The Vegetable World: Being a History of Plants, with Their Botanical Descriptions and Peculiar Properties

Fig. 122.—Dionea musvipula. liquid which attracts the insects. If a fly lights on this singular apparatus, the trap raises itself quickly by means of its long hinge. They approach, and it closes, rapidly crossing its long cilia, and the insect is a prisoner. The efforts of the fly to escape, increases the irritability of the plant, whose fangs only open when the movements of the animal have ceased, with its life.

Who that knows, who that has seen the Sensitive plant, Mimosa sensitiva, Fig. 123, has not also remarked on the strange sensibility of its leaves? The lightest touch suffices to make its folioles close upon their supports, the petiolar twigs upon the common petiole, and the common petiole upon the stem. If we cut with scissors the extreme end of one foliole, the others immediately approach in succession. De Candolle was in the habit of placing a drop of water upon one of the folioles of the Sensitive plant, applying it with so much delicacy as to excite no movement whatever. But

when he substituted for the water a drop of sulphuric acid, he observed that the folioles shrivelled up, the partial petioles as well as the common petiole was lowered, and gradually submitted to its influence, without the folioles situated below participating at all in the movement. This experiment shows very clearly that the irritation is not local, but communicates from circle to circle in the various elements of a leaf, and propagates itself from one leaf to another.

During the time that these movements are in operation, it will be observed that the limb of the foliole neither curves itself nor shrivels. In short, the contractile power resides at the point of insertion of the folioles upon the secondary petioles, these upon the common petiole, and these again upon the stem. These various points of insertion correspond to the very perceptible cylindrical cushions which during the season of rest swell below, while in the state of irritation they are distended above. The movement which we provoke in the case of the Sensitive plant manifests itself with much greater rapidity when irritated upon this cushion than in any other part of the plant.

We have remarked that the more vigorous the Sensitive plant is in its habit, the more susceptible is it; the higher the temperature, the more promptly does it respond to the touch. We may observe besides that the Sensitive plant can, up to a certain point, get accustomed to the movement. Desfontaines, carrying a Sensitive

plant in a carriage, observed that the plant closed its folioles, and all its leaves drooped as soon as the carriage began to roll over the pavement, but by degrees it seemed to recover from its fright, became habituated, so to speak, to the movements, its leaves resumed their erect position, and its folioles their full expansion. Desfontaines now caused the carriage to stop for a time. When it resumed its motion the plant responded by dropping its leaves as before, but after a time they expanded again, and so continued during the remainder of the journey. Shall we not say that here there is reflected impression and motive on the part of this singular plant? These phenomena of irritability under the influence of direct chemical or mechanical action, the plant repeats of itself during the night. The Sensitive plant closes its folioles when the obscurity of night sets in.

This habit of folding up its loayés during the night is not confined to the Sensitive plant exclusively; it appertains to other plants whose leaves occupy different positions during day and night. These are the plants to which Linnzus alludes when he writes of the sleep of plants. ‘‘ But we must remark,” says De Candolle, “that this term, borrowed from the animal kingdom, does not represent the same idea in both. In animals sleep indicates a flaccid drooping state of the members, of limpness in the articulations; in vegetables it indicates a changed state; but the nocturnal state majD-tains the same degree of rigidity and the same constancy as the diurnal position. We may break the sleeping leaf rather than maintain it in the position which belongs to it during the day.”

It was in the Bird’s-foot trefoil, the pretty Trigonella ornithopodioides, that Linnzeus remarked for the first time the difference between the altitude of the leaves during the day and night. Scarcely had he made this remark when he came to the conclusion that this phenomenon would be found not to be confined to this single plant, but would be found general in vegetable life. From that time, every night Linnzus tore himself from sleep, and in the silence of nature studied the plants in his garden. At each step he discovered a new fact. Each natural fact, when put in evidence by a first observation, has been rapidly confirmed by crowds of facts quite analogous to the first, and Linnzeus very 8002 satisfied himself that the change in the position of leaves during

the night was observable in a considerable number of vegetables, and that in the absence of light, plants quite changed their physiognomy, so that it became very difficult to recognise them from their bearing. He further states that it was the absence of light:and not the nocturnal cold which was the principal cause of the phenomena, for plants in hot-houses closed themselves during the night just like those which were exposed in the open air. He recognised also that this difference is much less apparent in young plants than in more matured ones. The illustrious Swedish botanist made many observations on the diversity of position that leaves affect during the night, and he has even attempted a classification of these differences. The most general idea which he sought to establish, was, that the positions differed according as the leaves were simple or compound. Linnzus thought that the object in these circumstances was to place the young shoots under shelter from nocturnal cold and from the effects of the air. It is among the composite leaves, in short, that the difference between the waking and sleeping is most clearly indicated. The folioles of the Trefoil stand erect, curving in a longitudinal direction in such a manner as to form a sort of cavity or cradle. The folioles of the Melilotus are half erect, but divergent at their summits. In the Oxalis, Fig. 124, the folioles usually rest upon a common petiole, in such a manner as to turn their lower surfaces inwards, and show only their upper surfaces. In the Bladder Senna (Colutea arborescens) the folioles rise vertically in such a 3 manner as to rest perpendicularly upon the common petiole, the

A upper surfaces turned towards each other. The Cassias have, on‘ 74 the contrary, the folioles depressed and folding back on the lower 4 surfaces. The folioles of the Mimosa rest the,length of their petioles in the direction of their summits in such a manner that the two extreme folioles are directed forward, folding back upon their upper surfaces, and the others falling upon the back of the folioles which range near the summit.

The leaves of the Orach root (Atriplex hortensis) fall back upon the young shoots and enclose them, as if to protect them from the effects of the atmosphere. The Chickweed closes its leaves during the night, and _ opens them in the morning. The Evening H

Primrose (C@nothera) has similar properties, and like the Trefoil forms during the night a sort of cradle for the reception of the leaves. On the contrary, the genus Sida and the Lupinus reverse their leaves. On the other hand, many of the Mallows roll their leaves into the form of a coronet. The Vetch, the Sweet Pea, the Broad Bean, rest their leaves during the night one against the other, and seem to sleep. This strange sleep of plants vaguely recalls to us the sleep of animals. Inits i sleep the leaf seems, by its to disposition, _ the approach &.\ age of infancy. “ It folds itself : up, nearly as it lay folded in the bud before it opened, when it slept the lethargic sleep of winter, sheltered under the robust and hardy scales, or shut up in its warm down. We may say that the plant seeks every night to resume the position which it occupied in its early days, just as the animal rolls itself up, lying as if it lay mn its mother’s bosom. What is the cause of the

Fig. 124.—Closed leaves of Oxalis. which we desig- phenomenon nate the sleep of plants? It occurs in all hygrometrical conditions of the atmosphere, and the hours during which it affects them ise not influenced by any change of temperature. De Candolle supposed that the absence of light was the direct cause of the pheno menon. To assure himself of this, he subjected plants whose leaves are disposed to sleep, to the action of artificial light, furnished by two lamps which were, when united, equal to five-sixths of daylight. The results were very varied. “When I exposed the Sensitive

plants to the light during the night, and to the shade during the day,” says De Candolle, “I observed that at first the plants opened. _ and closed their leaves without any fixed rule, but after a few days they seemed to submit to their new position, and opened their leaves in the night, which was day to them, and closed them in the morning, which was their night. When exposed to a continuous light they had, as in their ordinary state, alternations of sleeping and waking; but each of the periods were shorter than ordinary. On the other hand, when exposed to continued obscurity, they still presented the alternations of sleeping and waking, but very irregularly.” De Candolle adds that he was unable to modify the sleep of two species of Oxalis either by light or darkness, or by light at other than the natural periods. We may conclude with him, from these facts, that the movements of sleeping and waking are connected with some disposition inherent in the vegetable, but which is thrown into special activity by the stimulating action of light, which acts with different intensity on different vegetables, so that the same amount of light produces different results in different species. Having thus minutely studied the exterior character of leaves we shall endeavour to penetrate a little into their structure, and unveil the delicacy of their arrangements. A cellular tissue, to which we give in this case the name of parenchyma, from the soft, pulpy, and closely approximating cells of which it is composed, and which have been aptly compared to a mass of soap bubbles pressing against each other, fills all the interstices of the leaves left by the spreading xervures. It is covered, consolidated as it were, and protected against all external influence by the epidermis, a covering which spreads itself like a protecting mantle over the whole surface of the plant. Let us submit this parenchyma, nervure and epidermis, to microscopical examination, first considering the leaves of those vegetables which live on the air. If we pull to pieces with some care any leaf whatever, a fragment of transparent membrane without colour will be observed to detach itself from the leaf. If we place this moist shred upon a glass plate, and subject it to a magnifying glass, it will be found composed of large flattened cellules, having a contour, sometimes : H 2

rectilinear and figuring as a square, sometimes irregular and sinuous, as in Fig. 125. The granular contents of these cellules are neither very apparent nor very important, but we find in them betimes an aqueous liquid variously coloured. The cellular elements of this epidermous membrane are intimately united and pressed one against the other, in such a manner as to give it a certain solidity and power of resistance. The cells present an exterior wall with the air, which in connection is much thicker than the lateral or interior walls. Some of these cells to be elon- observed are sometimes gated, ramified, partitioned off, as it were, so as to constitute hairs or down of various forms. membrane is not The epidermic continuous or perfectly close. It