The Dahlia is a native of Mexico, and was discovered by Humboldt, growing in sandy soils, between four and five thousand feet above the level of the sea. It was first introduced into Europe from that country, and one variety flowered in 1789, under the care of that eminent botanist Cavanilles, Professor and Director of the Botanic Garden at Madrid, where he soon after flowered two others, figured and described in his Scenes. The first plant was received in England by the Marchioness of Bute, which appears to have been soon lost; and this species of Dahlia was unknown in England until it was reintroduced in 1804, by Lady Holland, who brought seeds of it in that year from Madrid. The name of Dahlia was given to this flower by Cavanilles, of Madrid, in 1789, in honor of Professor Dahl, a Swedish botanist. This name was afterwards changed by Wildenow to Georgina, in honor of a German botanist, who resided many years in St. Petersburg, in consequence of the genus dalea having been previously established by Thunberg. As, however, the name is neither 6pelled nor pronounced the same as Dahlia, and as the name of Dahlia was given long before that of Georgina, the plant is now restored to its original appellation.

During the first few years of their introduction into Europe, few varieties were originated, and those of very inferior merit. We find this circumstance to have been noticed in the most scientifically-managed gardens of France and Germany, as well as in the Madrid garden, where they were first introduced. De Candolle, in his memoir, published in 1810, - that is, eight years after their introduction into the Jardin des Plantes, - describes only eight varieties, and from what can be deduced from that paper, it appears that he had not, up to that period, produced a double flower, although be evidently expected such a result In the Berlin garden, long noted for the success of its cultivation, and into which the Dahlia had been received as early as 1800, no new varieties were obtained from seeds till six years afterwards.

The Dahlia is readily increased by seeds, cuttings, dividing the roots, and grafting: by seeds, only with the view of obtaining new and better varieties; by cuttings, to perpetuate good varieties already obtained in the greatest numbers, and also to have small roots or tubers convenient for exportation; by dividing the root to increase known sorts to a limited extent; and by grafting, to obtain plants with large roots more rapidly than the other methods. This last method is but seldom adopted. When seeds are procured, they should be carefully dried and preserved during winter, and sown in March or April in seed-pans or pots, in a hotbed or other temperature of about fifty-five or sixty-five degrees. If the plants come up quickly, it will be well to transplant them into single pots of the small sixty size, one plant in each pot, and keep them in a temperature of from forty-eight to fifty-five degrees, till the first week in May, when they may be planted out in a border of rich, deep, mellow, good soil. They will require particular attention against late spring frosts till towards the 20th of this month; afterwards their only culture is to stake them as they grow up, and select the most favorable-looking flowers as they appear, throwing away those that are single, or of inferior merit.

The best and by far the most general mode of propagating Dahlias is by cuttings, and in order to obtain these, the old roots are put into a state of growth early in the season, and the young shoots which spring from them, and which are produced in abundance, are employed as cuttings. Some extensive growers excite their Dahlia roots as early as December or January, and continue taking cuttings off them as long as they continue to afford them, or until a sufficient number of plants of the desired kind is obtained. For ordinary purposes, the beginning of March is a good time to put the rest into a situation to grow, which is usually done, when on a small scale, by putting them in light mould in larger pots, placing them in a hotbed, frame, vinery, plant-stove, or in any other convenient place where there is a temperature of about sixty or sixty-five degrees, or even more. Upon a larger scale, the old roots are placed together, without porting, upon a hotbed, shaking in a little light, sandy mould, or rotten tanner's-bark, amongst them.

In either case the young shoots soon begin to appear, and should then, as well as before, be occasionally sprinkled with water. ' When the shoots are from two to three inches in length, they may be cut off close to the old tuber, but not so as to injure its top or crown, because many more shoots will arise from it if required. The cuttings are prepared by cutting the shoot smoothly across under the first joint, and without-shortening the leaves, planting it in the smallest-sized pots (thirties) into a light soil, chiefly composed of decayed leaves and sand, or, as many practice with success, in pure white sand alone. The sand should be well wetted, and the cutting inserted just as far as will enable it to stand upright, for with most other cuttings, the shallower they are planted the better. When the cuttings are so planted, they should be plunged into a brisk bottom-heat, covered with a hand-glass, and regularly shaded; whilst in this situation they should be regularly watered, but not over the leaves, and kept close shut up, unless to inspect the plant.

In ten or twelve days they will have sufficiently rooted to be taken from under the glasses, and should be accustomed to the air by degrees; when they will stand without flagging or drooping their leaves, they may then be transplanted into larger pots, and into richer mould, the sand being previously shaken from them. They must now be accustomed to a lower temperature, and progressively hardened, by removing them from the propagating pit or house to other situations, unless they will stand the air and temperature of a close frame or cold pit; but in these they will require to be covered at night with mats till May or June, when they must be accustomed to stand the open air, so that they may be planted out where they are to flower by the middle of the month. Sometimes Dahlia plants are forward enough by the first of May to be planted out; but as it would be unsafe to expose them too early in the season, it would be well, if planted too early, to cover them with handglasses for a week or two, or in want of those with pots, which being left off during the day, and put on during the night, is the best substitute for the former.

Cuttings are also taken off the plants during the growth, any time from June to September. The shoots best calculated for this sort of propagation are those small lateral branches, which in general abound upon the plants, that may be successfully struck by being planted in sand in small pots, and placed behind a wall or other shaded situation, and otherwise treated as above.

The process of multiplying by dividing the root is simple, and requires nothing beyond the careful separation of the tubers, each piece having a portion of the crown of the root attached to it, in which there are two or more eyes or buds, without which they would not grow.

The process of increasing by grafting is applicable not only to Dahlias, but to most strong-growing herbaceous plants having solid stems and large tuberous roots.