Plant Response as a Means of Physiological Investigation

Localised polar effects on Pulvinus. — It is usually believed that the responsive effect seen in a pulvinus is entirely due to the excitability of the lower half of the organ. I have already shown that it is really due to the differential excitability of the upper and lower halves, and that the upper half is also contractile under stimulus, though in a less degree than the lower. This will be seen fully demonstrated in heliotropic response, where the localised action of moderate light on the upper surface will be found to induce a movement upwards. In the present case, a given electrode, carrying current, will be applied locally on the upper half of a pulvinus, the second electrode being applied at a considerable distance on the stem, and the specific result recorded.

For such local application, it will be understood that a large pulvinus is an advantage, and therefore I used for my experiment the pulvinus of Erythrina indica, the sensitiveness of which is not so marked as that of Mimosa. As preventing the diffusion of stimulus to the lower side, this lessened sensitiveness of the organ constitutes an added advantage for the purposes of my experiment. The responsive effect, it must be remembered, can be magnified to any extent that is desired by the Optic Lever, and in the present investigation a magnification of 200 times was employed. If the effect of the given electrode (laid on the upper surface), then, be to induce a responsive contraction, we can see that the normally horizontal leaf will be moved upwards, whereas a responsive expansion or relaxation would bring about a movement downwards. In order, however, to show the essential unity of all the different kinds of responses, whether by mechanical or growth movements, we shall regard them as cases respectively of the two fundamental phenomena of expansion and contraction. Expansion thus causes convexity or acceleration of growth, represented in these records by up curves, while contraction, concavity, or retardation of growth, is shown by down curves.

In my experiment on the upper half of the pulvinus of Erythrina, I first made it anode, the E.M.F. used being twenty volts. In previous experiments on the anodic and kathodic reactions of the pulvinus, it was the differential effect that was observed, and the magnification employed for the record was only slight. Hence the pure effect of anodemake was found to be inconspicuous, whereas the anodebreak induced the usual marked contractile action. In the

present case, however, no complicating differential action being involved, and the magnification being considerable, we are able to detect a make-effect, which is contrary to the break-effect at anode — that is to say, it induces an expansion and consequent convexity. This anode-make expansion is quick, and soon reaches its maximum. At break the usual contractile effect is induced, but this is not here very clearly distinguishable from the movement of a natural recovery (fig. 230).

The current was now reversed, making the upper half of the pulvinus kathode. This induced a contraction and concavity which, unlike the quickly exhausted effect of the anode-make, went on increasing for some time. At kathode-break we see not only the cessation of the contractile effect, but probably also an expansion, aided by the natural process of recovery. Other experiments will be described presently, which will clearly show that not only do anode and kathode induce opposite effects, but that each at its own make and break exhibits reactions, which, though not of equal intensity, are of contrary signs. These interesting opposite effects, at make and break respectively, are not easily observed in muscle under ordinary conditions, inasmuch as muscle which is in the usual state of expansion cannot be further expanded ; but Biedermann has found that smooth or cross-striated muscles, which are partially contracted, exhibit local expansion at the anode-make. Again, he found in the case of cardiac muscle that the kathode-break also gave rise to local expansion.

Fig. 236. Polar Effects ot Currents due to Localised Application on Upper Half of Pulvinus of Erythrina indica Up curve, in this and in fig. 232, represents expansion and convexity. Down curve represents contraction and concavity. Continuous curve represents the action at make. The dotted curve shows the effect at break. Am = convexity induced at anodemake. Ad = responsive concavity at anode-break. Km - induced concavity at kathode-make. Kb = expansion induced at kathodebreak. The time-marks in this and following curves represent minutes.

These opposite effects at make and break, which I have already demonstrated in the case of the pulvinar response of vegetable tissue, will next be exhibited in a still more striking manner in the case of growth-response. Anodic and kathodic effects on longitudinal growth. — We thus pass from the question of the motile response of pulvinated organs to the polar effects of currents on the rate of growth. From a demonstration of the fundamental action here, we shall be able to infer the curvature which will be induced by the unilateral application of polar currents in the growing organ. Experiments on growth, especially when conducted by the Method of Balance, have the unique advantage that the opposite responsive effects, of expansion and contraction, are clearly distinguishable, and not liable, under any circumstances, to be confused with the natural process of recovery ; for, as I have explained before, when a balanced horizontal record is taken of growth, an expansion or acceleration of the rate of growth will give rise to a deflection, represented, say, by an up curve. Recovery to the normal condition will now be indicated by a horizontal record. Contraction or retardation, similarly, will be represented by a deflection of the record downwards. Using this method, then, I have studied the effects of anode and kathode on the growing regions of both root and shoot, the second electrode being placed at a very great distance from the first, so that its effect may be considered nonexistent. It may be said here that the results obtained were the same for both root and shoot, and I shall now describe the effects observed in the case of an experiment on the root of Bindweed, using an E.M.F. of twenty volts. One of the electrodes — a moist strip of cloth — is wrapped about the growing region ; the second, as said before, being at a great distance. The balanced horizontal record of growth is first taken, and the growing region is made kathode. From the down curve in the record (fig.

231), it will be seen that a responsive retardation of growth is induced, which goes on increasing for a considerable time during the maintenance of the current. The current was now interrupted, and as an effect of the kathode-break we obtain an expansion and acceleration of growth above the normal, as seen from the up curve. This acceleration persisted for nearly three minutes, after which the growth-rate became again normal. The current was now sent in a reversed direction, and the result of this anode-make was a sudden expansion and acceleration of growth, which, as in the case of motile response, persisted for a relatively short time, when the growth-rate, as seen from the return to the horizontal, became once more normal. The current was now interrupted, and as the result of anodebreak we have a contraction and retardation of growth, which persisted for nearly a minute and a half, after which the growth-rate became again normal. From this experiment we again see that not only are the anodic and kathodic effects opposite, but that the effects on each of make and break are also opposite. It is also seen that the expansional effect caused by anodemake is relatively smaller, and occurs, and is completed, more rapidly than the kathode-make contraction, whose effect is more persistent. From these data we are able to arrive at a more comprehensive law of polar excitation in the case of vegetable tissues than was given in Chapter XVL, including not only ordinary mechanical responses, but also the modifications induced in growth. It will be understood that with regard to growth, expansion means acceleration of growth, and contraction means retardation.

Fig. 231. Effects of Anode and Kathode on Variation of Rate of Growth in Root of Bindweed exhibited by Balanced Growth-record Down curve represents contraction and retardation of growth, here and in fig. 233. Up curve represents expansion and acceleration of growth. Km - contraction at kathode-make ; Kb = expansion at kathode-break ; Am = expansion at anodemake, and Ad = contraction at anode-break. It will be noticed in this and other figures that the contraction at kathode-make is stronger and more persistent than the expansion at anodemake.

The generalised law of polar excitation in plants is as follows : The kathode induces contraction at make, and expansion at break. The anode induces expansion at make, and contraction at break. Galvano-tropic response. — From the demonstration just made of the fundamental polar effects on growth, we can easily infer the responsive curvature that would be induced in a growing organ by the unilateral action of anode or kathode. I shall, however, show that the effects directly observed are in strict conformity with these deductions. For these experiments I took the long scape of Crinum Lily, and on its growing region I applied one electrode, the other being connected with a distant point On now making one side of the growing region anode, there was induced on that side a sudden and short-lived responsive expansion and convexity, seen in the up curve (fig. 232). At break, the opposite effect occurred, consisting partly of contraction and partly of natural recovery. The current was now reversed, and the effect of the unilateral kathode-make was the induction of a very active and relatively longcontinued contraction and concavity, to be seen from the down curve. The kathode-break next gave rise to the opposite effect, which was made up partly of expansion, and partly of natural recovery. Thus in these galvano-tropic effects of currents on growing organs, we find what is simply a special case of the fundamental polar effects of currents on growth in general.

The indirect effect of polar excitation. — For the purpose of clearly demonstrating the fundamental action of currents, I have taken in the first place the simplest cases of the direct action of anode and kathode on the growing Fig. 232. Responsive Curvature in Scape of Crinum Lily by Unilateral Application of Anode and Kathode aw = expansion at anodemake ; Ad m contraction at anode-break ; Km = contraction at kathode-make, and Kb = ' expansion at kathode-break.

responding region itself ; but when similar applications are made, at a point distant from the responding region, we can foresee the fact that variations of these effects will occur. Thus the kathode-make, whose direct action induces contraction and concavity, will now, acting from a distance, bring about, by means of expelled water, the indirect effect of expansion and convexity. This can be demonstrated by selecting a growing and undetached petal of Champaca, in which the growing region is diffuse. The lower half of this petal is held in a clamp, the free upper half being attached to the Optic Lever for observation of its responsive movement. If now we make one side of the clamped half kathode (the other electrode being connected with the rest of the plant at a distance), then, at make of kathode, the indirect effect of stimulation, reaching the same side of the free end of the petal, gives rise to expansion and convexity — that is to say, the opposite effect to that which would have occurred had that region been directly subjected to kathodic action. The effect of the kathode on a distant growing organ is thus an acceleration of growth.

The effect on growth of ' electrification ' of soil. — Empiric attempts have been made to discover whether the maintenance of an electrical current through the soil might or might not be made to have the effect of accelerating the growth of plants. This problem can only be satisfactorily solved, however, from accurate knowledge of the direct and indirect effects on growth of currents under the given conditions. Since growth is determined by the direction of current, it is not clear that currents flowing through the soil from right to left, or left to right, could give rise to a single effect. Looking, again, at the root, on which the current acts, we can see that one side will be anode and the other side kathode, and as the actions of these are known to be antagonistic, it is at first sight difficult to see how there can be any resultant excitatory effect at all.

We are, however, enabled to obtain a clear understanding of the subject by an attentive comparison of the responsive effects of anode and kathode (figs. 230, 231). It is to be borne in mind that the growth of a plant, other factors remaining the same, is dependent on its suctional activity. And this again depends on the excitation of the root. Now, we saw that though the effects of anode and kathode at make are opposite, yet they are at the same time not equal. The anodic effect is relatively small and short-lived, and the kathodic effect is stronger and more persistent. Hence, though the effects on the two halves of the root may be electrically opposed, yet there will be a resultant differential effect of excitation, due to the predominance of kathodic action. Hence an electrical current, whatever be its direction, would excite the roots, causing a greater suctional activity, with consequent enhancement of growth. This deduction I have been able to verify by experimenting on the variation of the rate of growth of seedlings of vr^ „ w . • a 1 fI)l Oryza sativa when the soil

of Growth of Seedling of Oryza sativa in which they grow was effect on cessation of current. Record direction. 1 he growthof growth taken under balanced conrecord of a single Seedling was first taken under balanced conditions (fig. 233). The electrical current was next sent through the soil from left to right, and during the continuation of this current, we see from the up curve the acceleration of growth above the normal that took place. The current was now stopped, and the former induced acceleration was replaced by a brief retardation, as seen in the down curve, after which the normal rate was restored. The current was next reversed, and yet the response was one of accelerated growth, and on the stoppage of the current after a brief retardation there was again a restoration of the normal rate. This experiment

was carried out using an E.M.F. of ten volts, the two electrodes being applied at a distance of 10 cm. from each other. This meant a potential gradient of one volt per cm. This relatively strong voltage was applied for the sake of obtaining measurable response within a short time. But a much smaller E.M.F. is found to produce similar effects, though the action is slower. The application of a strong E.M.F. has the disadvantage of inducing fatigue, which is a drawback not present in the use of a feeble E.M.F. It is, however, shown that, during the continuation of a current through the soil, the rate of growth of a plant is enhanced.

The effects of unilateral chemical and galvanic excitation may be studied, both from pulvinar and growth movements. The unilateral application of alkali gives rise to positive curvature, that is to say, a movement towards the stimulating agent. Acids, under similar circumstances, give rise to negative curvature. The unilateral application of copper sulphate gives rise, by retardation of growth, to a concavity or positive responsive movement. The unilateral action of dilute solution of sugar, by enhancing the rate of growth, induces negative response. A strong solution, however, gives the reversed or positive response.

Effects fundamentally similar are expressed by swimming organisms in appropriate movements to and from sources of chemical stimulation. The assumption that the variation of curvature which occurs when an already curved organ is placed in strong solutions of salt is due to the action of plasmolysis is not always justified, for such solutions also exert characteristic excitatory effects. reacts to the action of salt, at least for a time, by an erectile responsive movement. Under a less favourable tonic condition, it responds by depression. That this latter is not entirely due to plasmolytic action is seen from the fact that the leaflet subsequently becomes erected, as is the case under continuous stimulation.

The localised and unilateral action of anode and kathode on a pulvinated organ is as follows : The anode-make causes expansion, inducing convexity. This effect attains its maximum in a short time. The kathode-make induces contraction and concavity. This effect is stronger and more persistent than the opposite effect of expansion at anode-make. The anode-make causes expansion and acceleration of growth. This effect attains its maximum in a short time*

The anode-break causes contraction and transient retardation of growth. The kathode-make causes contraction and retardation of growth. This effect is stronger and more persistent than the opposite effect of acceleration at anode-make. The kathode-break induces an expansion and transient acceleration of growth. The localised and unilateral action of anode and kathode on a growing organ, results from the unilateral exhibition of those growth-variations which have just been described. Since a growing organ is virtually a diffuse pulvinoid, the effects are exactly similar to those seem in a pulvinated organ, already described.

Owing to the fact that kathodic action is relatively stronger than anodic, a feeble or moderate current flowing through the soil exerts an excitatory action on roots, by which the suctional activity of the plant is increased. The result is an increased rate of growth, which is independent of the direction of flow of the current through the soil. Diversity of movements induced by light— Differentiation of responsive movements —Action of light on tissues in sub-tonic conditions— Effect of light on pulvinated

organs— Effect of diffuse stimulation of light on non-growing radial organs Retarding effect of light on longitudinal growth - Phenomenon of oscillation under long-continued stimulation — Similarity of responsive reaction under light and under other forms of stimulation. There is perhaps no other phenomenon in the plant-world which is at once so striking and so universal as that of the response which is evoked from plants by the stimulus of light. Under this influence the plant as a whole, and every part of it, is tremulous. Not only does the growing stem curve towards or away from the incident rays, but every leaf under their action is thrown into a state of periodic daily rhythm. By the absorption of light, again, the plant is energised. Thus the two factors, of external stimulus and internal energy, whose manifestations are so opposite in character, are brought into play

Diversity of movements induced by light. —Light, as a form of external stimulus, evokes responsive movements, which appear to be extremely diverse in their nature. Radial organs, for example, such as stems, under certain conditions, direct themselves towards the light, and under others, again, away from it. Leaves, however, behave quite differently. These Ire said to possess the peculiarity of placing themselves with leir surfaces at right angles to the light ; but even this ; not a property universally exhibited, for there are certain

that of light, either towards or away from it. Some plants, again, close their leaves or leaflets on the approach of night, in the so-called 'position of sleep ;' while a further complication arises from the fact that an apparently similar ' sleep ' movement is produced, in these or others, by the action of the noonday sun. Light, again, appears in some cases to initiate movement, as was seen in Desmodium at standstill, and in others to arrest it, as is said to happen with the spontaneous movements of Trifolium pratense. Certain swarm-spores, moreover, appear to be attracted by light, swimming towards it, with energetic beats of their cilia ; while others, on the contrary, are affected in the opposite manner, and swim away. Or the same specimen may be found to swim, now towards, and again away from, light, swinging backwards and forwards like a pendulum.

We thus see that not a single responsive effect of light has been observed in the case of plant-organs of which an example directly to the contrary may not be found. For this reason it has appeared hopeless to attempt to unify these phenomena, and this fact has left investigators with little option but to tend towards a ' belief in the individuality of the plant in deciding what shall be the effect on it of external conditions.' l So far we have been considering only the diversity of the responsive effects which are induced by light in plants. When we come, however, to the further question of the responsive mechanics by which the stimulus of light evokes these movements, we are confronted at the outset of our inquiry by the fact that, as Pfeffer says, ' the precise character of the stimulatory action of light has yet to be determined.1 2

Differentiation of responsive movements. — It is customary, in treating of plant physiology, to draw sharp lines of demarcation between the different classes of movements which are to be attributed to the action of light, ascribing each to 9 Pfeffer, Physiology of Plants, English translation, 1903, vol. ii. p. iqi. some unknown specific sensibility. Thus, Sachs differentiates some of the principal effects as follows : * In the case of that stimulation of light which produces waking and sleeping, the stimulus lies in the variations of the intensity of light ; it is not the light as a constant force which effects these movements, but the varying intensity. A further great difference between the heliotropic curvatures and those which bring about the sleep movements, lies in the fact that the organ can make heliotropic curvatures in all directions. The movement of waking and sleeping, on the contrary, only takes place in one plane, which divides the leaf and motile organ symmetrically, and it is thus unimportant here in what direction the rays of light fall upon the motile organ, but only important that light is present at all, or increases or decreases in intensity. The above will suffice for the distinction of the movement of waking and sleeping from the heliotropic curvatures.' *

Finally, he summarises the differences of motile effects as follows : 'We may thus say shortly, the movements of waking and sleeping are called forth by paratonic light stimulus, whereas the spontaneous movements of the same leaves are independent of any light stimuli, but probably dependent on phototonus. Heliotropic curvatures, on the contrary, have nothing to do with phototonus.' It will be found, however, that all these effects, sharply differentiated as they are, may be seen in one and the same organ. We may take for example the terminal leaflet of Desmodium gyrans. This exhibits under favourable conditions autonomous movements, whose period is short. It exhibits also daily periodic movements, with the very long period of twenty-four hours. It further, as I shall show, exhibits positive heliotropic curvature when exposed to onesided illumination. Of these effects, it is supposed that the autonomous movement is independent of the paratonic action of light, but probably dependent on phototonus. The daily

periodic movement is held to be dependent on paratonic effects and phototonus. The heliotropic effect is ascribed to the continuous action of light, independent of phototonus. Thus in the same organ we have to postulate various irritabilities and mechanisms, in order to account for its multifarious movements. Are there, then, independent or different irritabilities, coexisting simultaneously in the same organ ? Such a state of things is so difficult to imagine, that it prompts us to try to look at the problem in a fresh light, divested of all assumptions which are incapable of experimental proof.

Approaching the matter thus directly, then, we see that, instead of so many different irritabilities, there may possibly be, fundamentally, but a single phenomenon of irritability, finding expressions apparently diverse, in consequence of the anatomical or physiological differentiations of the responding organ. If this should be so, the question will resolve itself into three separate inquiries. First, what is that responsive action which constitutes the characteristic effect of stimulus of light ? Second, is such response to light unique in character, or is it a single instance of that universal phenomenon of contraction which we have seen to be the response of all excitable cells to stimulus in general ? And, lastly, in what manner do the various anatomical and physiological differentiations of responding organs operate to modify the expression of this response ?

Action of light on tissues in sub -tonic condition. — Before proceeding to a decisive demonstration of the nature of the effect of light on excitable tissues in a normal condition, however, I shall briefly refer to the suggestion which has been offered, that light in some unknown way induces a lessening of turgor, which brings about diminution of growth. This theory could not hitherto find acceptance for want of a precise knowledge of the exact nature of the stimulatory action of light, and of the relative significance of absorbed energy in promoting growth. Against the assumption that light diminished turgor, it was urged that the pileus of