Plant Response as a Means of Physiological Investigation

bud moving at a rate of '13 mm. per minute, ice-cold water was applied on the upper surface, by means of a strip of cloth, at the point marked in the figure by a downward arrow (I) (fig. 210). In consequence of this the movement is seen to be retarded, and in the course of five minutes it came almost to a stop. The cloth was now removed, at the Fig. 210. Effect on Apogeotropic Movement of Application of Ice-cold Water to Upper and Lower Surfaces alternately of a Horizontally laid Crinum Lily

The first part of the curve shows normal up movement, which is arrested in consequence of the application of cold on the upper surface, at the moment marked with a downward arrow (I). On removal of the application at point marked ( ^\ the normal apogeotropic movement is renewed, and continues unaffected by the application of cold below at the moment marked with an upward arrow ( t )• moment marked in the curve, and the movement of response to gravitation recommenced and tended gradually to attain its original value, with the return of the upper surface to the normal temperature. Ice-cold water was next applied on the lower surface, at the moment marked by an upward arrow (t), and it will be seen that this produced no perceptible effect on the rate of responsive curvature. A similar

experiment was performed with a long flower-scape of Uriclis Lily laid horizontally. The attachment to the recording lever was made with the upper end of the specimen, the lower end being held in a clamp. The specimen was 30 cm. long, and the responsive movement was found to be •23 mm. per minute (fig. 211). Here, too, ice-cold water was applied to the upper and lower surfaces alternately, four times in succession, and it will be seen from the figure that an application of cold to the upper surface caused arrest of

Fig. 211. Effect on Apogeotropic Movement of Temporary Applications . of Cold alternately to Upper ( | ) and Lower ( f ) Surfaces of Horizontally laid Scape of Uriclis Lily Application above is seen to produce arrest of movement, while application below has no perceptible effect. the responsive movement, while a similar application below produced no effect that could be detected. These experiments conclusively prove that the fundamental responsive effect induced by stimulus of gravitation is not acceleration, but contraction, or retardation of growth, precisely similar to the action of other forms of stimulus.

Though, under cooling, there cannot be any exhibition of mechanical response to gravitation, yet it appears that the effect of gravitation may be held latent in the organ, as will be seen from the following experiment. I took three long scapes of Uriclis Lily and laid them horizontally, packed in ice. As long as they were in the ice there was no responsive curvature. After an hour they were taken out of the ice and held erect, and by the time that they were restored to the temperature of the room it was found that the top of the scape in each case was bent from 1*3 to 1*5 cm. in the direction of what had been the upper surface when they were horizontal. This was evidently due to the fact that the cold brought about an arrest of growth ; but the geotropic stimulus remained latent, to express itself in a responsive movement later, when growth was renewed, under the action of a favourable temperature.

Responsive curvature of acellular organs. — There is one point, in connection with the induction of gravitational curvature, which might at first sight appear anomalous. In the case of the negative geotropic curvatures of multicellular organs, the fact that it is the upper side which is relatively effective, and that the curvature is the result of its responsive contraction and concavity, is evident from the experiments already described on the local application of cold. When we take an acellular apogeotropic organ, however, such as the stalk of the sporangium of Mticor, we find that it is the irritated lower side, differentially acted on by weight, whether of sap or statoliths, that becomes convex. This looks at first sight as if the effect of irritation were, as it is generally supposed to be, to induce acceleration of growth, and attendant convexity.

We must in this case, however, bear in mind the position of the surface on which the stimulus acts. In our experiment on the irritation produced by the pressure of magnetic particles it was the outside surface of the organ that was acted on by stimulus, and it was that side that became concave. In the case of the organ with a single row of cells as described, however, it is the internal surface which is so irritated, and if we take as our object of observation that internal surface, we shall find that, as in other cases, so here

also, the response is by contraction and concavity of the excited surface. The convexity of the outer is thus to be taken as the inevitable result of the concavity of the inner. The fact that in the acellular organ response actually takes place by the concavity to stimulus of the surface acted upon, is further seen in the response of such organs to stimulus of light. In the case of geotropic stimulus it is the internal surface of the lower side of the organ which is irritated by the differential weight of the cell-contents, and becomes concave to the stimulus thus acting upon it. In the case of light, on the other hand, stimulus acts from the outside, and it is thus the outer or external surface, say, of the same side, which becomes concave. Thus stimulus, acting on the same side in one case from within, and in the other from without, induces responsive curvatures in opposite directions.1

1 The explanation of the responsive curvatures of acellular organs has hitherto offered many difficulties. In a multicellular organ, acted on unilaterally by stimulus, there is a difference induced in hydrostatic pressure as between the two opposite sides. The diminished turgidity of the proximal, and increased turgidity of the distal, explains the induced curvature. In an acellular organ, however, there cannot be this difference of hydrostatic pressure on the two opposite sides. But the considerations which I shall now offer may perhaps be found to meet the difficulties of the case. The fundamental effect of stimulus is, as we know, to induce protoplasmic contraction. Hence unilateral stimulation acting on an acellular organ may be expected to induce contraction and concavity of the proximal side of the ectoplasmic layer, the result of which will be a curving over of the organ. As a result of this, the ectoplasmic layer of the distal side will be subjected to tension, which is, as we know, an influence that accelerates growth. Hence the retardation of growth on the proximal, due to contraction, and its acceleration on the distal under increased tension, will combine to produce growthcurvature.

A problem of somewhat greater complexity arises in the case of stimulus of light traversing a transparent acellular organ. Let us suppose such a vertical organ to be acted upon horizontally by rays of light from the right-hand side. We have in this case to consider the separate effects of stimulus of light on four different surfaces: (1) the outer ectoplasmic layer of the proximal side Vo; (2) the inner ectoplasmic layer of the proximal side Pz ; (3) the inner ectoplasmic layer of the distal side Dz ; and (4) the outer ectoplasmic layer of the distal side Do. The contractions of the outer surface of the proximal Vo, and the inner surface of the distal Dz, would induce a curvature to the right ; those of the inner surface of the proximal Pz and the outer surface of the distal Do a curvature to the left. But it is evident that as light passes through the organ there must be loss of intensity by absorption. Hence the sum of effective intensities at Vo and

Curvature of grass haulms under gravity. — We shall now take up the consideration of the curvatures induced in grass haulms, laid horizontally, when growth had originally been at standstill. It will be remembered that it was the appearance of curvature under such circumstances in the pulvinus of the grass haulm that gave the strongest support to the theory, that negative gravitational curvature in general was due to the increase in the rate of growth on the convex side, rather than to its retardation from active contraction on the concave. In the present case it was argued that since, at the beginning of the experiment, the upper side of the pulvinus was not undergoing growth, it was clear that growth there could not be retarded. The curvature, therefore, must be due to the induction of growth on the convex side, under the stimulation of gravity.

This misconception has arisen from the supposition that all curvatures must be induced by differential growth. I have shown, however, (i) that contraction takes place in a stationary organ in response to stimulus ; (2) that the unilateral stimulation of such an organ induces concavity ; and (3) that retardation of growth in a growing organ is itself the result of the contractile effect of stimulus. Now, in a horizontally laid grass haulm in which growth has ceased, the upper side — which we have found to be relatively the more effective — will contract under stimulus of gravity That this is the case is seen from the fact that this upper surface is found to become actually shorter than it was before. But, as regards the convexity of the lower surface, the water expelled from the actively contracting upper side will reach the lower, and the increased turgidity thus produced is sufficient, as we have already seen, to reinitiate growth in a dormant tissue. This explains the renewed growth and convexity of the lower side. Thus the curvature of the grass haulm cannot be held to support the

at D/ will be greater than the corresponding sum of effects at Vi and Do. As the result of this difference we shall have a right-handed or positive heliotropic curvature, view that the fundamental action of gravitational stimulus is to increase the rate of growth ; it shows, on the contrary, that contraction under stimulation is the active factor. Growth of grass haulms on a klinostat. — I shall here adduce certain considerations which may further serve to explain the difference between the stationary or feebly growing grass haulms, and other normally growing organs, as regards the effects induced in them by the rotation on the klinostat. It is found that in grass haulms, when subjected to the rotation of the klinostat, growth is recommenced, or increased ; while in other normally growing plants there is no such increase of rectilinear growth. For an explanation of this difference we have to recall the effect on growth of increased internal hydrostatic pressure, with the consequent increase of turgidity, which has already been described (p. 428). It was there shown that, when the plant was growing at a moderate rate, the curve of relation between increase of turgidity and increase of growth was practically a straight line ; that is to say, any increase or diminution of turgidity would then produce a proportionate increase or decrease of growth. But this relation did not hold good when the natural rate of growth was feeble or absent. In the latter cases, increase of pressure induced an effect that was disproportionately large. And this was specially the case when the growth of an organ had come to a temporary condition of standstill. In that case, when the pressure was gradually increased, growth was found at a certain point to be abruptly renewed, and to go on increasing with the increase of pressure, at a rate disproportionately large. If, now, the pressure be once more brought down to what it was just before the point was reached at which growth was started, we find that growth is not arrested, but persists. Thus the net result of these alternations of pressure is a positive resultant growth.

We have, thus, two distinct cases: (1) that in which the normal rate of growth is moderate, and in which alternate increase and diminution of pressure, acting for equal lengths of time, will induce equal increase and decrease of growth alternately, the total growth during the whole of the time being the same as it would have been without alternation ; and (2) that in which, the original rate of growth having been feeble or absent, equal alternations of pressure have the net effect of causing a positive increase of rectilinear growth. Now, in an ordinary growing plant, rotating on the klinostat, we have an instance of the first of these two cases ; for if at any given moment the side A be below, and the side B above, then B under stimulus of gravity will undergo contraction ; hence there will be a diminution of local turgidity, and the expelled water will produce an increase of turgidity on the opposite side A. At the end of a semirevolution of the klinostat, however, this state of things will be exactly reversed, and the alternating effects being thus equal and opposite, there will be no resultant increase of rectilinear growth ; but in the second case — that is to say, of stationary or feebly growing grass haulms — the resultant effect is not nil, but a positive increase of rectilinear growth.

It has been shown that the effect of gravitational stimulus on an apogeotropic organ is fundamentally the same as that of any other form of stimulus, namely, a responsive contraction. A rational explanation of the mode in which geotropic stimulus acts, is afforded by the radial-pressure theory of Pfeffer and Czapek, or by the statolithic theory of Noll, Haberlandt, and Nemec — the essential element of both lying in the hypothesis that stimulus is caused by means of the weight of the cell-contents acting differentially on the inner wall of the horizontally placed cell.

That the unilateral pressure of particles is competent to induce responsive curvature of the organ, has been shown experimentally by pressure resulting from the magnetic attraction of iron particles. It has generally been supposed that the active factor in apogeotropic curvature was the accelerated rate of growth on the convex side of the organ ; but it has here been shown by crucial experiments on the unilateral application of cold, that the active factor is really the responsive contraction and retardation of growth on the concave side.

Difference between shoot and root in their response to stimulus of gravity — •Difference in character of response between tip and growing region of root — Scope of the investigation — Electrical investigation — Responsive results of: I. Longitudinal transmission of effect of stimulus from tip ; {a) Moderate unilateral stimulation ; (b) effect of stronger unilateral stimulation— 2. Direct unilateral stimulation of growing region— Moderately strong stimulus— 3. Transverse transmission of effect of stimulus ; (a) moderate stimulation ; (b) stronger stimulation — Mechanical response inferred from observed electrical response — Tabular statement.

We have seen in the last chapter that the responsive effect of gravitational stimulus in an apogeotropic organ is of the same nature as that of any other form of stimulation. In positively geotropic organs like roots, however, this would seem not to be the case, for here the responsive curvature is in the opposite direction. Thus, a root placed horizontally bends in the direction of gravity, and not away from it. It may be urged that there is some polar difference between shoot and root, on account of which, if the response of the one be regarded as positive, that of the other must be negative ; but I have shown that, so far from this being the case, the response of the root to stimulus is precisely the same as that of all other organs, the shoot included, for all alike under stimulation exhibit' contraction (p. 76).

We shall first see, then, whether in the root there is any difference, as regards the action of gravity, from, for instance, the stem. We know that the growing stem with regard to the gravitational stimulus is both the perceptive and responding organ ; for we may cut and isolate any portion of it, ! and it will still show an apogeotropic curvature. But the case is quite different with the root. Here the perceptive organ and the responding organ are, as will be seen in the I next chapter, distinct and separate from one another.

Difference in character of response between tip and growing region of root. — We shall next turn our attention to the peculiar characteristics of the tip of the root as distinguished from the responding region of growth. Darwin, on applying moderate artificial stimulus unilaterally to the tip of the root, found that it moved away from the source of stimulus ; whereas Sachs, on applying similar unilateral stimulus to the responding growing region directly, found that it moved towards it. Thus we see that two different effects are induced, according as stimulation is applied on the responsive zone itself, or transmitted to it from the distant tip ; and in this fact we may perhaps find a clue to the explanation of the opposite effects produced by stimulus of gravity on positive and negative geotropic organs.

No explanation has yet been offered of the opposite characters of these responsive effects induced by similar stimuli, according as they are applied on the responsive zone or on the tip of the root. It was suggested by Darwin that 'the tip of the radicle is endowed with diverse kinds of sensitiveness ; and that the tip directs the adjoining growing parts to bend to or from the exciting cause, according to the needs of the plant' l These diverse kinds of sensitiveness have in his view been acquired by the tip of the root, for the final advantage of the plant.

The question, then, which we must investigate is, as to whether the peculiar sensitiveness of the tip has been specially evolved by the burrowing root, or is characteristic of the tips of growing organs in general. Should the latter prove to be the case, we have next to account for this characteristic itself. I have already suggested, as a possible explanation of the difference between the responsive actions in apogeotropic and geotropic organs, that in the one case stimulus acts

directly, producing a movement towards, while in the other the effect of stimulus is transmitted from a distance, producing a responsive movement in the opposite direction. As against this assumption, however, we are confronted with the fact, which we shall find later, that in some instances of transmitted stimulus of light, the responding organ bends towards, and not away from, the source of stimulation. Scope of the investigation.— We have therefore to determine (i) whether or not the tips of all growing organs behave alike ; (2) why the behaviour of the tip is different from that of the responsive zone of growth ; and (3) under what circumstances the transmitted effect of stimulus causes an organ to move towards, and under what, to move away from, the source of stimulation, at the same time ascertaining clearly the mechanics of such movements. For the purpose of this investigation I shall first use the electrical method of inquiry, which I have already fully described, since it has the unique advantage of offering unerring indications under the most difficult experimental conditions ; and shall study by its means the characteristic differences of response as between the tip and growing region of a single organ, in this case the shoot.

Electrical investigation. — It will be shown that growthcurvature is produced by unequal variations of turgidity on the two sides of the responding organ. This variation can be detected with great certainty by electrical means. I have already explained how a positive turgidity-variation gives a concomitant electrical variation of galvanometric positivity ; and that the true excitatory effect of negative turgidityvariation gives rise to a concomitant electrical change of galvanometric negativity. For the present experiments I took specimens of various growing plants — such as Bryopkyllum, Cucurbita, and others — and the results obtained from all were alike. In order to obtain unmistakable indications of the effect produced in the responding zone, one electrical connection was made at a point in the growing region, A (fig. 212, a), and the other with a point so distant

that the effect of stimulation could not reach it. As parenchymatous tissues offer great resistance to the conduction of stimulus, it is an advantage to make this second contact with a leaf. In the present investigation we have to study the effect of unilateral stimulation of varying intensity and duration, when applied either at the tip T, or at C, near the responding point A, the latter being in the same longitudinal line as the excited points T and c. We shall also study the effect of stimulation of A, on the transverse point

Fig. 212. Experimental Connections for obtaining Electrical Response due to Direct and Indirect Effects of Stimulation (a) electrical connections for detection of electrical changes at A, caused by indirect effect of stimulus, from excitation of same side of distant tip, T, and direct effect of stimulus from excitation of adjacent point, c ; {b) electrical connections for detection of electrical changes at B, caused by excitation of diametrically opposite point, A ; (c) electrical connections for detection of relative electric variations of diametrically opposite points, A and B, due to excitation of c, adjacent to A.

B (fig. 2 1 2, b\ and* the resultant effects on A and B, when the point C, near A, is stimulated (fig. 212, c). Before proceeding further, it will be well to consider the theoretical conclusions to which we are led from the demonstrations already made of the turgidity-variations caused by stimulation. The tip T consists, as we know, of undifferentiated tissue, which is a relatively bad conductor of stimulation. Moderate stimulus, then, at T, might be expected to induce local excitatory contraction, and the water thereby expelled would originate a wave of positive turgidity-variation ; this would reach A, a point on its own line, with greater effect than the transversely placed B. The moderate stimulation of T would thus produce a positive turgidity-variation

at A on its own side. This transmitted effect of increased turgidity we have already distinguished as the indirect effect of stimulation. But all cells conduct stimulus more or less efficiently, the difference being one only of degree. Hence an indifferently conducting tissue, such as that of the tip, will conduct the true excitatory state only if the stimulation be sufficiently strong and long continued. The transmitted effect of this true excitation may then be expected to produce negative turgidity-variation, and concomitant galvanometric negativity, at A.

When the stimulus is applied, however, at or near the responding point A, we may expect to obtain the direct effect of stimulation — that is to say, a negative turgidity-variation and the concomitant galvanometric negativity. To sum up, then, it may be expected that moderate unilateral stimulus applied at the tip T will give rise on the same side of the responding region to the indirect effect of stimulation, which is an increase of turgidity exhibited by a positive electrical variation. The direct effect of stimulus, whether immediate or transmitted, always produces a negative turgidity-variation, evidenced by galvanometric negativity. This effect may be obtained either by the local application of moderate, or by the distant application of strong, stimulus.

In making the electrical investigation 'which is now to be described, I employed various forms of stimulation — thermal, mechanical, chemical, and the stimulus of light. Mechanical stimulus may be applied by friction of emery paper, or by means of a pin-prick. Chemical stimulation is applied by touching the point with a brush which has been moistened in hydrochloric or sulphuric acid. Very dilute acid produces moderate, strong acid a more intense, stimulation. The most perfect mode of stimulation is by means of incident light, the intensity of which may be varied at will. The effect ot stimulus of light, however, will be fully described in the chapter on heliotropism. Another form of stimulation which is also very suitable is the thermal. A short piece of platinum