All the plants belonging to the genera Cistus, the Rock-rose, and Helianthemum, the Sun-rose, have showy flowers, each having five petals, which are crumpled in the bud like those of the Poppy; they also resemble the petals of the Poppy tribe in falling almost as soon as they have expanded, as every one must have observed who has noticed the flowers of a Gum-Cistus. The calyx in both Cistus and Helianthemum generally consists of five sepals, two of which are larger and of a paler green than the others, and grow a little below them; and this calyx remains on after the petals have fallen, and, indeed, till the seed is ripe. In the Gum-Cistus, however, and some other species, the two outer sepals are wanting. There are a great many stamens, which are rather short, and form a tuft in the centre of the flower, surrounding the pistil, which has a round flat-headed stigma, a rather long style, and an ovary divided into five cells. The seeds are numerous, and each has a separate footstalk, by which it is attached to the placentas, which, in the Cistus tribe, are in the centre of the ovary, and not proceeding from its sides, as in the Mignonette. The capsule, which remains covered with the calyx till it is quite ripe, divides into five or ten concave valves, each having a placenta, to which the seeds were attached, in its centre. The seed of any plant belonging to the order Cistaceæ, is remarkable when cut open for the great size of the embryo enclosed in it, and the curious manner in which it is curled up. The embryo is the germ of the future plant, and it is usually buried in a great mass of albumen, or floury matter intended for the nourishment of the young plant, till its roots are in a fit state to supply it with nourishment. In the seed of the Cistus, there is scarcely any albumen; but in its stead a long narrow embryo, coiled up like a sleeping snake. The Gum-Cistus is generally called, in the nurseries, Cistus ladaniferus; but it differs materially from the plant so named by Linnæus, as that has a ten-celled capsule, while the capsule of the common Gum-Cistus (which botanists call _C. Cyperius_) has only five cells. The leaves also differ, the under surface of those of the one kind being woolly, and of the other smooth; the one is also a native of Spain and Portugal, and the other, as the specific name imports, of Cyprus. Both species, and also _C. Ledon_, exude from their stems and leaves, a kind of gum or resin called Ladanum or Labdanum, which is used in medicine. It is from this gum having been formerly always mixed with opium when that drug was dissolved in spirits of wine, that the name of laudanum is given to the tincture of opium.