Habit and leaf form. Herbs (procumbent or creeping); bearing essential oils. Plants unarmed. Leaves cauline. Young stems obtusely tetragonal, or cylindrical to oval in section. To 0.3 m high (flowering stems, remainder prostrate); rhizomatous. Leaves small to medium-sized; not fasciculate; opposite; decussate; ‘herbaceous’, or fleshy (somewhat); not imbricate; petiolate to subsessile; foetid, or without marked odour; simple; epulvinate. Leaf blades entire; flat; obovate (or narrowly so), or oblong, or elliptic, or rhombic, or trullate, or obtrullate, or triangular, or obtriangular; pinnately veined; cross-venulate; cuneate at the base. Leaves without stipules. Leaf blade margins entire, or serrate to dentate; flat. Leaves without a persistent basal meristem. Stem anatomy. Secondary thickening developing from a conventional cambial ring.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Plants heterostylous. Entomophilous.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’. Inflorescence many-flowered. Flowers in spikes, or in heads. The terminal inflorescence unit racemose. Inflorescences axillary; axillary, pedunculate, many-flowered, dense, spikes, usually capitate-globose at first but becoming ovoid to oblong-cylindrical; not pseudanthial. Flowers sessile; bracteate; minute to small; very irregular; zygomorphic; cyclic; tetracyclic. Free hypanthium absent. Hypogynous disk present, or absent. Perianth with distinct calyx and corolla; ostensibly 7; 2 -whorled; anisomerous. Calyx ostensibly 2; 1 -whorled; gamosepalous; lobed; deeply blunt-lobed. Calyx lobes markedly longer than the tube. Calyx erect; ovoid campanulate (or compressed); strongly bilabiate; persistent; non-accrescent. Calyx lobes broadly ovate. Corolla present; 5; 1 -whorled; gamopetalous; lobed; lobulate. Corolla lobes markedly shorter than the tube. Corolla contorted; hypocrateriform and tubular (tube cylindrical); weakly bilabiate, or regular; largely glabrous abaxially, or hairy abaxially (with a few hairs outside on or below base of lobes); glabrous adaxially; with contrasting markings; white to purple (or purplish), or purple (mauve or lilac), or white to pink. Corolla lobes ovate (to broadly so). Corolla members entire. Androecium present. Androecial members definite in number. Androecium 4. Androecial sequence not determinable. Androecial members adnate (to the corolla tube); markedly unequal; free of one another; 1 -whorled. Androecium exclusively of fertile stamens. Stamens 4. Staminal insertion midway down the corolla tube, or in the throat of the corolla tube. Stamens inserted at markedly different levels to all inserted at the same level; scarcely becoming exserted, or remaining included; didynamous; all more or less similar in shape; reduced in number relative to the adjacent perianth; fertile stamens representing the posterior-lateral pair and the anterior-lateral pair; oppositisepalous. Filaments glabrous; filiform (short). Anthers separate from one another; dorsifixed; dehiscing via longitudinal slits; introrse; tetrasporangiate (but the posterior member aborting). Gynoecium 2 carpelled. The pistil 2 celled (representing the anterior carpel). Carpels reduced in number relative to the perianth. Gynoecium syncarpous; eu-syncarpous; superior. Ovary plurilocular; 2 locular, or 1 locular (with the posterior member aborting). Locules secondarily divided by ‘false septa’. Gynoecium median. Ovary summit glabrous. Gynoecium stylate. Styles 1; simple; attenuate from the ovary; apical; shorter than the ovary at anthesis, or about as long as the ovary at anthesis; not becoming exserted; hairless. Stigmas 1; 1 - lobed, or 2 - lobed; (sub-) peltate, or capitate. Placentation basal to axile. Ovules 2 per locule, or 1 per locule (per cell); ascending; non-arillate; anatropous.
Fruit and seed features. Fruit 1–2 mm long; non-fleshy; a schizocarp. Mericarps 2; comprising nutlets. Fruit 2 celled. Seeds 1 per mericarp. Seeds non-endospermic. Cotyledons 2 (expanded, flat). Embryo straight.
Special features. Calyx limb deeply 2 lobed. Upper lip of calyx entire, or lobed; 2 lobed (P.nodiflora), or 4 lobed (P. canescens). Lower lip of calyx entire, or lobed; 2 lobed (P. nodiflora), or 4 lobed (P. canescens). Corolla tube exceeding the calyx; straight. The upper lip of the corolla incorporating 2 members, the lower 3; (posterior, adaxial) lip of the corolla entire to bilobed. Lower (abaxial) lip of the corolla 3 lobed; not concave. The anterior pair of stamens exceeding the posterior pair.
Geography, cytology, number of species. Adventive. Not endemic to Australia. Australian states and territories: Western Australia, South Australia, Northern Territory, Queensland, New South Wales, Victoria, and Australian Capital Territory. Northern Botanical Province, or South-West Botanical Province. A genus of ca 11 species; 2 species in Western Australia; 0 endemic to Western Australia.
Etymology. From the Greek for "a union among the citizens of a state, a tribe"; refers to the numerous flowers ‘from a common calyx’, i.e. in a dense, head-like spike.