The electric method of investigation described in the last chapter, holds out the possibility of discovering the character of the responsive reaction induced in the root by its displacement from vertical to horizontal position; we shall, moreover, be able to make an electrical exploration of the root-tip and the zone of growth, and thus determine the qualitative changes of response, induced in two regions of the root under the action of gravitational stimulus. For the detection of geotropic action in the shoot, electric contacts were made at two points diametrically opposite to each other. Displacement of the shoot from vertical to horizontal position induced excitatory change of galvanometric negativity at the upper side of the organ, demonstrating the effect of direct stimulation of that side; this excitatory reaction of the upper side finds independent mechanical expression in the induced contraction and concavity of that side of the organ.

I employ a similar electric method for detection of geotropic excitation of the root, responses to geotropic stimulus being taken at the root-tip and also at the zone of growth in which geotropic curvature is effected.

I shall now proceed to give a detailed description of the characteristic electric responses of the tip and of the growing region.

The two diametrically opposite contacts at the tip will be distinguished as _a_ and _b_, the corresponding points higher up in the growing region being A and B. When the root is vertical the electric conditions of the two diametrically opposite points are practically the same. But when the root is rotated in a vertical plane through +90 a geo-electric response will be found to take place; the direction of the responsive current disappears when the root is brought back to the vertical. Rotation through -90 gives rise again to a responsive current, but its direction is found reversed.

GEO-ELECTRIC RESPONSE OF THE ROOT-TIP.

_Experiment 182._--I took the root of the bean plant and made two electric contacts with the diametrically opposite points, _a_ and _b_, of the root-tip at a distance of about 15 mm. from the extreme end.

Owing to the very small size of the tip this is by no means an easy operation. Two platinum points tipped with kaolin paste are very carefully adjusted so as to make good electric contacts at the two opposite sides, without exerting undue pressure. For geotropic stimulation the root has to be laid horizontal, and as the root of the bean plant is somewhat long and limp, displacement from the vertical position is apt to cause a break of the electric contact. This is avoided by supporting the root from the top and also from the sides; for the latter purpose, I use paddings of cotton wool.

[Ill.u.s.tration: FIG. 172.--Diagrammatic representation of geo-electric response of root-tip. The middle figure shows root in vertical position.

Rotation through +90 places _a_ above, which becomes galvanometrically negative. Rotation through -90, places _b_ above and makes it negative.]

After due observance of these precautions the electric response obtained is found to be very definite; when the root is made horizontal, by rotation of the root through +90, the point _a_ is above, and the responsive current is found to flow from _b_ to _a_, _the upper side of the tip_ becoming galvanometrically negative; when the root is brought back to the vertical, the responsive current disappears; rotation through -90 makes the point _b_ occupy the upper position, and the responsive current is from _a_ to _b_; the upper side thus exhibits in every case, an excitatory electric change of galvanometric negativity (Fig. 172). The root-tip thus exhibits the characteristic response to direct stimulation. Experiments carried out with 12 different specimens gave concordant results. The following table gives the absolute values of electro-motive force induced at the tip under geotropic stimulus.

TABLE x.x.xVII.--GEO-ELECTRIC RESPONSE OF THE ROOT TIP (_Vicia Faba_).

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

Specimen.

Induced E. M. F.

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

1

00005 volt.

2

00011 "

3

00010 "

4

00015 "

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

ELECTRIC RESPONSE IN THE GROWING REGION.

_Experiment 183._--I next undertook an investigation on the electric variation induced in the growing region under the stimulus of gravity.

The experimental difficulties are here greatly reduced, since the available area of contact for galvanometric connection is not so restricted as in the case of the root-tip. The specimen is securely mounted so that the root is vertical. It is next rotated in the vertical plane through +90, so that the point A in the growing region occupied the upper position. The electric response in the growing region took place in a short time and was very distinct. The induced electric change at A was now galvanometric _positivity_ indicative of increase of _turgor and expansion_.

The series of experiments were carried out in the following order. The specimen was first rotated through +90 so that A was above. The responsive electric variation rendered it galvanometrically positive.

The root was rotated back to neutral position when the current disappeared. The root was next rotated through -90 and the responsive current became reversed, the upper B becoming electro-positive (Fig.

173). The alternative rotations through +90 and -90 were carried out six times in succession with consistent results. The interval allowed between one stimulation and the next was determined by the period of complete recovery. Growing fatigue was found to increase this period; at first it was seven minutes, at the second repet.i.tion it was ten minutes, and at the third time it was prolonged to fifteen minutes.

[Ill.u.s.tration: FIG. 173.--Diagrammatic representation of geo-electric response of growing region of root. (_a_) Rotation through -90 makes B, galvanometrically positive. (_b_) Vertical and neutral position. (_c_) Rotation through +90 places A above and renders it galvanometrically positive. (_d_) Additive effect on current of response, root-tip a negative, and growing region A positive.]

I give below the series of electric responses induced by alternate rotations through +90 and -90. The upper position was occupied by A in the odd series, and by B in the even series. In every case the upper side became galvanometrically positive.

TABLE x.x.xVIII.--GEO-ELECTRIC RESPONSE OF ROOT IN THE REGION OF GROWTH.

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

Odd

Galvanometer deflection

Even

Galvanometer deflection

series.

A, positive.

series.

B, positive.

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

1

20 divisions.

2

18 divisions.

3

16 "

4

18 "

5

10 "

6

12 "

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

ADDITIVE ACTION-CURRENT AT THE TIP AND THE GROWING REGION.

It has been shown that under geotropic stimulus the upper side of the tip, _a_, becomes galvanometrically negative, while the point A, higher up in the growing region, becomes galvanometrically positive. If now we make the two galvanometric connections with _a_ and A, the induced electric difference is increased, and the galvanometric response becomes enhanced.

_Experiment 184._--The root was at first held vertical, and two electric contacts made with _a_ and A. In this neutral position there is little or no current. But as soon as the root was laid horizontal, an electro-motive response was obtained which showed that _a_ was galvanometrically negative, and A galvanometrically positive (Fig.

173d). The induced electric response disappeared on restoration of the root to the vertical position. I give below the results of typical experiments with a vigorous specimen which gave strong electric response. It was possible to repeat the geotropic stimulation six times in succession, the results being perfectly consistent. The responses taken in succession exhibited slight fatigue, the first deflection being 140 divisions, and the sixth 115 divisions of the galvanometer scale.

TABLE x.x.xIX.--INDUCED E. M. F. VARIATION BETWEEN THE TIP AND THE GROWING REGION (_a_ NEGATIVE AND A POSITIVE).

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

Geotropic stimulation.

Resulting electric response.

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

First stimulation

140 divisions.

Second "

130 "

Third "

130 "

Fourth "

123 "

Fifth "

127 "

Sixth "

115 "

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

The results of experiments 182 and 183 are summarised as follows:--

(1) the induced galvanometric negativity at root tip indicates direct stimulation of the tip, and

(2) the induced galvanometric positivity of the growing region shows that it is the effect of indirect stimulus that reaches it.

From these facts it will be seen that the tip perceives the stimulus and thus undergoes excitation, and that owing to the intervening tissue being a semi-conductor of excitation, it is the positive impulse that reaches the growing region and induces there an expansion and a convex curvature.

GEO-PERCEPTION AT THE ROOT TIP.

The results given above fully confirm Charles Darwin's discovery that it is the root tip that perceives the stimulus of gravity[37]; he found that removal of the tip abolished the geotropic response of the root.

Objection has been raised about the shock-effect of operation itself being the cause of abolition of response. But subsequent observations have shown that Darwin's conclusions are in the main correct.

[37] "This view has been the subject of a considerable amount of controversy. Wiesner denies the localisation of geotropic sensitiveness. Czapek, on the other hand, supports Darwin's theory. Recently Picard has attacked the problem in a new way (and) concludes that not only the root tip but also the entire growing zone is capable of perceiving gravitational stimuli.... As both Picard's experimental method and his interpretation are open to criticism, the author has repeated his experiments with a more satisfactory apparatus. He finds that in _Vicia Faba_, _Phaseolus multeflorus_ and _Lupinus albus_, both apex and growing zone are geotropically sensitive, the former being by far the more sensitive of the two, and the curvature of the growing zone being without a doubt largely induced by secondary stimuli transmitted from the apical region. Charles Darwin's views were therefore in the main correct."--Haberlandt--_Ibid_, p. 748.

The experiments which I have described on the geo-electric response of the root tip and of the growing region offer convincing proof of the perception of the stimulus at the tip, and the transmission of the effect of indirect stimulus to the growing region. These experiments exhibit in an identical _uninjured_ organ: the excitatory reaction at the upper side of the tip, the cessation of excitation, and the excitation of the opposite side of the tip, following the rotation of the organ through +90, 0 and -90. The effect at the growing zone is precisely the opposite to that at the tip, _i.e._, an expansive reaction which results from the effect of indirect stimulus, in contrast to the contractile reaction due to direct stimulation.

We may now proceed a step further and try to obtain some idea of the difference in the mechanics of geotropic stimulation of the shoot and of the root, to account for the different responses in the two organs. The reason of this difference lies in the fact that in the shoot the perceptive and responding region is one and the same; every cut-piece of stem exhibits the characteristic geotropic curvature. In the root the case is different; for the removal of the sensitive root-tip reduces or abolishes the geotropic action; the region of maximum geotropic perception is thus separated from that of response. It must be borne in mind _that this holds good only in the case of gravitational stimulus_, for the decapitated root still continues to respond to other forms of stimulation such as chemical or photic.

The cause of this difference in the reactions to geotropic and other stimuli lies in the fact that in the latter case, energy is supplied from outside. But in geotropism the force of gravity is by itself inoperative; it is only through the weight of the cell contents that the stimulus becomes effective. Want of recognition of this fundamental difference has led many observers in their far-fetched and sweeping attempt, to establish an ident.i.ty of reaction of the root to geotropic and photic stimulations, in spite of facts which plainly contradict it.

Thus the root moves away from the incident vertical line of gravity; but under light, the root very often moves towards the stimulus. The negative phototropic response of the root of _Sinapis_ is an exceptional phenomenon for which full explanation has been given in page 376.

We shall next consider whether the particular distribution of the falling starch-grains (which offers a rational explanation of geotropic stimulation) in the shoot and in the root, is capable of furnishing an explanation of the different geotropic responses in the two organs. In this connection, the results of investigation of Haberlandt and Nemec are highly suggestive. Haberlandt finds statoliths present in the responding region of the stem; the geotropic stimulation of the stem is therefore direct. Nemec's investigation on the distribution of statoliths in the root show, on the other hand, that it is the central portion of the root cap that contains the falling starch grains, and this would account for the indirect geotropic stimulation of the root.

The theory of statoliths is, however, not essential for the explanation of the opposite geotropic effects in the shoot and in the root. The observed fact, that the perceptive region in the root is separated from the responding region, is sufficient to explain the difference of geotropic action in the two organs. Through whatever means the stimulus of gravity may act, it is inevitable, from the fact that the stimulation of the shoot is direct and of the root indirect, that an identical stimulus should in two cases induce responsive reactions of opposite signs.

It will thus be seen that the postulation of two different irritabilities in the shoot and in the root is wholly unnecessary and unwarranted by facts. For the irritability of the root has been shown to be in no way different from that of other organs; an uniformity is thus found to exist in the reaction of all vegetable tissues.

SUMMARY.

On subjection of the tip of the root to the stimulus of gravity, the upper side exhibits excitatory reaction of galvanometric negativity.

This shows that the root-tip undergoes direct stimulation.

The electric response in the growing region above the stimulated point of the root-tip is positive, indicative of increase of turgor and expansion. This is due to the effect of indirect stimulus.

The stimulus of gravity is perceived at the root-tip; it is the effect of indirect stimulus that is transmitted to the responding region of growth.

In contrast with the above is the fact that the growing region of the shoot is both sensitive and responsive to geotropic stimulus.

As the effects of direct and indirect stimulation on growth are ant.i.thetic, the responses of shoot and root to the direct and indirect stimulus must be of opposite signs.

There is no necessity for postulating two different irritabilities for the shoot and the root, since tissues in general exhibit positive or negative curvatures according as the stimulus is direct or indirect.