Life Movements in Plants Part 20

SATYENDRA CHANDRA GUHA, M.Sc.

The experiments that have been described in the preceding chapter show that the upper side of a horizontally laid shoot undergoes excitatory contraction, in consequence of which the organ bends upwards. The fundamental geotropic reaction is, therefore, not expansion, but contraction which results from all modes of stimulation.

In confirmation of the above, I wished to discover and employ new means of detecting excitatory reaction under geotropic stimulus. In regard to this, I would refer to the fact which I have fully established that the state of excitation can be detected by the induced electromotive change of galvanometric negativity. This electrical indication of excitation may be observed even in plants physically restrained from exhibiting response by mechanical movement.[34]

[34] "Comparative Electro-Physiology," p. 20.

ELECTRIC RESPONSE TO STIMULUS.

Before giving account of the results of investigations on the detection of geotropic excitation by means of electric response, I shall describe a few typical experiments which will fully explain the method of the electrical investigation, and show the correspondence of mechanical and electric responses. I have explained how tropic curvatures are brought about by the joint effects, of contraction of the directly excited proximal side A, and the expansion of the distal side B. In the diagram of mechanical response to stimulus (Fig. 164a) the excitatory contraction is indicated by - sign, and the expansion, by + sign. The resulting movement is, therefore, towards the stimulus as shown by the curved arrow.

I shall now describe the corresponding electric effects in response to unilateral stimulus. We have to determine the induced electrical variation at the proximal side A, and at the distal side B.

[Ill.u.s.tration: Fig. 164.--Diagrammatic representation of the mechanical and electrical response to direct unilateral stimulation indicated by arrow:--

(_a_) Positive mechanical response (curved arrow) due to contraction of directly stimulated A, and expansion of indirectly stimulated B.

(_b_) Electric response of induced galvanometric negativity of A under direct stimulation.

(_c_) Electric response of induced galvanometric positivity at the distal point B.

(_d_) Additive effects of direct and indirect stimulations; galvanometric negativity of the directly stimulated proximal A, and galvanometric positivity of the indirectly stimulated distal point B.]

_Electric response to direct stimulation: Experiment 168._--For the determination of electric response at the directly excited proximal side A, we take a shoot with a lateral leaf. The point A, which is to undergo stimulation, is connected with one terminal of the galvanometer, the other terminal being led to an indifferent or neutral point N on the leaf. Application of any form stimulus at A, gives rise to an electric current which flows through the galvanometer from the neutral to the excited point A (Fig. 164b). _The directly stimulated point A thus becomes galvanometrically negative._ The "action" current lasts during the application of stimulus and disappears on its cessation.

_Electric response to indirect stimulation: Experiment 169._--We have also seen that application of stimulus at A causes indirect stimulation of the distal point B resulting in an increase of turgor and expansion.

The corresponding electric change of the indirectly stimulated point B is found in the responsive current, which flows now through the galvanometer from the indirectly stimulated B to the neutral point N (Fig. 164c). _The indirectly stimulated point thus becomes galvanometrically positive._

Having thus obtained the separate effects at A and B, we next modify the experiment for obtaining the joint effects. For this purpose the neutral point N is discarded and A and B connected directly with the indicating galvanometer. On stimulation of A that point becomes negative and B positive, and the current of response flows through the galvanometer from B to A. The deflection is increased by the joint electrical reactions at A and B (Fig. 164d).

The results may thus be summarised:--

TABLE x.x.xIII.--ELECTRIC RESPONSE TO DIRECT UNILATERAL STIMULUS.

+---------------------------------------------------------+ Electrical change at the Electrical change at the proximal side A. distal side B. +----------------------------+----------------------------+ Galvanometric negativity Galvanometric positivity indicative of contraction indicative of expansion and diminution of turgor. and increase of turgor. ---------------------------------------------------------+ The corresponding tropic curvature is positive movement towards stimulus. +---------------------------------------------------------+

Galvanometric negativity is thus seen to indicate the effect of direct stimulus, and galvanometric positivity that of indirect stimulus. We thus see the possibility of electric detection of the effects of geotropic stimulation. This method would, moreover, enable us to discriminate the side of the organ which undergoes greater excitation.

EXPERIMENTAL ARRANGEMENTS FOR OBTAINING GEO-ELECTRIC RESPONSE.

Returning to the investigation on electric response to geotropic stimulus, the specimen of plant is at first held erect; two electrodes connected with a sensitive galvanometer are applied, one to an indifferent point, and the other to one side of the shoot. The sensitiveness of the galvanometer was such that a current of one millionth of an ampere produced a deflection of the reflected spot of light through 1,000 divisions of the scale. An action current is produced on displacement of the plant from vertical to horizontal position.

_Non-polarisable electrodes._--The electrical connections with the plant are usually made by means of non-polarisable electrodes (amalgamated zinc rod in zinc-sulphate solution and kaolin paste with normal saline).

I at first used this method and obtained all the results which will be presently described. But the employment of the usual non-polarisable electrodes with liquid electrolyte is, for our present purpose, extremely inconvenient in practice; for the plant-holder with the electrodes has to be rotated from vertical to horizontal through 90.

The reliability of the non-polarisable electrode, moreover, is not above criticism. The zinc-sulphate solution percolates through the kaolin paste and ultimately comes in contact with the plant, and seriously affects its excitability. The name non-polarisable electrode is in reality a misnomer; for the action current (whose polarising effect is to be guarded against) is excessively feeble, being of the order of a millionth of an ampere or even less; the counter polarisation induced by such a feeble current is practically negligible.

The idea that non-polarisable electrodes are meant to get rid of polarisation is not thus justified by the facts of the case. The real reason for its use is very different; the electrical connections with the plant has to be made ultimately by means of two metal contacts. If we take two pieces of metal even from the same sheet, and put them in connection with the plant, a voltaic couple is produced owing to slight physical differences between the two electrodes. Amalgamation of the two zinc rods with mercury reduces the electric difference but cannot altogether eliminate it.

I have been able to wipe off the difference of potential between two pieces of the same metal, say of platinum, and by immersing them in dilute salt solution from a voltaic couple. The circuit is kept complete for 24 hours, and the potential of the two electrodes by this process is nearly equalised. A perfect equality is secured by repeated warming and cooling of the solution and by sending through the circuit, alternating current which is gradually reduced to zero. I have by this means been able to obtain two electrodes which are iso-electric. The specially prepared electrodes (made of gold or platinum wire) are put in connection with the plant through kaolin paste moistened with normal saline solution. Care should be taken to use opaque cover over the plant-holder, so as to guard against any possible photo-electric action; moistened blotting paper maintains the closed chamber in a uniform humid condition.

The direct method of contact described above is extremely convenient in practice; the resistance of contact is considerably reduced, and there is no possibility of its variation during the necessary process of rotation of the plant for subjecting it to geotropic action.

[Ill.u.s.tration: FIG. 165.--Diagrammatic representation of geo-electric response. The middle figure represents vertical position. In figure to the right rotation through +90 has placed A above with induced electric change of galvanometric negativity of A. In the figure to the left, rotation is through -90 A being below; the electric response is by induced galvanometric positivity of A. For simplification of diagram, vertical position of sepal is not always shown in the figure.]

GEO-ELECTRIC RESPONSE OF THE UPPER AND LOWER SIDES OF THE ORGAN.

We have next to discover the electric change induced by geotropic stimulus on the upper and lower sides of the organ. For this purpose it is necessary to find a neutral point which is not affected by the inclination of the organ from vertical to horizontal position. For the present experiment, I employed the flower of the water lily _Nymphaea_, the peduncle of which is sensitive to geotropic action. One electrical contact is made with a sepal, which is always kept vertical; the other electric contact is made at the point A, on one side of the flower stalk (Fig. 165). On making connections with a sensitive galvanometer a very feeble current was found, which was due to slight physiological difference between the neutral point, N, and A. This natural current may be allowed to remain, the action current due to geotropism being _superposed on it_; or the natural current may be neutralised by means of a potentiometer and the reflected spot of light brought to zero of the scale.

_Induced electric variation on upper side of the organ: Experiment 170._--While the sepal is held vertical, the stalk is displaced through +90 so that the point A is above. Geotropic stimulation is at once followed by a responsive current which flows through the galvanometer from N to A, the upper side of the organ thus exhibiting excitatory reaction of galvanometric negativity (Right-hand figure of 166). When the stalk is brought back to vertical position geotropic stimulation disappears, and with it the responsive current.

_Electric response of the lower side: Experiment 171._--The stalk is now displaced through -90; the point A, which under rotation through +90 pointed upwards, is now made to point downwards. The direction of the current of response is now found to have undergone a reversal; it now flows from A on the lower side to the neutral point N; thus under geotropic action _the lower side of the organ exhibits galvanometric positivity_ indicative of increase of turgor and expansion (Left-hand figure 166).[35]

[35] For detailed account cf. Chapter XLIII.

Having thus found that the upper side of the organ under geotropic stimulus becomes galvanometrically negative, and the lower side, galvanometrically positive, we make electric connections with two diametrically opposite points of the shoot A and B, and subject the organ to alternate rotation through +90 and -90. The electro-motive changes induced at the two sides now became algebraically summated. I employ two methods for geotropic stimulation: that (1) of Axial Rotation, and (2) of Vertical Rotation.

[Ill.u.s.tration: FIG. 166.--Diagrammatic representation of the Method of Axial Rotation H, and of Vertical rotation V (see text).]

METHOD OF AXIAL ROTATION.

In the method of Axial Rotation, the organ is held with its long axis horizontal (Fig. 166 H). We have seen that the geotropic action increases with the angle which the responding surface of the organ makes with the vertical lines of gravity. When the organ is held with its length horizontal, the angle made by its two sides, A and B, with the vertical is zero and there is thus no geotropic effect. There is, moreover, no differential effect, since the two sides are symmetrically placed as regards the vertical lines of force. The plant is next rotated round its long axis, the angle of rotation being indicated in the circular scale. When the rotation is through +90, A is above and B below; this induces a differential geotropic effect, the upper side exhibiting excitatory electric change of galvanometric negativity.

_Experiment 172._--I shall, as a typical example, give a detailed account of experiments with the petiole of _Tropaeolum_ which was found so highly excitable to geotropic stimulus (p. 434). The specimen was held horizontal with two symmetrical contacts at the two sides, the electrodes being connected in the usual manner with the indicating galvanometer. When the plant is rotated through +90 there is an immediate current of response, the upper side becoming _galvanometrically negative_. This excitatory reaction on the upper side finds, as we have seen, mechanical expression by contraction and concavity, with positive or up-curvature.

[Ill.u.s.tration: FIG. 167.--Diagrammatic representation of the geo-electric response of the shoot (see text).]

The differential stimulation of A and B disappears on rotation of the axis back to zero position, and the induced electro-motive response also disappears at the same time. If now the axis be rotated through -90, A will become the lower, and B the upper and the excited side. The electro-motive change is now found to have undergone a reversal, B becoming galvanometrically negative. This induced electro-motive variation under geotropic stimulus is of considerable intensity often exceeding 15 millivolts. The characteristic electric change is shown diagrammatically in figure 167 in which the middle figure shows the symmetrical or zero position. On rotation through +90 (figure to the right) A occupies the upper and B the lower position. A is seen to exhibit induced change of galvanometric negativity. Rotation through -90 reverses the current of response, as B now occupies the upper and A the lower position.

CHARACTERISTICS OF GEO-ELECTRIC RESPONSE.

There are certain phenomena connected with the electric response under geotropic stimulus which appear to be highly significant. According to statolithic theory

"Geotropic response begins as soon as an organ is deflected from its stable position, so that a few starch-grains press upon the ectoplasts occupying the walls which are underneath in the new position; an actual rearrangement of the starch-grains is therefore not an essential condition of stimulation. As a matter of fact, the starch-grains do very soon migrate on to the physically lower walls, when a positively or negatively geotropic organ is placed horizontally, with the result that the intensity of stimulation gradually increases attaining its maximum value when all the falling starch-grains have moved on to the lower region of the ectoplast. The time required for the complete rearrangement of the statoliths may be termed the period of migration; its average length varies from five to twenty minutes in different organs."[36]

[36] Haberlandt--_Ibid_--p. 598.

Stimulation, according to the statolithic theory, is induced by the displacement of the particles. The diameter of the geotropically sensitive cells is considerably less than 01 mm.; and the stimulus will be perceived after the very short interval taken by the statoliths to fall through a s.p.a.ce shorter than 01 mm. This may be somewhat delayed by the viscous nature of the plasma, but in any case the period for perceptible displacement of the statoliths should be very short, about a second or so, and the latent period of perception of stimulus should be of this order.

The mechanical indication of response to stimulus is delayed by a period which is somewhat indefinite; for the initiation of responsive growth variation will necessarily lag behind the perception of stimulus.

[Ill.u.s.tration: FIG. 168.--Geo-electric response of the petiole of _Tropaeolum_.]