Habit and leaf form. Prostrate, scrambling or erect herbs (in W.A.). Annual, or perennial; plants with neither basal nor terminal concentrations of leaves. Mesophytic (in vine thickets). Leaves opposite; simple. Leaf blades entire; pinnately veined. Leaves without stipules. Leaf blade margins entire. Stem anatomy. Nodes unilacunar. Secondary thickening developing from a conventional cambial ring, or anomalous.
Reproductive type, pollination. Fertile flowers hermaphrodite (and sterile). Unisexual flowers absent. Plants hermaphrodite.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’. Inflorescences compound. The terminal inflorescence unit cymose. Inflorescences of terminal and axillary spikes, bearing partial inflorescences (burrs), each of which consists of one bisexual flower and two sterile flowers, the burrs falling entire at maturity; (burrs) pseudanthial. Flowers regular; 5 merous; cyclic; tricyclic. Free hypanthium absent. Hypogynous disk absent. Perianth sepaline (greenish); 5; 1 -whorled; free; green; non-fleshy; persistent; non-accrescent. Calyx present (the perianth being thus interpreted); 5; 1 -whorled; polysepalous; erect; imbricate; regular; green; with the median member posterior. Sepals narrowly elliptic. Corolla absent. Androecial members definite in number. Androecium 5. Androecial sequence not determinable. Androecial members free of the perianth; all equal, or markedly unequal; basally coherent; 1 - adelphous (the filaments basally connate into a somewhat succulent cup); 1 -whorled. Androecium exclusively of fertile stamens. Stamens 5; oppositisepalous. Anthers dorsifixed; dehiscing via longitudinal slits; introrse; bilocular; bisporangiate, or tetrasporangiate. Pollen shed as single grains. Gynoecium 2–3 carpelled. The pistil 1 celled. Carpels reduced in number relative to the perianth. Gynoecium syncarpous (pseudomonomerous); eu-syncarpous; superior. Ovary unilocular; 1 locular. Gynoecium stylate. Styles 1; attenuate from the ovary; apical. Stigmas 1; 1 - lobed; capitate; dry type; papillate; Group II type. Placentation basal. Ovules differentiated; in the single cavity 1; pendulous; non-arillate; campylotropous.
Fruit and seed features. Fruit non-fleshy; dehiscent to indehiscent; a capsule to capsular-indehiscent (a utricle). Capsules if dehiscing, splitting irregularly. Dispersal unit part of the inflorescence (the burr). Fruit 1 seeded. Seeds non-endospermic. Perisperm present. Seeds slightly compressed; small; with starch. Embryo well differentiated. Cotyledons 2. Embryo curved. Testa smooth (shiny).
Geography, cytology, number of species. Adventive. A genus of 4 species; 1 species in Western Australia.
Additional comments. The two sterile flowers of the burr each consist of a bracteole and 3–5 stipitate clusters of uncinate spines on a common stipe, the spines accrescent on the fruit.
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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/