Plant Response as a Means of Physiological Investigation

Unilateral light exerts a directive action on responsive swimming movements, because the only stable position is one in which pairs of cilia are equally excited. The axis of the organism is thus orientated till parallel with incident light. Strong stimulation of light, for reasons described, causes a reversal of the normal movement. Alternate periodic fatigue causes a responsive movement of the organism to and fro. Under moderate unilateral thermal stimulus the organism exhibits a positive swimming movement, and under stronger a negative movement.

Similar responses to stimulus are exhibited under galvanic excitation. The normal response here also undergoes reversal under long-continued stimulation ; since the actions of acids and alkalis are antagonistic, organisms reared under acid and alkali cultures tend to exhibit opposite reactions under galvanotaxis. Under chemical stimulation, swimming organisms exhibit multiple responses. Acid and alkali, owing to their antagonistic actions, bring about opposite responses. The same reagent again occasions opposite responses, according to the amount of the dose. Thus antherozoids of ferns are attracted by the dilute solution of a malate, but stronger solutions exert a repellent action.

Responsive contraction — Kunchangraphic records— Direct and indirect effects of stimulation— Various forms of responsive expression; (a) Lateral motile response by differential contraction; (d) Suctional response; (c) Growthresponse ; (d) Torsional response ; (e) Death response ; {/) Thermographs of regional death ; {g) Electrical response— Different types of response : {a) Uniform response ; {b) Fatigue ; (c) Staircase response— Excitability- Conductivity— Polar effects of currents— Multiple response— Continuity of multiple and autonomous responses — The ascent of sap.

We have now studied in detail the manner in which a plant reacts to the varied forces of its environment, and although its response to them finds modes of expression which appear highly diverse, yet we have found that on analysis these are all reducible to two very simple and well-defined factors, of responsive contraction and expansion. Responsive contraction.— In vegetable cells, as in other protoplasmic bodies, it has been shown that the impact of all external stimulus evokes responsive contraction ; and this we found to be true, not in the so-called sensitive plants alone, but in all plants. Taking the simple case of a radial vegetable organ, such as a stem, style, stamen, or other filamentous structure, we find that it undergoes longitudinal contraction under stimulation. Here, then, we have a phenomenon which is analogous to the contraction of muscle, The amount of contraction of these ordinary vegetable organs, further, is sometimes very considerable, as we saw in the case of the coronal filament of Passiflora, where it was as great as 20 per cent, of the original length. Such responsive contraction takes place, moreover, under all forms of stimulation, mechanical, thermal, electrical, photic, and chemical ; and we have found that all the various move-

merits of plants which are seen in nature, under the action of external stimulus, are but different expressions of a single fundamental response by contraction. Kunchangraphic records. — By taking advantage of this responsive contraction, we are able to study all the physiological modifications induced in the vegetable tissue by various reagents, with as great ease and certainty as similar phenomena can be studied in animal muscle, using myographic records. By such study, again, we are once more led to see how misleading has been the superficial distinction between sensitive and non-sensitive plants, since the latter, or so-called ordinary, plants also exhibit contraction under stimulation.

Direct and indirect effects of stimulation. — The living organism is thus a delicately responding machine, whose responsive movements are brought about by external stimulus ; but this complex machinery has also the power of holding part of the energy of the external stimulating shock latent, for a longer or shorter time, so that part only may find immediate expression, while the rest is stored up as internal energy to be given out after the lapse of an intervening period. These two factors, of external stimulus and internal energy, again, induce opposite effects, of contraction and expansion respectively. And the infinite multiplicity of responsive processes in the life-cycle of the plant is brought about by their mutual play. That the combination of these two elements in varying degrees of each, finding expression in different ways, creates a tangle which would at first sight appear inextricable, can be easily understood. And it was the bewilderment which this fact imposed upon the observer, that drove us to postulate the existence of an unknown and indefinable vital force, whose mysterious working was to be held to account for the occurrence of all those phenomena that we were otherwise unable to explain.

It is possible, however, as we have found, going back step by step, to trace out the different expressions of these two distinct factors, of external stimulus and internal energy, and to show, moreover, how the latter may be derived from the former. This may be clearly and easily seen by taking a simple case, in which we excite the stem by the application of external stimulus, on a zone a few centimetres below its upper end. As the direct effect of this stimulation, a contraction takes place in that zone. By this contraction an active expulsion of water, with local negative turgidity-variation, is brought about, and the expelled water is driven outwards in both directions from the excited zone. Following the course of the water which is thus forced upwards, we have found that it produces an increase of turgidity, or positive turgidityvariation, with consequent distension or expansion of all the cells above the stimulated area. Work is thus performed on these cells, in consequence of external stimulus, by which their latent energy is increased.

The energy of the stimulus applied in one has thus been conveyed to another region, hydraulically, there to give rise to an effect of increased turgidity and expansion, which was designated as the indirect effect. Thus the effect of external stimulus is seen to be twofold— namely, first by its direct action to induce local contraction, and secondly by its indirect effect, of increasing the internal energy, to bring about an expansion. The expressions of direct and indirect stimulation are thus seen to be opposite in character, and we have seen how they find opposite modes of expression in the case of each form of response.

Various forms of responsive expression. — The next question to be passed in review is that of the various modes in which the response of the plant was seen to find expression ; and here we found various responsive phenomena which are characteristic of life, and apparently entirely unrelated, to be ultimately dependent upon this fundamental inter-action between, on the one hand, the contraction due to external stimulus, and on the other the expansion due to internal energy.

(a) Lateral motile response by differential contraction. — Taking first the responsive mechanical movement of motile organs, we find that its gradual evolution can be traced from the simplest case, namely, the longitudinal response of a radial organ. In such a radial organ, if one side be rendered in any way the less excitable — say by the unilateral application of cold or anaesthetics, or by the fatigue induced by long-continued unilateral stimulation— we have an induced anisotropy. On now subjecting the organ to diffuse stimulation, the relatively more excited side will undergo the greater contraction, and the response will thus be by that movement which results from the concavity of the more excited. We find many instances again of the various stages through which this anisotropy passes before it culminates in the dorsi-ventral pulvinus ; thus, for example, in a spirally formed tendril, where concavity has been induced by the unilateral excitation of that side, the concave surface is less excitable than the convex ; and such a tendril, when diffusely excited by strong electrical stimulus, exhibits the excitatory effect by extraordinary writhing movements due to the relatively greater excitation and concavity of the originally convex side (p. 92). A plagiotropic stem, again, whose upper side is fatigued by the action of sunlight, exhibits on diffuse stimulation a downward movement, due to the greater excitatory contraction of the more excitable lower half (p. 86). The phenomenon of differential response, then, whose various preliminary stages we have thus traced, comes to its greatest perfection in the dorsi-ventral pulvinus of such plants as Mimosa, for we have found that in the last-named organ the characteristic responsive movement is not brought about by the action of excitable cells restricted to the lower half. That the upper half also is excitable is shown by the fact that on applying to it localised stimulus, say of light, its cells contract and raise the leaf (p.

63 1). The fall of the leaf, on .the application of diffuse stimulation, is thus the result of the greater excitability of the lower. This fall, agajn, is not due to mere flaccidity caused by the expulsion of water from the excited organ, but to the actual differential contraction of the lgwer half, whose activity may be gauged by the tension it can be made to exert on a spiral spring (p. 26). As regards mechanical response, then, a single law— that response is always by concavity of the more excited side— will be found universally applicable. In a dorsi-ventral organ, like the pulvinus of Mimosa, we have seen that response to diffuse stimulation is always by the contraction of the more excitable lower half. The same law, however, is also applicable even in the case of a radial organ excited unilaterally, for here the side acted on is relatively the more excited, and response is by concavity of that side.

A radial organ, acted upon from one side, undergoes concavity of that side, and consequent movement towards the stimulus. The same is true of pulvinated organs, when either the upper or lower half is acted upon locally by stimulus. As in radial organs, then, so also here, under these circumstances, we obtain instances of the directive action of stimulus ; but when stimulus is either internally or externally diffused, we obtain from a dorsi-ventral pulvinus the greater contraction of the more excitable half, and movement is thus made, for anatomical reasons, to occur in the direction at right angles to the plane which separates the two anisotropic halves.

Even in Mimosa, the contraction of the pulvinus itself is not very great, but the contractile movement is highly magnified by the attached petiolar index. When a radial organ is diffusely stimulated there is no lateral movement at all, and from this fact it has been erroneously inferred that ordinary plants are not sensitive. In fact, however, every plant is sensitive, and exhibits contractile response, which is shown, in the case of radial organs, by longitudinal contraction under diffuse stimulus, and by lateral response under unilateral stimulus.

The direct effect of stimulus on the pulvinus of Mimosa, causing a negative turgidity-variation of the organ, finds appropriate expression in the responsive movement of fall. But we have seen that the expression of the indirect effect of stimulus is opposite in character to that of the direct effect. The indirect effect of stimulus, or the increase of internal energy, thus results in an erectile response, as seen in the erection of the leaf in Mimosa, or Biophytum, when their internal energy is in any way enhanced, say by rise of temperature (p. 400). Taking all kinds of response, it will be found universally true that if the increase of internal energy give rise to one form of expression, the impact of external stimulus evokes the opposite.

(&) Suctional response. — It has been shown that contractile response gives rise to a forward impulsion of water ; hence by the excitatory contraction of the root-cells the movement of water upwards is initiated. When such contractile movement is not single, but repeated or multiple, continuous propulsion of water is maintained. The rate of this propulsion therefore affords us a means of measuring the rhythmic activity of the plant-tissue. (c) Growth response. — This pumping-in of water causes the transmission of energy to the distant growing region, and as by this the internal energy of the plant is increased, it finds expression there in a pulsatory expansion, which is the movement of growth. Following each pulse of expansion, there is a recovery which is incomplete, owing to fixation of growth-material. The resultant growth is thus the irreversible effect of the entire process. This growth-movement is another expression of that indirect effect of stimulation which we have already considered, by which a distant excited point gives rise to a progressive train of waves of positive turgidityvariation. The action of these waves may be seen not only in the movement of expansion at the growing point, but also by the erectile response of an intervening motile organ. This will be understood from the following diagram of an artificial plant (fig. 277), which shows how contractile action at the base, giving rise to an hydraulic wave, causes two different expressions of (a) motile response of the lateral organ, or leaf, and (b) growth-expansion of the terminal growing point. The pulvinus of this artificial motile organ consists of an

india-rubber tubing, the lower half of which is thinner, and The upper end of the tube, representing the end of the plant, is closed over with a thin elastic strip of india-rubber. When We further see in the case given that growth-response is not, strictly speaking, ' of its own accord ' or spontaneous ; for the energy of a definite stimulus, causing contractile movement at the base, is transmitted hydraulically, and performs the work of growth. It would be as accurate to describe the work done by hydraulic machinery as spontaneous, ignoring the energy that had set the pump in action, as to call growth

Sudden compression of indiarubber, bulb, r, causes hydraulic wave upwards, downward movement being prevented by valve v. This wave of increased pressure causes erection of the leaf, L, and the expansion of the terminal septum into G. spontaneous, without recognising or tracing that energy which in the form of stimulus must have been supplied to some part of the plant machinery. We have seen that a plant in a state of growth-standstill has its activity renewed when a stimulus is applied to the distant root, and that when the amount thus supplied is exhausted, the activity ceases. In growth, then, which is regarded as so characteristic a phenomenon of vital action, we see the law of the conservation of energy holding good, as in an ordinary inorganic system. The plant thus expresses the absorbed energy, by a responsive expansion, either of erection or of increased rate of growth, according to the particular organ of response which is concerned, both of these constituting cases of work done by internal energy, or indirect effect of stimulus ; but where the responding organ is directly excited by external stimulus, we obtain a contractile response of the organ, with expulsion of water, or negative turgidity-variation. The hydraulic current in our model is now reversed, and opposite responsive movements take place, by the fall of growth below the normal rate, and by depression of the leaf. If now we take a balanced record of growth, the impact of a uniform series of external stimuli will be found to give rise to a series of responses by depression of the rate, followed by recovery, exactly similar to those records of depressions of the leaf, with recoveries, which are obtained from pulvinated organs.

We may also see the expression of external stimulus and cessation of stimulus, in the appropriate periodic variations of growth, and movements of motile organs, which occur under the stimulating action of daylight, and the withdrawal of such stimulus at night. Thus in the daytime we see a response consisting of depression of the rate of growth, which corresponds to the depression of the leaf, say of Mimosa, and at night a recovery, or enhancement of growth, and erection of the motile organ. The daily periodic curves obtained of this growth-variation and responsive mechanical movement are very similar.

(d) Torsional response.— Another interesting type of response occurs when an anisotropic or dorsi-ventral organ is stimulated laterally. Under such conditions, a responsive torsion is induced, by which the less excitable side is made to face the external stimulus. The extent of the response depends on the intensity of stimulus, and the differential excitability of the anisotropic organ. In dorsi-ventral organs, the plane of anisotropy is fixed. But in certain climbing plants, this plane revolves in a positive or negative direction, and under the internal activity of growth, an autonomous torsional movement is thus observed. The opposition of the effects of internal energy and external stimulus is here seen, when such an organ is uni-laterally acted on, say by light. The autonomous torsional movement is then found to be retarded, or even reversed.

(e) Death response. — There is another curious phenomenon of response, which takes place at a certain definite point. When an organ is gradually raised in temperature, the internal energy is increased, and the organ exhibits a responsive expansion, if radial by elongation, or if pulvinated by erection ; but when the death-point is reached, a sudden and irreversible molecular change takes place, attended by an excitatory contraction. In the curve of thermo-mechanical response we here find a sharply defined point of reversal, which affords us an exact index of the death-point. This death-point is very definite in plant-organs under normal conditions ; in phanerogamous plants it is very near 6o° C. Physiological modification of the tissue, moreover, may be gauged by the transposition of the otherwise definite deathpoint (p. 185).

(/) Thermographs of regional death. — Just as there is a definite point of reversal in the thermo-mechanical curve, so there is also a point of discoloration which is, under standard conditions, at a determinate interval from the death-point. A particular region, physiologically changed, may thus be thermally 'developed/ and made to exhibit as a thermograph, a picture of localised physiological variation (p. 184). (g) Electrical response. — We have, lastly, to consider briefly the electrical mode of responsive expression. The excitatory contraction, with negative turgidity-variation, of a vegetable, as of an animal tissue, is accompanied by an electrical variation of galvanometric negativity. The indirect effect of stimulus with its positive turgidity-variation, moreover, has also a concomitant electrical expression of galvanometric positivity (p. 37).

Different types of response : (a) Uniform response. — These various expressions of response are brought about by the fundamental molecular change induced by stimulus. When stimulus is moderate, and sufficient time allowed for recovery from molecular distortion to the original condition, a series of responses to uniform stimuli will be uniform ; but if very strong stimulus be applied, recovery will only be completed after a long interval. (b) Fatigue. — If successive stimuli be applied before complete recovery has taken place, the successive responses will exhibit diminution, or fatigue. Under strong and longcontinued stimulation the plant-tissue exhibits, as in the case of tetanised muscle, a fatigue-reversal — that is to say, the contracted tissue passes into what is apparently its original expanded condition ; but the difference between the normal condition and the condition of fatigue-reversal is seen in the fact that, while the former is sensitive to fresh stimulation, the latter is insensitive. The fatigued tissue, however, resumes its original excitability after a period of rest. This fatigue-reversal explains the erection of the Mimosa leaf under continuous stimulation (p. no). We observe similar fatigue-reversals, even in inorganic substances like india-rubber, where the normal contraction under thermal stimulation passes into relaxation under the long-continued action of such stimulation ; and the india-rubber becomes sensitive again only after a sufficient period of rest (p. 120). In connection with this, we sometimes meet with the very curious case of alternate or periodic fatigue, both in living and in inorganic substances. The simplest type of this

occurs when, under uniform stimuli, responses are alternately large and small. These alternations sometimes show themselves in groups. Under continuous stimulation, again, this periodic fatigue exhibits itself by response of an oscillatory character. (V) Staircase response. — The tissue is sometimes in a relatively sluggish condition, and by the absorption of stimulus the molecular mobility is gradually increased. The effect of this is seen in the amplitude of successive responses, increasing in a staircase manner.

It is usually supposed that response is brought about by a chemical run-down of energy, of an explosive character. The external stimulus is thus supposed to act as it were on a trigger, to release the latent energy. The response is hence assumed to be disproportionately larger than the stimulus. That this cannot, however, hold good in all cases is clear ; for the tissue is often found to absorb a certain proportion of the incident stimulus, the immediate expression of response being thus disproportionately smaller than stimulus. The energy of the system is now found, instead of being lowered, to be raised above par. The internal energy thus held latent is sometimes seen, as in the case of strongly stimulated Biophytum, to find expression later by multiple response. In the case of growth-response, again, it is a variable fraction of incident stimulus that finds immediate expression in the direct effect of retardation of growth, whereas the absorbed component gives rise to the subsequent responsive effect of an enhanced rate of growth. Referring to the former of these as the direct, and to the latter as the indirect, effect of stimulus, it is found that the sum of the two remains approximately constant. Below the optimum tonic condition it is found that the indirect effect is relatively the larger, but near the optimum this relation is reversed, and the direct effect is the larger. In a sub-tonic condition stimulus produces little or no direct effect, it being utilised to produce the indirect effect of enhanced growth. At the optimum, on the other

hand, practically the whole of the incident energy is expressed in direct response, there being little or no absorbed element (p. 460). A similar series of considerations may be applied to the response of mature pulvinated organs. In this case, however, the indirect effect of stimulus will find expression in an enhancement of the rate of recovery of the organ. It is, however, difficult always to discriminate with certainty between the natural and an enhanced rate of recovery ; but on turning to growth-response we find that, by using the balanced method of record, it is easy to distinguish between the direct and indirect effects of stimulus, since these are here shown by curves in opposite directions. This method moreover affords us some means of measuring the relative magnitudes of the two factors in the response.

Excitability. — It is interesting to find that an agency which induces a variation of excitability produces a similar modification of all the different forms of response. In this way the long-continued application of cold has the effect of lessening excitability, and the response of a motile organ is thus found to be temporarily diminished or abolished. At the moment of application, however, owing to the fact that any sudden variation of environment acts as a stimulus, its effect is the induction of an excitatory movement. These phenomena are repeated with curious exactness in the case of suctional response. On the application of ice-cold water to the root, the immediate effect is a transient exaltation of suction, followed later by depression and arrest (p. 375). In growth-response, also, growth is diminished or arrested by this agency.

Another effect of the moderate application of cold is to induce a molecular sluggishness by which the latent period is increased. A moderate rise of temperature, on the other hand, increasing the molecular mobility, has the contrary effect, of reducing the latent period. Anaesthetics, again, induce a diminution or abolition of excitability, as is seen by their effect on the various forms of response. The excitability of a tissue which has not recovered fully from previous strong stimulation is found to be impaired. The fatigue-effect only disappears after the lapse of a period of rest. If, then, the resting intervals between successive stimuli be gradually shortened, the motile responses will be found to be progressively diminished, and a time arrives when the succeeding stimulus evokes no response, the tissue having become as it were refractory. The minimum interval during which the tissue remains thus irresponsive is known as the refractory period. In Biophytum, under normal conditions, this period is about ten seconds in duration (p. 273).

Conductivity. — It is usually supposed that the transmission of the excitatory effect, as seen in sensitive plants like Mimosa, is merely the transmission of a hydromechanical disturbance, and therefore unlike the transmission of the excitatory effect in animal tissues. It has, however, been shown that this is not the case, for here, as in the animal, transmission of excitation takes place by the propagation of protoplasmic changes. It was shown further that the hydrostatic disturbance was transmitted with a relatively great rapidity, whereas the true excitatory effect has a slower and definite velocity, characteristic of the particular specimen. This velocity, moreover, is found to be modified, and that in a manner precisely similar, by all those agencies which modify the velocity of transmission of excitation in animal tissues. Thus, cold, anaesthetics, and fatigue are all influences which reduce the velocity of transmission. As an example of this, we saw, in a certain specimen of Biophytum, in which the normal velocity was 38 mm. per second, that slight cooling reduced it to 1*3 mm. per second, or almost to one-third. Conversely, the raising of the temperature from 300 C. to 370 C. increased the velocity from 37 to 91 mm. per second. Strong stimulus is found to be conducted further and more rapidly than feeble or moderate. It is found in the case of animal tissues, again, that on account of its physiologically depressing effect, the anode acts as a block to the transmission of excitation ; and the

same statement holds good in the case of the plant. The passage of mere hydro-mechanical disturbance could not have been affected in any way by the anode. Nor is this transmission of excitation confined to sensitive plants alone. The fact that it occurs in all plants alike I have been able to demonstrate by various other methods, in using which we are rendered independent of the motile indications afforded by lateral leaflets. Thus by the Electrotactile Method we are enabled to detect, in any zone of the plant, the moment of arrival of the state of excitation from a distant point (p. 259). The Electromotive Method, again, displays the moment of arrival of the wave by the induced galvanometric negativity of the point (p. 261). The Chemical Method, again, shows the arrival of the wave of excitation by the projection of a dense precipitate, produced in a suitable solution (p. 255). All these different methods give us results which are in mutual agreement.

As showing how ill founded is the common distinction as between sensitive and ordinary plants, it was demonstrated that the velocity of transmission in some of the latter is greater than in the former. Thus in Ficus religiosa the velocity was determined at 9/4 mm. per second, whereas in the 'sensitive' Neptunia oleracea it was only ri mm. per second. The velocity of transmission of the excitatory impulse in plants is found, again, to be of the same order of magnitude as in the nerves of lower animals (p. 252).