Plant Response as a Means of Physiological Investigation

I have shown further that excitation is transmitted in the plant in both directions. It is, however, interesting to note that, generally speaking, the facility of this transmission is greater centrifugally — that is to say, in the direction of the ascent of sap — than centripetally. Thus, for example, in a petiole of Biophytum, while the centripetal velocity was i*8 mm. per second, the centrifugal velocity was 3 27 mm. per second. Since conduction of stimulus takes place by transmission of protoplasmic change, such change is naturally conducted

most easily along those paths in which there is most protoplasmic continuity. Parenchymatous tissues, in which the cells are divided from each other by more or less complete septa, are thus relatively inefficient as conductors of the excitatory effect to a distance. Hence, the lamina of the leaf does not transmit its local excitation to any distance ; but tissues which contain fibro-vascular elements, and which are thus characterised by a greater protoplasmic continuity, are therefore better conductors of excitation. Thus stems, petioles, and peduncles are better conductors than the laminae of leaves, and parallel-veined leaves, again, are better in this respect than reticulated. As regards stems, petioles, and peduncles, moreover, the conducting power is greater longitudinally than transversely. In the peduncle of Musa, for example, I find the conductivity lengthwise to be three times as great as that crosswise. In consequence of this difference, it is found that the transmitted excitatory effect of a stimulus unilaterally applied is greater on the same than on the opposite side. This explains the appearance of responsive concavity at a distance from the point of stimulation, but on the same side.

It is to be remembered that, owing to the fact that this conduction of the effect of stimulus is an excitatory process, we find that in autumn and winter, when the physiological excitability is low, the conducting power of the tissue is also very much reduced. Various degrees of conductivity are possessed by different tissues, and the distance to which the excitatory effect is conducted depends not only on the conducting power, but also on the strength and duration of stimulus. Thus, while in a feebly conducting tissue the effect of moderate stimulation is not transmitted to any distance, strong and longcontinued stimulation is transmitted to a certain extent. Even in a better conducting tissue the excitatory effect of a moderate stimulus, on account of gradual enfeeblement, can only reach up to a certain distance. It is to be borne in mind that, while no tissue is absolutely non-conducting,

neither is any a perfect conductor, the difference between extreme examples being one only of degree. The true excitatory effect, whether due to direct or transmitted excitation, consists, as has been shown, of contraction, with concomitant negative turgidity-variation. Theresult of this contraction and concomitant expulsion of water is, however, the sending out of a wave of positive turgidityvariation. Thus, up to the point reached by the true excitatory effect, we obtain contraction, with negative turgidity-variation; and beyond this point, a positive turgidity-variation, with consequent expansion. This latter effect we have designated as the indirect effect of stimulus. It is thus seen that, whereas the direct effect of unilateral stimulus is a concavity, its indirect effect is a convexity.

We have also seen that it is possible by electrical means to determine whether it is the direct or indirect effect of stimulus which has in any given instance reached a point from a distance ; for the indirect effect of stimulus, with its positive turgidity-variation, is always attended by galvanometric positivity, whereas the true excitatory effect with its negative turgidity-variation is characterised by galvanometric negativity. A tissue may conduct without exhibiting any motile indication of its state of excitation. With reference to this it is to be borne in mind that certain advantageous circumstances are necessary for the display of motile response ; for since the fall of an excited leaf, such as that of Mimosa, takes place in consequence of the expulsion of water, it follows that when this is in any way impeded, as by overturgidity of the tissue, there may be excitation without any responsive movement ; for this reason, the leaflets of Biophytum, in the morning, when they are most tense, are not so sensitive as later in the day. Motile excitability is as a rule found to be abolished earlier than conductivity ; hence a strong stimulus may be conducted through a region which exhibits, through narcotisation, no motile excitability (p. 229).

Polar effects of currents. — Another observation by which the fundamental identity of excitatory phenomena in the animal and vegetable may be seen, lies in the respective effects induced at anode and kathode ; for example, employing the leaflets of Biophytum as experimental specimens, and using a moderate E.M.F., we find that the excitatory depression of the leaflets takes place at the kathode at make, and at the anode at break. The antagonistic effects of anode and kathode are further seen in the fact that while the kathode-make excites, the anode-make depresses. It is owing to this latter fact that an excitatory wave is blocked during transit at an anodic area (p. 233).

Another very interesting difference between anode and kathode, both at make and break, is seen in the fact that at make, while an induced contraction takes place at the kathode, an induced expansion occurs at the anode ; at break both these effects are reversed, there being now an expansion at the kathode, and contraction at the anode. Expansion at make, moreover, attains its maximum in a short time, while the kathodic contraction is relatively strong and persistent. These fundamental effects find appropriate expression in the response of growth. Thus, the unilateral application of the anode induces expansion, acceleration of growth, and resultant convexity, while the effect of the kathode is to induce contraction, retardation of growth, and resultant concavity (p. 558).

Owing to the fact that kathodic action is stronger than anodic, a feeble or moderate current flowing through the soil exerts a predominant excitatory action on the roots, by which the suctional activity of the plant is increased. The result is an increased rate of growth of the plant, which is independent of the direction of the current through the soil (p. 560). The normal polar effects which have been described take place under the action of a moderate electromotive force. When this is excessively high, however, the normal eff< are, or tend to be, reversed. In this reversal there appear to be two stages— the A stage and the B stage. In the A stage both anode and kathode excite at make ; but in the B stage,

under a still higher E.M.F., there is a complete reversal, inasmuch as the anode here excites at make, and the kathode at break. This reversal, further, is facilitated by fatigue of the tissue (p. 215). These polar effects may also, as I have shown, be demonstrated in the case of animal tissues by means of the glowresponse of the firefly. An excitatory reaction is here shown by an increase of the intensity of luminescence, and a depressing reaction by its diminution.

Multiple response. — When response is observed by means of the electromotive or electrotactile method, we obtain a single response to a single moderate stimulus ; but on the application of strong stimulus a multiple series of responses is found to be evoked. In the case of the retina, similarly, a single intense stimulation by light gives rise to recurrent visual impulses. In the same way, in the leaf of Biophytum, while a single moderate stimulus gives rise to a single mechanical response, a strong stimulus gives rise to a multiple series of responses. In this case certain other peculiarities may also be observed ; for instance, a certain minimal intensity of stimulus induces the maximal mechanical response, which is not increased by any increase of intensity of the stimulus. The excess of such stimulus is held latent by the tissue for the time being, to find subsequent expression as rhythmic multiple response. These multiple responses are evoked by all forms of stimulation, mechanical, thermal, chemical, photic, and electrical. In this respect, of the minimally effective stimulus inducing maximal response, we have an important point of resemblance between the actions of a rhythmic plant-tissue and the cardiac muscle of the animal. Both, again, are characterised by the exhibition of a long refractory period, which is an expression of fatigue, or temporary loss of excitability after excitatory discharge. The periodic oscillation of excitability which is thus induced, imparts a rhythmic character to the mechanical expression of the excess of stimulus which is held latent in the tissue.

The sum total of the energy derived by the plant from the various stimuli of its environment determines what is known as its tonic condition. Continuity of multiple and autonomous responses.— There is no line of demarcation between the phenomena of multiple and autonomous response. When the latent or internal energy of the plant is above par, it finds expression in the form of multiple response, which is apparently automatic. Taking the typical case of a multiply-responding plant which is furnished by Biophytum, we find, on supplying it with excess of energy, by maintaining it at the temperature of say 350 C, and thus exalting its tonic condition, that it displays autonomous response. Conversely, when the tonic condition of an autonomously responding plant, such as Desmodium, is in any way reduced, by reason of low temperature, unfavourable season, or other circumstances, it becomes converted into an ordinarily-responding plant like Biophytum. A single moderate stimulus now gives rise to a single response, and a strong stimulus to multiple responses.

It is in accordance with this, that a Desmodium leaflet in a state of temporary standstill has its multiple or autonomous response renewed by any circumstance, or combination of circumstances, which sufficiently enhances the internal energy of the plant. Amongst such circumstances are: (1) The action of light ; (2) favourable temperature ; (3) the presence of stimulating chemical substances ; (4) an increase of internal hydrostatic pressure. The energy which expresses itself in pulsatory movements, then, may be derived by the plant either directly from immediate external sources ; or from the excess of such energy already accumulated and held latent in the tissue, aided by the incidence of external stimulation ; or from an excessive accumulation of such latent energy alone. Thus there is, strictly speaking, no such thing as automatism, for only when acted upon by stimulus can a living tissue give responsive indications. The impact of an external stimulus may give rise to immediate response, or it may be held

latent, in whole or in part, for subsequent expression. ' Inner stimuli ' are simply external stimuli absorbed previously, and held latent. A plant or an animal is thus an accumulator, which is constantly storing up energy from external sources ; and in the case of the plant, its suctional activity, determining the ascent of sap, its growth, and its spontaneous motile indications, are some of the principal forms in which this accumulated energy finds expression.

The ascent of sap. — The ascent of sap has been shown to be due to a multiple excitatory reaction of the planttissue, the movement of the water being a secondary effect of the rhythmic activity. The excitatory nature of the phenomenon has been demonstrated by the fact that various agencies which induce increase or diminution of excitability, have also the effect of bringing about an enhanced or diminished rate of suction, above or below the normal. The effects induced by these agencies, together with their timerelations, can be easily and accurately recorded, as has been shown, by means of the Balanced Shoshungraph. The transient excitation due to a sudden application of cold, and the abolition of excitation under its prolonged application, are seen in a transient enhancement of suction, followed by arrest. The excitatory effect of the application of hot water, again, is shown by an enhanced rate of suction. Poisonous chemical reagents arrest suction quickly in specimens where, owing to a less favourable tonic condition, the power of resistance is low, and slowly in other cases. As. the tissue of the plant exhibits this suctional activity throughout its length, the local death of a given portion, by scalding or by poison, would not necessarily arrest the suction of the entire plant. Such an arrest can only occur definitely when the entire plant is killed.

The internal energy, on which the activity of suction depends, may fall so much below par as to bring it to a standstill ; but the activity is renewed on the application of fresh stimulus. transpiration from the leaves is seen from the fact that in a saturated atmosphere it continues to take place. That it is not, again, fundamentally, due to the osmotic action of the concentrated cell sap in the leaves is seen from the fact that the ascent continues to take place on the removal of leaves. It is seen again from the further fact that under favourable circumstances, on the application of an osmotically strong solution of sodium chloride to the root, the cell sap, instead of being withdrawn by osmotic action, is made, by the excitatory effect of the salt, to ascend more vigorously.

The ascent of sap is thus an excitatory phenomenon, and its uni-directioned flow is due to the graduated passage from point to point of the co-ordinated excitatory reaction, propelling water forward. This rhythmic excitation is initiated in the intact plant at its root, by the stimulus of contact with soil, the friction of the growing organ against rough surfaces, the excessive turgidity caused by the absorption of water, and possibly by the chemical stimulus of substances present in the soil. In the case of cut branches placed in water, the excessive turgidity at the cut end initiates rhythmic activity, which drives the water upwards ; but if such a branch be placed upside down, with its foliage in water, the now turgid anatomically upper end becomes the seat of excitation, and the direction of the flow of sap is reversed.

The connection between the conduction of stimulus and conduction of water is seen from the fact that the movement of water takes place preferentially along those channels which are also good conductors of excitation. Hence it is transported more easily along the plant than across it ; and while the movement is possible either upwards or downwards, yet it is quicker in the upward direction, which is also preferentially the direction of conduction of stimulus.

The same movement of water which is produced by the co-ordinated rhythmic activity of cells throughout the plant appears either as suctional or as pressure movement, according to the point of view which we adopt. When the removal of water from the plant is in any way arrested, a positive pressure is produced, owing to its excessive accumulation. Similarly, when loss is greater than supply, the pressure will be negative. The ascent of sap, primarily due to cellular activity, may be secondarily aided by evaporation from the leaves, and by the osmotic action of the concentrated cell sap there. Owing to the distribution of unequally active cells, an irregular variation of pressure may be induced in the stem. The excitatory movement may be transmitted to a distance by conduction, or there may be conduction by ' relays.' An isolated mass of highly excitable tissue may thus be excited de novo. The excretion of water and of nectar are phenomena of cellular activity, analogous to that which brings about the ascent of sap. The translocation of foodmaterial is also probably due, at least in part, to excitatory reaction.

The internal activity of the plant, causing increase of turgidity, may be detected mechanically by that erection of the leaf which is characteristic of the positive turgidityvariation. Any increase of internal activity is exhibited in dorsi-ventral organs, such as the petioles of Mimosa, Biophytum, and Artocarpus, by the erection of the leaf. Thus, when the internal energy of the plant is increased by a rise of temperature, the leaves become erected. Conversely, under the action of cold, on account of the diminution of the latent energy, the opposite effect, or droop, is induced. This explains the drooping of various leaves during frost, and their subsequent erection, when brought into a warmer atmosphere.

Longitudinal growth and its variations— Effect of temperature on growth— Re- sponsive growth-curvature under unilateral stimulation : — i. Direct unilateral stimulus on the responding organ : {a) Positive response under moderate stimulation ; (/>) Intermediate or neutral response ; (c) Negative response ; {d) Dorsiventral positive response; (e) Dorsi-ventral response which may hecome negative— 2. Indirect effect of unilateral stimulation: (a) Negative response ; (b) Positive response — Responsive action under stimulus of gravity — Heliotropic action in radial organs — Heliotropic action in plagiotropic and dorsiventral organs — Phototactic movements— Nyctitropic movements.

We shall next pass in review the responsive growthcurvatures induced in plants by various agencies, and shall then in the following chapter consider at some length the extended range of those similarities which exist as between the physiological responses of plant and of animal tissues. Longitudinal growth and its variations. — It was shown by means of the highly magnified continuous record which was obtained with the ordinary and Balanced Crescographs, that growth was a phenomenon of multiple response ; and it was further shown that these multiple responses of growth exhibited the same characteristics as had previously been observed in the multiple motile responses of Biophytum and Desmodium. Each of the constituent responses consisted of a sudden elongation due to a pulse of increased turgidity, followed by an incomplete recovery. The irreversible orrowth-effect consisted of the difference between this elon-

gation and its recovery. These pulses of positive turgidityvariation were mainly due to excitatory reactions occurring about the zone of growth, which delivered from within, upon the plastic material of that zone, repeated hydrostatic blows. The consequent expansive response was thus the indirect effect of stimulation. It is thus the internal energy, ultimately derived from external stimulus, that gives rise to those rhythmic activities by which the pulsations of growth are maintained. When the sum total of the latent stimulating factors that determine the tonic condition is below par, there is an arrest of the multiple response of growth, corresponding to the similar arrest of multiple motile response in Desmodium. In a plant in which growth is at standstill, it may be renewed by a fresh supply of energy. Thus, if hot water be applied to the root of such a plant, energy is hydraulically transmitted to the growing region, and there re-initiates growth.

If moderate stimulus be thus imparted, the responsive growth-movement persists for a short time, and then comes to a standstill, to be again renewed by a fresh supply. Again, the movement of growth being due to the indirect effect of stimulus, we might renew or accelerate it by applying stimulus, say, on the stem or its top, at such a distance from the growing region that the direct excitatory effect would not be transmitted to it. Stimulus applied directly on the growing region would, however, by its true excitatory effect, induce contraction and retardation of growth.

The longitudinal growth thus described takes place in a strictly radial organ. If the organ, however, be bilateral, instead of radial, it will exhibit lateral oscillation, owing to the alternate growth of the two sides. Or growth may proceed in a spiral line, giving rise to circular or elliptical movements. A very good example of the last is afforded by the torsional growth-movements of climbing plants. These various circummutating autonomous movements of growth, passing from regular movements in a circle, through ellipses, to a straight line, are exactly paralleled by different examples of autonomous mechanical responses in Desmodiumy where also we find circular, elliptical, and rectilinear movements.

Effect of temperature on growth.— That growth is an excitatory phenomenon is seen, again, in the fact that it is increased by any circumstance that tends to increase excitability. Thus, for example, in the case of most tropical phanerogamous plants, it is found that responsive excitatory contraction is greatest at a temperature of about 35° C. ; and this is also found to be the optimum temperature, at which the natural rate of growth is at its maximum.

I have described a method of obtaining a Thermo- CRESCENT CURVE for the determination of the various rates of growth which correspond to different temperatures. The continuous record thus obtained in the course of about half an hour affords us not only the rate of growth at any temperature, but also a means of determining its optimum and maximum points. The optimum temperature may also be determined, with an accuracy within one-tenth of a degree, by means of the Balanced Crescographic record. The results obtained by all the different methods employed are found to concur. The optimum point is thus shown, under normal conditions, to be very constant (p. 451). It may be said here that in the case also of plants which exhibit torsional growth-response, the rate of torsional movement is greatest at this optimum point.

The arrest of growth which occurs at the maximum temperature does not appear to be due to any cessation of activity as brought on by rigor ; for we found in a record taken from a seedling of Balsam at 440 C. that at this temperature the constituent growth-pulsations had actually become more frequent than before, the resultant abolition of growth being due to the fact that response and recovery were now equal. It was likewise shown that the apparent arrest of the pulsatory movements of Desmodium at certain high temperatures was not due to the cessation of activity, but that at such temperatures the pulsations had become more frequent and very minute (p. 431). The fact that at the maximum temperature growth is not arrested by rigor receives curious illustration, again, when the application of

doses of poison at such a temperature brings about, at least temporarily, a renewal of resultant growth (p. 487). Another important point in the effect of temperature has already been alluded to. It has been shown that a plant below the optimum temperature, being in proportionately sub-tonic condition, will to a very great extent, or even entirely, hold the incident stimulus latent, thus increasing its own latent energy. In this sub-tonic condition, then, the stimulus induces little direct contractile effect, but is utilised to induce the indirect acceleration of growth. At the optimum temperature, however, almost the whole of the incident stimulus finds expression in direct contractile response, there being now little or no absorbed component ; and beyond the optimum, the tissue not only possesses little or no power of holding stimulus latent, but its receptivity also appears to undergo great diminution

Responsive growth-curvature under unilateral stimulation.— I have shown that the response of a growing is not essentially different from that of a pulvinated organ. The direct effect of unilateral stimulation gives rise in both cases alike to negative turgidity-variation, with consequent concavity of the side acted upon ; and the indirect effect, on the other hand, consequent on the unilateral stimulation of a distant point, gives rise, in both cases alike, to a positive turgidity-variation, or convexity of the same side of the responding region. This fact was demonstrated in the case of Mimosa by applying stimulus : (1) near the motile organ, in which case we obtained the direct effect by fall of the leaf; and (2) at a considerable distance, when the indirect effect gave rise to the erection of the leaf (p. 531). It was found, however, that when the stimulus applied at a distance was very strong and long-continued, true excitation was ultimately transmitted by conduction, inducing excitatory contraction, with fall of the leaf.

In growth-curvatures, similarly, we obtain responsive movements appropriately due either to the direct or indirect effect of stimulus. These have been shown to be classified as follows : 1. Direct unilateral stimulus on the responding organ : (a) Positive response under moderate stimulation. — The proximal, by the direct action of stimulus, contracted ; and the distal, by the indirect action of stimulus, expanded. The result was a concavity of the proximal, and convexity of the distal, conspiring to bring about movement towards stimulus.