Common name. Sheeps' Burrs. Family Rosaceae.
Habit and leaf form. Herbs; evergreen. Plants unarmed. Perennial (often more or less woody at base). Leaves basal, or cauline. Plants with neither basal nor terminal concentrations of leaves; to 0.2–0.6 m high; rhizomatous (or stoloniferous). Mesophytic. Not heterophyllous. Leaves medium-sized; not fasciculate; alternate; spiral; not decurrent on the stems; ‘herbaceous’; imbricate; petiolate. Petioles wingless. Leaves non-sheathing (stipules sheathing at base and fused to the petiole); compound; epulvinate; pinnate; imparipinnate. Leaflets 7–25; 0.8–2.3 cm long. Lateral leaflets opposite. Leaflets not stipellate; epulvinate; elliptic, or oblong, or ovate, or obovate, or orbicular; attenuate to the base, or rounded at the base; flat; without lateral lobes. Leaflet margins flat. Leaf blades pinnately veined; cross-venulate. Mature leaf blades adaxially glabrous, or pilose, or villous; abaxially pilose, or villous. Leaves with stipules. Stipules intrapetiolar; adnate to the petiole; free of one another; leafy; persistent. Leaf blade margins serrate, or dentate; not prickly; flat. Vegetative buds scaly. Leaves without a persistent basal meristem. Leaf anatomy. Hydathodes absent. Hairs present; glandular hairs absent. Stem anatomy. Secondary thickening developing from a conventional cambial ring.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Plants not viviparous; homostylous. Floral nectaries present. Entomophilous; via beetles, or via lepidoptera.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; not crowded at the stem bases. Inflorescence many-flowered. Flowers in spikes, or in heads. Inflorescences simple. The terminal inflorescence unit racemose. Inflorescences terminal, or axillary; ascending; with involucral bracts, or without involucral bracts. Flowers sessile; ebracteate; ebracteolate; small; regular; 4–5 merous. Floral receptacle markedly hollowed. Free hypanthium present; tubular. Perianth sepaline; (3–)4–5(–7); 1 -whorled. Calyx present; (3–)4–5(–7); 1 -whorled; polysepalous; usually imbricate; regular; green; mainly persistent. Sepals elliptic. Corolla absent. Androecium present. Androecial members definite in number. Androecium 1–10. Androecial members free of the perianth; all equal; free of one another; 1 -whorled. Stamens 1–10; attached on the rim of the hypanthium; all more or less similar in shape; reduced in number relative to the adjacent perianth, or isomerous with the perianth, or diplostemonous; inflexed in bud. Filaments not geniculate; glabrous; filiform. Anthers all alike; dorsifixed; versatile; dehiscing via longitudinal slits; tetrasporangiate. Gynoecium 1 carpelled, or 2–4 carpelled (rarely). The pistil 1 celled. Carpels reduced in number relative to the perianth. Gynoecium monomerous, or apocarpous (rarely); of one carpel, or eu-apocarpous; superior. Carpel stylate; apically stigmatic. Style curved (dilated into a fringed, plumose stigma). Carpel 1 ovuled. Placentation apical. Ovary summit glabrous. Styles simple; not becoming exserted; persistent; hairless. Stigmas plumose. Ovules pendulous; non-arillate; hemianatropous, or anatropous.
Fruit and seed features. Fruit 1.5–8 mm long; non-fleshy; hairy (prickles or awns barbed with reflexed hairs); spinose. The fruiting carpel indehiscent; an achene. Fruit 1 celled. Dispersal unit the fruit. Dispersal in the fur of animals or the soft down of young sea birds. Fruit 1 seeded. Seeds 1 per locule. Seeds non-endospermic. Cotyledons 2. Embryo straight. Testa hard; non-operculate. Seedling. Cotyledon hyperphyll assimilatory.
Geography, cytology, number of species. World distribution: Australia, New Zealand, New Guinea, sub-antarctic islands of the southern Indian and Atlantic oceans, South Africa, Hawaii, California, South America. Native of Australia, or adventive. Not endemic to Australia. Australian states and territories: Western Australia, or South Australia, or Queensland, or New South Wales, or Victoria, or Australian Capital Territory, or Tasmania. South-West Botanical Province. X=21; ploidy levels recorded up to 6. A genus of ca. 100 species; 3 species in Western Australia; Acaena agnipila Gand., Acaena echinata Nees, Acaena novae-zelandiae Kirk; 0 endemic to Western Australia.
Etymology. From the Greek akaina, "thorn", alluding to the prickles 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/