THE ACACIA.

THE genus Acacia contains trees and shrubs both useful and ornamental. The common Acacia, which, however, is included in the genus Robinia, is a thorny tree of rapid growth, of middling stature, and though the adult tree possesses no great beauty, yet when young it is very ornamental, and well suited for copse wood and rough timber. In North America the wood is highly valued, being close-grained, hard, and finely veined; and more valued by the cabinet-maker than any other native timber whatever. Fine pinnated leaves and pendulous white odorous flowers add greatly to the beauty of this tree.

The Rose Acacia is a very handsome shrub, but requires a very sheltered situation, as the branches are very liable to be shattered or blown off by high winds. In young trees grafted above ground, the fracture generally takes place at the graft, so that a good means of prevention is to graft in the root, a little below the surface.

A. Catechu, the Medicinal Acacia, and A. vera, the Egyptian Thorn, are both used in medicine. The Catechu is obtained in the following manner from the inner wood of the former, which is of a brown colour. After felling the trees, the manufacturer carefully cuts

off all the exterior white part of the wood. The interior coloured part is cut into chips, with which he fills a narrow-mouthed unglazed earthen pot, pouring water upon them until he sees it among the upper chips; and when this is half evaporated by boiling, the decoction, without straining, is poured into a flat earthen pot, boiled to one-third part, and then set in a place to cool for one day. The decoction is afterwards evaporated by the heat of the sun, stirring it several times in the day; and when it is reduced to a considerable thickness, it is spread upon a mat or cloth, which has previously been covered with the ashes of cowdung. The mass is lastly divided into square or quadrangular pieces by a string, and these are completely dried by burning them in the sun, until they are fit for sale. This extract, when first introduced into Europe, was named Terra Japonica, from the supposition that it came from Japan, and was an earth. As a medicine it is one of the most valuable of the vegetable astringents.

The

From A. vera we derive the gum Arabic of the shops. The tree is found in almost every part of Africa, but those which yield the gum, which is exported from Barbary to Great Britain, grow principally in the Atlas Mountains. It is a hard, withered-looking, low tree, with a crooked stem and a grey bark. gum exudes naturally from the bark of the trunk and the branches, in a soft, nearly fluid state, and hardens in the air without losing its transparency. It is gathered about the middle of December. It has a faint smell when first stowed in the warehouses, and is heard to crack spontaneously for many weeks. In the

Deccan, where this tree is common, it is called the Babool-tree. It thrives there equally well in a black or red soil. It grows rapidly, and requires no water. When covered with its round heads of yellow flowers it is very ornamental. Moore, in his Lalla Rookh, has introduced it in an Arabian scene:

"Our rocks are rough, but smiling there

The Acacia weaves her yellow hair,
Lonely and sweet, nor loved the less

For flowering in a wilderness."

Nearly allied to this genus is the Mimosa, in which we find the Sensitive Plant, of which Darwin writes—

"Weak with nice sense Mimosa stands,

And from each touch withdraws her timid hands;
Oft as light clouds o'erpass the summer glade,
Alarmed she trembles at the moving shade ;
And feels, alive through all her tender form,

The whispered murmurs of the gathering storm."

Dr. Dutrochet made a series of experiments upon this wonderful plant, the main result of which we state as follows. The principal point of locomotion, or of mobility, exists in the little swelling which is situated at the base of the common and partial petioles of the leaves: this swelling is composed of a very delicate cellular tissue, in which is found an immense number of nervous corpuscles; the axis of the swelling is formed of a little fascicle of tubular vessels. It was ascertained, by some delicate experiments, that the power of movement, or of contraction and expansion, exists in the parenchyma and cellular tissue of the swelling, and that the central fibres have no specific action connected with the motion. It also appeared that the energy of the nervous powers of the leaf depended wholly upon an abundance of sap, and that

a diminution of that fluid caused an extreme diminution of the sensibility of the leaves. Prosecuting his remarks yet further, the author ascertained that in the motion of the Sensitive Plant two distinct actions take place: the one of locomotion, which is the consequence of direct violence offered to the leaves, and which occurs in the swellings already spoken of; the other of nervimotion, which depends upon some stimulus applied to the surface of the leaflets, unaccompanied by actual violence, such as the solar rays concentrated in the focus of a lens. As in all cases, the bending or folding of the leaves evidently takes place from one leaf to another with perfect continuity, it may safely be inferred that the invisible nervous action takes place in a direct line from the point of original irritation, and that the cause by which this action of nervimotion is produced must be by some internal uninterrupted agency. This was, after much curious investigation, determined by the author to exist neither in the pith nor in the bark, nor even in the cellular tissue filled with nervous corpuscles, and on which he supposes the locomotion of the swelling at the base of petioles to depend. It is in the ligneous part of the central system, in certain tubes supplied with nervous corpuscles, and serving for the transmission of the sap, that Dr. Dutrochet believes he has found the true seat of nervimotion, which he attributes to the agency of the sap alone, while he considers the power of locomotion to depend upon the nervous corpuscles alone.

PROPAGATION.-This

valuable genus very

of green

house exotics is propagated by cuttings and by division of the roots.

CUTTINGS.-The greater number of the species will make roots from cuttings. From those kinds which grow the strongest large cuttings are taken off at a joint, and being set in a pot of sand, the pot is plunged in a bark bed, and the cuttings are covered with a hand-glass. In propagating the smaller sorts, younger cuttings are taken, and in like manner placed under a bell-glass and plunged in heat. When the cuttings have formed roots, the sooner the plants are potted off the better will they thrive, indeed, if allowed to remain too long in the sand the roots will be injured. When potted a glass is placed over them, and they should be kept close and shaded for a few days after, and be gradually exposed to the air.

The vital energy of the sap of plants, and its power to reproduce members of which the plant has been deprived, by whomsoever discovered, is made the means of increasing and multiplying innumerable trees and plants with the greatest facility. This power resides in the most remote branches, and is exerted with the greatest success in the production of roots by those extremities of the shoots which have the least substance, and consequently require the least amount of nutriment from without, or rather can longer sustain that privation of supply which its separation from the rooted tree causes. When a portion of a plant is separated from it, and is consequently deprived of its first and natural roots, it immediately seems susceptible of its loss, and being