In the last chapter we studied the electric response of the shoot to the stimulus of gravity, and found that the excitatory effect of that stimulus is similar to that of other forms of stimulation. Before taking up the subject of the geo-electric response of the root to gravitational stimulus, I shall describe the effects of other forms of stimuli on the mechanical and electrical response of the root.

In connection with this subject, it should be borne in mind that the responsive curvature in the root takes place in the sub-apical growing zone which is separated by a certain distance from the tip. The stimulus is therefore direct when applied at the responding growing region; it is indirect when applied at the tip of the root. The intervening distance between the root-tip and the responsive zone of growth is semi-conducting or non-conducting.

I shall proceed to give an account of my investigations on the response of the root to direct and indirect unilateral stimulation. We shall study:--

(1) The Mechanical response to Direct unilateral stimulus.

(2) The Electrical response to Direct unilateral stimulus.

(3) The Mechanical response to Indirect unilateral stimulus.

(4) The Electrical response to Indirect unilateral stimulus.

MECHANICAL RESPONSE TO DIRECT STIMULUS.

As the geotropic responses of the shoot and the root are opposed to each other, the object of the investigation is to find out; whether the response of the root to various stimuli is specifically different from that of the shoot. We have seen that tissues in general respond to direct unilateral stimulus by contraction of the proximal and expansion of the distal side, the tropic curvature being thus _positive_. We shall now determine whether direct unilateral stimulation of the root induces a tropic movement which is similar or dissimilar to that exhibited by the shoot.

_Experiment 178._--In experimenting with roots of various plants I obtained results which are precisely similar to that of the shoot. The movement of the root was observed by means of a reading microscope focussed on the tip of the organ. I employed various forms of stimuli, mechanical, thermal, and chemical. Unilateral application of these on one side of the growing region gave rise to a _positive_ tropic curvature, resulting in a movement towards the stimulus. These experiments confirm Sachs' observation that unilateral application of stimulus in the region of growth induces positive curvature of the root.

ELECTRICAL RESPONSE TO DIRECT STIMULATION.

I next undertook an investigation on the electric response of the root to direct unilateral stimulation.

_Experiment 179._--The terminals of the galvanometer were suitably connected with the two diametrically opposite points A and B in the growing region of the root. Stimulus was now applied very near the point A, the various stimuli employed in different experiments being: (1) mechanical, (2) chemical, and (3) thermal. In every instance the excited point A becomes galvanometrically negative. This shows that the response of the root is in no way different from that of the shoot.

MECHANICAL RESPONSE TO INDIRECT STIMULUS.

Before describing the effect of indirect stimulus on the root, I shall recapitulate its effects on ordinary tissues. I have shown that the effect of indirect unilateral stimulus is to induce a movement away from stimulus. This was shown to be the case with the bud of _Crinum_ (p.

275) and the tendril of _Pa.s.siflora_ (p. 291). The mechanical and electric response to indirect stimulation in the shoot is shown in the diagrammatic representation (Fig. 170). I shall now proceed to describe the mechanical response induced by unilateral stimulation of the root tip. As the responding region of growth is at some distance from the tip, the stimulation is therefore indirect.

[Ill.u.s.tration: FIG. 170.--Mechanical and electrical response to indirect stimulation at dotted arrow. In figure to the left, the point A, on the same side undergoes expansion, with responsive mechanical movement away from stimulus indicated by continuous arrow. In figure to the right, indirect stimulus at dotted arrow induces electric response of galvanometric positivity at A, indicative of increase of turgor and expansion.]

_Experiment 180._--I employed at first mechanical stimulus of moderate intensity by rubbing one side of the tip of the root of _Bindweed_; this induced a movement away from stimulus. Unilateral application of dilute acid gave rise to a similar response. Thermal stimulus of moderate intensity also induced responsive movement away from the stimulus (Fig.

171).

Darwin in his _Movements of Plants_ described experiments on the responsive behaviour of the tip of the radicle. He produced unilateral stimulation in three different ways, first by attaching minute fragments of cardboard to one side of the root-tip; this moderate and constant irritation was found to induce a convexity on the same side of the growing region, with the resulting negative movement, _i.e._, away from stimulus. His second method was chemical, one side of the tip being touched with silver nitrate; the third method of stimulation was a slanting cut. All these methods induced a movement away from stimulus.

ELECTRICAL RESPONSE TO INDIRECT STIMULATION.

The next investigation was for the determination of the electrical change induced in the growing region by application of unilateral stimulus at the root-tip.

[Ill.u.s.tration: FIG. 171.--Diagrammatic representation of mechanical and electric response of root to indirect stimulus applied at the tip _a_.

Figure to the left shows responsive movement away from stimulus. The electric response to indirect stimulus is indicated in the figure to the right; the point on the same side exhibiting galvanometric positivity.

The shaded part indicates the responsive region of growth at some distance from the tip.]

_Experiment 181._--One of the two electrical connections with the galvanometer is made at one side of the growing region A, the other connection being made with the diametrically opposite point B.

Unilateral stimulus was applied at the root tip _a_, of the bean plant and on the same side as A. I subjected the tip to various modes of unilateral stimulation. Mechanical stimulation was effected by emery-paper friction or by pin-p.r.i.c.k; chemical stimulation was produced by application of dilute hydrochloric acid. Thermal stimulation was caused by the proximity of electrically heated platinum wire. In every case the response was by _induced galvanometric positivity at A_ (Fig.

171). This electrical variation took place within about ten seconds of the application of stimulus; the interval would obviously depend on the length of path to be traversed by the transmitted effect of indirect stimulation.

The galvanometric positivity at A indicated that there was induced at that point an increase of turgor and expansion, in consequence of which the organ would move away from stimulus. Thus both by the mechanical and electrical methods of investigation we arrive at an identical conclusion that the effects of unilateral stimulus at the tip of the root gives rise to a movement, by which the organ is moved away from the source of stimulus; since tropic movement towards stimulus is termed _positive_, this opposite response must be regarded as _negative_.

TABLE x.x.xVI.--EFFECT OF INDIRECT STIMULUS UNILATERALLY APPLIED AT THE ROOT-TIP.

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

Effect at the proximal side A in the

Effect at the distal

growing region.

side B.

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

Galvanometric positivity, indicative

Negligible.

of increase of turgor and expansion.

--------------------------------------------------------------

The corresponding tropic curvature is negative, _i.e._, a

movement away from stimulus.

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

The root-tip when burrowing its way underground comes in contact with hard substances and moves away from the source of irritation. The irritability of the root-tip is generally regarded as being specially evolved for the advantage of the plant. But reference to experiments that, have been described shows that this reaction is not unique but exhibited by all plant organs, growing and non-growing. Indirect stimulus has been shown to give rise, in both shoot and root, to a _negative_ tropic curvature in contrast to the _positive_ curvature brought about by direct stimulation; the response of the root is therefore in no way different from that of vegetable tissues in general.

It will also be seen that an identical stimulus induces two opposite effects, according as the stimulus is applied at the tip or at the growing region itself. In the former case, the stimulus is indirect, and in the latter case it is direct. The results are in strict conformity with the laws of effects of direct and indirect stimulations that have been established regarding plant response in general (p. 231).

SUMMARY.

In the root, the responsive region is in the zone of growth. The tip of the root is separated from the region of response by a semi-conducting or non-conducting tissue.

Direct unilateral stimulus (applied at the region of growth) induces a positive curvature by the contraction of the proximal and expansion of the distal side.

The electrical response to direct unilateral stimulus is galvanometric negativity of the proximal, and galvanometric positivity of the distal side.

Indirect unilateral stimulus induces expansion of the proximal side resulting in negative curvature and movement away from stimulus.

The corresponding electric response induced is galvanometric positivity of the proximal side.

The responses of the root, to both direct and indirect stimulations, are precisely similar to those in the shoot. The a.s.sumption of specific irritability of the root as differing from that of the shoot, is without any justification.

XLII.--GEO-ELECTRIC RESPONSE OF ROOT

_By_

SIR J. C. BOSE,

_a.s.sisted by_

SATYENDRA CHANDRA GUHA.

The effects of various stimuli, direct and indirect, on the response of the root have been described in the last chapter. These responsive reactions have been found to be in no way different from those of the shoot. But the shoot and the root exhibit under the stimulus of gravity, responsive movements which are diametrically opposite to each other.

These opposite effects of an identical stimulus have been regarded as due to specific differences of irritability in the two organs, specially evolved for the advantage of the plant. The root is thus supposed to be characterised by "positive" and the shoot by "negative" geotropism.

As regards response to other forms of stimuli, the root has been shown to behave like the shoot. We have now to inquire whether the reaction of the root to gravitational stimulus is specifically different to that of the shoot.