Fig. 400.—Transverse section of seed of Sisymbrium alliaria.
Many are biennial, forming in the first year a close leaf-rosette. By cultivation the tap-root can readily be induced to swell out into the form of a tuber (Turnips, Swedes, etc.). Stipules are found indicated by small glands on the very young leaves; in Cochlearia armoracia they are fairly large triangular scales. Stellate hairs often occur. Floral-leaves are occasionally developed. Terminal flowers are never found in the inflorescences. Iberis and Teesdalia have zygomorphic flowers. Subularia (Fig. [403]) is perigynous. The 2 external sepals (Fig. [397]) stand in the median plane; it may therefore be supposed that there are two bracteoles outside these which, however, are suppressed, and can only in a few instances be traced in the young flower; the two lateral sepals are often gibbous at the base, and serve as reservoirs for the nectar secreted by the glands placed above them; they correspond in position to the external petals of the Fumariaceæ. The 4 petals which follow next arise simultaneously, and alternate with the 4 sepals; if it could be shown that these are merely 2 median petals, which have been deeply cleft and the two parts separated from each other and displaced to the diagonal position, there would be a perfect correspondence with the Fumariaceous flower; then the petals would be followed in regular alternation by the 2 lateral small stamens, the 2 median long stamens, which it has been proved are split into 4 and placed in couples, and the 2 laterally-placed carpels,—in all 6 dimerous whorls. But the formation of the corolla by the splitting of 2 petals does not agree with the development of the flower or bear comparison, and hence the only fact in favour of this theory is the otherwise prevailing correspondence with the Fumariaceæ. Yet it may be observed that in special cases each pair of long stamens clearly enough arises from one protuberance and even later on may be considerably united or entirely undivided (e.g. Vella); in other instances they are quite distinct from the beginning, and it is possible that this latter condition has become constant in the corolla. Lepidium ruderale and others have no corolla. Senebiera didyma has only 2 median stamens. Megacarpæa has several stamens, no doubt by dédoublement, as in Capparidaceæ.—The number of carpels may also be abnormally increased; Tetrapoma barbareifolium has normally 4 carpels with an equal number of placentæ and repla. It is supposed to be a variety of Nasturtium palustre.—The 2–4–8–10 greenish glands, which are found at the base of the stamens, are nectaries, morphologically emergences, and not rudimentary stamens. The forms of fruits are of great systematic significance, see the genera. In some species dimorphic fruits are present, e.g. Cardamine chenopodiifolia which has both ordinary Cardamine-siliquas and 1-seeded siliculas.
The curved embryo appears in five forms, which have systematic importance: 1. To the Pleurorhizæ belong those genera whose radicle (with the hypocotyl) lies bent upwards along the edge of the flat cotyledons (Fig. [399]); to this group belong Cardamine, Nasturtium, Cheiranthus, Matthiola, Cochlearia, Draba, Iberis, Thlaspi, etc.; diagrammatic transverse section: ◯=.—2. To Notorhizæ belong those whose radicle lies in an upward direction along the back of one of the flat cotyledons (Figs. [400], [413]); e.g. Hesperis, Sisymbrium, Lepidium, Capsella, Camelina: ◯‖.—3. Orthoploceæ differ from the Notorhizeæ in having the cotyledons folded (not flat) (Fig. [398] E, F); to this belong Brassica, Sinapis, Raphanus, Crambe, etc.: ◯>>.—4. Spirolobeæ: the radicle lies as in the Notorhizæ, but the cotyledons are so rolled together that a transverse section of the seed cuts them twice; Bunias: ◯‖‖.—5. Diplecolobeæ: the cotyledons are folded forward and backward so that a transverse section cuts them several times; Subularia, Senebiera: ◯‖‖‖.
On germination the cotyledons appear above the ground as green leaves; in the Orthoploceæ they are bilobed, in the Lepidium-species divided.
1. Silicula, broad replum (Siliculosæ latiseptæ), valves flat or slightly vaulted, and the replum extends through the greatest width of the silicula (Fig. [404]). The seeds are situated in two rows.
◯=: Cochlearia (Horse-radish): the siliqua is nearly spheroid; glabrous herbs, generally with fleshy, stalked leaves, and white flowers.—Draba has an oblong, lanceolate, somewhat compressed silicula; herbs with small rosettes of leaves, most frequently with stellate and long-stalked racemes.—Alyssum and Berteroa are whitish, on account of the stellate hairs; they have a more compressed and round or elliptical silicula. Vesicaria; Aubrietia. Lunaria (Honesty, Fig. [401]): very broad and flat silicula with long stalk (the receptacle as in Capparidaceæ).
◯‖: Camelina (Gold-of-pleasure) has a spheroid, pear-shaped siliqua with a small rim passing right round (Fig. [402]). Subularia (Awlwort), an aquatic plant with perigynous flower (Fig. [403]) and folded cotyledons.
2. Silicula, narrow replum (Siliculosæ angustiseptæ), i.e. the replum is much shorter than the arched, more or less boat-shaped valves (Figs. [405], [406], [407]).
Fig. 401.—Lunaria biennis. Fruit, the valves of which have fallen off.