THE remarkable diversity of curious forms exhibited by cactus plants, and their growth and retention of vitality under apparently adverse circumstances, combine to render the group perhaps the most interesting in the vegetable kingdom. The gigantic cereus rears its massive columnar stem to a height of sixty feet, while the beautiful mamilla-rias are but a few inches high. Some, as the old man cactus (Pilocereus seniiis), are densely covered with strange long hairs that impart a weird aspect to the plants; others, as in the genus echino-cactus, are covered with gigantic spines, which not only appear formidable, but actually pierce the flesh, as would needle-pointed spikes of steel. These spines are sometimes barbed, and when any such enter the skin their extraction is no easy or painless matter. In other species again, they are very minute, barely visible to the naked eye, and these are the most troublesome of all. The least touch is sufficient to cause them to adhere to one's person. With gloves and proper caution, however, there need be no difficulty about handling the plants.

The flowers are usually large and showy, and frequently deliciously scented. One species, Cereus McDonaldiae, produces flowers which measure fifteen inches across, and as much in length. The colors are varied - scarlet, crimson, salmon, purple, rose, pink, yellow and white being the most desirable tints.

The majority of cactus flowers retain their full beauty but a short time, and the most conspicuous are of shortest duration. The night-blooming cereuses afford the largest and most showy flowers, and these last but a few hours. They expand a short time before sunset, and close, never to open again, before sunrise. One glimpse of their nocturnal beauty, however, compensates for this in overflowing measure. The genera phyllocactus and epiphyllum are the most useful as flowering plants.

The latest authorities have arranged cactuses under thirteen genera, and each of these is placed in one of two tribes. The first tribe, named echinocactea, is distinguished by having the calyx tube elongated beyond the ovary, the stems, usually destitute of leaves, being traversed by longitudinal ridges, or beset with tubercules. The genera melocactus, mamillaria, pelecyphora. leuchtenbergia, echinocactus, disco-cactus, cereus. phyllocactus, and epiphyllum come under this head. The genera rhipsalis, nopalea, opuntia and pereskia are placed in the second tribe, which is termed opuntiae. Here the stems are branched or jointed, and the calyx does not extend beyond the ovary. Under the genus melocactus, commonly called melon or turk's cap cactus, there are about thirty species, which inhabit the dry hot parts of Mexico, Brazil and the West Indian Islands. The stems of these species attain a height of three feet, being fleshy, globular and regularly ridged, the ridges bearing stout spines in clusters. Each plant bears at the top a thickly-set mass of spines and hairy matter, which, in common language, is termed the cap. Upon this the small reddish flowers are borne, and are succeeded by bright red fruits.

M. communis is the most desirable species.

The mamillarias, or nipple-bearing cactuses (see Figs. E and F), are exceedingly beautiful and interesting little plants. They are seldom more than a foot high, and most frequently attain a size of only three or four inches. The tubercules, exhibiting various oblong or globular outlines, are arranged spirally, and bear a cluster of spines at the apex. There are over three hundred species and the genus occurs in North and South America and the West Indies. Some of the North American species inhabit mountainous regions, where they are subject to several degrees of frost. The flowers proceed from the base of the tubercules, but are not so interesting as the arrangement and general appearance of the spines.

The following description, by Dr. Lindley, of M. tenuis, comparatively a poor species, will be sufficient to show that even the most insignificant forms are worthy of study. He says: "Gentle reader, hast thou never seen in a display of fire-works a crowd of wheels all in motion at once, crossing and intersecting each other in every direction, and canst thou fancy those wheels arrested in their motion by some magic power, their rays retained, but their fires extinguished and their brightness gone? Then mayest thou conceive the curious beauty of this little herb, a plant so unlike all others that we would fain believe it the re-animated spirit of a race that flourished in former ages with those hideous monsters whose bones alone remain to tell the history of their existence in the quarries of our sandstone, slate and clay." This extract is taken from the Botanical Register, where it appeared in 1832. Many new species of mamillaria, even more beautiful than the above, have been discovered since then.

The genus pelecyphora is a near relative of the mamillarias, and contains only one species, namely, P. aselliformis (Fig. U). It is commonly called the hatchet cactus. The stems are about six inches high, and usually appear in clusters when the plant is old.

The flattened tubercules have been supposed to bear some resemblance to a hatchet, and, instead of spines at the apex, they are furnished with two rows of hard scales, which appear like those on the back of a woodlouse; and these suggested the specific name. The flowers, borne at the apex of the plant, are large and of a showy purple color. There are several varieties, and all are natives of Mexico.

The leuchtenbergia, as in the preceding genus, has but a single representative, L. principis (Fig. O). If we except the flowers, it bears very little resemblance to any other form of cactus, being, in fact, more like an aloe. The erect, cylindrical, and woody stem is surmounted by triangular tubercules, about six inches in length, which bear tufts of dry, rough vegetable fibre at their extremities. The stem varies in height from one to twelve inches, the lower part of the old plant being clothed with the scale-like remains of former tubercules. The large yellow flowers are borne from the axils of the younger tubercules. It is a native of Mexico.