic

response.

SEASONAL CHANGE OF PHOTOTROPIC ACTION.

Sachs observed a positive phototropic curvature in the stems of

_Tropæolum majus_ in autumn; but this was reversed into negative in

summer; similarly in the hypocotyl of Ivy, the positive curvature in

autumn is converted into negative curvature in summer.

Certain organs are apparently insensitive to the action of light. Thus

no phototropic response is found in the tendril of _Passiflora_ even

under the action of strong light. The tendrils of _Vitis_ and

_Ampelopsis_ exhibit, according to Wiesner, positive phototropism under

feeble, and negative phototropism under strong light.

The anomalies referred to above may be explained by taking into

consideration the modifying influence of temperature on the

excitability, and the conductivity of the organ.

EFFECT OF TEMPERATURE ON EXCITABILITY.

The excitability of an organ is abolished at a low temperature; it is

enhanced by a rise of temperature up to an optimum. The temperature

minimum and optimum varies in different tissues. The following table

shows the enhancement of excitability of _Mimosa_ at different

temperatures, the testing stimulus being the same.

TABLE XXXII--SHOWING VARIATION OF EXCITABILITY OF PULVINUS OF

_Mimosa_ AT DIFFERENT TEMPERATURES.

+-----------------------------------+

|Temperature.|Amplitude of response.|

+------------+----------------------+

| 22°C. | 2 divisions. |

| 27°C. | 16 " |

| 32°C. | 36 " |

+-----------------------------------+

Below 20°C. the excitability of the pulvinus of _Mimosa_ is practically

abolished. The excitability increases till an optimum temperature is

reached, above which it undergoes a decline.