Pogostemon Desf.

Scientific Description

Family Lamiaceae.

Habit and leaf form. Herbs, or shrubs; evergreen; bearing essential oils. Plants unarmed. Annual, or perennial. Leaves cauline. Plants with neither basal nor terminal concentrations of leaves. Young stems usually tetragonal. To 0.3–0.6 m high. Hydrophytic to helophytic; rooted. Leaves emergent; medium-sized; not fasciculate; opposite, or whorled; 4–10 per whorl; not decurrent on the stems; ‘herbaceous’; not imbricate; sessile, or subsessile, or petiolate; aromatic, or foetid (depending on opinion); simple; epulvinate. Leaf blades entire; flat; linear; pinnately veined; cross-venulate; attenuate at the base. Mature leaf blades adaxially glabrous; abaxially glabrous. Leaves without stipules. Leaf blade margins entire, or serrate (obscurely to distinctly); flat. Leaves without a persistent basal meristem. Leaf anatomy. Hairs absent; glandular hairs present. Urticating hairs absent. Stem anatomy. Secondary thickening developing from a conventional cambial ring.

Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Plants heterostylous. Entomophilous, or ornithophilous; usually via hymenoptera, or via lepidoptera, or via diptera.

Inflorescence and flower features. Flowers aggregated in ‘inflorescences’. Inflorescence many-flowered. Flowers in verticils. The terminal inflorescence unit cymose. Inflorescences terminal; spike-like, cylindric, units crowded, densely hairy. Flowers bracteate; minute; somewhat irregular; zygomorphic; cyclic; tetracyclic. Floral receptacle developing a gynophore, or with neither androphore nor gynophore. Free hypanthium absent. Perianth with distinct calyx and corolla; 4–10; 2 -whorled; isomerous, or anisomerous. Calyx present; 5; 1 -whorled; gamosepalous; blunt-lobed; imbricate, or open in bud; campanulate; regular to unequal but not bilabiate (the lobes subequal); non-fleshy; persistent; with the median member posterior. Calyx lobes triangular. Corolla present; ostensibly 4, or 5 (disguisedly, the upper pair of members scarcely distinguishable); 1 -whorled; gamopetalous; blunt-lobed; imbricate; tubular; regular to unequal but not bilabiate (the abaxial and lateral lobes broad, the lateral lobes smaller than the abaxial one, the adaxial lobe-pair larger than abaxial lobe); plain, or with contrasting markings; pink, or pink to purple. Corolla lobes broadly ovate, or ovate (lobes pubescent). Corolla members entire (or notched). Androecium present. Fertile stamens present. Androecial members definite in number. Androecium 4. Androecial members adnate; all equal, or markedly unequal; free of one another; 1 -whorled. Androecium exclusively of fertile stamens. Stamens 4. Staminal insertion midway down the corolla tube to in the throat of the corolla tube. Stamens becoming exserted; not didynamous, not tetradynamous; 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; all alternating with the corolla members. Filaments hairy; filiform. Anthers separate from one another; dorsifixed; versatile, or non-versatile; dehiscing via longitudinal slits; introrse; unilocular to bilocular; tetrasporangiate; unappendaged. Pollen shed as single grains. Fertile gynoecium present. Gynoecium 2 carpelled (the carpels deeply lobed to mimic G4). The pistil 4 celled. Carpels reduced in number relative to the perianth. Gynoecium syncarpous; synstylovarious to eu-syncarpous; superior. Ovary plurilocular; 2 locular (originally), or 4 locular (by intrusions of the ovary wall constituting ‘ false septa’). Locules secondarily divided by ‘false septa’. Gynoecium median; stylate. Styles 1; simple; from a depression at the top of the ovary (the ovary deeply lobed); ‘gynobasic’; becoming exserted. Stigmas 2; 2 - lobed. Placentation basal. Ovules 2 per locule, or 1 per locule (two per original loculus, but one per locellus); ascending; apotropous; non-arillate; anatropous, or hemianatropous.

Fruit and seed features. Fruit non-fleshy, or fleshy; a schizocarp. Mericarps (2–)4; comprising nutlets. Seeds endospermic to non-endospermic; minute. Embryo well differentiated. Cotyledons 2. Embryo straight.

Special features. The flowering nodes aggregated in dense spicate inflorescences. (1–)2–25 flowers subtended by each floral leaf (usually ‘many’). Calyx limb 5 lobed. Corolla tube usally not exceeding the calyx; straight. The upper lip of the corolla incorporating 2 members, the lower 3.

Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, Northern Territory, and Queensland. Northern Botanical Province. A genus of ca 50 species; 1 species in Western Australia; 0 endemic to Western Australia.

Etymology. From the Greek for "beard" and "thread, stamen"; the staminal filaments are bearded.