_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 x.x.xI.--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

_c.l.i.toria 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 _c.l.i.toria 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_.

x.x.xV.--EFFECT OF TEMPERATURE ON PHOTOTROPIC CURVATURE

_By_

SIR J. C. BOSE,

_a.s.sisted 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 _Tropaeolum 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 _Pa.s.siflora_ 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 x.x.xII--SHOWING VARIATION OF EXCITABILITY OF PULVINUS OF _Mimosa_ AT DIFFERENT TEMPERATURES.

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

Temperature.

Amplitude of response.

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

22C.

2 divisions.

27C.

16 "

32C.

36 "

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

Below 20C. 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.

Though rise of temperature enhances excitability up to an optimum, there is an antagonistic reaction induced by it which opposes the excitatory contraction. The physiological reaction of a rise of temperature, within normal range, is expansion and this must oppose the contraction induced by stimulus. Hence the effect of rise of temperature is complex; it enhances the excitability which favours contraction, while tending to oppose this contraction by the induced physiological expansion. As a result of these opposite reactions there will be a critical temperature, below which the contractile effect will relatively be greater than expansion; above the critical point, expansion will be the predominant effect. The critical temperature will obviously be different in different organs. The positive curvature may thus be increased by a slight rise, while it may be neutralised, or even reversed by a greater rise of temperature.

The induced variation of excitability due to change of temperature is not the only factor in modifying tropic curvature, for variation of conductivity also exerts a marked effect.

EFFECT OF TEMPERATURE ON CONDUCTION.

The conducting power of an organ is greatly enhanced with rise of temperature. Thus in _Mimosa_ the velocity of transmission of excitation is doubled by a rise of temperature through 9C. (p. 100). An organ which is practically non-conducting at a low temperature will become conducting at a higher temperature.

Thus at a low temperature the organ may be non-conducting, and the excitatory contraction under unilateral stimulus will remain localised at the proximal side; this will give rise to a positive curvature. But under rising temperature, the power of transverse conduction will be increased and the excitation will be conducted to the distal side. The result of this will be a neutralisation or reversal into negative curvature (p. 139). A positive curvature is thus reversed into negative by change of excitability and conductivity, induced by rise of temperature; examples of this will be given presently.

PHOTOTROPIC RESPONSE OF TENDRILS.

I shall here adduce considerations which will show that the apparent anomalies regarding the response of tendrils to light is due to the variation of transverse conductivity of the organ. In a semi-conducting tissue, while the excitatory effect of feeble stimulus remains localised at the proximal side, the effect of stronger stimulus is conducted to the distal side. This explains the positive phototropic curvature of tendrils of _Vitis_ and _Ampelopsis_ under feeble light, and its reversal into negative curvature under intense light.

As the conducting power is increased with rise of temperature it is evident that at a certain temperature the tropic effect will be exactly neutralised by transverse conduction. Lowering of temperature, by reducing the transmission of excitation to the distal side, will restore the positive curvature. Enhancement of conduction under rise of temperature will, on the other hand, increase the antagonistic reaction of the distal side and give rise to a negative curvature.

I shall in verification of the above, describe experiments which I have carried out on the phototropic response of the tendril of _Pa.s.siflora_, supposed to be insensitive to the action of light.

_Phototropic response of the tendril of_ Pa.s.siflora: _Experiment 145._--The tendril was cooled by keeping it for a long time in a cold chamber, maintained at 15C. The effect of unilateral light on the cooled specimen was found to be positive; the tendril was next allowed to a.s.sume the temperature of the room which was 30C. The response was now found to have undergone a change into negative. The positive and negative phototropic curvatures of an identical organ at different temperatures is seen in the two records given in figure 145.

Neutralisation takes place at an intermediate temperature, and the organ thus appears insensitive to light.

SEASONAL VARIATION OF PHOTOTROPIC CURVATURE.

[Ill.u.s.tration: FIG. 145.--(_a_) Positive curvature of tendril of _Pa.s.siflora_ at 15C.; (_b_) negative phototropic curvature at 30C.]

Reference has been made of the phototropic curvature of _Tropaeolum_ and of Ivy undergoing a change from positive in autumn to negative in summer. The experiment described above shows that rise of temperature, by enhancing transverse conductivity, transforms the positive into negative heliotropic curvature. The reversal of the phototropic curvature of _Tropaeolum_ and Ivy, from positive in autumn to negative in summer, finds a probable explanation in the higher temperature condition of the latter season. This inference finds independent support from the fact previously described (p. 100) that while the velocity of conduction of excitation in the petiole of _Mimosa_ is as high as 30 mm.

per second in summer, it is reduced to about 4 mm. in late autumn and early winter.

ANTAGONISTIC EFFECTS OF LIGHT AND OF RISE OF TEMPERATURE.

I have explained the complex effect of rise of temperature on phototropic curvature. Rise of temperature, within limits, enhances the excitability, and therefore the positive curvature under light. Its expansive reaction, on the other hand, opposes the contraction of the proximal side, which produces the normal positive curvature. Rise of temperature, as previously stated, introduces another element of variation by its effect on conductivity. Transverse conduction favoured by rise of temperature promotes neutralisation and reversal; the resultant effect will thus be very complicated. I give below account of an experiment where the induced positive curvature under light underwent a reversal during rise of temperature.

_Reversal of tropic curvature under rise of temperature: Experiment 146._--The specimen employed for this experiment was a seedling of pea, enclosed in a gla.s.s chamber, the temperature of which could be gradually raised by means of an electric heater. Provisions were made to maintain the chamber in a humid condition. The temperature of the chamber was originally at 29C., and application of light on one side of the organ gave rise to positive curvature, followed by complete recovery on the cessation of light (Fig. 146a). The next experiment was carried out with the same specimen; while the plant was undergoing increasing positive curvature under the continued action of light, the temperature of the chamber was gradually raised from 29 to 33C. at the point marked with arrow. It will be seen that the positive curvature became arrested, neutralised, and finally reversed into negative (Fig. 146b).

[Ill.u.s.tration: FIG. 146.--Effect of rise of temperature on phototropic curvature. (_a_) normal positive curvature followed by recovery, (_b_) reversal of positive into negative curvature by rise of temperature at (H). (Pea seedling).]