Habit and leaf form. Somewhat glaucous herbs. Annual, or biennial. Leaves cauline. Stem internodes solid. To 0.15–0.6(–0.9) m high. Mesophytic. Leaves medium-sized to large; alternate; spiral; ‘herbaceous’; petiolate; non-sheathing; simple; epulvinate. Leaf blades dissected to entire (the cauline leaves reducing); (basal leaves) lyrate pinnatifid; pinnately veined; cross-venulate. Leaves without stipules. Leaf blade margins coarsely dentate (to irregular). Leaves without a persistent basal meristem. Leaf anatomy. Hairs present (hispid); glandular hairs 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; small to medium-sized; 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; erect to spreading (‘half spreading’); decussate; regular. Sepals inner pair saccate. Corolla present; 4; 1 -whorled; alternating with the calyx; polypetalous; imbricate, or contorted; regular; yellow (dark veined). Petals clawed (claws 5–10 mm long). 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; 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 1, or 2; commissural; 1–2 - lobed; capitate. Placentation parietal. Ovules 2–7 per locule (few); with ventral raphe; non-arillate; anatropous.
Fruit and seed features. Fruit 5–10 mm long; non-fleshy; lomentaceous (disarticulating between the beak and the valve); a silicula; 2 celled; 1 seeded, or 2 seeded (the upper article globular and one-seeded, the lower smaller and with or without a smaller seed). Seeds scantily endospermic, or non-endospermic; not mucous; small to medium sized; wingless (ovoid, the one in the upper segment larger). Embryo well differentiated. Cotyledons 2; folded; incumbent; conduplicate. Embryo bent.
Physiology, biochemistry. Mustard-oils present.
Special features. Fruit body distinctly differentiated into valve and beak regions. Beak containing seeds. Replum present and complete. The inner (lateral) pair of sepals somewhat saccate basally for nectar storage, or not noticeably saccate. Petals not peculiarly elongated as in Stenopetalum. Nectariferous glands lateral and median.
Etymology. From the Latin for "turnip, rape" and -aster, suffix denoting similarity with an implication of inferiority; imitation.