CRUCIFERAE

cruciferae, or crucifer family, a natural order of flowering plants, which derives its name from the cruciform arrangement of the four petals of the flower. it is an order of herbaceous plants, many of which, such as wallflower, stock, mustard, cabbage, radish and others, are well-known garden or field-plants. many of the plants are annuals; among these are some of the commonest weeds of cultivation, shepherd's purse (_capsella bursa-pastoris_), charlock (_brassica sinapis_), and such common plants as hedge mustard (_sisymbrium officinale_), jack-by-the-hedge (_s. alliaria_ or _alliaria officinalis_). others are biennials producing a number of leaves on a very short stem in the first year, and in the second sending up a flowering shoot at the expense of the nourishment stored in the thick tap-root during the previous season. under cultivation this root becomes much enlarged, as in turnip, swede and others. wallflower (_cheiranthus cheiri_) (fig. 1) is a perennial. the leaves when borne on an elongated stem are arranged alternately and have no stipules. the flowers are arranged in racemes without bracts; during the life of the flower its stalk continues to grow so that the open flowers of an inflorescence stand on a level (that is, are corymbose). the flowers are regular, with four free sepals arranged in two pairs at right angles, four petals arranged crosswise in one series, and two sets of stamens, an outer with two members and an inner with four, in two pairs placed in the middle line of the flower and at right angles to the outer series. the four inner stamens are longer than the two outer; and the stamens are hence collectively described as tetradynamous. the pistil, which is above the rest of the members of the flower, consists of two carpels joined at their edges to form the ovary, which becomes two-celled by subsequent ingrowth of a septum from these united edges; a row of ovules springs from each edge. the fruit is a pod or siliqua splitting by two valves from below upwards and leaving the placentas with the seeds attached to the _replum_ or framework of the septum. the seeds are filled with the large embryo, the two cotyledons of which are variously folded. in germination the cotyledons come above ground and form the first green leaves of the plant. [illustration: fig. 1.--wallflower (_cheiranthus cheiri_), reduced. 1, flower in vertical section. 2, horizontal plan of arrangement of flower in _barbarea_.] [illustration: fig. 2.--_cruciferae._ floral diagram (_brassica_).] [illustration: fig. 3.--_cardamine pratensis._ flower with perianth removed. (after baillon.)] [illustration: fig. 4.--cruciferous fruits. (after baillon.) a, _cheiranthus cheiri._ b, _lepidium sativum._ c, _capsella bursa-pastoris._ d, _lunaria biennis_, showing the septum after the carpels have fallen away. e, _crambe maritima._] [illustration: fig. 5.--seeds of _cruciferae_ cut across to show the radicle and cotyledons. (after baillon.) a, _cheiranthus cheiri._ b, _sisymbrium alliaria._ figures 2-5 are from strasburger's _lehrbuch der botanik_, by permission of gustav fischer.] pollination is effected by aid of insects. the petals are generally white or yellow, more rarely lilac or some other colour, and between the bases of the stamens are honey-glands. some or all of the anthers become twisted so that insects in probing for honey will touch the anthers with one side of their head and the capitate stigma with the other. owing, however, to the close proximity of stigma and anthers, very slight irregularity in the movements of the visiting insect will cause self-pollination, which may also occur by the dropping of pollen from the anthers of the larger stamens on to the stigma. cruciferae is a large order containing nearly 200 genera and about 1200 species. it has a world-wide distribution, but finds its chief development in the temperate and frigid zones, especially of the northern hemisphere, and as alpine plants. in the subdivision of the order into tribes use is made of differences in the form of the fruit and the manner of folding of the embryo. when the fruit is several times longer than broad it is known as a siliqua, as in stock or wallflower; when about as long as broad, a silicula, as in shepherd's purse. [illustration: fig. 6.--honesty (_lunaria biennis_), showing flower and fruit. reduced.] the order is well represented in britain--among others by _nasturtium_ (_n. officinale_, water-cress), _arabis_ (rock-cress), _cardamine_ (bitter-cress), _sisymbrium_ (hedge mustard, &c.; _s. irio_ is london rocket, so-called because it sprang up after the fire of 1666), _brassica_ (cabbage and mustard), _diplotaxis_ (rocket), _cochlearia_ (scurvy-grass), _capsella_ (shepherd's purse), _lepidium_ (cress), _thlaspi_ (penny-cress), _cakile_ (sea rocket), _raphanus_ (radish), and others. of economic importance are species of _brassica_, including mustard (_b. nigra_), white mustard, used when young in salads (_b. alba_), cabbage (q.v.) and its numerous forms derived from _b. oleracea_, turnip (_b. campestris_), and swede (_b. napus_), _raphanus sativus_ (radish), _cochlearia armoracia_ (horse-radish), _nasturtium officinale_ (water-cress), _lepidium sativum_ (garden cress). _isatis_ affords a blue dye, woad. many of the genera are known as ornamental garden plants; such are _cheiranthus_ (wallflower), _matthiola_ (stock), _iberis_ (candy-tuft), _alyssum_ (alison), _hesperis_ (dame's violet), lunaria (honesty) (fig. 6), _aubrietia_ and others.