French horticulture, always seeking to increase our enjoyments, has accomplished a valuable result for the ornament of our parks and landscape gardens. After several years of experiment, it has succeeded in the open air culture of the Acacia (or Mimosa) deal-baia.

This tree, so well known in all green-houses, a native of Van Dieman's Island, was first introduced into England in 1818, and into France in 1824. It has proved since then, every year, the greatest ornament of the green-house and the conservatory, growing so luxuriantly, that in a few years it reached the roof, whatever may have been its height elsewhere.

Few green-houses are high enough to allow it to accomplish the development of which it is susceptible; and such is the vigor of its growth, that very often it forces itself through the roof, struggling to attain that freedom of development that nature has granted it. There is something so wonderfully beautiful in its perfect inflorescence, that few persons familiar with good exotic collections, have not frequently paid a just tribute of admiration to this remarkable tree. What can be more graceful than its smooth branches, of a beautiful glaucous green, clad with its delicate persistent foliage of the same color, its myriads of gay, golden flowers, lighter than down, and seeking to envelop it with a floating cloud, gilded by the first beams of the morning. No description can do justice to the lightness, the elegance, and the grace of this truly lovely tree.

Mr. Andes Leroy, whose taste in horticulture is universally known, could not behold this tree, upon which nature seems to delight to lavish her gifts, without regretting that it should not be able to attain all its proportions in his extensive nurseries, so as to embellish with its masses of beauty, the numerous parks and plantations which he forms every year. Being desirous to know to what extent it could endure the rigor of the winter, he planted several in the open air, which not only resisted without injury, the most intense cold known in the south of France, but which soon established a growth such as we were not acquainted with in any other tree.

Several of these Acacias, planted three years since, merely against a wall, with a northern exposure, are now sixteen feet high; their branches nearly six feet long; the flowers which cover them are so abundant, and so finely relieved against the back-ground of glaucous foliage, that they resemble a large golden sheaf of the most graceful and elegant form. Others, planted at the same time, and entirely in the open air, that is to say, without any protection, are not at all inferior to the first, in luxuriance and vigor of growth.

The first tree of this kind planted in the open air at Angers, is now eight or nine years old, and more than twenty-six feet high; its branches are nearly eight feet in length, and extend in every direction, bending under the astonishing mass of its flowers. It is not possible to do justice to the beauty and the brilliancy of the bloom of this tree; and whoever has not Been it in all its splendor, can form but a very imperfect idea of it.

One remarkable fact about this Acacia is, that it continues to grow all the year round, and even during the winter months, the vegetation scarcely seems to be arrested. The flower-buds begin to appear at the end of summer; they remain in perfect preservation until the first fine days of spring; and towards the end of March, when the gardens begin to throw off their winter garb, the tree rapidly bursts into its greatest beauty.

Angers, which seems the chosen country of Flora, and for that reason, doubtless, has been called " the Nursery of France," is situated between the 47th and 48th degrees of north latitude; the temperature is tolerably uniform, although the centigrade thermometer sometimes falls there below 12°, (5° above zero of Fahrenheit;) but there are none of those sudden changes which are so injurious to vegetation, in disorganizing the tissues. The severe frosts occur generally in December and January, and a thaw almost always takes place during a cloudy and foggy season, owing, no doubt, to the influence of the four rivers which surround Angers, and whose fogs counteract, by intercepting them,th* rays of the sun. Owing to these favorable circumstances, we are able to cultivate in the open air, so great a number of plants, such as Camellias, and evergreen Magnolias, which do not succeed so well even in countries farther south than the city of Angers. Besides the above mentioned frosts, they also occur in March and April, of three, four, and five degrees, and sometimes more; these, although less severe than those .of winter, are, however, much more injurious, because they occur at a season when vegetation is already under way, and when all the sap-vessels are much more sensitive to atmospheric influences.

The soil of Angers and its environs is argillaceous, resting upon a stratum of silicioua rock of great depth, but permeable to water; it is easily warmed,; the deposit of vegetable earth varies from eighteen to twenty-four inches in thickness.

I have entered into all these details of the nature of the climate and soil, in order to give a clear idea of the circumstances, in the midst of which the open air cultivation of this Acacia is accomplished; a tree which, for a long time, it was not thought possible to cultivate except in the hot-house or conservatory.

When I take into consideration the vast extent of the territory of the United States of America - when I recall the luxuriant and varied vegetation covering the different portions of that western soil, and which I was never weary of admiring when I bad the happiness of visiting it, I do not doubt that the southern and temperate latitudes are as favorable as our own, and that the open air cultivation of this superb Acacia may be equally successful,, which will add still more to the natural riches of those pretty country seats, whose beauty is only equalled by the vegetation that surrounds them.

I shall be happy, if, in introducing to the readers of that excellent Journal, the Horticulturist, so beautiful an acquisition to horticulture, I might be able to induce some of those similarly located as to climate, to attempt experiments, the success of which will repay them for their efforts. Baptists Desportes, Nurseryman at Angers, (France).

[We hope some of our readers in the southern states will profit by M. Desportes' valuable hints. No doubt this, and other Acacias, would be found perfectly hardy as far north as Columbia, in South Carolina].