Weedy plants, containing among other genera, Knot-grass (_Illecebrum_), and Strapwort (_Corrigiola_). The new order Scleranthaceæ has been separated from this; and it takes its name from the British weed, Knawel (_Scleranthus_).
ORDER XCI.--CRASSULACEÆ.--THE HOUSELEEK TRIBE.
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 Houseleek, the anthers sometimes produce seeds instead of pollen.
