Animal Behaviour

by C. Lloyd Morgan

IV.—The Behaviour of Plants

A short parenthetic section on the behaviour of plants may serve further to illustrate the nature of organic behaviour. We have seen that Paramecium is apparently attracted by faintly acid solutions, and have briefly considered Dr. Jennings’s interpretation of the facts disclosed by careful observation. In the ferns the female element, or ovum, is contained in a minute flask-shaped structure (archegonium), in the neck and mouth of which mucilaginous matter, with a slightly acid reaction, is developed; and this is said to exercise an attractive influence on the freely swimming ciliated male elements, or spermatozoids, which are necessary for fertilization. “Now, it has been shown by experiment that the spermatozoids of ferns are attracted by certain chemical substances, and especially by malic acid. If artificial archegonia are prepared (consisting of tiny capillary glass-tubes) and filled with mucilage to which a small quantity of this acid has been added, they are found, when placed in water containing fern-spermatozoids, to exercise the same attraction upon them which the real archegonia exercise in nature. The malic acid gradually diffuses out into the water, and the spermatozoids are influenced by it, so that they move in the direction in which the substance is more concentrated, i.e. towards the tube. Although it cannot be proved that the archegonia themselves contain malic acid, as they are too small for a recognizable quantity to be obtained from them, yet there can be little doubt that the natural archegonia owe their attractive influence to the same chemical agent which has proved efficacious in experiment.”[5] In the light of Dr. Jennings’s observations, it is perhaps not improbable that this so-called attractive influence is similar to that seen in Paramecium; and that the spermatozoids enter the organic acid in the course of their random movements, and there remain. Be that as it may, the male elements collect in the mucilaginous mass, and pass down the neck of the flask until one reaches and coalesces with the female element, or ovum, and effects its fertilization. Here we have organic behaviour unmistakably directed to a biological end—behaviour which may indeed be accompanied by some dim form of consciousness, but which is due to a purely organic reaction. It is scarcely satisfactory to say that the spermatozoids “possess a certain power of perception, by which their movements are guided.”[6] If consciousness be present, it is probably merely an accompaniment of the response, and has no directive influence on its nature and character.

In the higher plants, as in the higher animals, the differentiation and the orderly marshalling of the cell-progeny arising from the coalescent male and female elements, afford, during development, examples of corporate organic behaviour which can be more readily described than explained, but which not less clearly subserve definite biological ends, and in many cases, such as the direction of growth in radicles and roots, the curling of tendrils, and the reaction to the influence of light and warmth, are related to and evoked by the environing conditions. More closely resembling familiar modes of behaviour in animals are such movements as are seen in the “tentacles” which project from the upper surface and margin of the Sun-dew leaf. Their knobbed ends secrete a sticky matter, which glistens in the sun, and to which small foreign bodies readily adhere. If particles of limestone, sand, or clay, such as may be blown by the wind, touch and stick to these knobs, there follows an exudation of acid liquid, but no marked and continuous change occurs in the position of the tentacles. But should an insect alight on the leaf, or a small piece of meat be placed upon the tentacles, not only is there a discharge of acid juice, but a ferment is also produced, which has a digestive action on the nitrogenous matter. Slowly the tentacle curves inwards and downwards, as one’s finger may bend towards the palm of one’s hand; neighbouring tentacles also turn towards and incline on to the stimulating substance; then others, further off, behave in a similar way, until all the tentacles, some two hundred in number, are inflected and converge upon the nitrogenous particle. Nay, more: “When two little bits of meat are placed simultaneously on the right and left halves of the same Sun-dew leaf, the two hundred tentacles divide into two groups, and each one of the groups directs its aim to one of the bits of meat.”[7]

Fig. 6.—Sun-dew (Drosera). Leaf (enlarged) with the tentacles on one side inflected over a bit of meat placed on the disc. (From Darwin’s “Insectivorous Plants.”)

The movements, though slow, are orderly, methodical, and effective, the secretions of many glands being brought to bear on just those substances which are capable of digestion and absorption by the plant. The seemingly concerted action is moreover due to an organic transmission of impulses from cell to cell—a transmission accompanied by visible changes in a purple substance contained within the cells. In the Sun-dew any tentacle may form the starting-point of the spreading wave of impulse. But in the Venus’s Fly-trap there are six delicate spines, the slightest touch on any one of which causes the two halves of the specially modified leaf-end to fold inwards on the midrib as a hinge. The transmission of impulse is more rapid, the trap closing in a few seconds; and electric currents have been observed to accompany the change. Tooth-like spines at the edge of the trap interlock, and serve to prevent the escape of small insects, while short-stalked purple glands secrete an acid digestive juice. Division of labour has been carried further; and organic behaviour, not less purposive, is carried out in a manner even more effective.

Fig. 7.—Venus’s Fly-trap (Dionæa). Leaf viewed laterally in its expanded state. (From Darwin’s “Insectivorous Plants.”)

In other plants adaptive movements are well known. “Few phenomena have such a peculiar appearance as the movements which occur in the sensitive Oxalis when rain comes on. Not only do the leaflets on which the finest rain-drops fall fold together in a downward direction, but all the neighbouring ones perform the same movement, although they have not themselves been shaken by the impact of the falling drops. The movement is continued to the common leaf-stalk bearing the numerous leaflets. This also bends down towards the ground. The rain-drops now slide over the bent leaf-stalk and down over the depressed leaflets, and not a drop remains behind on their delicate surfaces.”[8] The waves of impulse are said to be transmitted along definite lines, and to cause the expulsion of water from certain cells at the point of insertion of the leaflets or leaf-stalks, rendering them flaccid.

Fig. 8.—Flower of Valisneria.

Appealing even more strongly to the popular imagination, though probably not of deeper biological significance, is the behaviour of plants in relation to the essential process of fertilization. Only two examples can here be cited. Valisneria spiralis is an aquatic plant, with long submerged strap-like leaves, which grows in still water in Southern Europe. The female flower is enclosed in two translucent bracts, which form a protective bladder so long as the flower is beneath the surface of the water; but the flower-stalk continues to grow until the flower reaches the surface, when it becomes freely exposed by the splitting of the bracts. There are three boat-shaped sepals, which act as floats; three quite minute petals; and three large fringed stigmas, which project over the abortive petals in the space between the boat-like sepals. The flower is now ready for fertilization.

The male flowers, which are developed on different individuals from those which produce the female flowers, grow in bunches beneath an investing bladder. The stalk does not elongate, so that the bladder never rises far above the bottom, and remains completely submerged. Here the bladder bursts, and the male flowers, with short stalks, are detached. Each has three sepals, which enclose and protect the stamens. The separated flower now ascends to the surface, the sepals open and form three hollow boats, by means of which the flower floats freely, while the two functional stamens project upwards and somewhat obliquely into the air, exposing the large sticky pollen-cells. Blown hither and thither by the wind, these little flower-boats “accumulate in the neighbourhood of fixed bodies, especially in their recesses, where they rest like ships in harbour. When the little craft happen to get stranded in the recesses of a female Valisneria flower, they adhere to the tri-lobed stigma, and some of the pollen-cells are sure to be left sticking to the fringes on the margins of the stigmatic surface.”[9]

This is a good example of purely organic behaviour admirably adapted to secure a definite and important biological end. Few will be likely to contend that it is even accompanied by, still less under the guidance of, any conscious foresight on the part of the plant. And the lesson it should teach is that, in the study of organic behaviour, adaptation to the conditions of existence is not necessarily the outcome of conscious guidance.

It is well known that the orchids exhibit, in their mode of fertilization, remarkable adaptations by which the visits of insects are rendered subservient to the needs of the plant. In the Catasetums, for example, the male flower may be described as consisting of two parts—a lower part, the cup-like labellum (Fig. 9, l), which constitutes a landing-stage on which insects may alight; and an upper part, the column (Fig. 9, c), surrounded by the upper sepal and petals. In the upper part of the column the pollen-masses are borne at one end of an elastic pedicel, at the other end of which is an adhesive disc, and the rod is bent over a pad so as to be in a state of strain. The disc is retained in position by a membrane with which two long tubular horns (Figs. 9, h; 10, an) are continuous. These project over the labellum, where insects alight to gnaw its sweet fleshy walls, and if they be touched, even very lightly, they convey some stimulus to the membrane which surrounds and connects the disc with the adjoining surface, causing it instantly to rupture; and as soon as this happens, the disc is suddenly set free. The highly elastic pedicel then flirts the disc out of its chamber with such force that the whole is ejected, sometimes to a distance of two or three feet, bringing away with it the two pollen-masses. “The utility of so forcible an ejection is to drive the soft and viscid cushion of the disc against the hairy thorax of the large hymenopterous insects which frequent the flowers. When once attached to an insect, assuredly no force which the insect could exert would remove the disc and pedicel, but the caudicles [by which the pollen-masses are attached] are ruptured without much difficulty, and thus the balls of pollen might readily be left on the adhesive stigma of the female flower.”[10]

Fig. 9.—Flower of Catasetum; c, column; h, horns; l, labellum.

Here again we have adaptive behaviour of exquisite nicety, and we have the transmission of an impulse very rapidly along the cells of the irritable horns, followed by the sudden rupture of a membrane. Beautiful, however, as is the adaptation, effective as it is to a definite biological end, the organic behaviour does not afford any indication of the guidance of consciousness. Among plants we have many interesting and admirable examples of organic behaviour; but nowhere so much as a hint of that profiting by individual experience which is the criterion of the effective presence of conscious guidance and control.

Fig. 10.—Catasetum; C, diagram of column; a, anther; an, horn; d, adhesive disc; f, filament of anther; g, ovarium; ped, pedicel; D and E, pollinium; p, pollen-mass. (From Darwin’s “Orchids.”)

V.—Reflex Action

It is sometimes said that the tentacles of the Sun-dew leaf indicate a primitive kind of reflex action in plants, and that they afford evidence of discrimination. “It is,” says Romanes, “the stimulus supplied by continuous pressure that is so delicately perceived, while the stimulus supplied by impact is disregarded.”[11] And, comparing this with what is observed in the Venus’s Fly Trap, he says: “In these two plants the power of discriminating between these two kinds of stimuli has been developed to an equally astonishing extent, but in opposite directions.”[12] It is well, however, to avoid terms which carry with them so distinctively a conscious implication as “discrimination” and “perception” do for most of us. Just as the photographer’s film reacts differently according to the quality of light-rays, violet or red, which reach it, so do many organic substances react differently to stimuli of different quality, irrespective of their intensity. The “discrimination” of plants and of some of the lower animals is of this kind, and it is better to speak of it simply as differential reaction. There can then be no chance of its being confused with conscious choice.

Nor should the movements of the Sun-dew tentacles or of those of the Sea-anemone be termed in strictness reflex action. As originally employed by Marshall Hall, and, since that time, by common consent, reflex action involves a differentiated nervous system. There is, first, an afferent impulse from the point of stimulation passing inwards to a nerve-centre; secondly, certain little-understood changes within this centre; and thirdly, an efferent impulse from the centre to some organ or group of cells which are thus affected. In plants there is no indication of anything analogous to this specialized mode of response. The impulse passes directly from the point of stimulation to the part affected without the intervention of anything like a nerve-centre. In the sensitive Oxalis the impulse passes directly to the point of insertion of the leaflet or leaf-stalk; in Catasetum, from the horn to the retaining membrane; in the Sun-dew, from the affected tentacle to those in its neighbourhood. Even in the Sea-anemone, though there is a loosely diffused nervous system, the passage of the impulse from one part of the circlet of tentacles to other parts, seems to follow a direct rather than a reflex course, and there do not appear to be any specialized centres by which the impulses are received and then redistributed.

In all animals in which well-differentiated nervous systems are found, in which there are distinct nerve-fibres and nerve-centres, reflex actions, simple or more complicated, occur. They form the initial steps leading up to the highest types of organic behaviour. So long as the nervous arcs—afferent fibres, nerve-centre, and efferent fibres—remain intact reflex acts may be carried out with great precision and delicacy, even when the higher centres, which we believe to be those of conscious guidance and control, have been destroyed. When, for example, the whole of the brain of a frog has been extirpated and the animal is hung up by the lower jaw, if the left side be touched with a drop of acid the left leg is drawn up and begins to scratch at the irritated spot, and when this leg is held, the other hind leg is, with seemingly greater difficulty, brought to bear on the same spot. “This,” says Sir Michael Foster, “at first sight looks like an intelligent choice.... But a frog deprived of its brain so that the spinal cord only is left, makes no spontaneous movements at all. Such an entire absence of spontaneity is wholly inconsistent with the possession of intelligence.... We are therefore led to conclude that the phenomena must be explained in some other way than by being referred to the working of an intelligence.”[13] But if we concede that intelligence is absent, may there not at least be some consciousness? Sir Michael Foster’s reply to such a question goes as far as we have any justification for going, even when we give free rein to conjecture. “We may distinguish,” he says, “between an active continuous consciousness, such as we usually understand by the term, and a passing or momentary condition, which we may speak of as consciousness, but which is wholly discontinuous from an antecedent or from a subsequent similar momentary condition; and indeed we may suppose that the complete consciousness of ourselves, and the similarly complete consciousness which we infer to exist in many animals, has been evolved out of such a rudimentary consciousness. We may, on this view, suppose that every nervous action of a certain intensity or character is accompanied by some amount of consciousness which we may, in a way, compare to the light emitted when a combustion previously giving rise to invisible heat waxes fiercer. We may thus infer that when the brainless frog is stirred by some stimulus to a reflex act, the spinal cord is lit up by a momentary flash of consciousness coming out of the darkness and dying away into darkness again; and we may perhaps infer that such a passing consciousness is the better developed the larger the portion of the cord involved in the reflex act and the more complex the movement. But such a momentary flash, even if we admit its existence, is something very different from consciousness as ordinarily understood, is far removed from intelligence, and cannot be appealed to as explaining the ‘choice’ spoken of above.”[14]

These sentences indicate with sufficient clearness the distinction, more than once hinted at in the foregoing pages, between consciousness as an accompaniment, and consciousness as a guiding influence. We shall have more to say in this connection in subsequent chapters. The experiment with the frog shows, at any rate, that reflex actions, of a distinctly purposive nature, may be carried out when the centres, which are believed to exercise conscious control and guidance have been destroyed. It is said that in man, when, owing to injuries of the spine, the connection between the brain and the lower part of the spinal cord have been severed, tickling of the foot causes withdrawal of the limb without directly affecting the consciousness of the patient. But in all such cases we are dealing with a maimed creature. The living frog or man, healthy and intact, is, presumably in the one case, certainly in the other, conscious of these reflex actions, and can exercise some amount of guidance and control over them. In man this is unquestionably the case. But granting that the brain is the organ of conscious control, granting that it can receive impulses from and transmit impulses to the reflex centres, no more is here implied, and no more can be legitimately inferred, than that the kind of organic behaviour we call “reflex action” is in the higher animals in touch with the guiding centres. We have no ground for assuming that in reflex action there is any power of intelligent guidance independent of that which is exercised by the brain or analogous organ. In brief, reflex acts, in animals endowed with intelligence, may be regarded as specialized modes of organic behaviour; which are in themselves often characterized by much complexity; which subserve definite biological ends; which are effected by subordinate centres capable of transmitting impulses to, and receiving impulses from, the centres of intelligent guidance; and which, as responses confined to certain organs or parts of the body, form elements in the wider behaviour of the animal as a whole.