The Cacteae are a very numerous and exceedingly interesting tribe of plants, which we think ought all perhaps to be included in one genus, divided into several sections. At the same time, it must be admitted that they are so numerous and varied in their forms, that it is more handy in the system divided into the numerous genera, as we now find them; but in our practice we have found that such genera are merely nominal, as they hybridize with one another - many of them at least - as we have found by experience. We had under our charge for ten years, a collection containing upwards of 15,000 plants of the order Cacteae, which contained Mammillaria, Melocactus, Echinocactus, Cereus, Cactus, Epiphyl-lum, Pereskia, and Opuntia. From Epiphyllum Ackermanii and Cereus spe-ciosissimus we raised several hundreds of seedlings, many of them quite distinct, most of which produced flowers of the size and shape of Epiphyllum with the beautiful color of the Cereus. Those who cultivate Epiphyllum Macoyii, can not fail to recognize in the flower the form of the Epiphyllum Ackermanii, with the color of Cereus speciosissimus.

The Melocactus and Echinocactus, we have hybridized with perfect success. The progeny are, almost without exception, most grotesque, and in the highest degree interesting to the admirer of succulent plants.

All the Cacteae grow in similar situations; hence they require nearly the same treatment. The differences between them are those of shape, size, and color. With the exception of about two, which are found in the south of Europe and one in Missouri, they are all natives of South and Central America. There they inhabit the most dry and barren situations on the slopes of the mountains, and many of them upon rocks where water never stagnates. During the dry season, which is by far the greater part of it, they remain quite inert, many of them retaining, if not their freshness, at least their plumpness, I presume by absorption through the epidermis; while many others become flaccid, and may be twisted about in any shape without sustaining the slightest injury. During this season they show but few of the ordinary characters of vegetation. In this state they are merely pieces of matter, the vegetable quality and life of which are inferred from the texture of the surface, and which is not unfrequently quite glossy, of various shades of green, red, brown, and sometimes with beautiful tints of crimson and carmine. Many of them are covered with spurs and prickles, usually in tufts, some straight, others curved or hooked, quite formidable in appearance.

In habit, these interesting plants differ from every other class, in having no distinct difference of petal, wood, and bark, or of leaf and stem. How, therefore, can the name Epiphyllum be a correct one? As we understand the term, it implies that the flowers are produced or grow upon the leaves, whereas there are no leaves. The entire substance of the plant is always a mass of matter, which might be called a stem or frond, but never a leaf.

In all the tribe, so far as we are informed, the principle of vegetable life is not only stronger and more generally diffused, but more indestructible than in almost any other tribe of plants. The smallest portion of a stem will not only emit roots, but in due time become a perfect plant; and a piece from the side or bottom is equally adapted for the purpose as from the end. If taken from the end, it matters not which end is placed in the earth to root.

The capacity of remaining inactive for such lengthened periods of time, and then being so easily restored to growth and flowering, accords well with the natural situation of the plant, and is, moreover, of the highest practical service in their artificial culture. It is not unusual, upon the hot, desolate locations in which they grow, that they are deprived of rain for a whole year, which is caused by adverse currents and other circumstances; in this parched up state they do not die, but merely remain inactive till the wet season returns, when they immediately start into growth, and flower.

During the summer of 1841, we had placed, by way of experiment, some plants of Melocactus Ottii and communis, Echinocactus subgibbosus, . melocacti-formus and intricatus, with several plants of Cereus, Mammillaria, and Cactus, upon the upper shelf, where they received the direct rays of the sun in the pine stove. In this torrid situation they remained for twelve months, without any water whatever, after which they received a copious supply, retaining them in the same situation. The result was five plants dead; forty-five, after having been in a perfect state of inactivity for twelve months, upon having a good supply of moisture, began to grow immediately, and many of them produced very fine flowers. After this we never experienced any difficulty in flowering the beautiful Echinocactus Eyresii, formosus, nobilis, with many others. I do not wish to be understood that we placed 15,000 plants upon the shelf of a pine stove for twelve months, to starve them into subjection. Our practice was, after a plant had flowered, or refused to flower, after gradually withholding water, it was then placed in the dry stove (a hothouse for plants requiring no moisture, the temperature of which was about 80° Fahr.) for three, four, or more months, according to circumstances, such as the natural habit of the plant, the time when the plant would be required to produce its flowers, etc., all of which the practical man will understand, and all of which can be understood by any intelligent mind, by observation and inquiry, by becoming acquainted with the geographical distribution of plants, the all-absorbing study of botany, with a thousand other delightful researches in the volume of Nature.

We believe the point has been fully proved, at least to some extent, particularly with the growers of the beautiful order Orohideaa, that the growth and flowering of many plants are more energetic in proportion as they have been in a state of inactivity. We believe the rare flowering of the night-blooming Cereus to be entirely owing to keeping it in a state of activity throughout the entire year.

A case in point. We had a plant of Cereus grandiflorus attached to the back fruiting pine house, covering a space of 30 by 4 feet, or thereabout; in this house the air was moist from the watering of the plants and other'causes. Here the Cereus grew and flourished amazingly, producing its singular roots from all along the stems in the wildest confusion, but seldom produced its flowers. In using the same house as a dry stove, the Cereus, instead of being of a fine, lively green color, suddenly became brown, and somewhat rigid. Water was withheld for a season; when again applied by the syringe upon its stem, with a good supply of manure-water in the tub wherein it was growing, it produced its splendid odoriferous flowers without stint or stay for several weeks in succession-The inference to be drawn was, in cultivating the Cacteae, of whatever species they might be, to place them in a house by themselves; or, if that is not practicable, to keep them as far apart as possible (during their period of rest) from other plants that require frequent watering all the season. Unless kept quite dry for a season, their awakening into growth and flowering will be found to be much less vigorous.

During this time, wherever they are kept, all water should be withheld, not only from the soil wherein they are growing, but in the air by which they are surrounded. In this state they are subject to little injury from variations of temperature, unless it should be near freezing, which they can not bear. A rather low temperature during the time of rest, is more favorable than otherwise. As soon as it is perceived that their vigor is coming into action, the temperature should be raised, and water given in such quantities as to completely drench them, (but not in such quantities as to turn them into aquatics.) By such timely treatment most all the species can be brought into flower every year; and as many of the flowers of the Melocacti and Echinocacti are exceedingly beautiful, the proper management of them would contribute very much to the general appearance and richness of collections of flowering plants.

There is another circumstance to which we would invite the attention of those who have no hot or green-house, and which should enhance the value and interest of this singular and interesting class of plants, viz.: for the portable parlor plant case, wherein they may be grown the entire year, they are invaluable.

As we shall have occasion to revert to the Cactese, as well as to the plant case, soon, we shall say no more upon them now.

[The whole treatment here indicated is based upon one special requirement, a season of rest. This is often overlooked in many other plants besides the Cactus. There is no plant better fitted for growth in rooms than this; very few do so well there. - Ed].