The common House-leek (Sempervivum tectorum) grows, as is well known, on the tiles of houses, or on walls, where there does not appear a single particle of earth to nourish its roots. The leaves are, however, so contrived as to form a cluster of flat scaly circles, and thus to shade and keep moist the roots beneath them. The flowers, which are produced on a tall flower-stem rising from the leaves, are pink, and usually consist of a green calyx, cut into twelve segments, and a corolla of twelve petals, with twelve stamens and twelve carpels, which spread out like a star in the middle of the flower. The number of petals, &c., is by no means constant, as it varies from six to twenty; but the other parts of the flower vary in the same manner, and always agree with each other, except as regards the stamens, which are sometimes twice the number of the petals, and arranged in two series, those in one series being abortive. At the base of each carpel is a kind of scale or gland, and this is the case with most of the genera included in the order. There are several species of Sempervivum, natives of the Canary Isles, which are very ornamental, and which have yellow flowers; but this genus, and that of Sedum, the Stone-crop, have been lately remodelled by Mr. Philip Barker Webb, and some new genera formed out of them. The principal other genera in the order are Crassula and Kalosanthes; the latter having been formed out of the former, and including those species of Crassula which have a tube-shaped corolla, with a spreading limb, divided into five segments, while the flowers of those species which have been left in Crassula have five distinct petals. All the plants belonging to the order have succulent leaves; and in all of them the number of the petals, sepals, and carpels, is the same, and of stamens either the same, or twice as many. In the common House-leek, the anthers sometimes produce seeds instead of pollen.
ORDER XCII.—FICOIDEÆ.—THE FIG-MARIGOLD TRIBE.
The principal genus in this order is that of Mesembryanthemum, the Fig-marigold. In the species of this genus, the leaves are always thick and fleshy, and sometimes in very singular shapes; and sometimes they are covered with a sort of blistery skin, which makes them look as though covered with ice, as in the Ice-plant (M. crystallinum). The leaves, when this is the case, are said to be papulose. Some of the species are annuals, others shrubby, and others perennials; and they are all natives of the Cape of Good Hope. The flowers, which are generally showy, have a green, fleshy, tubular calyx, with a four or five cleft limb, the tubular part of which encloses the ovary; and a corolla of numerous very narrow petals, which are arranged in two or more series. The stamens are very numerous; and the capsule has four or more cells, each of which contains numerous seeds. The valves of the capsule open when the seeds are ripe, if the weather should be dry; but remain firmly closed, so long as the weather continues wet.
The genera Reaumuria and Nitraria, which were formerly included in this order, have been removed from it, and made into separate orders, the latter of which is introduced here; and the genus Grielum, which was formerly included in Rosaceæ was first removed to Ficoideæ, and afterwards made into a separate order, under the name of Neuradiaceæ, which precedes Nitrariaceæ.
ORDER XCIII.—CACTACEÆ.—THE CACTUS TRIBE.
There is perhaps no order in the vegetable kingdom which embraces plants so singular in their forms as those comprehended in this tribe. All the genera, with the exception of Pereskia, are destitute of leaves, but they have all succulent stems which answer the purposes of leaves. The flowers of all the genera are extremely showy; the calyx and corolla are coloured alike, and confounded together; the stamens are numerous, with versatile anthers and very long filaments; the style is generally long and slender, and the stigmas are numerous, and either spreading or collected into a head. The ovary is in the tube of the calyx, and it becomes an eatable fruit, very similar to that of the gooseberry. The genera are all natives of tropical America. The principal kinds are the following: viz. Mammillaria, the stems of which are subcylindrical, and covered with tubercles, which are disposed in a spiral manner; and each of which is crowned with a little tuft of radiating spines mixed with down. The flowers are without stalks, and they are disposed in a kind of zone round the plant. The Melon Thistle or Turk’s-cap (Melocactus communis) has a globose stem with deep furrows, the projecting ribs having tubercles bearing tufts of spines. The stem is crowned with a woolly head, from which the flowers are protruded, the flowers themselves resembling those of Mammillaria, but being larger. The Hedgehog Thistles (Echinocactus) have stems resembling those of the different species of Melocactus, but they have not the woolly head; and the flowers rise from the fascicles or tufts of spines on the projecting ribs. The Torch-Thistle (Cereus) has generally an angular stem with a woody axis, and it has tufts of spines on the projecting angles. It has not a woolly head, and the flowers, which are very large and showy, either arise from the tufts of spines, or from indentations in the angles. The limits of this genus are very uncertain; and several plants which are included in it by some botanists, are placed in other genera by others. The Old-man Cactus was once called Cereus senilis, but it is found to have a woolly head of great size, which has very much the appearance of a sable muff, and as, consequently, it cannot belong to that genus, it has been called Pilocereus. This plant is covered with long white hairs, and, when of small size, it looks very much like an old man’s head. In its native country, however, it grows to a great height, and specimens have been imported fifteen feet long, and not more than a foot in circumference. The Peruvian Torch-Thistles (C. hexagonus and peruvianus), in their native country, are upwards of forty feet high, though not thicker than a man’s arm. They grow close together without a single branch, and form a singular sort of prickly crest on the summit of some of the mountains in South America. The creeping Cereus (C. flagelliformis) has slender cylindrical trailing stems, which hang down on every side when the plant is grown in a pot. The flowers, which are very numerous, are pink. The night-flowering Cereus (C. grandiflorus) only opens during the night, and fades before morning; the rays of the calyx are of a bright yellow when open, and the petals are snow-white. The stem is angular, branched, and climbing, throwing out roots at every joint. The common Torch-thistle (C. speciosissimus) is an erect plant, with a three or four angled stem, and very large bright crimson flowers, which are purplish inside; and C. speciosa, sometimes called Epi phyllum phylanthoides, has thin leaf-like stems with beautiful pale rose-coloured flowers. C. Jenkinsonii is a hybrid between the last two species. C. truncatus is another well-known species. Opuntia has stems consisting of round, flat, leaf-like bodies, united together by joints, and generally covered with tufts of spines. The most remarkable species are O. communis, the Prickly Pear, grown to a great extent in the South of Europe, and also in Brazil, as hedges, the fruit of which is very good to eat; O. Tuna, the Indian Fig, common in South America, and much cultivated there, both as a hedge plant and for its fruit; and O. cochinillifera, the Nopal-tree, very much cultivated in Mexico and South America, for the cochineal insect, which feeds upon it. Rhipsalis has slender cylindrical jointed stems, which look like samphire. All these genera have only leaves when quite young, and as soon as the plants begin to grow, the leaves fall off. Pereskia, however, is a genus belonging to this order which has leaves like ordinary plants, which it retains during the whole period of its existence. The principal species are P. aculeata, the Barbadoes Gooseberry, and P. Bleo, which has beautiful rose-coloured flowers.