parasitic plant by the attraction of the body on which it is placed, rather than by the much more powerful attraction of the earth. In order to ensure this particular tendency, without possessing which the existence of the miseltoe would be put in hazard, its root has received from the same all-powerful hand a disposition so much greater than other plants to avoid light, and to bury itself in the obscurity of the interior of a tree, as to be sufficient to overcome the influence of its green colouring matter.

The next direction of the parts of plants, which may be called special, is that of the upper surface of the leaves towards the sky, and of the lower towards the earth. This disposition is so powerful, that, if the usual direction of a leaf is inverted, the petiole will twist so as to enable it to recover itself. This phenomenon has been noticed by Bonnet, whose explanation has been already given (p. 278.), but which is obviously inadmissible. There is always a natural difference between the two faces of the leaf: the upper is always the most deeply coloured; a difference which will be found constant in all cases. The face with the deepest colour turns towards the sky or light, and, with the weakest colour, towards the earth or obscurity; and this is so constant a law, that it will be found that, if that surface of the leaf which is naturally inferior is more deeply coloured than the superior, the petiole will be twisted round by the greater affinity of the lower surface for the light, which will thus become uppermost, the leaf presenting the appearance of an inverted leaf. This may be seen in many grasses, but not in Zea Mays, Triticum repens, and Agrostis rubra. Hence it is to be concluded, that the upper surface of the leaf is not turned towards the heavens merely in consequence of its quality of being the upper surface, but because it is generally the most deeply coloured.

The same law influences the directions of the petals, in which the upper surface, — that which is turned towards the heavens, is always the most highly coloured: this, indeed, is sometimes not very apparent, but is nevertheless constant. Even in white petals, — such, for example, as those of Lilium album, the upper face will be found of a dense but brilliant white, while the lower is of a much paler hue. The white

colour of the petals, Dutrochet proceeds to remark, like all the other colours of plants, is due to a particular kind of colouring matter deposited in the parenchyma lying below the cuticle. Thus the whiteness of the flowers of plants is not dependent upon the absence of colour, like the roots and etiolated stems: in the former a white colouring matter exists; in the latter the whiteness is caused by absence of colour. Some apparent exceptions to this law,- such as the outside of many monopetalous flowers being paler than the inside, as in Digitalis purpurea, Fritillaria latifolia, and others,- Dutrochet thinks may be explained thus: - They, no doubt, are due to the tendency of the less coloured part to avoid the light, which is manifested by bearing down the flower so as to approach the seat of obscurity as nearly as possible: all such flowers being always nodding. This tendency is aided by the weakness of the peduncle, which seems to have been specially provided for enabling such flowers to retire from the light. In papilionaceous plants, the inside of the vexillum, which is most deeply coloured, always turns itself towards the light; and the alæ twist themselves half round, to effect the same object. The ovaria often take a different direction after the fall of the corolla than they had before. Thus, during flowering, the ovarium of Digitalis purpurea was nodding like the flower, the direction of which it was compelled to follow. Immediately after the fall of the corolla, it turns upwards towards the light, to which it is attracted by its green colour. A contrary phenomenon is presented by the ovarium of Convolvulus arvensis. The flower is turned towards the sky: as soon as it has fallen, the ovarium takes a direction towards the earth, bending down the peduncle. This cannot be due to the weight of the ovarium, which is much lighter than its peduncle, but must depend upon its disposition to avoid the light, on account of its pallid hue, which is nearly the same as that of the root. In Convolvulus sepium, on the contrary, in which the ovarium is equally pale, its erect position is maintained, and the influence of decoloration counteracted by the greater affinity to the light of two large green bracteæ in which it is enveloped.

From the following and some other experiments, Du

trochet infers that the direction of leaves to the light is not mechanically caused by the operation of an external agent, but is due to a spontaneous motion, put in action by the influence of external agency. He took a leaf and cut off its petiole, the place of which was supplied by a hair, hooked by one end upon the leaf, and having a piece of lead attached to its opposite extremity. They were plunged in a vessel of water: the weight of the lead carried the leaf to the bottom of the water, where it stood erect in consequence of its lightness inducing it to attempt to ascend. Being exposed in a window, so that the under surface was turned to the light, no alteration took place in its position. Now, as from Bonnet's experiments, it is certain that leaves immersed in water act exactly as if surrounded with air, it is to be inferred that the external influence of the light is of no effect, unless aided by a spontaneous power within the vegetable which was destroyed by the removal of the petiole. Leaves immersed in water under similar circumstances, with their petioles and stem uninjured, turned towards the light as they would have done in the open air. The power thus supposed to exist in all probability depends upon the same nervous matter which has elsewhere been shown to exist, in a greater or less degree, in all plants, and especially in those called sensitives.

Those who desire more information upon this very curious subject should consult Dutrochet's work, Sur la Motilité des Végétaux.

CHAPTER XII.

OF IRRITABILITY.

THE vitality of plants seems to depend upon the existence of an irritability, which, although far inferior to that of animals, is, nevertheless, of an analogous character.

This has been proved by a series of interesting experiments by M. Marcet, of Geneva, upon the exact nature of the action of mineral and vegetable poisons. The subject of his observations was the common kidney-bean; and in each experiment a contrast was formed between the plant operated upon and another watered with spring water. A vessel containing two or three bean plants, each with five or six leaves, was watered with two ounces of water, containing twelve grains of oxide of arsenic in solution. At the end of from twenty-four to thirty-six hours the plants had faded, the leaves drooped, and had even begun to turn yellow. Attempts were afterwards made to recover the plants, but without success. A branch of a rose tree was placed in a solution of arsenic; and in twenty-four hours ten grains of water and 0.12 of a grain of arsenic had been absorbed. The branch exhibited all the symptoms of unnatural decay. In six weeks a lilac tree was killed, in consequence of fifteen or twenty grains of moistened oxide of arsenic having been introduced into a slit in one of the branches. Mercury, under the form of corrosive sublimate, was found to produce effects similar to those of arsenic; but no effect was produced upon a cherry tree, by boring a hole in its stem, and introducing a few globules of liquid mercury. Tin, copper, lead, muriate of barytes, a solution of sulphuric acid, and a solution of potash, were found to be all equally destructive of vegetable life; but it was ascertained, by means of sulphate of magnesia, that those mineral substances which are innocuous to animals are harmless to vegetables also. In the experiments with vegetable poisons, the bean plants were carefully taken from the earth, and their roots immersed in

the solutions used. It had been previously ascertained that plants so transplanted and placed in water under ordinary circumstances would remain in excellent health for six or eight days, and continue to vegetate as if in the earth. A plant was put into a solution of nux vomica at nine in the morning: at ten o'clock the plant seemed unhealthy; at one the petioles were all bent in the middle; and in the evening the plant was dead. Ten grains of an extract of Cocculus suberosus, dissolved in two ounces of water, destroyed a bean plant in twenty-four hours; Prussic acid produced death in twelve hours, laurel water in six or seven hours, a solution of belladonna in four days, alcohol in twelve hours. From the whole of his experiments, M. Marcet concludes, 1st, That metallic poisons act upon vegetables nearly as they do upon animals: they appear to be absorbed and carried into different parts of a plant, altering and destroying the vessels by corrosive powers. 2dly, That vegetable poisons, especially those which have been proved to destroy animals by their action upon the nervous system, also cause the death of plants: whence he infers that there exists in the latter a system of organs which is affected by poisons, nearly as the nervous system of animals.

These facts have been confirmed by other experiments of M. Macaire, which will be mentioned presently under the head of Poisons.

Irritability, in the common acceptation of the term in botany, means those extreme cases of excitability in which an organ exhibits movements altogether different from those we commonly meet with in plants. Of this kind of irritability there are three distinct classes; namely, those which depend upon atmospheric phenomena, spontaneous motions, and such as are caused by the touch of other bodies.

Among the cases of irritability excited by particular states of the atmosphere, the singular phenomenon called, by Linnæus, the sleep of plants is the most remarkable. In plants with compound leaves, the leaflets fold together while the petiole is recurved at the approach of night; and the leaflets again expand and raise themselves at the return of day.

In others the leaves converge over the flowers, as if to

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