Habit and leaf form. Shrubs; non-laticiferous and without coloured juice. Plants succulent, or non-succulent. Mesophytic, or xerophytic. Leaves minute to medium-sized; alternate; spiral, or distichous; midrib and margins prominent and slightly thickened; petiolate; non-sheathing; gland-dotted, or not gland-dotted; simple. Leaf blades entire; pinnately veined, or palmately veined. Leaves with stipules. Stipules scarious, narrowly triangular; caducous, or persistent. 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, or with pistillodes (vestigial in S. polyandrus). Entomophilous.
Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’; when solitary, axillary; when aggregated, in apparently whorled clusters. Inflorescences axillary. Flowers pedicellate to sessile; bracteate; minute to small; regular. Floral receptacle developing an androphore, or with neither androphore nor gynophore (? depending on interpretation of staminal column). Free hypanthium absent. Hypogynous disk present, or absent. Perianth sepaline; 6; 1 -whorled. Calyx 6 (rather rigid, narrow); 1 -whorled; polysepalous; imbricate; regular. Fertile stamens present, or absent (female flowers). Androecium 10–25 (or ‘numerous’). Androecial members branched, or unbranched; free of the perianth; free of one another (if filaments interpreted as arising from an androphore), or coherent (outer free, inner connate into a column in S. polyandrus; filaments short, unequal, on a shallowly convex receptacle in S. axillaris; or anthers sessile on column). Androecium exclusively of fertile stamens. Stamens 10–25; diplostemonous to polystemonous; erect in bud, or inflexed in bud; filantherous (usually), or with sessile anthers (in S. brachyphyllus and S. vermicularis, if the sessile anthers are interpreted as arising from an androphore). Anthers dehiscing via longitudinal slits, or dehiscing via pores; 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; superior. Ovary plurilocular; 2 locular, or 3 locular. Gynoecium stylate (then greatly reduced), or non-stylate (if carpel interpreted as possessing sessile stigmatic branches). Styles 2, or 3; apical. Stigmas 2, or 3 (divergent); dry type; papillate, or non-papillate; Group II type. Placentation axile, or apical. Ovules 1 per locule, or 2 per locule; pendulous; epitropous; with ventral raphe, or with dorsal raphe; when two, collateral; arillate; orthotropous, or anatropous, or hemianatropous.
Fruit and seed features. Fruit non-fleshy; dehiscent; a capsule (ovoid); 1 celled (by abortion); 1 seeded. Seeds endospermic. Endosperm oily. Seeds arillate. 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. Endemic to Australia. Australian states and territories: Western Australia. South-West Botanical Province.
Additional comments. Further study of specimens is required before the proposal by Radcliffe-Smith (1993) to subsume Stachystemon under Pseudanthus can be accepted (P.G. Wilson, pers. comm.).