O. arborescens (tree-like).—­This species is known as the Walking-Stick or Elk-Horn Cactus, from its cylindrical, woody stems being made into very curious-looking walking-sticks (examples of which may be seen in the Museum at Kew), whilst the arrangement of the branches is suggestive of elk horns.  Habit erect; joints cylindrical, branching freely, and forming trees from 8 ft. to 30 ft. high.  Stems covered with oblong tubercles and tufts of long, needle-like spines, which give the plant a very ferocious aspect.  Flowers on the ends of the young branches, 2 in. to 3 in. in diameter, bright purple in colour, developing in June.  It is a native of Mexico, &c., and requires greenhouse or stove treatment.  The skeletons of this species, as seen scattered over the desert places where it is wild, have a very singular and startling appearance.  They stand in the form of trees, quite devoid of leaves, spines, or flesh, and, owing to the peculiar arrangement of the ligneous layers, nothing remains except a hollow cylinder, perforated with mesh-like holes, indicating the points where the tubercles and small branches had been.  These skeletons are said to stand many years.

O. arbuscula (small tree).—­Another of the cylindrical kinds, with a solid, woody trunk, about 4 in. through, and clothed with smooth, green bark; it grows to a height of 7 ft. or 8 ft.  Branches very numerous, slender, copiously jointed, the ultimate joints about 3 in. long and 1/2 in. thick; they are slightly tuberculated, and bear tufts of spines nearly 1 in. long.  Flowers 11/2 in. in diameter, produced in June; petals few, greenish-yellow, tinged with red.  It is a native of Mexico, and requires stove treatment.  A pretty plant, or, rather, a very remarkable one, even when not in flower, the thin branches, with their hundreds of long, whitish spines, being singular.  Unfortunately, it is not easily grown.

O. arenaria (sand-loving).—­Stems spreading, forming a tuft 3 ft. through and about 1 ft. high.  Joints 11/2 in. to 3 in. long, and a little less in width, terete, with very prominent tubercles and numerous tawny bristles; upper spines 1 in. to 11/2 in. long, white, with a yellow point, shorter ones hair-like and curled.  Flowers 2 in. in diameter, produced in May.  Fruit 1 in. long, bearing a few short spines.  Mexico.  A strong-rooted plant, which should be grown in very loose, sandy soil.  It would probably thrive best when planted out on a stage near the glass in a stove.

O. Auberi (Auber’s).—­An erect-growing plant, 8 ft. or more high, not unlike O. Ficus-indica in the form of its joints, but with long spines springing from the cushions, whereas the latter has none.  The joints are oblong-ovate, glaucous-green, the cushions few and scattered; spines white, flattened, of various lengths.  Flowers tawny yellow, small for the size of the plant.  A native of Cuba, and requiring stove treatment.  Being very brittle, this plant should be supported with stakes.