Читать книгу Animal Behaviour - C. Lloyd Morgan - Страница 10

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.”)