Common name. Mustard. Family Brassicaceae.
Habit and leaf form. Herbs. Annual (usually), or biennial. 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 (0.05–)0.1–0.9 m high. Mesophytic and xerophytic. Leaves small to large; alternate; spiral; ‘herbaceous’; petiolate to sessile; non-sheathing; foetid, or without marked odour; simple; epulvinate. Leaf blades dissected (mostly, at least the lower), or entire; mostly lyrate or sinuate pinnatifid, or runcinate; pinnately veined; cross-venulate. Leaves without stipules. Leaf blade margins entire, or serrate, or dentate. Leaves without a persistent basal meristem. Leaf anatomy. Hairs present, or absent; 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; bracteate, or 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 to annular (glands confluent). Perianth with distinct calyx and corolla; 8; 3 -whorled (K 2+2, C 4). Calyx present; 4; 2 -whorled; polysepalous; erect (in ours); decussate; regular. Corolla present; 4; 1 -whorled; alternating with the calyx; polypetalous; imbricate, or contorted; regular; white (rarely), or yellow (often), or pink (often). Petals 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; 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; more or less 2 - lobed; capitate. Placentation parietal. Ovules 3–10 per locule (several?); with ventral raphe; non-arillate; anatropous.
Fruit and seed features. Fruit 7–100 mm long; non-fleshy; dehiscent; a siliqua. Capsules valvular. Fruit 2 celled; 10–100 seeded (several to many). Seeds 5–50 per locule (to ‘many’). Seed rows per locule 1. Seeds scantily endospermic, or non-endospermic; mucous; ovoid, elliptic or ellipsoidal; small to medium sized; wingless. Embryo well differentiated. Cotyledons 2; ‘usually’ flat; incumbent. Embryo bent.
Physiology, biochemistry. Mustard-oils present.
Special features. Fruit body with no clear differentiation into valve and beak regions. Replum present and complete; broad. Fruit bilaterally compressed to terete; if compressed, compressed parallel with the septum. The inner (lateral) pair of sepals not noticeably saccate. Petals not peculiarly elongated as in Stenopetalum. Nectariferous glands lateral and median (the medians confluent with the laterals). Valves of the fruit neither winged nor keeled; conspicuously longitudinally veined; longitudinally 1 veined, or 3 veined (commonly with a strong median and weaker laterals).
Etymology. From the Latin name used by Pliny for mint and also for watercress.