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shelter those more delicate organs from the chill air of night. The flowers of the crocus and similar plants expand beneath the bright beams of the sun, but close as soon as they are withdrawn. The Enotheras unfold their blossoms to the dews of evening, and wither away at the approach of day. Some Silenes roll up their petals in the day, and expand them at night. The florets of numerous Compositæ, and the petals of the genus Mesembryanthemum, are erect in the absence of sun, but become reflexed when acted upon by the sun's beams; and many other such phenomena are familiar to every observer of nature. It is probable, indeed, that a different effect is produced upon all plants by day and night, although it is less visible in some than in others: thus plants of corn, in which there is little indication of sleep when grown singly, exhibit that phenomenon very distinctly when observed in masses; their leaves become flaccid, and their ears droop at night. These effects have been generally attributed to the action of light; and it is probable that that agent contributes very powerfully to produce them; for a flower removed from the shade will often expand beneath a lamp, just as it will beneath the sun itself. De Candolle found that he could induce plants to acknowledge an artificial day and night, by alternate exposure to the light of candles. There must, however, be some cause beyond light, of the nature of which no opinion has yet been formed many flowers will close in the afternoon while the light of the sun is still playing upon them, and the petals of others will fold up under a bright illumination.

Spontaneous movements are far more uncommon than those which have just been described. In Megaclinium falcatum, the labellum, which is connected very slightly with the columna, is almost continually in motion; in a species of Pterostylis, shown me by Mr. Brown, I observed a kind of convulsive action of the labellum; the filaments of Oscillatorias are continually writhing like worms in pain; several other Confervas exhibit spontaneous movements: but the most singular case of the kind is that of Hedysarum gyrans. "This plant has ternate leaves: the terminal leaflet, which is larger than those at the side, does not move, except to sleep; but the lateral ones, especially in warm weather, are in continual motion, both day and night, even when the terminal

leaflet is asleep. External stimuli produce no effect; the motions are very irregular; the leaflets rise or fall more or less quickly, and retain their position for uncertain periods. Cold water poured upon it stops the motion, but it is immediately renewed by warm vapour."

To this class of irritability ought, perhaps, to be referred the curious phenomenon well known to exist in the fruit of Momordica elaterium, the spirting cucumber. In this plant the peduncle, at a certain period, when the fruit has attained its perfect maturity, is expelled, along with the seeds and the mucus that surrounds them, with very considerable violence. Here, however, endosmosis appears to offer a satisfactory explanation. According to Dutrochet, the fluid of the placentary matter in this fruit gradually acquires a greater density than that which surrounds it, and begins to empty the tissue of the pericarpium: as the fruit increases in size the same operation continues to take place; the pulpy matter in the centre is constantly augmenting in volume at the expense of the pericarpium; but, so long as growth goes on, the addition of new tissue, or the distention of old, corresponds with the increase of volume of the centre. At last growth ceases, but endosmosis proceeds; and then the tissue that lines the walls of the central cell is pressed upon forcibly by the pulp that it encloses, until this pressure becomes so violent that rupture must take place somewhere. The peduncle, being articulated with the fruit, at length gives way, and is expelled with violence; at the same time the cellules of tissue lining the cavity all simultaneously recover their form, the pressure upon them being removed, and instantly contract the space occupied by the mucous pulp; the consequence of which is that it also is forced outwards at the same time as the peduncle. It has been found by measurement, that the diameter of the central cavity is less after the bursting of the fruit than before.

Movements produced by touch, or by external violence, are very frequent. The sensitive plant (Mimosa pudica), which will rapidly fold up its leaves as if in a state of sleep, is, perhaps, the most familiar instance: but many others also exist. If the centre of the leaf of the Dionaea muscipula is irritated, the sides collapse, so as to cross the ciliæ of their

margin, like the teeth of a steel-trap for catching animals. Roth is recorded to have seen something of the same kind in Drosera rotundifolia. If the bottom of the stamens of the common berberry is touched on the inside with the point of a needle, they spring up against the pistillum. The valves of Impatiens noli-tangere, when the fruit is ripe, separate and spring back with great elasticity when touched. In this case the phenomenon is apparently capable of explanation upon a similar principle to the Momordica elaterium. In the fruit of Impatiens the tissue of the valves consists of cellules, that gradually diminish in size from the outside to the inside; and the fluids of the external cellules are the densest. The latter gradually empty the inner cellules and distend themselves, so that the external tissue is disposed to expand, and the internal to contract, whenever any thing occurs to destroy the force that keeps them straight. This at last happens by the disarticulation of the valves, the peduncle, and the axis; and then each valve rapidly rolls inwards with a sudden spontaneous movement. M. Dutrochet proved that it was possible to invert this phenomenon by producing exosmose: for that purpose he threw fresh valves of Impatiens into sugar and water, which gradually emptied the external tissue, and, after rendering the valves straight, at length curved them backwards.

The column of the genus Stylidium, which in its quiescent position is bent over one side of the corolla, if slightly irritated, instantly springs with a jerk over to the opposite side of the flower. In Kalmia the anthers are retained in little niches of the corolla; and, as soon as they are by any cause extricated, the filaments, which had been curved back, recover themselves with a spring. In certain orchideous plants, of the tribe called Vandeæ, the caudicula to which the pollen masses are attached will often, upon the removal of the anther, disengage themselves with a sudden jerk.

An elaborate exposition of the phenomena accompanying the movements of the sensitive plant has been given by Dutrochet, in his Recherches, &c. sur la Structure intime des Animaux et des Végétaux, et sur leur Motilité, which should be consulted by all readers desirous of studying the irritability of that very remarkable plant.

CHAPTER XIII. '

OF THE EFFECT OF POISONS UPON VEGETATION.

WHEN treating of irritability, some experiments of M. Marcet were quoted which had a reference to this subject. It is one, however, of sufficient importance to demand a chapter by itself.

At present we know scarcely any thing of the causes of the diseases of the vegetable kingdom; but it can scarcely be doubted, that to ascertain what are the specific effects of deleterious matters upon the vital powers of plants, is to lay the foundation of an acquaintance with their pathology.

M. Marcet's experiments proved that narcotic and irritating poisons produced an effect upon vegetables altogether analogous to that which they produce upon animals. The very valuable experiments with gases by Drs. Turner and Christison, mentioned formerly (p. 252.), lead to the same conclusion. These gentlemen remark, that "the phenomena, when compared with what was observed in the instances of sulphurous and hydrochloric acid, would appear to establish, in relation to vegetable life, a distinction among the poisonous gases nearly equivalent to the difference existing between the effects of the irritant and the narcotic poisons on animals. The gases which rank as irritants in relation to animals seem to act locally on vegetables, destroying first the parts least plentifully supplied with moisture. The narcotic gases,

-including under that term those that act on the nervous system of animals, destroy vegetable life by attacking it throughout the whole plant at once. The former, probably, act by abstracting the moisture of the leaves; the latter, by some unknown influence on their vitality. The former seem to have upon vegetables none of that sympathetic influence upon general life, which in animals follows so remarkably injuries inflicted by local irritants.

A similar result was arrived at by M. Macaire, whose very curious and instructive experiments are recorded in the Bibliothèque Universelle, xxxi. 244., and which I think of sufficient importance to be detailed at length.

The first plant used was the Berberis vulgaris. The six stamina of the flowers of this plant have the property of rapidly approaching the pistil when touched by the point of an instrument. The motion occurs at the base of the stamens. When cold, the motion is sometimes retarded. When put into water or solution of gum, the flowers may be preserved many days, possessing their irritability. The petals and stamens close at night to open again in the morning. Putting the stem of this plant into dilute prussic acid for four hours, occasioned the loss of the contractile property by irritation; the articulation became flexible, and might be inclined in any direction by the instrument. The leaves had scarcely begun to fade. On placing the expanded flowers on the prussic acid, the same effect took place, but much more rapidly.

The experiment being repeated, with an aqueous solution of opium, a similar effect was produced in nine hours.

Dilute solutions of oxide of arsenic and arseniate of potash were used: the stamens lost the power of approaching the pistil; but they were stiff, hard, withdrawn backwards, and could not have their direction altered without fracture. It seemed like an irritation, or a vegetable inflammation. Solution of corrosive sublimate more slowly produced the same effects.

Sensitive plant (Mimosa pudica). Experiments were now made with this vegetable. When a leaf of this plant is cut, and allowed to fall on pure water, the leaflets generally contract rapidly; but after a few moments expand, and are then susceptible of contraction by the touch of any other body. They may thus be preserved in a sensible state two or three days. If the section be made with a very sharp instrument, and without concussion, the leaves may be separated without any contraction. The branches of this plant may be preserved for several days in fresh water. Gumwater also effects the same purpose.

When a cut leaf of this plant falls upon a solution of cor

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