Common name. Thread Petal. Family Brassicaceae.
Habit and leaf form. Shrubs (rarely), or herbs. Annual, or perennial. Leaves cauline, or basal and cauline. Stem internodes solid. To 0.3–1 m high (or straggling). Mesophytic, or xerophytic. Leaves small to medium-sized; alternate; spiral; ‘herbaceous’; petiolate, or subsessile, or sessile; non-sheathing; foetid, or without marked odour; simple; epulvinate. Leaf blades dissected to entire; when dissected, pinnatifid; one-veined, or 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 (usually), or absent (rarely); glandular hairs absent; complex hairs present, or absent. Branched hairs present, or absent. Complex hairs when present, stellate. 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 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. Perianth with distinct calyx and corolla; 8; 3 -whorled (K 2+2, C 4). Calyx present; 4; 2 -whorled; polysepalous; erect; decussate; regular. Corolla present; 4; 1 -whorled; alternating with the calyx; polypetalous; imbricate, or contorted; regular; white, or orange (orange-yellow). 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; 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 non-stylate, or stylate. Styles 1; apical. Stigmas 1 (or 2); 1 - lobed (depressed-capitate); more or less capitate. Placentation parietal. Ovules 4–20 per locule; with ventral raphe; non-arillate; anatropous.
Fruit and seed features. Fruit 3.5–10(–13) mm long; non-fleshy; dehiscent; a silicula, or a siliqua. Capsules valvular. Fruit 2 celled; 8–40 seeded. Seeds 4–20 per locule. Seed rows per locule 2. Seeds scantily endospermic, or non-endospermic; mucous; compressed (oblong, elliptic, orbicular or reniform); small to medium sized. Embryo well differentiated. Cotyledons 2; accumbent (rarely), or 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, or terete; when compressed, compressed parallel with the septum. The inner (lateral) pair of sepals saccate basally for nectar storage, or not noticeably saccate. Petals drawn out into a long, narrow apex or ligulate, often much longer than the sepals, sometimes twisted. Nectariferous glands lateral only. Valves of the fruit neither winged nor keeled. Fruit apically notched, or not apically notched.
Etymology. From the Greek "narrow" and "leaf, petal".