Habit and leaf form. Erect shrubs (with opposite, virgate branches), or trees; evergreen; laticiferous (latex white). ‘Normal’ plants, or switch-plants; with the principal photosynthesizing function transferred to stems, or ‘cactoid’ (the photosynthetic stems appearing succulent although actually somewhat woody). Leaves well developed, or much reduced, or absent (at flowering). Plants succulent (apparently), or non-succulent. Mesophytic, or xerophytic. Leaves minute to medium-sized; opposite; ‘herbaceous’, or leathery, or fleshy; more or less sessile; gland-dotted, or not gland-dotted; simple. Leaf blades entire; penninerved. Leaves with stipules, or without stipules. Stipules when present, entire, inconspicuous; caducous. Leaves without a persistent basal meristem. Leaf anatomy. Urticating hairs present, or absent. Stem anatomy. Nodes tri-lacunar, or unilacunar. Secondary thickening developing from a conventional cambial ring, or anomalous; from a single cambial ring.
Reproductive type, pollination. Fertile flowers functionally male, or functionally female. Unisexual flowers present. Plants monoecious. The unisexual flowers aggregated in different parts of the same inflorescence. Female flowers solitary (in the centre of the involucre). Male flowers aggregated in ‘inflorescences’ (in 4 clusters of 3–16 flowers within the involucre and opposite its lobes, each cluster subtended and more or less embraced by bracts, the outer 1 or 2 much enlarged and enclosing the cluster); without pistillodes. Entomophilous.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in heads. Inflorescences axillary; with involucral bracts (involucres (cyathia) campanulate, resembling a calyx, 4-lobed and with small glands alternating between the lobes); pseudanthial. Flowers bracteate, or ebracteate; minute to small; regular. Free hypanthium absent. Hypogynous disk absent. Perianth sepaline (in female flowers), or absent (in male flowers); when present, 4, or 6; 1 -whorled. Calyx 4, or 6; 1 -whorled; gamosepalous; regular. Fertile stamens present, or absent (female flowers). Androecium 1. Androecium exclusively of fertile stamens. Stamens 1; erect in bud, or inflexed in bud. Filaments articulated with the pedicel, filiform to somewhat flattened. Anthers dorsifixed; dehiscing via longitudinal slits (cells oblong); extrorse, or introrse; bisporangiate, or tetrasporangiate. Fertile gynoecium present, or absent (male flowers). Gynoecium 2 carpelled, or 3 carpelled. The pistil 2 celled, or 3 celled. Gynoecium syncarpous; synovarious, or synstylovarious (depending on interpretation of partially connate styles); superior. Ovary plurilocular; 2 locular, or 3 locular; sessile. Gynoecium stylate. Styles 2, or 3; free, or partially joined (shortly connate at the base); forked; apical. Stigmas 4, or 6; dry type; papillate, or non-papillate; Group II type. Placentation axile, or apical. Ovules 1 per locule; pendulous; epitropous; with ventral raphe, or with dorsal raphe; arillate; orthotropous, or anatropous, or hemianatropous.
Fruit and seed features. Fruit non-fleshy, or fleshy; a schizocarp (capsular, trilobate). Mericarps 3; 2-valved. Fruit elastically dehiscent (schizocarpic capsules often splitting elastically), or passively dehiscent. Seeds 1 per mericarp. Seeds oblong to subglobose-obloid; endospermic. Endosperm oily. Seeds arillate. Cotyledons 2 (usually wider than the radicle). Embryo straight, or curved. Testa crustaceous. Seedling. Germination phanerocotylar, or cryptocotylar.
Physiology, biochemistry. Mustard-oils present, or absent.
Geography, cytology, number of species. Native of Australia. Endemic to Australia. Australian states and territories: Western Australia, Northern Territory, and Queensland. Northern Botanical Province and South-West Botanical Province.