Plant Response as a Means of Physiological Investigation

These considerations show the mechanical action of heliotropic stimulus in causing : (i) positive heliotropic curvature under moderate illumination ; (2) the neutralisation of this action under stronger illumination ; and (3) the conversion of the normal positive into negative* when illumination is excessive. Thus the observation made by Oltmanns, on young seedlings of Lepidium^ subjected to varying intensities of light, an observation of which there has been hitherto no satisfactory explanation, is fully accounted for. Oltmanns subjected a row of seedlings of Lepidium to the action of sunlight, diverging from the focus of a lens. The seedlings nearest the focus were thus subjected to the strongest stimulation, those further from this point being under gradually decreasing intensities of light It was then found that the seedlings nearest the focus, which were subjected to the strongest degree of light, exhibited negative heliotropic curvature, while others, further away, and therefore subjected to less intense illumination, did not show any effect at all (neutralisation), and others again, which were still further away, and therefore under only moderate intensity of illumination, exhibited positive curvature.

Gradual transition from positive to negative, through intermediate phase of neutrality. — I shall, however, give a still more conclusive verification of the theoretical inferences which I have just set forth regarding the gradual transition of positive heliotropic response into negative, through the intermediate neutral, in consequence of the increasing internal diffusion of stimulus with increasing intensity of stimulating light. This I shall do by a continuous record taken from a single plant under changing conditions of increasing illumination, in which record, further, we shall be able to follow all the phases of responsive change, from positive to negative. Taking a hypocotyl of Sinapis nigra, I subjected it to the unilateral action of light from a sixteen-candle-power incandescent electric lamp, placed at a distance of 10 cm. from the specimen. The plant, hitherto quiescent, began to move towards the light, as shown by the up curve in the record (fig. 247), the maximum being attained in the course of fifty minutes. The intensity of incident stimulus was now increased, by bringing the lamp to a distance of 6 cm. from the specimen, at the moment marked with the downward arrow. It will be seen that this resulted in a process of neutralisation of the preceding response, and that this became complete in the course of a further exposure of seventy minutes, the hypocotyl being then erect and free from curvature, having thus placed itself at right angles to the incident light. Still stronger illumination of sunlight was now applied, at the point marked x . This induced, as is seen in the down curve, a very marked reversed or negative heliotropic response.

Thus, in other words, in an identical organ, under different conditions of illumination, the plant turning towards the light exhibits the positive heliotropic, at right angles to the light the dia-heliotj-opic, and away from the light the negative The first curve shows positive response induced by incandescent electric lamp at distance of 10 cm. Increased intensity of light applied at arrow ([) by bringing lamp to distance of 6 cm. causes neutralisation. Reversal, or negative response, when sunlight applied at x .

hcliotropic effect, thus conclusively proving that the induction of these three effects is not due to the presence of three distinct and characteristic sensibilities. These different responsive movements are thus traced to a single phenomenon of contractile response. This may be made equally clear from another point of view, as illustrations namely of the general law that response takes place by the contraction of the more excited, or the relatively more excited. With moderate stimulation it is the proximal side of the organ that is excited, and, becoming concave, gives rise to a positive heliotropic curvature. On the application of somewhat stronger stimulation, however, when excitation is transmitted to the distal side, there is a case in which the excitation of the proximal and distal are equal, and the differential excitation being thus zero, there is no resultant response. The organ, standing thus at right angles to the light, and moving neither towards nor away from it, will now appear to be either dia-heliotropic or else irresponsive to heliotropic stimulus.

But under still stronger stimulation two effects are induced simultaneously: (i) a physiological anisotropy, by the fatigue and loss of excitability of the proximal, in consequence of which the distal becomes the more excitable ; and (2) the internal diffusion of the stimulus, which, now acting on the physiologically anisotropic organ, induces concavity of the distal, that is to say a negative heliotropic curvature. 1 shall now proceed to give further examples of that transverse transmission of excitation, in consequence of which an organ appears either to be irresponsive to heliotropic stimulus or to give negative response.

Organs apparently insensitive to light. — Many vegetable organs exhibit no resultant movement under the action of light, and are therefore supposed to be insensitive to it ; but this inference could be accepted only on the theory of a specific sensibility, in which case some organs would be without it, while in others it would be characterised by certain inherent positive or negative properties. We have seen, however, that such a theory is untenable. The apparent absence in some organs of sensibility to light may perhaps, then, be explicable, not as want of sensitiveness, but as the neutralisation of effect, by the equal excitation of the two opposite sides.

In the case of certain seedlings of Avena, it has already been shown that the so-called insensitiveness of the lower part of the organ was due to this cause. Many tendrils, again, according to Mohl and others, are heliotropically insensitive. Thus, for example, on subjecting the tendril of Passiflora to lateral sunlight, there is practically no responsive movement ; but as the tendril is a highly conducting organ, we might expect that its responsive movement would be neutralised by the transverse transmission of excitation. It occurred to me that this question, as between a characteristic insensitiveness, and a sensitiveness with equal excitation of two sides, might be tested by artificial reduction of the conducting power by cooling. Under such circumstances, if any sensitiveness existed, a one-sided excitation by light would remain localised, and induce concavity, or positive heliotropic movement. On carrying out this experiment, I found that the selected tendril of Passiflora now exhibited a marked positive heliotropic movement by the induced concavity of the side acted upon.

Negative heliotropism of tendril of Vitis. — The tendril of Vitis is adduced as the type of those organs which exhibit the negative heliotropic effect. 1 therefore undertook an investigation on this organ, to determine whether it would not be possible to explain its negative movement without postulating the existence, in its case, of a specific heliotropic sensibility of negative sign. When sunlight strikes it on one side it is found that it moves away from the light. If, now, this movement be really due to the intensity of stimulus, causing it to be rapidly conducted to the distal side, and at the same time giving rise to the fatigue of the proximal, then we should expect that the application of moderate unilateral illumination would induce the positive heliotropic movement.

The crucial test would thus lie in the observation of the responsive movement under moderate unilateral stimulation. I placed a tendril in a dark room, and subjected it to light of moderate intensity from a sixteen-candle-power electric lamp, placed at a distance of 1 5 cm. This induced an active movement of the tendril towards the light, or positive heliotropic response. I then brought the lamp nearer, to a distance of 5 cm., thus increasing the intensity of light. The active positive movement was now quickly reversed into a movement away, or negative heliotropic response. This experiment once more demonstrates the fact that positive and negative heliotropic responses are not due to two specific sensibilities of opposite sign.

The negative heliotropic curvature in an organ originally radial, which we have just studied, was due to the physiological anisotropy induced by the stimulus itself; but there are organs in which anisotropy is already developed in various degrees of perfection, and in them we shall be able to observe varying intensities of this negative heliotropic effect. This will be discussed in detail in succeeding chapters. Negative curvature is induced under the action of light in two different ways : (1) by the indirect effect of the moderate stimulation of the tip of root or shoot ; and (2) by the transversely transmitted effect of strong or long-continued stimulus acting on the distal side of an organ, when the proximal has become fatigued. The parallelism between geotropic and heliotropic effects is thus incomplete.

The direct effect of stimulus, unilaterally applied, may be longitudinally transmitted, and cause responsive movement in the distant growing region. The unilateral effect of stimulating light on the growing region varies with its intensity, as follows : 1. Moderate intensity induces the normal positive responsive movement. 2. (a) Stronger or long-continued stimulus, on account of its internal diffusion by transverse transmission, causes neutralisation of effect. In other words, the organ, after first moving towards the light, returns to its original position at right angles to it, or assumes the dia-heliotropic attitude. This neutralisation, depending as it does on transverse conduction of stimulus, can only take place when the stimulus is very strong, or when the organ is highly conducting. The first of these considerations explains the fact that while moderate stimulation causes positive curvature, stronger stimulation has no resultant effect. The effect of the second factor (the higher conductivity of the tissue), which is brought about by a warmer season, is seen when the transversely transmitted effect causes neutralisation. The apparent absence of heliotropic effect in various tendrils is due not to any want of sensibility, but to this neutralisation by transverse conduction of the effect of stimulus.

(J?) The transverse transmission of stimulus to the distal side is sometimes attended by oscillatory responsive movements, owing to periodic or alternate fatigue. 3. With still stronger unilateral stimulation, the organ becomes for the time being anisotropic, owing to the fatigue of the proximal side. The internally diffused stimulus then induces negative curvature, through the relatively greater excitability and contraction of the distal half of the' organ. The negative heliotropic movement of the tendril of Vitis is explained by these considerations. This tendril, under moderate unilateral stimulus, exhibits positive heliotropic movement ; but stronger stimulation, in consequence of transverse transmission and unilateral fatigue, gives rise to negative heliotropic movement.

The statement that the different responsive curvatures brought about by light are not due to different sensibilities possessed by different organs, is proved by the fact that the same organ exhibits continuous changes, from positive to negative through neutral, under different intensities of stimulation. Effect of temperature and its variations — Demonstration of fundamental effect of thermal radiation on growth — Response to successive uniform stimuli of thermal radiation— Effect of continuous unilateral stimulation— Effect of electrical waves on growth — Response of Mimosa to electric radiation— Action of high frequency Tesla current.

We have now studied the curvature effects induced in plants by those ethereal vibrations that lie within narrow limits, and are known as visible light. There are, however, other vibrations outside this range, the ultra-violet and the infra-red. The excessively quick vibrations beyond violet are known to produce very marked heliotropic effects. But below the red, again, we have comparatively long waves which give rise to thermal, and others, still longer, to electrical radiation.

In studying the curvature-effects on plants of invisible radiations of low frequency,- it is necessary to distinguish carefully between the action of radiation as such and the subsidiary effect of temperature. Effect of temperature and its variation. — In this investigation it becomes especially important to distinguish the temperature from the radiation-effect, and I shall presently describe a very decisive experiment by which the effects of the two may be clearly distinguished. The effect of temperature up to the optimum is, as we have seen, to increase the internal energy of the plant, in consequence of which there is an acceleration of the rate of growth ; but variation of temperature acts as an external stimulus, and would thus be effective in inducing a transient retardation of growth. A part of the stimulus, however, is held latent, as

we have seen, in the tissue, so long as the temperature is below the optimum, to give rise later to an acceleration of the rate of growth. Hence, frequent variations of temperature below the optimum will, by reason of these alternate retardations and accelerations, produce little total effect on the rate of growth ; but above the optimum, the stimulating action of variation of temperature will retard growth, and as there is here no latent factor, this will not be made up by any subsequent acceleration (p. 461). Hence, the total effect of such variation will be a retardation. This consideration explains the different conclusions to which observers have been led as to the effect of frequent variation of temperature on growth.

Demonstration of fundamental effect of thermal radiation on growth. — In the usual experiments on the effect of thermal radiation on the induction of growth-variation, a difficulty arises in distinguishing between the effect of thermal radiation and that of temperature. In order, then, to determine the fundamental effect of thermal radiation, the experiment must be so arranged that the radiation whose effect is to be. observed causes no change of temperature. I have been able to accomplish this by mounting the growing organ in a plant chamber surrounded by a wide heating coil of platinum wire. Between the coil and the specimen there is a cylinder of mica, which is opaque to thermal radiation. By means of a string attached to it from above, this cylinder may be alternately lifted and lowered. The plant is attached to the Crescograph, and a balanced record is taken of its growth when the shield is down, and when, by maintaining a current through the heating coil, the chamber has already been brought to a steady temperature of 340 C. As everything inside the chamber has now attained a steady temperature, the movement of the shield up and down will produce no change in this condition. By now raising, the mica shield, we can subject the specimen to the action of thermal radiation, which proceeds from the heated spiral wire, without producing any variation of temperature. When the

mica shield is again dropped the action of radiation on the organ is cut off. Experimenting in this manner, and obtaining a horizontal record with the shield down, we find, when the shield is lifted, that there is at once an upsetting of the balance, which indicates a retardation of growth. When the shield is once more allowed to drop, the deflected record becomes again horizontal, indicating the restoration of the original rate of growth. This experiment conclusively shows that radiation by itself acts as an external stimulus, retarding the rate of growth. It will be remembered that this is quite distinct from the effect of temperature, which, up to the optimum, always induces the opposite result, namely, an enhancement of the rate of growth. The same distinction is also to be borne in mind in dealing with the excitatory effect of light, for incident light also has the twofold effect of causing external stimulation and at the same time raising the temperature of the tissue.

Response to successive uniform stimuli of thermal radiation. — Having thus demonstrated the fundamental action of thermal radiation, we shall now study the effect of the unilateral application of this form of stimulus, which will be found similar to that caused by visible light. The responsive effect of successive uniform Fig. 248. Responses to Successive Uniform .. , ,. , Stimuli of Thermal Radiation in Pistil of Musa Stimulations may De studied by placing a V-shaped platinum wire with its point opposite to the region of growth, and heating it periodically by definite currents of short duration ; the flashes of thermal radiation thus produced bring about the usual responsive concavity. These responses and recoveries are recorded in the usual manner. I give in fig. 248 a series of such responses

of the pistil of Musa, in some specimens of which the growing region is found to be narrow and sharply defined. In this case the V-shaped radiator was placed opposite to this region. In these responses we see that there is a considerable amount of recovery after each transient stimulation, and also a certain degree of fatigue. Effect of continuous unilateral stimulation. — We shall next study the effect of continuous unilateral stimulation. For this experiment I took the hypocotyl of Tamarindus indica. As the growing region in this case is extended,

Fig. 249. Response of Hypocotyl of Tamarindus indica to Continuous Stimulation of Thermal Radiation I used a thermal radiator, which consisted of a linear platinum wire, 5 cm. in length, placed parallel to one side of the growing region. Under the action of continuous unilateral radiation, an increasing positive curvature—that is to say, towards the stimulus — was induced, till a maximum effect had been attained (fig. 249). With specimens of other plants I obtained either the neutralisation or the reversal of this curvature, as it might happen, in consequence of the internal diffusion of stimulus to the distal side. This neutralisation is brought about by equal excitation of the proximal and distal sides, in precisely the same manner as in the

case of light. It is seen in the return of the organ to its original position, or by oscillations about that position as the mean ; but in cases where the proximal becomes fatigued, the responsive contraction of the distal gives rise to a negative curvature. In all these cases we see thermal radiation inducing the same curvature effects — positive, neutral, and negative — as were found to be induced by visible light. Effect of electric waves on growth. — I shall next describe the effect of Hertzian waves in inducing responsive curvatures ; and first, in order to obtain the fundamental effect on growth itself, I took an electric radiator consisting of a rod 5 cm.

Connection with electric vibrator at downward arrow ( ! ) is seen to arrest growth and to induce contraction, by which the specimen undergoes an actual shortening. in length, excited by oscillatory discharge from a Ruhmkorff's coil. The specimen was a flower-bud of Crinum Lily, whose upper and lower ends were connected by means of thin wires with the two ends of the electric radiator. The natural growth-record under unbalanced conditions was first taken (fig. 250), and the specimen was now subjected to the action of electric waves at the point marked in the record with a downward arrow. It will be seen that this gradually diminished the rate of growth, till at the end of five minutes it was completely arrested ; and as the action of the electric waves continued, the contractile effect by which growth is arrested is seen to be carried further, actually bringing about a shortening in the length of the specimen.

Response of Mimosa to electric radiation.— In the last case, in order to subject the specimen to the intense action of electric waves, the radiator was electrically connected with the responding organ, which was diffusely stimulated by it, and exhibited a longitudinal response of contraction. If, however, we wish to observe the effect of the unilateral action, it will be necessary that the radiation shall act on one side only. A difficulty is here met with, however, owing to the relatively greater length of these electric waves, which, like those of sound, do not cast shadows, but curl round corners. In order to produce unilateral action, then, the length of these waves has to be reduced to a minimum. By using small spheres of platinum as the source of radiation, I succeeded in obtaining short electric waves of about 1 cm. in length ; but the intensity of such radiation is somewhat feeble, and with them I could only occasionally obtain responsive curvatures of growth. I was, however, more successful in obtaining responsive effects from the pulvinus of Mimosa. When the small radiator which I have described was placed at a distance of a few centimetres from the lower half of a pulvinus, and when the radiator was allowed to act, a depression of the leaf of this plant occurred after an exposure of nearly half a minute to the continuous action. It should be mentioned that for the demonstration of this effect it is the younger leaves which are suitable, the older not being sufficiently sensitive. With regard to the electric waves, it is to be borne in mind that to them water is opaque, while ebonite is highly transparent.

Hence the interposition of a sheet of ebonite, between the radiator and the plant, does not stop the responsive action, while a parallel-sided trough of water in the same place would effectually prevent the passage of the rays, and thereby put an end to the action. Action of high frequency Tesla current— If one electrode of a Tesla coil be put in connection with the growing organ, this electric variation of high frequency is found to cause an arrest of growth. Even the contiguity of wires carrying

these high frequency currents is often found to retard growth. It is probable that this is due to certain material emanations that proceed from the wire. A coil of iron wire was made to surround the growing organ, and during the excitation of this coil the normal growth-rate was found to be retarded ; but there was no such retardation when a glass cylinder was interposed between the specimen and the coil. During the short period of the experiment there was but little rise of temperature within the coil, and the retardation of growth could not have been due to any thermal radiation.

Variation of temperature acts as a stimulus. Below the optimum the direct effect of variation of temperature is a retardation of growth, and the negative after-effect, owing to absorption of stimulus, is an acceleration of growth. Hence repeated variations of temperature below the optimum have little resultant effect on growth. Above the optimum, stimulus is not held latent, and there is no negative after-effect of accelerated growth. Hence, repeated variations of temperature above the optimum will retard growth.

While rise of temperature up to the optimum accelerates growth, thermal radiation, as such, acts as a stimulus, and retards growth. The unilateral action of thermal radiation is similar to that of luminous radiation — that is to say, thermal radiation, when of moderate intensity, gives rise to positive, and when strong, to neutral, or negative movement. Electric waves, induce retardation of growth. The unilateral application of electric radiation is also found to induce responsive movements.

Photonasty and para-heliotropism — Response of Troptcolnm tnajus — Responses of plagiotropic stems: (a) Mimosa— (b) Ipomo:a—{c) Cucurbita— Daily periodic movements of plagiotropic stems— Responsive movements of pulvinated organs — Pulvinated organs showing positive heliotropic movement : (a) Response of terminal leaflet of Desmodium—{b) Response of leaflet of Robinia—(c) Re- sponsive movements of leaflets of Erythrina indica and Clitoria ternatea — The negative heliotropic type of response : (a) Response of pulvinus of Mimosa — (b) Diurnal sleep of Oxalis— (c) Diurnal sleep of Biophytum —Directive versus non-directive action of light — General view of responsive curvatures induced in different organs by unilateral application of light.

I SHALL now enter upon the investigation of a very large class of phenomena, brought about by the action of light, which have hitherto been regarded not only as obscure, but also as totally unrelated to each other. These phenomena may be described in a general way as the exhibition of the differential effects of light on anisotropic or dorsi-ventral organs. Such effects may again be divided for convenience into two classes, according as they are exhibited either by growing or by mature and pulvinated organs. The responsive curvature in the former of these cases, due to the differential growth induced by light, we shall designate as photonastic. It must be understood that there is in reality, as we shall see, no fundamental difference between the responsive curvatures induced in growing organs, whether radial or dorsi-ventral, and those in pulvinated organs ; but it is nevertheless suitable for the purposes of this investigation to treat them under separate headings.