The two genera, Cistus and Helianthemum, differ chiefly in the capsule, which in the latter genus is triangular and one-celled, opening into three valves, each of which has a narrow dissepiment down its centre. To prevent any confusion arising from the use of these terms, I may here observe that when a capsule is divided into several cells, having no communication with each other, the membranes that separate them are called dissepiments; while the nerve-like part of it to which the seeds are attached is named the placenta. Sometimes the placenta is merely a nerve running down the side of the capsule, when the capsule is one-celled, without any dissepiment or division; and sometimes the dissepiment does not spread across the capsule so as to divide it into different cells, but only projects a little way from the side towards the centre, as in the one-celled capsule of the Poppy, (see p. 260,) and in that of the Helianthemum, when the seed-vessel opens naturally into different parts, as in the Gum Cistus, these parts are called valves, as are also the parts of pods, as shown in the curled-up valves of the silique, _fig._ 115, in p. 268. To return to the Helianthemum, the species of this genus are generally used for rockwork, as they are all dwarf plants, though many of the genus Cistus are large shrubs four or five feet high. The English name of the Helianthemum, Sun-rose, is very appropriate, as the flowers will only expand in sunshine, and will even decay in the bud without opening at all, when gloomy weather lasts for several days. ORDER XX. VIOLACEÆ—THE VIOLET TRIBE.