Separated from Mitrasacme sens lat along with Schizacme and Phyllangium by C.R.Dunlop (1996).
Habit and leaf form. Herbs. Annual. Leaves cauline, or basal (appearing rosulate in diminutive plants). Plants with neither basal nor terminal concentrations of leaves. Young stems cylindrical. To 0.25 m high. Self supporting. Hydrophytic (P. distylis), or helophytic, or mesophytic. Leaves in aquatic forms of P. distylis submerged. Heterophyllous. Leaves small; opposite; decussate; not decurrent on the stems; ‘herbaceous’; sessile; simple; epulvinate. Leaf blades dorsiventral; entire; flat; linear, or ovate, or obovate, or elliptic (often narrow); pinnately veined; cross-venulate. Mature leaf blades adaxially glabrous, or pilose (in P. paradoxum); abaxially glabrous. Leaves with stipules (reduced to a connate sheath). Stipules interpetiolar (or represented by a stipular sheath); ochreate; much reduced. Leaf blade margins entire; flat. Leaf anatomy. Hairs absent, or present (in P. paradoxum); glandular hairs absent. Unicellular hairs present (in P. paradoxum).
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Plants homostylous. Pollination mechanism unknown.
Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’ (of solitary peduncles or peduncles several to many in umbellate clusters); not crowded at the stem bases; terminal, or axillary. Inflorescence few-flowered to many-flowered. Flowers in umbels (when not solitary). Inflorescences simple. The terminal inflorescence unit racemose. Inflorescences terminal; ascending; with involucral bracts (involucres 2-lobed, foliaceous, calyx-like). Involucral bracts persistent. The involucres usually accrescent. Flowers very shortly pedicellate; ebracteolate; minute, or small; regular; 3 merous; cyclic; tricyclic. Free hypanthium absent. Hypogynous disk absent. Perianth petaline; 4; 1 -whorled; isomerous. Calyx absent. Corolla present; 4; 1 -whorled; gamopetalous; lobed; lobulate. Corolla lobes markedly shorter than the tube. Corolla regular; glabrous abaxially; glabrous adaxially; plain; white; deciduous. Corolla lobes valvate. Androecium present. Androecial members definite in number. Androecium 4. Androecial members free of the perianth; all equal; free of one another; 1 -whorled. Stamens 4; remaining included; all more or less similar in shape; isomerous with the perianth; all opposite the corolla members. Filaments glabrous; filiform. Anthers basifixed; appendaged; minutely apiculate, or non-apiculate (in P. sulcatum, but obtuse). Gynoecium 2 carpelled. The pistil 2 celled. Carpels reduced in number relative to the perianth. Gynoecium syncarpous; synovarious to eu-syncarpous; partly inferior. Ovary plurilocular; 2 locular. Gynoecium stylate. Styles 2; partially joined, or free (in P. palustre and P. distylis); apical; deciduous. Stigmas 2. Placentation axile. Ovules 30–50 per locule.
Fruit and seed features. Fruit 2–4 mm long; persistent; non-fleshy; not hairy (walls membranous); dehiscent; a capsule. Capsules loculicidal. Dispersal unit the seed. Seeds angular. Cotyledons 2.
Geography, cytology, number of species. Australian. Native of Australia. Endemic to Australia. Australian states and territories: Western Australia, South Australia, New South Wales, Victoria, and Tasmania. South-West Botanical Province. A genus of 5 species; 4 species in Western Australia; 2 endemic to Western Australia (P. paradoxa and P. palustre).
Additional comments. Name from the Greek phyllon (a leaf), and angion (a vessel), in reference to the foliaceous involucre surrounding the flowers.
Etymology. From the Greek for "leaf" and "vessel"; refers to the foliaceous involucre surrounding the flowers.
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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/