ORDER XIII. CRUCIFERÆ.--CRUCIFEROUS PLANTS.
[Illustration: FIG. 113.--THE FLOWER AND STAMENS AND PISTIL OF THE SEA-KALE, TO ILLUSTRATE THE CRUCIFERÆ.]
The Cruciferous plants form so natural an order, that when one of them has been described the others may be easily recognised. They have all a separate calyx and corolla, each in four divisions; the four sepals being placed alternately with the four petals, the latter forming a cross (as shown at _a_ in _fig._ 113), whence the name of Cruciferous, which signifies cross-bearing. There are six stamens (_b_), two of which are much shorter than the others; and two carpels with one style, and a capitate or divided stigma. The seed-vessel is a kind of pod, either short and broad, like that of the Shepherd's Purse (_fig._ 114), where it is called a sillicle; or long and narrow, like that of the Cabbage, which is called a silique. The two valves of the silique open naturally when ripe, from the bottom curving upwards, (see _fig._ 115,) and the seeds are deposited on a thin membrane between the cells, which is the dissepiment. All the Cruciferæ, from abounding in nitrogen, have an unpleasant smell when decaying, like putrid flesh; and when cultivated, they even in a wild state require abundance of animal manure; hence, they are generally found near human habitations, or where cattle are kept. When wild, they have generally acrid properties; and though these are in most cases softened by cultivation, yet they are still perceptible in the roots of the Horse-Radish, and the common Radish, and in the leaves and seeds of Mustard, and the different kinds of Cress, &c. This acridity, however, is never so great as to be injurious; and Cruciferous plants, particularly if their texture be succulent and watery, may always be eaten with perfect safety. Even those which, in a wild state, are tough and stringy, such as the wild Cabbage and the root of the wild Turnip, become excellent by cultivation; and all Cruciferous plants are so extremely nourishing as to be considered next in this quality to animal food.
Among the many garden flowers which belong to this order, few are more popular than the common Wallflower (_Cheiranthus Cheiri_). Its hardiness, and the facility with which it is raised and cultivated--the gaiety of its flowers, their profusion, and their delightful fragrance, combine to make it a general favourite; and I think I cannot take a flower to illustrate the order which is more generally known and liked. The flowers of the Wallflower (see _fig._ 116, _a_) consist of four petals, each of which is furnished with a long tapering point, called the claw (_b_), and a broad flat part called the limb (_c_). The claws of the petals are buried in a calyx of four sepals, placed alternately to the petals, and somewhat swelled out at the base, (see _d_). The stigma (_e_) is two-lobed, and forms a kind of notched head. There are six stamens, which appear at first to be all nearly of the same height, but on examination it will be found that two are somewhat shorter than the others. The seed-vessel is of course the lower part of the pistil; which, after the petals drop, becomes elongated into a somewhat cylindrical silique, which contains several flattish seeds. [Illustration: FIG. 116.--FLOWERS OF THE WALLFLOWER.]