Habit and leaf form. Herbs. Annual. Leaves cauline, or basal and cauline. Plants with a basal concentration of leaves, or with neither basal nor terminal concentrations of leaves. Stem internodes solid. To 0.3–0.8 m high. Mesophytic. Leaves medium-sized to large; alternate; spiral; ‘herbaceous’; petiolate to sessile; non-sheathing; simple (cauline, sometimes), or compound; epulvinate; when compound, pinnate. Leaf blades when simple, dissected, or entire; pinnately veined; cross-venulate. Leaves without stipules. Leaf blade margins dentate. Leaves without a persistent basal meristem. Leaf anatomy. Hairs present, or absent; complex hairs absent. Branched hairs absent. Extra-floral nectaries absent.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in racemes, or in corymbs. The terminal inflorescence unit racemose. Flowers pedicellate; ebracteate; ebracteolate; minute to small; regular; 2 merous; cyclic. Floral receptacle with neither androphore nor gynophore. Free hypanthium absent. Hypogynous disk present; of separate members. Nectariferous glands 4. Perianth with distinct calyx and corolla; 8; 3 -whorled (K 2+2, C 4). Calyx present; 4; 2 -whorled; polysepalous; spreading; decussate; regular. Corolla present; 4; 1 -whorled; alternating with the calyx; polypetalous; imbricate, or contorted; regular; yellow. Petals clawed (shortly clawed). Androecial members definite in number. Androecium 6. Androecial members branched (in that the inner whorl of 4 is derived from only 2 primordia); free of the perianth; markedly unequal; free of one another; 2 -whorled (2+4). Androecium exclusively of fertile stamens. Stamens 6 (2 outer, 4 inner); tetradynamous; all more or less similar in shape; hypogynous, on receptacle, outer stamens lateral. Filaments not appendiculate. Anthers basifixed; non-versatile; dehiscing via longitudinal slits; introrse; unilocular to bilocular; tetrasporangiate; appendaged, or unappendaged. Pollen shed as single grains. Gynoecium 2 carpelled. The pistil 2 celled. Gynoecium syncarpous; eu-syncarpous; superior. Ovary plurilocular; 2 locular. Locules secondarily divided by ‘false septa’. Gynoecium transverse. Ovary sessile. Gynoecium stylate. Styles 1; apical. Stigmas 2; commissural; 1–2 - lobed; capitate. Placentation parietal. Ovules (1–)3–50 per locule; with ventral raphe; non-arillate; anatropous.
Fruit and seed features. Fruit 20–55 mm long; non-fleshy; dehiscent; a siliqua. Capsules valvular. Fruit 2 celled; 2–24 seeded. Seeds 1–12 per locule. Seed rows per locule 1. Seeds scantily endospermic, or non-endospermic; not mucous; spherical; small to medium sized; wingless (spherical). Embryo well differentiated. Cotyledons 2; folded; incumbent; conduplicate. Embryo bent (folded, the cotyledons against the radicle).
Physiology, biochemistry. Mustard-oils present.
Special features. Fruit body distinctly differentiated into valve and beak regions. Beak containing seeds (usually 1–seeded). Replum present and complete. Fruit terete. The inner (lateral) pair of sepals not noticeably saccate. Petals not peculiarly elongated as in Stenopetalum. Nectariferous glands lateral and median. Siliquae moniliform to not moniliform. Valves of the fruit neither winged nor keeled; conspicuously longitudinally veined; longitudinally 3–7 veined.
Etymology. From the Latin name used by Pliny for Brassica alba and B. niger.