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

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

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

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

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

XXVII.--ON GALVANOTROPISM

_By_

SIR J. C. BOSE,

_a.s.sisted by_

GURUPRASANNA DAS.

Before describing the effect of unilateral application of an electrical current in inducing tropic curvature, I shall give an account of the polar effect of anode and cathode on the pulvinated and growing organs.

In my previous work[6] on the action of electrical current on sensitive pulvini I have shown that:--

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

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

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

I have also carried out further investigations on the polar effect of current on the autonomous activity of the leaflet of _Desmodium gyrans_.

These rhythmic pulsations can be recorded by my Oscillating Recorder.

Each pulsation consists of a sudden contractile movement downwards, corresponding to the systole of a beating heart, and a slower up movement of diastolic expansion. Application of cathode at the pulvinule was found to exert a _contractile_ reaction, exhibited either by the reduction of normal limit of diastolic expansion, or by an arrest of movement at systole. The effect of anode was precisely the opposite; the induced _expansion_ was exhibited either by reduction of normal limit of systolic contraction, or by arrest of pulsation at diastole.

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

(1) contraction is induced at the cathode, and

(2) expansion is brought about at the anode.

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

POLAR EFFECT OF ELECTRICAL CURRENT ON GROWTH.

The object of this investigation was to determine whether anode and cathode exerted similar discriminative and opposite effects on growth.

For this experiment I took a specimen of _Kysoor_ and determined the region where growth was maximum. A piece of moist cloth was wrapped round this region to serve as one of the two electrodes. The second electrode was placed in the neighbouring indifferent region where there had been a cessation of growth.

_Effect of Cathode: Experiment 111._--The particular specimen of _Kysoor_ had a normal rate of growth of 048 per second. On application of the cathode the rate was reduced to 014 per second, or to less than a third. This will be seen in record (Fig. 107), where N is the normal rate of growth and K, r.e.t.a.r.ded rate under the action of the cathode.

[Ill.u.s.tration: FIG. 107.--r.e.t.a.r.dation of rate of growth under the action of cathode (_Kysoor_).]

[Ill.u.s.tration: FIG. 108.--Acceleration of rate of growth under anode (_Kysoor_).]

_Effect of anode: Experiment 112._--If the cathode induced a r.e.t.a.r.dation, the anode might be expected to induce an acceleration of growth. But in my first experiment on the action of anode, I could detect no perceptible variation of rate of growth. In trying to account for this failure, I found that the specimen employed for the experiment had normally a very rapid rate of growth. It appeared that an induced acceleration would be brought out more conspicuously by choosing a specimen in which the growth-rate was low, rather than in one in which it was near its maximum. Acting on this idea, I took another specimen of _Kysoor_ in which the normal rate was as slow as 010 per second. On applying the anode to the growing region, there was an enhancement to one and half times the normal rate (Fig. 108).

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

Specimen A Normal rate 010 per sec.

Acceleration under anode 0155 per sec.

Specimen B Normal rate 048 per sec.

r.e.t.a.r.dation under cathode 014 per sec.

The effects given above take place under the action of a feeble current.

Strong current on the other hand induces a r.e.t.a.r.dation or an arrest of growth.

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

SUMMARY.

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

XXVIII.--ON THERMONASTIC PHENOMENA

_By_

SIR J. C. BOSE,

_a.s.sisted by_

SURENDRA CHANDRA DAS.

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

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

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

We have therefore to find:

(1) The physiological effect of variation of temperature.