which are produced by these shoots are entirely colourless, like roots; and, instead of taking a direction towards the sky, as coloured stems would do, they lead downwards, pointing towards the centre of the earth. In order to take this position, the young shoot forces its way through the substance of the petiole which covers it; thus overcoming a mechanical obstacle, in its tendency towards the earth. This subterranean stem next takes a horizontal course, and does not assume any tendency towards the sky until the points become green."

Dutrochet has also remarked a similar phenomenon in roots. It is well known that exposed stems of many plants produce roots when green, they turn upwards, as in Pothos and Cactus phyllanthus; when colourless, they point downwards. Hence it is to be inferred, that stems do not descend merely because they are stems, but because their parenchyma is coloured; and that roots descend, not in their quality of roots, but because their parenchyma is colourless. It seems, however, that although this law is uniform in its operation in all terrestrial plants, yet that a deviation, or apparent deviation, from it exists in the parasitic Mistletoe. The radicle of this plant, which is of a paler green than the other parts, instead of turning towards the light, avoids it with so much pertinacity that it is impossible to induce it to take such a direction; so that it seems to be repelled by light. Dutrochet does not seem to be able to satisfy himself of the reason of this exception but it appears to be by no means difficult to account for. We have seen that, in the direction of its radicle, nature has enabled it to fulfil its functions as a 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 insure this particular tendency, without possessing which the existence of the Mistletoe 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 be 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, but which is obviously inadmissible. There is always a natural difference between the two faces of the leaf: the upper is always the more deeply coloured; a difference which will be found constant in all cases. The face with the deeper colour turns towards the sky or light, and that with the weaker colour towards the earth or obscurity; and this is so constant a law, that it will be found that if the 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 being inverted. This may be seen in many grasses, but best 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 epidermis. Thus the whiteness of the flowers of plants is not dependent upon the absence of colour, as in 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:-These cases, 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 ovaries often take a different direction after the fall of the corolla from what they had before. Thus, during flowering, the ovary of Digitalis purpurea is nodding like the flower, the direction of which it is 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 ovary of Convolvulus arvensis. The flower is turned towards the sky: as soon as it has fallen, the ovary takes a direction towards the earth, bending down the peduncle. This cannot be due to the weight of the ovary, 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 Calystegia sepium, on the contrary, in which the ovary 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 bracts in which it is enveloped.

From the following and some other experiments, Dutrochet 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.

I am unwilling to give more space to this subject, which belongs less to practical physiology than to speculative philosophy. The reader who wishes to study it will find abundant illustrations, explanations, and speculations in Dutrochet's Mémoires pour servir à l'Histoire Anatomique et Physiologique des Végétaux: Paris, 1837,

EE

CHAPTER XV.

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.

and

This has been proved by a series of interesting experiments, by 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 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