Plant Autographs and Their Revelations

This will be better understood from the following illus¬ tration. If we place some sand-grains on a flat board and begin to tilt it, the particles start sliding down only after a certain critical angle has been reached. If the board is rough, this critical angle will be large; if it is smooth, it will be small. Moreover, by the scouring action of the sand, the rough surface may become smoothed down after numerous repetitions of the experiment, the result being a diminished degree of the critical angle.

their critical angle for the immediate perception of geo¬ tropic stimulation. Experiments which I carried out with various plants show this angle to be about 31°; there is no electrical response to geotropic stimulation until the organ is tilted to this angle. But when the experiment is repeated with the same specimen, the critical angle is reduced to about 25°. The results of these experiments is, therefore, to strengthen the evidence in favour of the statolith-theory.

The plant is constantly adjusting itself, by appropriate movements, to the various forces of the environment which are acting upon it. Such movements are very slow and the final results become marked only after a lapse of time. When taking a walk in the garden we are struck by the constant changes that are taking place in the posture of various organs of plants. Here is a tendril that has touched the twig of a neighbouring shrub (fig. 6o) ; the contact has been perceived by the tendril, which has clasped the support and twined round it, thereby helping the plant to climb up so as to get as much light as possible. There, a potted plant in a dark shed, which has so bent and twisted its leaves that their upper surfaces are fully exposed to the incident light coming through a hole in the shutter (fig. 6i ). In yet another place, the plant has turned away so as to avoid excess of light. Here a plant has been accidentally upset and the horizontally laid stem is bending upwards so as to grow erect once more.

These movements, induced by the stimuli of the environ¬ ment, are extremely varied and complicated. The forces in operation are manifold: — the influence of changing temperature, the stimulus of contact, of gravity, and of light, visible and invisible. Again, an identical stimulus produces sometimes one effect and at other times precisely the opposite. It thus appears hopeless to unify these very diverse phenomena, and there has been a tendency towards a belief that there is not any general physiological reaction, but that it is the individuality of the plant that determines the choice of the movement for its own advantage. Such a teleological assumption is no explanation at all.

Various descriptive phrases have been in use to designate the diverse movements, such as positive heliotropism, when the organ turns towards the light, negative heliotropism when it moves away from it. Similarly, the terms positive and negative geotropism have been employed to denote the movement of different or¬ gans towards or away from the earth. These merely descriptive phrases must not, however, be regarded as of¬ fering any real explanation of the phenomena. Such misuse of descriptive phrases ‘mav even be mischievous,’ protests Bayliss in his book on physiology, ‘leading to the belief that new knowl¬ edge has been obtained when a phenomenon is described by a name derived from the classical tongue instead of in English.’

It may perhaps be possible to offer a rational explana¬ tion of the varied move¬ ments of plants. The baf¬ fling character of the prob¬ lem is, in reality, due to the combined action of differ¬ ent factors, the individual reactions to which are unknown to us. In this connection there are two questions to be investi¬ gated, namely, the reactions under different kinds of stimu¬ lation, and the curvature induced by one-sided stimulation. Much confusion has arisen from the uncertainty as to the effects of different modes of stimulation ; the stimulus of contact, for example, may produce a specific reaction different from that of light. Such specific reactions have, in fact, been assumed for different stimulations. In reality.

there Is no such difference, for results described in a previ¬ ous chapter show that all kinds of direct stimulation of a certain intensity give rise to contraction, actual or incipient ; for instance, this incipient contraction is expressed in a growing organ by retardation of the rate of growth. I have already explained how there are various contrivances by which external stimulation reaches the sensitive tissue in the interior, often with increased intensity (see p. 107).

I shall next consider the question of the bending or curva¬ ture of an organ induced by one-sided stimulation. Let us take a concrete example and imagine the right side of a stem to be acted upon by a stimulus, say mechanical con¬ tact or light. The right side of the stem is stimulated directly and consequently there is induced a local contrac¬ tion, actual or incipient. The normal rate of growth of the right side is now retarded, so that if the left side grows as usual, the stem will become curved, the stimulated side being concave. The stem will thus become bent towards the stimulus, a bending which is often described as positive tropism. I shall presently show that there is a second con¬ tributory factor which accentuates the curvature.

In the case of positive tropism described above, we centred our attention on the right side of the organ which was directly stimulated. What happens on the left or distal side which was not directly stimulated? Does it remain passive, or is some indirect effect transmitted to it? This indirect effect, if it exists at all, will either help or hinder the production of curvature due to the direct action of stimulus. Nothing has hitherto been known about the effect of indirect stimulation.

With regard to the effect of indirect stimulation on nor- rnal growth in length, I have shown (fig. 52) that while direct stimulation induces a diminution of turgor, with con¬ traction and retardation of growth, indirect stimulation produces the diametrically opposite reaction of increase of turgor, with expansion and enhancement of rate of growth. This is diagrammatically represented in fig. 62, a, b. Direct stimulation induces contraction and shortening of the organ, while indirect stimulation causes expansion and elongation

(a) Stimulus applied directly at the growing region induces retarda¬ tion of growth or contraction, as represented by dotted line. Stimu¬ lated area represented in this and following by shading. (b) Stimulus applied indirectly (at some distance from growing region) gives rise to acceleration of growth and expansion. (c) Stimulus applied to right side of organ causes contraction of that side, thus giving rise to positive curvature towards stimulus.

(d) Excitation transmitted to the opposite side causes neutralisation. (e) Excitation caused by intense stimulation is transmitted across and thus reverses the positive curvature to negative, i.e. away from stimulus. Parallel effects may be expected when a stimulus acts on one side of the organ. It is the change of turgor induced by stimulation which is the basis of all responsive movements. I have succeeded in devising a striking experi¬ ment demonstrating the opposite reactions induced by direct and indirect stimulation.

We shall now consider the opposite effects of direct and indirect stimulations. In Mimosa, diminution of turgor is visually manifested by the fall of the leaf, increase of turgor by a movement of erection. Figure 63 gives a dia¬ grammatic view of the stem of iMimosa with an attached leaf, at the base of which is the motile pulvinus. If the stimulus, say strong light, be applied on the right-hand Fig. 63. Effect of one-sided Indirect Stimulation of stem of Mimosa.

a Diagrammatic representation of experiment. h Record showing preliminary erectile response due to indirect stimulation, and subsequent excitatory fall caused by trans¬ verse conduction of excitation after continuous stimulation. side, the pulvinus is directly stimulated and the leaf under¬ goes a fall. If then the stimulus be transferred to the oppo¬ site side of the stem at a point diametrically opposite to the motile leaf, the record shows an erectile movement of the leaf, indicative of increase of turgor at the distal side. The effects of direct and indirect stimulation are thus opposite. The effect of continuous indirect stimulation in causing the fall of the leaf will be presently explained.

Thus one-sided stimulation by light causes a contraction of the proximal and expansion of the distal side. The two effects conspire to produce a positive curvature towards the stimulus (fig. 62 c). This normal bending towards the light is described as positive heliotropism. Similar effects are produced under other modes of stimu¬ lation. Thus when a tendril is stimulated on one side by contact, then the side touched grows more slowly, and the oposite side faster, than the normal. The effects on the two opposite sides thus conspire to bring about a movement towards the stimulus and the curling of the tendril round the support.

It is, therefore, not necessary to assume diverse sensi¬ bilities and reactions to different modes of stimulation. All stimuli only produce contraction when they are direct and sufficiently intense. The simple law that direct stimu¬ lation induces contraction and retardation of growth, and indirect stimulation expansion and acceleration, explains all the 'positive’ tropisms or turning of the organ towards the stimulus. When an organ turns away from light, the phenomenon is called ‘negative heliotropism,’ the assumption being that the organ is possessed of a specific sensitiveness of a quite different kind. But an identical organ cannot be possessed of both positive and negative sensitiveness. And yet, if an organ is continuously exposed on one side to strong light, it is found that it moves at first towards the light, then the over-excited organ begins to turn away, as if to avoid an excess of light. The same organ is thus positively helio¬ tropic for a time and negatively heliotropic afterwards.

The organ bends towards a light of certain intensity, it bends away from light of greater intensity. This latter result is due to the percolation of excitation from the proximal to the distal side of the organ. In regard to the transmission of excitation, the ordinary tissue of a plant is not such a good conductor of excitation as a nerve : but the difference is one of degree only, not one of kind. Nervous tissue transmits excitation to a distance, even under feeble stimulation, while conduction takes place in ordinary tissues with difficulty and under strong and con¬ tinued stimulation. Thus the effect of moderate stimulation remains more or less localised at the directly stimulated side of the stem. But the excitation caused by excessive or long continued stimulation may overcome the block, and thus percolate and traverse the organ in a transverse direction.

The effect of this transversely transmitted excitation is demonstrated by the record of response of the leaf of Mimosa (see fig. 63), in which a point of the stem directly opposite to the responding leaf was subjected to continuous stimulation by light. The first effect was the erectile response of indirect stimulation; excitation was then con¬ ducted across the stem and caused the sudden fall of the leaf. In the case of the bending of the stem under one-sided action of light, the first effect is a positive curvature. But under continued stimulation, the transversely conducted excitation induces contraction of the further side, causing neutralisation of the former bending (fig. 62 d).

Sometimes the near or proximal side becomes fatigued by over-excitation, while the far or distal side is still fresh. The result of this is not merely neutralisation, but an actual bending away from the light by the greater contraction of the distal side (fig. 62 ^). In an actual experiment carried out with a seedling of Indian corn {Zea Mais), the plant, under continuous action of strong light from one side, exhibited at first a move¬ ment towards light. This reached its maximum after fifty minutes. The plant then began to turn back and return to the position from which it started. This neutralisation was completed in a further period of forty-three minutes. After this, the response of the plant was a negative one.

The movements of life, then, are not capricious, but are determined by very definite physiological reactions. The elusiveness of the problem which had baffled us was due to ignorance of the numerous factors that are in operation. It has been shown how seemingly opposite results are brought about by the mere difference of the intensity and duration of stimulus. In actual conditions of life, numerous other complicating factors are also present. Thus the sign of the response is modified by the point of application of stimulus, by the transverse conductivity of the organ, and by the tonic condition of the plant. It is the different combinations of these factors that produce the numerous variations in the resulting response, which at first sight appear so perplexing.

The movements described in this chapter are such as are induced by one-sided stimulation ; these, however, by no means exhaust all the phenomena of plant-movement, for there are others which are induced, not by one-sided, but by diffuse stimulation exercised by the environment. A strik¬ ing example of this is found in the periodic opening and closing of the flower of Water-Lily, fully described in the next chapter. The poets have forestalled the men of science. Why does the Water-Lily keep awake all night and close her petals during the day? Because, say they, the Water-Lily is the lover of the Moon, and as the human soul expands at the touch of the beloved, so the Lily opens out her heart at the touch of the moonbeam and keeps watch all night long; she shrinks affrighted from the rude touch of the Sun and closes her petals during the day. The outer floral leaves of the Lily are green, and in the day-time the closed flow¬ ers are hardly noticeable among the broad green leaves which float on the water. In the evening, the scene is trans¬ formed as if by magic, and myriads of glistening white flowers cover the dark water (fig. 64). This phenomenon, recurring every day, has not only been observed by the poets, but an explanation has been offered for it : the Lily loves the moon and is frightened by the sun !

Had the poet taken out a lantern in the dark night, he would have noticed that the Lily opened even in the total absence of light from the moon! But the poet is not ex¬ pected to carry a lantern and peer about in the dark; that inordinate curiosity is characteristic only of the man of science. The Lily, on the other hand, does not close with the appearance of the sun, for the flower often remains awake up to eleven in the forenoon.^ A French dictionary-maker consulted Cuvier, the zoolo¬ gist, upon his definition of a crab as ‘a little red fish which

^ The Water-Lilies of Europe close at night and open in the day-time. The cause of this difference will be explained in the course of the chapter. walks backwards.’ ‘Admirable!’ said Cuvier; ‘only the crab is not necessarily little, nor is it red till boiled ; it is not a fish, and it cannot walk backwards ; with these exceptions, your definition is perfect.’ And so with the poet’s descrip- tion of the movement of the Lily; it does not open to the moonlight nor yet close to the sun.

The sleep and waking of the Water-Lily is by no means an isolated occurrence. My attention was first drawn to another remarkable floral display by the folksong which begins : For now behold the gold-starred fields Of opening Jhinga flower. Now I witness, every evening, a glorious transformation in my experimental garden at Sijbaria on the Ganges. The gardener has planted a large plot with Jhinga (Luff a acutan- gula). The flowers when closed in the day-time are very inconspicuous, the outer floral leaves being dull green; in my afternoon walk, I can hardly recognise the old familiar field; yet a little later, it is covered with masses of flowers in their golden glory. They remain open throughout the night, but close early in the morning; the fairy field of cloth of gold seems suddenly to have vanished.

We will now try to understand the phenomenon of the opening and dosing of the Water-Lily. Numerous envi¬ ronmental stimuli act upon plants, and one of these is the action of light, of which the familiar example is furnished by the turning of the Sunflower towards the light. It has been found that of all the rays present in white Irght which are effective in producing the movement of plant-organs, the blue and violet rays are the most potent; the yellow and the red are practically ineffective. Now the moon¬ light, besides being feeble, contains but little of the effective rays; hence the light of the moon cannot cause the move¬ ment of the floral leaves.

The only effective light is that of the sun, but the open¬ ing and closing of the Water-Lily has little connection with the rising or setting of the sun. The opening cannot be due to the setting sun, for the flower is open in the fore¬ noon; neither can it be due to the rising sun, since the flowers are already open when it rises. The daily move- ments of the flower cannot, therefore, be due to the alternat¬ ing action of light and darkness. The plant is affected not merely by a single form of stimulation. The phenomena of plant-movement, as pre¬ viously stated, have remained obscure on account of the numerous factors which contribute to induce them. This will be better understood if we consider only two out of numerous factors operating on the plant — the stimulus of gravity and that of light. Certain organs are highly sen¬ sitive to geotropic stimulation, while others are feebly sen¬ sitive to it. The stronger reaction will be represented by G and the feeble by g. In regard to light, there are two dis¬ tinct classes of effect: positive heliotropism when the organ turns towards light, and negative heliotropism when the organ turns away from it. Their effects when strong will be represented by + L and — L ; when feeble, by -|- / and

What will be the resulting effect when a horizontal stem is exposed to combined geotropic and heliotropic stimula¬ tion ? In geotropic response the stem will bend upwards ; should the organ be positively heliotropic, the curvature under vertical light will also be upwards. Geotropism and heliotropism will thus conspire, the joint effect being G -j- L : but should the organ be negatively heliotropic, the result¬ ant will be G — L. If further account be taken of the relative sensitiveness of the organ to the stimuli of gravity and light, we shall have the following possible combina¬ tions :

Eight different effects can thus be produced by the com¬ bination of only two factors ; there are, however, other factors present, such as the effects of rise and fall of tem¬ perature. x\dditional complications are introduced by the unequal sensitiveness of the two sides of the organ ; in some, it is the upper side, in others it is the lower side that is more excitable and therefore reacts more effectively. There are thus at least ten factors in operation, and the different combinations possible would exceed a thousand.

It is no wonder that the movements of plants appear so extraordinarily complex. Efforts to discover the true ex¬ planation have long been baffled by the fact that it has hitherto been impossible to isolate and study the effect of each of the factors so as to analyse their complex result. I stated that the different possible variations of results produced by various combinations would exceed a thousand. Life would not be long enough to go through all of them one by one, on the chance of discovering the solution of a particular problem. It is possible, however, by simple pre¬ liminary tests, to eliminate the ineffective factors, and thus to bring the inquiry within narrow limits. Let us now consider the various factors which are likely to be present ; these are the effects of stimulation by gravity and by light, and of variation of temperature.

Does the stimulus of gravity exert any marked effect on the movement of opening and closing of the flowers? The petals close up in the middle of the day, each of the petals standing erect. If the flower were susceptible to the stimulus of gravity, then, on turning the flower upside down, the closed petals in their inverted position would start to curl upwards and outwards, thus opening the flower. But no such effect takes place. M^e may next enquire whether variation of the intensity of light could induce the move¬ ment of the petals. Light appears in the morning and dis¬ appears in the evening. If the movement of the petals were entirely dependent on light, two opposite effects would be produced in the morning and evening respectively. But the flower is open at both these periods. Moreover, the course of the opening and closing movements does not