Life Movements in Plants Part 32

DIURNAL MOVEMENT IN INVERTED POSITION.

I have already referred to the distinction that is made between nastic and paratonic movements. In the former the movement is autonomous and in relation to the plant, and in the latter it is due to an external force which determines the direction of movement. In nastic reaction, closure movement would persist as a closure movement[42]; but should the direction of movement be determined by the stimulus of gravity, closure movement would, on inversion, be reversed into an opening movement.

Viewed from an external point of view an up-movement in the latter case would, after readjustment on inversion, become an up-movement, though in so doing, the expansion should be transferred from the upper to the lower side of the organ. It is to be understood in this connection, that some time must lapse before this readjustment is possible, and that the former movement may continue, in certain cases, as a persistence of after-effect.

[42] By closure is meant movement of opposite pairs of leaf-organs towards each other.

I succeeded in demonstrating the paratonic effect of geotropic stimulus on the periodic movement of the palm leaf, by holding the plant in an inverted position (p. 24). On the first day of inversion, the diurnal record was erratic, but in the course of 24 hours, the leaf readjusted itself to its unaccustomed position, and became somewhat erected under geotropic action. After the attainment of this new state of geotropic equilibrium, the leaf gave the record of down-movement during rise, and up-movement during fall of temperature, movements which in reference to the plant are the very opposite to those in a normal position. But seen from an external point of view, rise of temperature caused in both normal and inverted positions, a down-movement indicative of diminished geotropic curvature; fall of temperature, on the other hand, brought about an erectile movement, thus exhibiting enhancement of geotropic curvature.

[Ill.u.s.tration: FIG. 204.--Effect of inversion of the plant on diurnal movement. (_a_) Normal record, (_b_) record 24 hours after inversion and (_c_) after 48 hours (_Tropaeolum_).]

_Experiment 213._--A still more striking result exhibiting the phase of transition was given by the geotropically curved stem of _Tropaeolum_.

Its diurnal curve and the subsequent changes after inversion are given in figure 204. In (_a_) is seen the normal diurnal curve; the specimen was inverted, and it took an entire day for the plant to readjust itself to the new geotropic condition. The record (_b_) was recommenced on the second day after inversion; the persistence of previous movement is seen in the reversed curve during the first half of the second day; but in the second half the record became true, and the third day the inverted plant gave a record which, from an external point of view, was similar to that given by the plant in the normal position.

SUMMARY.

A continuity is shown to exist between the thermo-geotropic response of rigid trees, stems, and leaves of plants.

The diurnal record exhibits an erectile movement from thermal-noon to thermal-dawn, and a movement of fall from thermal-dawn to thermal-noon.

In contrast with thermonastic movement which takes place in growing organs, thermo-geotropic movement takes place in fully grown organs including rigid trees. The thermonastic movement is independent of the direction of gravity, while in thermo-geotropic reaction, the stimulus of gravity exerts a directive action.

The effect of variation of temperature on the diurnal movement is demonstrated by induced change of normal rhythm, by artificial transposition of periods of thermal inversion, and by the abolition of periodic movement under constant temperature.

The effect of stimulus of gravity on the diurnal movement is demonstrated by the effect induced on holding the plant upside down.

The direction of the daily movement is found to be determined by the directive action of the stimulus of gravity.

L.--THE AFTER-EFFECT OF LIGHT

_By_

SIR J. C. BOSE,

_a.s.sisted by_

SURENDRA CHANDRA DAS.

We have considered two types of diurnal movement, one due to the predominant action of variation of light, and the other, to that of changing temperature. There are, however, other organs which are sensitive to variations both of light and of temperature. The effect of light is, generally speaking, antagonistic to that of rise of temperature; hence the resultant of the two becomes highly complex.

Still greater complexity is introduced by the different factors of immediate and after-effect of light. This latter phenomenon is very obscure, and I attempted to determine its characteristics by electrical method of investigation. A fuller account of after-effect of light on the response of various plant-organs and of animal retinae will be found elsewhere.[43] I shall here refer only to one or two characteristic results which have immediate bearing on the present subject.

[43] "Comparative Electro-Physiology"--p. 392.

Direct stimulation under light induces excitatory reaction, which is mechanically exhibited by contraction, and electrically by induced galvanometric negativity. Under continuous stimulation, the excitatory effect, either of positive curvature or of induced galvanometric negativity, is found to attain a maximum. This is often found to undergo a decline and reversal; for under continuous stimulation there is a fatigue-decline, as seen in the relaxation following normal contraction in animal muscle. The positive tropic curvature, and the induced galvanometric negativity may thus undergo a decline, and neutralisation.

This neutralisation is also favoured, in certain cases, by transverse conduction of excitation to the distal side.

The character of the after-effect will presently be shown to be modified by the duration of previous stimulation, the different phases of which will for convenience, be distinguished as pre-maximum, maximum and post-maximum. Since stimulus simultaneously induces positive "A" and the negative "D" changes (p. 143), their intensities will undergo relative variation during the continuance and cessation of stimulus. The after-effect will therefore exhibit unequal persistence of the expansive "A" and contractile "D" reaction at different phases of stimulation.

ELECTRIC AFTER-EFFECT.

Confining our attention to the electric response, it is found that under continued action of light the excitatory galvanometric negativity increases to a maximum, after which there is a decline, and neutralisation. Figure 205 gives the galvanographic record of the electric response of the leaf stalk of _Bryophyllum_ under light; the up-curve represents increasing negativity which, after attaining a maximum, undergoes neutralisation as seen in the down-curve. I shall, with the help of the diagram given in the next figure, describe and explain the various after-effects I observed on sudden stoppage of light: before the attainment of maximum, at the maximum, and after the maximum.

[Ill.u.s.tration: FIG. 205.--Electric response of the leaf-stalk of _Bryophyllum_ under continuous photic stimulation. Increasing negativity represented by up-curve; neutralisation by down-curve.]

[Ill.u.s.tration: FIG. 206.--Diagrammatic representation of electric after-effect of stimulation. Pre-maximal stimulation produced by stoppage of light at _a_, gives rise to continuation of previous response followed by recovery. Stoppage of light at maximum _b_ gives rise to recovery to equilibrium position. Stoppage of light at post-maximum _c_, gives rise to over-shooting below zero line.]

_After-effect of pre-maximum stimulation: Experiment 214._--Light is applied at arrow and stopped in different experiments at _a_, _b_, and _c_ (Fig. 106). Continuous stimulation induces increasing galvanometric negativity; when stimulus is stopped at _a_ before the maximum, the after-effect is a persistence of excitatory galvanometric negativity, which carries the response record higher up; after a certain interval recovery takes place and the record returns to the zero line of normal equilibrium. The after-effect of pre-maximum stimulation is thus a short-lived continuance of response followed by recovery.

_After-effect at maximum: Experiment 215._--In this the photic stimulus was continued till the attainment of maximum, when light was suddenly removed at _b_. The after-effect was no longer a persistence of responsive movement, but disappearance of negativity and recovery to zero line of equilibrium.

_Post-maximum after-effect: Experiment 216._--In this light was continued till there was a complete neutralisation, the curve of response returning to zero line; to all outer seeming the responsive indication of the tissue is the same as before excitation. But stoppage of stimulus at _c_ causes an over-shooting at a rapid rate far _below_ the zero line; and it is after a considerable period that the curve returns to the zero line of equilibrium.

The condition at post-maximum _c_ is thus one of dynamic equilibrium where two opposite activities, "A" and "D," balance each other; for had the condition of the 'neutralised' tissue been exactly the same when fresh, cessation of stimulus would have kept the galvanometric spot of light at the zero position.

The electric investigation described above shows that the after-effect is modified by duration of stimulation, and that:

(1) the after-effect of pre-maximum stimulation is the continuation of response in the original direction (upward, and away from zero line), followed by recovery,

(2) the after-effect of the maximum is an electric recovery towards zero position, and

(3) the after-effect of post-maximum stimulation is an over-shooting _downward_ below the zero line.

TROPIC RESPONSE UNDER LIGHT AND ITS AFTER-EFFECT.

I shall now describe the after-effect of light as seen in mechanical response, and the results will be found parallel to those given by the electric response. The specimen employed is the terminal leaflet of _Desmodium gyrans_, the pulvinus of which is very sensitive to light.

Pulvinated organs, generally speaking, exhibit a diurnal variation of turgor in consequence of which the position of equilibrium of the leaf or leaflet undergoes a periodic change. But this equilibrium position of the organ remains fairly constant for nearly two hours about midday, the variation of temperature at this period being slight. We may therefore obtain the pure effect of light by carrying out the experiment at this period, and completing it within a short time to avoid complication arising from the autonomous variation of turgor.

The period of experiment of the plant may be shortened by a choice of suitable intensity of light; a given tropic effect induced by prolonged feeble light may thus be obtained by short exposure to stronger light.

The source of light for the following experiment was a 50 c.p.

incandescent lamp. The intensity was increased to a suitable value by focussing light on the upper half of the pulvinus by means of a lens.

The intensity was so adjusted that the maximum positive curvature was attained in the course of about 6 minutes, and complete neutralisation after an exposure of 17 minutes.

_Pre-maximum after-effect: Experiment 217._--Light was allowed to act on the upper half of the pulvinus for two minutes and twenty seconds; this induced an up-movement _i.e._, a positive curvature. On the stoppage of light the up-movement continued for one minute and twenty seconds, after which the down-movement of recovery was completed in six minutes and twenty seconds (Fig. 207). The immediate after-effect is thus a movement upward, away from the zero line of equilibrium. The result is seen to be the same as the electric after-effect of pre-maximum stimulation.

[Ill.u.s.tration: FIG. 207.--Light applied at arrow, and stopped at the second arrow within a circle. After-effect of pre-maximum stimulation is continuation of positive curvature followed by recovery.]

[Ill.u.s.tration: FIG. 208.--After-effect at maximum; recovery towards zero position of equilibrium.]

[Ill.u.s.tration: FIG. 209.--After-effect at post-maximum is a rapid overshooting below the position of equilibrium. Light was applied in all cases on upper half of pulvinus of terminal leaflet of _Desmodium gyrans_.]

_After-effect at maximum: Experiment 218._--Application of light for 5 minutes and twenty seconds induced a maximum positive curvature.

Stoppage of light was followed at once by recovery which was completed in about 10 minutes (Fig. 208).