Plant Response as a Means of Physiological Investigation

(a) Uni-directioned flow. — In connection with the transmission of multiple excitatory waves, such, for example, as those which we have seen in Biophytum, it is important to note that these repeated excitations are all initiated at the original point of stimulation, and are propagated in proper sequence from point to point outwards. Thus, after each wave of excitation is exhausted, it starts anew from the original point. This sequence of multiple excitations is exactly parallel to that which is observed in multi ciliated tissues in which the cilia repeatedly contract in sequence. Thus a single cilium at one end gives, as it were, a signal which is followed serially by the rest.

This being so, it is clear that if such a multi-ciliated tissue take the form of a hollow tube, with the ciliated surface inwards, and if the tube be filled with water, then, owing to this peculiarity of the multiply-responding cilia, the water will always be driven in one direction. A somewhat similar phenomenon occurs in the bloodcirculation of animals, where the sinus giving the signal, the rhythmic contraction of the heart proceeds towards the ventricle, and the pumping-action thus initiated produces a one-directioned flow of fluid.

In the leaf of Biophytum, strongly excited at, say, the inner end, we have similar rhythmic excitations, passing in regular succession from leaflet to leaflet, the innermost leaflet, which is near the seat of multiple excitation, giving the signal to the rest. And just as rigid sequence is observable in the movements of motile cilia, so it is also seen in the plant, by the depression in orderly series of its lateral motile leaflets. (b) Initiation of multiple rhythmic excitations. — We have just seen that in multiple rhythmic response, the multiple waves of excitation proceed serially outwards, from the point of excitation. We also saw in Chapter XXI. that the wave of excitatory contraction is attended by the propulsion of water in the direction of propagation of excitation. It remains to be determined, then, with regard to the ascent of sap in a plant, how this one-directioned propagation of excitation is initiated.

In the case of an intact plant, the root is acted on constantly by various forms of stimulation, among which are (1) contact with the soil; (2) friction of the growing organ against rough surfaces ; (3) turgor of its own tissues, due to absorption of water ; and there may also be in addition, stimulation by (4), chemical substances of various kinds in the soil, which possibly exert some excitatory influence. All these factors, separately or in combination, serve to set up multiple rhythmic excitation at the extremity of the plant, and the excitatory effect is transmitted upwards, preferably along certain better-conducting tissues. Similarly, in the case of a cut branch placed in water, rhythmic excitation is initiated at the lower end by excessive turgor, just as we found rhythmic movements to be initiated in Desmodium leaflets at standstill, by the artificial increase of internal hydrostatic pressure, that is to say, by excessive turgor (p. 348).

Connection between conduction of excitation and conduction of sap. — It was said in the case of the intact plant that excitation is transmitted upwards by conducting tissues and we have already seen, in studying transmission of excitation (p. 250), that the fibro-vascular elements are those which conduct best. It is therefore to be expected that the movement of water, which is itself an excitatory effect, should also follow by preference the length of the fibro-vascular elements ; and it is here worthy of note that the ascent of sap is known to take place preferably along such channels. Again, I have shown (p. 250) that the power of conducting excitation is greater along the length of the plant than across ; and we find also that the same is true as regards the transport of water, which is known to be greater in the direction of length. Conduction of excitation takes place with very great slowness across parenchymatous tissues, and the same is the case as regards the conduction of water through such a tissue. If, further, a plant were to be excited from above, instead of below, the transmission of excitation would be downwards, instead of upwards. We might then conceive of the possibility of reversing the direction of the normal transport of water. In such a case, rhythmic excitation would need to be initiated at the top, instead of at the bottom, of the plant. This may be seen in the experiment already referred to, of placing a leafy branch upside down, with its leaves in water. Rhythmic activity due to excessive turgor being now initiated at the upper extremity of the branch, the watermovement is reversed, and sap exudes from the cut end of the stem. Turning back, however, to the subject of the transmission of excitation, we have found, it will be remembered, in the case of Biophytum that this takes place with greater rapidity in a centrifugal than in a centripetal direction, that is to say, in the direction of the normal transport of water.

It is therefore interesting to note that a reversed watermovement is, in general, known to be somewhat less rapid than the normal flow. Rapidity of ascent of sap accounted for by stimulatory action. — One great difficulty with regard to the ascent of sap has lain in its relatively great rapidity. No theory hitherto suggested has been held to account for this. As, however, we now find that the ascent of sap is due to the propulsive energy of vigorous excitatory contraction proceeding from cell to cell, the rapidity of the movement is easily understood.

We thus see how by the action of this cellular machinery, set in motion by stimulus, an upward movement of water takes place. We have in fact an active chain of pumps, working throughout the length of the plant, partly carrying water themselves, and partly pumping it into the better conducting vessels of the xylem ; and there is no limit to the height to which it may, by such means, be lifted. Positive and negative pressures due to one cause. — Let us suppose an india-rubber pipe, open at its upper end, and provided throughout its length with a series of pumps, one above the other, each of these being independently engaged in raising water upwards. The individual activity of these several pumps may or may not be uniform, but, provided that they are sufficiently numerous, when the pipe is placed in connection with a supply of water, the result of their combined action will be that water will be sucked in at the lower end, and ejected at the upper, in a uniform stream.

If now we confined our attention to the lowermost pump, it would appear to us to he forcing water up ; if, on the other hand, we observed the uppermost pump alone, it would appear to be sucking water up ; and if, finally, we selected some intermediate point for scrutiny, we should discover that the pumps above were sucking, and those below pressing water upwards. Thus, one single effect, namely, the rhythmic activity of pumps, is made to appear various, by simply changing the point of view. Again, certain peculiarities of variation of pressure may appear in the pipe as a whole or in particular parts of it, depending on the rates of supply and removal of water.

(1) Positive pressure. — We may now suppose the aperture of escape at the upper end to be narrowed. The water pumped into the flexible pipe being thus in a state of compression, will produce a bulging or over-turgidity, and a manometric tube inserted laterally will indicate a positive pressure. Or if, under these circumstances, we make a cut in the pipe, we shall observe exudation under pressure. Now, this corresponds to the exudation pressure, causing so-called 'bleeding,' which we find on making incisions in plants in spring time, when the loss of water by transpiration is feeble, the buds being still unfolded ; or even in summer, if transpiration be by any means prevented.

In connection with this exudation of sap, we have to remember that the whole question is one of income and expenditure. Generally speaking, the loss by transpiration is less at the end of winter or the beginning of spring than in other seasons. Thus the wild date palm, or Phcenix sylvestris, yields considerable quantities of sugary sap from incisions in the stem, at the end of winter or the beginning of spring. In the case of the Palmyra palm {Borassus flabelliformis) of Bengal, however, the increase of cellular activity in summer more than compensates for the loss by transpiration, and sugary juice is collected in the height of summer at the top of the tree by incisions in the peduncle. The pressure exerted by the sap may be gathered from the fact that these trees are often more than a hundred feet high.

(2) Negative pressure. — We may next suppose that in the chain of pumps, those at the upper end are the most active, and the aperture of escape wide open, the removal of water being further aided by evaporation. The loss of water being thus greater than the supply, it is clear that there will be a negative pressure in the pipe, and the mercury in a testing lateral manometer will be sucked in. This corresponds to the negative pressure exhibited by an actively transpiring plant. In such a case it will be noted that the activity of the rhythmic cells, which is the fundamental cause of the ascent of sap, is further aided by the evaporation from the leaves. Concentration of cell-sap also, and the osmotic action thereby produced, may then constitute an additional auxiliary factor.

When the stem of an actively transpiring plant is cut across, the stump of the plant does not always immediately showbleeding, but often, on the contrary, will suck in water. This is generally ascribed solely to the existence of negative pressure in the stem. There is another element in the problem, however, which is generally overlooked. By such stimulus as that of amputation, excitation must be produced at the cut end of the stem, which is propagated downwards, and tends to induce a reversal of flow. But when the negative tension and the excitatory effect, due to stimulus of cut, have both subsided, exudation will begin to take place.

(3) Irregular variations of pressure. — It is evident from what has been said that the hydrostatic pressure at any given zone of the tissue will depend on the relative activities of cells below and cells above. Cells immediately above, by their activity, produce suction or negative pressure ; and those immediately below, an increase or positive pressure. The resultant pressure at any individual zone depends, therefore, on the algebraical summation of these. Now, though all the cells throughout the length of the plant may be active, yet there will be some difference in their activities. Or the same cells again, at different periods of life and different conditions, may undergo variations of their activity. There are, in nature, many disturbing influences which produce local variations of excitability ; hence, by the distribution of cells whose excitabilities are irregular, we may obtain a variation of internal pressure from the top to the bottom of the plant, which is not uniform but fluctuating.

Direct conduction and conduction by relays. — I have already said that the movement of water, being mainly brought about by excitatory reactions, will take place preferentially along conducting channels. We have seen that every tissue possesses the power of conduction to a greater or less degree ; in parenchymatous cells, however, owing to the presence of numerous more or less complete septa, transmission is enfeebled, whereas in prosenchymatous fibrovascular elements it is very much facilitated. Thus, for

conduction through long tracts, the fibro-vascular tissues are the most favourable. But even in this case the transmission of the excitatory effect may be much enfeebled by distance. Or an interposition of parenchymatous elements may offer a relative obstruction to the transmission. In such cases there might be pseudo-conduction by means of ' relays.' For as an example of the last case we may imagine a mass of excitable parenchymatous tissue, against which a conducting tissue abuts. This mass, being supplied with water by the conducting elements, may become over-turgid, and thus rhythmic activity may be initiated in it de novo.

That rhythmic activity may under favourable circumstances be started locally in a mass of excitable tissue, we have seen in the case of the pulvinus of Desmodium. For the fact that activity in this case was not due to any transmitted impulse was proved in the experiment on the localisation of the excitable area, when it was found that an isolated leaflet, if sufficiently turgid, could pulsate. Under natural conditions, the necessary turgidity is maintained by the cellular activity of the tissue below. It is worth while to remember, in this regard, that the characteristic pulsation of the leaflet has no immediate connection with that rhythmic activity of the plant-tissue which brings about the ascent of sap. The period of pulsation of the leaflet is determined by certain constants of its cell-complex. We may in fact have various vibration-periods in different organs of the same plant, the different oscillations being brought about by the turgidity caused by the ascent of the sap, just as the same electrical current may give rise to various frequencies of vibration of different electro-magnetic vibrators included in the same circuit.

Excretion of water. — The rhythmic activity of a mass of excitable cells is seen again in the case of such as actively excrete water. A very striking example is that of Colocasia esculentum, in which the successive expulsions of water-drops noticed by Musset were as many as eighty-five in one minute. A distinction is sometimes made between this excretion — due to the special local activity of a certain group of cells — and the somewhat passive excretion of water from waterpores, which is said to be caused by the general pressure of exudation. But this difference is really one of degree and not of kind. All cells are excitable, and the exudation pressure itself is produced by cellular activity. As regards excitability, however, we may have a transition from moderately to highly excitable cells distributed in a continuous or discontinuous manner. Even in the stem we have seen that there are cases of irregular distribution, bringing about irregularities of water-pressure. Extreme instances of these are found in Desmodium, and in the actively excreting cells of Colocasia, where highly excitable cells are localised in special areas. A test which is sometimes insisted on as a means of distinguishing between the active and so-called passive excretions is, that in the latter case the flow ceases from the excreting organ, as soon as the branch is cut off. But this is not by any means a satisfactory proof of the absence of active excreting cells in the latter. We saw that the only distinction between the activities of the multiply-responding tissues of Biophytum and Desmodium, lay in the fact that the latter had the capacity to hold latent a large amount of energy by which the rhythmic activity was maintained even on the cessation of a directly exciting cause. In the case of Desmodium, indeed, if the tonic condition be above par, and the leaf as a whole be cut off and isolated, without any supply of water, the rhythmic activity will be maintained for a considerable time, though the turgidity is undergoing constant diminution.

But when the tonic condition of the plant is below par, its rhythmic activity comes to a stop, and can only be maintained by an artificial increase of internal hydrostatic pressure. Similarly in the case of water-excreting organs, we have some which under a favourable condition can maintain their activity for a considerable period, even when the supply of water is cut off, while in other instances activity can be maintained only under favourable conditions of turgidity.

Excretion of nectar. — It is often assumed that the excretion of nectar is due to plasmolytic action. The excreted solution of sugar dries up by evaporation, and this by plasmolysis draws up more from behind. We have seen that the ascent of sap in the plant is brought about mainly by rhythmic activity, and that concentrated solutions in the leaves may help this movement osmotically. In the case of nectaries, the presence of concentrated sugar solution outside may thus help continued excretion, but this does not explain the initiation of excretion, which could only have been caused by the rhythmic activity of cells.

Translocation of organic food-substances.— Though the flow of organic food-materials towards places where there is a deficit may be brought about by diosmosis, yet such a mode of diffusion must be extremely slow. A more rapid transport than could be produced by this means would appear to be a necessity. There are certain considerations moreover which may be brought forward, tending to show that this translocation of foodmaterials receives considerable aid from stimulatory actions. Czapek's observation, too, that there is a cessation of translocation in a chloroformed leaf-stalk, points to the inference that it is a physiological process.

We know that an excitatory action proceeds from the more to the less stimulated. Now the accumulation of a large quantity of organic food-material may of itself act as a stimulating agent in a given case ; thus an excitatory movement would proceed from cell to cell, from places where there was excess to places where there was deficit. In this way large quantities of food-materials may be rapidly transported by excitatory reaction, through conducting channels. And such transport is also possible by stimulatory action even through unspecialised cells. These ordinary cells are known to be connected with each other, by means of pores and plasmic threads. Mr. Horace Brown has shown {Phil. Trans. vol. cxciii.) that transport of fluid may take place through such a 'multi-perforate septum ' with almost as great rapidity

as if no closing membrane were present. It is clear that by the contracting expulsive action of excited cells, transport of materials might be brought about more rapidly than by mere osmotic action. Mechanical response to suctional activity. — The effect of that internal rhythmic activity which, as we have seen, is caused by the absorption of energy from various forms of stimulus, is to induce an increase of turgidity throughout the plant. I shall next proceed to describe a mechanical means of obtaining some indication of this internal activity.

I have shown that owing to the dorsi-ventral differentiation in the pulvinus of Mimosa, when the turgidity of the organ is increased, the leaf is erected ; and when the turgidity is decreased, as by the direct action of an external stimulus, the leaf is depressed. Similar effects occur not in pulvinated organs alone, but in all dorsi-ventral organs, in which the lower half is more excitable than the upper. Thus, for example, the lower half of . the petiole carrying the lateral leaflets of Biophytum is more excitable than the upper half. It is true that the leaf possesses a slightly developed pulvinus ; the differential excitability on which the motile response depends is not, however, confined to this pulvinus alone, but extends throughout the length of the petiole. The petiole thus acts like a diffuse pulvinoid.

The action of stimulus on the diffuse pulvinoid of Biophytum and on the pulvinus of Mimosa is to produce the fall of the leaf. And just as the leaf of Mimosa was erected by increased turgidity, so in Biophytum also we should expect to have, with similar increase of turgidity, a similar responsive movement of erection ; and this I find to be the case. The erection of the leaf of Mimosa or Biophytum is therefore a mechanical indication of an increase of turgidity or of positive turgidity - variation, by whatever means this may have been induced. Such an increase may be brought about by an augmentation of suctional activity. And the up movements of dorsi-ventral

petioles in which the lower surface is the more excitable may thus be taken as mechanical indications of such added activity. Effect of warmth. — We have already seen that application of warm water to the root of a plant, increases its suctional activity. I was able to demonstrate this fact by means of the mechanical response of Mimosa and Biophytupi. On making the application, the leaves of both plants responded by erection to a position which in the case of Mimosa was 9 mm. and in the case of Biophytum 12 mm. above normal. A rise of temperature we have also seen to have the effect of enhancing the internal energy of the plant. By doing this, it brings about an erection of the leaves.

Effect of cold. — We have seen, on the other hand, that the effect of cold is to stop rhythmic activity, and thereby to arrest the ascent of sap. The motile indication given by the leaf would in this case be the opposite to that of positive turgidity-variation, that is to say, would consist of a fall or droop. On applying ice-cold water to the root of Mimosa I found that the effect of cooling was to induce a lowering or drooping of the leaf, to a position of 3 mm. below the normal. In Biophytum the corresponding fall was through 5 mm. Lowering of temperature, by depressing the internal energy of the plant, has the same effect.

Explanation of the drooping of leaves during frost. — These experiments offer an explanation of that drooping of leaves which is observed in frost, and of the disappearance of this drooping when a plant is restored to a warmer atmosphere indoors ; for we have seen that when the internal energy of the plant is normal, or above par, its suctional activity and consequent turgidity are high, this favourable internal condition being outwardly exhibited by the erection of the leaves. But when the internal energy is below par, the reverse effect is seen in their droop or fall.

Explanation of response and recovery. — It has been stated, when describing the effect of stimulus in inducing a fall of the leaf, and its subsequent erection, that the latter movement, namely, that of recovery, was an active and not a passive process. Certain experiments that I am about to describe will enable us to analyse response and recovery still more closely. When an organ is locally excited, the contraction induced results in an expulsion of water, thus reducing the turgidity of the organ, and the consequence of this is a mechanical fall of the leaf. Some internal activity now forces the water back into the organ from which it was expelled, and we have the recovery or erection of the leaf. That it is the internal energy of the plant to which this recovery is due, will be seen clearly from the following experiment. A leaf of Biophytum falls, as has been said before, when it is stimulated, whatever be the form of stimulus. I shall explain in my chapter on the effect of stimulus of light, that the effect of photic stimulus is the same as that of any other form, while its special advantage is that it may be applied without causing any mechanical disturbance, and that its intensity can be very easily regulated.

On subjecting a petiole of Biophytum, therefore, to the action of sunlight, the leaf responded by depression, the average rate of fall of the tip of the leaf being *i6 mm. per minute. In half an hour it had passed through almost 5 mm. On now shutting off the light, the after-effect persisted for another five minutes, when there followed recovery by slow Fig. 166. Record showing Recovery to be Hastened by the Increase of Internal Activity which is caused by Application of Warm Water to the Roots

The first part of the record shows the fall of the leaf, due to direct stimulation by sunlight during thirty minutes. Stimulus is next cut off at point marked by interruption of record. After-effect persists for five minutes, and there is a subsequent slow recovery. Warm water applied to root at moment x , with the result of quickening the rate of recovery by fifteen times. erection of the leaf at a rate of '12 mm. per minute. Warm water was now poured on the root, thus suddenly increasing the internal activity of the plant. Now, if it be true that recovery is brought about by this factor of internal energy, then the increase of internal energy ought to produce a sudden augmentation of the rate of the recovery. That this is the case will be seen from the record (fig. 166), where it will be noticed that the enhanced rate of recovery from this point is r8 mm. per minute, that is to say, fifteen times the normal rate.