Habit and leaf form. Herbs. Annual. Leaves basal and cauline. Plants with neither basal nor terminal concentrations of leaves (basal leaves non-persistent). Stem internodes solid. To 0.25 m high. Mesophytic, or xerophytic. Leaves small to medium-sized; alternate; spiral; almost fleshy, or ‘herbaceous’; petiolate, or subsessile, or sessile; sheathing to non-sheathing; foetid, or without marked odour; simple; epulvinate. Leaf blades dissected to entire; obovate to linear; when dissected, pinnatifid; pinnately veined; cross-venulate. Leaves without stipules. Leaf blade margins entire, or dentate. Leaves without a persistent basal meristem. Leaf anatomy. Hairs present; glandular hairs absent; complex hairs absent. Branched hairs absent (hairs flattened, terete or twisted). 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. Perianth with distinct calyx and corolla; 8; 3 -whorled (K 2+2, C 4). Calyx present; 4; 2 -whorled; polysepalous; decussate; regular. Corolla present; 4; 1 -whorled; alternating with the calyx; polypetalous; imbricate, or contorted; regular; white, or yellow, or pink. Petals sessile. 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 (the outer pair shorter); coherent; 2 -whorled (2+4). Androecium exclusively of fertile stamens. Stamens 6; tetradynamous; all more or less similar in shape; hypogynous, on receptacle. 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 stylate. Styles 1; apical. Stigmas 1 (or 2); commissural; 1 - lobed, or 2 - lobed; capitate. Placentation parietal. Ovules 3–10 per locule (several); with ventral raphe; non-arillate; anatropous.
Fruit and seed features. Fruit 2.5–5 mm long; non-fleshy; dehiscent; a silicula (round). Capsules valvular. Fruit 2 celled; 6–14 seeded. Seeds 3–7 per locule. Seeds scantily endospermic, or non-endospermic; mucous; small to medium sized. Embryo well differentiated. Cotyledons 2; 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; somewhat narrow. Fruit bilaterally compressed; compressed at right angles to the septum. The inner (lateral) pair of sepals not noticeably saccate. Petals not peculiarly elongated as in Stenopetalum. Nectariferous glands lateral and median. Valves of the fruit slightly keeled. Fruit not apically notched.
Etymology. From the Greek for "light weight" and "back"; refers to the lack of prominence of the keel of the valves.
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Western Australian Herbarium (1998–). FloraBase—the Western Australian Flora. Department of Biodiversity, Conservation and Attractions. https://florabase.dpaw.wa.gov.au/