Life Movements in Plants, Volume II

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

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.

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 identity 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 antithetic, 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.

XLIII.—LOCALISATION OF GEO-PERCEPTIVE LAYER
BY MEANS OF THE ELECTRIC PROBE

It is true that post-mortem examination of sectioned tissues under the microscope enables us to form a probable hypothesis as regards the contents of certain cells causing geotropic irritation; we have thus the very illuminating theory of statoliths propounded by Noll, Haberlandt and Nemec. But for the clear understanding of the physiological reaction which induces the orientating movement, it is necessary to get hold, as it were, of a single or a group of sensory cells in situ and in a condition of fullest vital activity; to detect and follow by some subtle means the change induced in the perceptive organ and the irradiation of excitation to neighbouring cells, through the entire cycles of reaction, from the onset of geotropic stimulus to its cessation.

The idea of obtaining access to the unknown geo-perceptive cell in the interior of the organ for carrying out various physiological tests would appear to be very extravagant; yet I could not altogether give up the thought that the obscure problem of geotropic action might be attacked with some chance of success, by means of an electric probe which would explore the excitatory electric distribution in the interior of the organ. But the experimental difficulties which stood in the way were so great that for a long time I gave up any serious attempt to pursue the subject. And it is only when the present volume is going through the press that the very first experiments undertaken proved so highly successful that I am able to give a short account of the more important results, which cast a flood of light on the obscurities of geotropic phenomena. The new method has opened out, moreover, a very extensive range of investigation on the activities of cells in the interior of an organ, and enabled me to localise the conducting 'nerve' which transmits excitation in plants. These and other results will be given in the next volume.

METHOD OF EXPLORATION BY THE ELECTRIC PROBE.

The principle of the new method will be better understood if I first explained the steps of reasoning by which I was led to discover it. The experiments described in Chapter XL showed that the upper surface of a horizontally laid shoot exhibits sign of excitation by induced galvanometric negativity; that this was due to the stimulus of gravity was made clear by restoration of the plant-organ to the vertical position, when all signs of electric excitation disappeared. Now the skin of the organ on which the electrode was applied could not be the perceptive organ, for the removal of the epidermis did not abolish the geotropic action; the perceptive layer must therefore lie somewhere in the interior. As every side of a radial organ undergoes geotropic excitation, the geo-perceptive cells must therefore be disposed in a cylindrical layer, at some unknown depth from the surface. In a longitudinal section of the shoot, they would appear as two straight lines G and G´ (Fig. 174). In a vertical position the geo-perceptive layer will remain quiescent but rotation through +90° would initiate the excitatory reaction. Let us first centre our attention to the geo-perceptive layer G, which occupies the upper position. This sensitive layer perceives the stimulus and is therefore the focus of irritation; the state of excitation is, as we have seen, detected by induced galvanometric negativity, and the electric change would be most intense at the perceptive layer itself. As the power of transverse conduction is feeble, the excitation of the perceptive layer will irradiate into the neighbouring cells in radial directions with intensity diminishing with distance. Hence the intensity of responsive electric change will decline in both directions outwards and inwards.

The distribution of the excitatory change, initiated at the perceptive layer and irradiated in radial directions is represented by the depth of shading, the darkest shadow being on the perceptive layer. Had excitation been attended with change of light into shade, we would have witnessed the spectacle of a deep shadow (vanishing towards the edges) spreading over the different layers of cells during displacement of the organ from vertical to horizontal; the shadow would have disappeared on the restoration of the organ to the vertical position.

Different shades of excitation in different layers is, however, capable of discrimination by means of an insulated electric probe, which is gradually pushed into the organ from outside. It will at first encounter increasing excitatory change during its approach to the perceptive layer where the irritation will be at its maximum. The indicating galvanometer in connection with the probe will thus indicate increasing galvanometric negativity, which will reach a maximum value at the moment of contact of the probe with the perceptive layer.

It will be understood that the surface electric reaction under geotropic stimulus, which we hitherto obtained, would be relatively feeble compared to the response obtained with direct contact with the maximally excited perceptive layer. When the probe passes beyond the perceptive layer the electric indication of excitation will undergo decline and final abolition. The characteristic effects described above are to be found only under the action of gravitational stimulus; they will be absent when the organ is held in a vertical position and thus freed from geotropic excitation.

I have hitherto spoken of the excitatory effect of the upper layer; there must be some physiological reaction on the lower perceptive layer, though of a different character, represented diagrammatically by vertical shading. Had the physiological reaction on the lower side of a radial organ been the same as on the upper, geotropic curvature would have been an impossibility, for similar reactions on opposite sides would, by their antagonistic effects, have neutralised each other.

After this preliminary explanation, I shall give a detailed account of the experiments and results. It is to be borne in mind that the investigation I am going to describe presupposes no hypothesis of geotropic action. I start with the observed fact that an organ under the stimulus of gravity, exhibits responsive movement. I ascertain the nature of the underlying reaction by electric tests; I have, in my previous works, fully demonstrated that the excitatory contractile reaction is detected by electro-motive change of galvanometric negativity, and the opposite expansive reaction by a change of galvanometric positivity. With the electric probe I ascertain whether geotropic irritation is diffuse, or whether it is localised at any particular depth of the organ. I map out the contour lines of physiological reaction with its heights and depths of excitation.

I shall now proceed to describe the results of electric exploration into the interior of the organ. The trouble I foresaw, related to the irritation caused by the passage of the probe, and the after-effect of wound on variation of excitability.

THE ELECTRIC PROBE.

The wound-irritation is, however, reduced to a minimum by making the probe exceedingly thin. A fine platinum wire 0·06 mm. in diameter passes through a glass tubing drawn out into a fine capillary, and fused round one end of the platinum wire which protrudes very slightly beyond the point of fusion; the exploring electrode is thus insulated except at the protruded sharp point of the platinum wire. The length of the capillary is about 6 mm., just long enough to pass the experimental plant-organ transversely from one end to the other; the average diameter of the capillary is about 0·15 mm. The other end of the platinum wire comes out of the side of the tubing and is led to one terminal of the galvanometer, the other being connected with an indifferent point in the organ. The probe can be gradually pushed into the plant-organ by rotation of a screw head, one complete rotation causing a forward movement through 0·2 mm. (Fig. 175).

Wound-reaction.—I have shown that a prick acts as a mechanical stimulus, and in normal excitable tissues induces an excitatory change of galvanometric negativity. This wound-reaction increases with the extent of the wound, and the suddenness with which it is inflicted. On account of the fineness of the probe, it insinuates itself into the tissue rather than make any marked rupture; the probe again is introduced very gradually; with these precautions the wound-reaction is found to be greatly reduced. The immediate effect of the prick is a negative deflection of the galvanometer, which declines and attains a steady value in the course of about 5 minutes.

Effect of wound on excitability.—I have shewn (p. 81) that severe wound caused by transverse section induced a temporary abolition of irritability in Mimosa, but that the normal excitability was restored in the course of an hour. A prick from a thick pin was shown to depress temporarily the rate of growth, the normal rate being restored after an interval of 15 minutes (p. 202). In the case of geo-electric excitability, the depressing effect of the passage of the probe, I find, to disappear in the course of about 10 minutes.

For a choice of experimental material we have to find specimens which are not merely geotropically sensitive, but also exhibit large electric response under stimulus. In both these respects the shoot of Bryophyllum and the flower stalk of Nymphæa give good results.

ELECTRIC EXPLORATION FOR GEO-PERCEPTIVE LAYER BY MEANS OF THE PROBE.

Experiment 185.—I shall now proceed to give a detailed account of the experiments. The first specimen employed was the shoot of Bryophyllum, one contact being made with the side of the stem, and the other with an indifferent point on the leaf which was always held vertical. In a particular experiment, the probe was introduced into the stem through 0·4 mm. and a feeble galvanometric negativity was induced as the wound-effect. After an interval of 5 minutes, this attained a steady value of -15 divisions. On the rotation of stem through +90°, the point A was above and a very much larger deflection of -82 divisions was obtained, being the result of summation of wound and geo-electric effects. On restoration of the plant to vertical position the geo-electric reaction disappeared, leaving the persistent wound reaction of -15 divisions unchanged. The true geo-electric reaction at a point 0·4 mm. inside the stem was thus -67 divisions which is the difference between -82 and -15 divisions. I obtained in this manner the excitatory reactions at different layers of the organ. The following table gives true values of geo-electric reaction at different layers of the stem as the probe entered it by steps of 0·4 mm.

TABLE XL.—SHOWING THE GEO-ELECTRIC REACTION AT DIFFERENT DEPTHS OF THE ORGAN (Bryophyllum).

The results given above, typical of many others, show that there is a definite layer in the tissue which undergoes maximum excitation under the stimulus of gravity, and that this excitation irradiates with diminishing intensity in radial directions inwards and outwards.

The geo-perceptive layer may thus be experimentally localised by measuring the depth of intrusion of the probe for maximum deflection of galvanometric negativity.

Localisation of geo-perceptive layer in Nymphæa: Experiment 186.—I employed the same method for the determination of the perceptive layer of a different organ namely, that of the flower stalk of Nymphæa. The electric reaction in Nymphæa, even under the prevailing unfavourable condition of the season, was moderately strong, being about three times greater than in Bryophyllum. A dozen observations made with different specimens gave very consistent results of which the following may be taken as typical. The probe was in this case, as in the last, moved by steps of 0·4 mm. at a time. Other examples will be given later where readings were taken for successive steps of 0·2 mm.

TABLE XLI.—SHOWING THE DISTRIBUTION OF INDUCED GEO-ELECTRIC EXCITATION IN DIFFERENT LAYERS (Nymphæa).

It will be seen that as in Bryophyllum, so in Nymphæa, the geo-electric excitation increased at first with increasing depth of the tissue till at a depth of 0·8 mm. of the particular specimen the induced excitation attained a maximum value. The excitatory effect then declines till it vanished at a depth of 2·4 mm.

The depth of layer at which maximum excitation takes place varies to some extent, according to the thickness of the shoot. Thus while in a thin specimen of Bryophyllum 3·6 mm. in diameter the geo-perceptive layer was found at a depth of 0·6 mm., it occurred at the greater depth of 0·8 mm. in a thicker specimen, 5 mm. in diameter. In Nymphæa also the perceptive layer was found at a depth of 0·8 mm. in a thin and at a depth of 1·4 mm. in a thick specimen.

Having thus succeeded in localising the geo-perceptive layer by experimental means, it was now possible to examine the anatomical characteristics of the layer by examining it under the microscope. I also wished to find out from microscopic examination, the cause of certain differences noticed in the determinations of the perceptive layer in Bryophyllum and in Nymphæa. In the former the probe always encountered the maximally excited geo-perceptive layer from whichever point of the surface it entered the organ; this indicated that the sensitive layer in Bryophyllum was continuous round the axis. In Nymphæa, however, the probe occasionally missed the sensitive layer; but a new point of entry led to successful localisation of the perceptive layer; this was probably due to the particular layer not being continuous but interrupted by certain gaps.

MICROSCOPIC EXAMINATION OF THE MAXIMALLY EXCITED LAYER.

The specimens were taken out after the electric test, and the transverse sections made at the radial line of the passage of the probe. Thus in a particular experiment with Bryophyllum the point of maximum geotropic excitation was found to be at a distance of 0·8 mm. from the surface. By means of the micrometer slide in the stage and the micrometer eye-piece, the internal layer 0·8 mm. from the surface was examined; the particular sensitive layer S was recognised as the continuous 'starch sheath' or endodermis containing unusually large sized starch grains (Fig. 176). These often occurred in loosely cohering groups of 8 to 10 particles, and their appearance is very different from the small sized irregularly distributed grains in other cells.

Examination of the microscopic section of the flower stalk of Nymphæa showed that the 'starch sheath' was not continuous but occurred in crescents above the vascular bundles which are separated from each other. The occasional failure of electric detection of the perceptive layer is thus due to the probe missing one of the crescents, which with intervening gaps, are arranged in a circle.

I give below a number of experimental determinations of the geo-perceptive layer in different specimens together with the micrometric measurement of the distance of the 'starch sheath' from the surface, the transverse section being made at the place where the probe entered the shoot. Eight different determinations are given, three for Bryophyllum and five for Nymphæa.