Habit and leaf form. Trees, or shrubs; laticiferous (latex milky). Plants succulent, or non-succulent. Mesophytic, or xerophytic. Leaves medium-sized to large; alternate; spiral, or distichous; ‘herbaceous’, or leathery, or fleshy; petiolate; non-sheathing; gland-dotted, or not gland-dotted; simple. Leaf blades entire; pinnately veined, or palmately veined. Leaves with stipules. Stipules conspicuous, leafy; 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. Male flowers without pistillodes. Entomophilous.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in racemes (with both male and female flowers in each raceme; male flowers in clusters, each cluster subtended by a bract which has 1 or 2 glands at the base; female flowers not clustered, each subtended by a similar bract). Inflorescences terminal. Flowers bracteate, or ebracteate; minute to small; regular. Free hypanthium absent. Hypogynous disk absent. Perianth sepaline; 1–3; 1 -whorled. Calyx 1–2 (in male flowers), or 2–3 (in female flowers); gamosepalous; 2–3- lobed, or entire; regular; not persistent (on female flowers of H. novo-guineensis). Fertile stamens present, or absent (female flowers). Androecium 5–50. Androecial members free of the perianth; free of one another. Androecium exclusively of fertile stamens. Stamens 5–50; polystemonous; erect in bud, or inflexed in bud. Filaments short. Anthers dehiscing via longitudinal slits; 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. Styles 2, or 3; partially joined (connate towards the base); simple, or forked; apical. Stigmas 2–6; 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; dehiscent, or indehiscent; a capsule (tardily dehiscent in H. novo-guineensis). Seeds ovoid; endospermic. Endosperm oily. Seeds arillate (caruncle succulent). Cotyledons 2 (usually wider than the radicle). Embryo straight, or curved. Seedling. Germination phanerocotylar, or cryptocotylar.
Physiology, biochemistry. Mustard-oils present, or absent.
Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, Northern Territory, and Queensland. Northern Botanical Province.