Plectranthus L'Her.

Scientific Description

Common name. Plectranthus. 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 0.3–1.5 m high. Leptocaul. Leaves small to medium-sized; not fasciculate; opposite; membranous (blade); not imbricate; petiolate; aromatic; simple; epulvinate. Leaf blades entire; flat; ovate, or elliptic; one-veined; cross-venulate; cuneate at the base (to subtruncate). Mature leaf blades adaxially woolly; abaxially woolly. Leaves without stipules. Leaf blade margins undulate- crenate; flat. Leaves without a persistent basal meristem. Leaf anatomy. Hairs present; glandular hairs present; complex hairs absent. Branched 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. 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; unbranched or branched; units cymose, arranged in pseudo-whorls. Flowers sessile, or subsessile; bracteate; minute to small; very 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, or toothed; imbricate, or open in bud; exceeded by the corolla; tubular, or campanulate; unequal but not bilabiate, or bilabiate (subequally 5-toothed or with abaxial lobe divided into 2 triangular teeth and the 2 triangular lateral lobes similar but with adaxial lobe usually broadly ovate); non-fleshy; persistent; accrescent; with the median member posterior. Calyx lobes broadly ovate, or triangular. Corolla present; disguisedly 5; 1 -whorled; gamopetalous; blunt-lobed; imbricate; bilabiate (the upper lip short, 4 lobed and recurved, the lower lip larger, entire or notched, long boat-shaped); with contrasting markings; white, or blue to violet. Corolla lobes more or less obovate. Corolla members entire. Fertile stamens present. Androecial members definite in number. Androecium 4. Androecial members adnate; markedly unequal; free of one another (connate at base for ca 1 mm); 1 -whorled. Androecium exclusively of fertile stamens. Stamens 4. Staminal insertion in the throat of the corolla tube. Stamens inserted at markedly different levels; becoming exserted; 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; all alternating with the corolla members. Filaments glabrous; filiform. Anthers connivent, or 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, or 1 (by reduction); 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. Embryo well differentiated. Cotyledons 2. Embryo straight.

Special features. The flowering nodes separated by extended internodes. 3–15 flowers subtended by each floral leaf. Calyx limb 5 lobed. Corolla tube exceeding the calyx; basally bent. The upper lip of the corolla incorporating 4 members, the lower 1; (posterior, adaxial) lip of the corolla 3 lobed (representing the adaxial and lateral members, the median sometimes itself bilobed); upper (adaxial) lip of the corolla not concave. Lower (abaxial) lip of the corolla entire to bilobed (entire to notched); markedly concave. Stamens projecting straight forwards or spreading, or descending.

Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia and Queensland (?NT). Northern Botanical Province. A genus of ca 300 species; 2 species in Western Australia; 0 endemic to Western Australia.

Etymology. From the Greek for "cock's spur" and "flower"; refers to the spurred or gibbous base of the corolla.