Habit and leaf form. Herbs; evergreen; bearing essential oils. Plants unarmed. Perennial. Leaves cauline. Plants with neither basal nor terminal concentrations of leaves. Young stems usually tetragonal. To 0.3–0.6 m high. Leaves small to medium-sized; not fasciculate; opposite; decussate; ‘herbaceous’; not imbricate; petiolate; foetid; simple; epulvinate. Leaf blades entire; flat; broadly ovate, or rhombic to ovate to orbicular; pinnately veined, or one-veined to pinnately veined; cross-venulate; more or less cordate to oblique at the base. Mature leaf blades adaxially sparsely cottony pilose; abaxially pilose (white-felty with dense stellate and simple cottony hairs). Leaves without stipules. Leaf blade margins irregularly crenate; flat. Leaves without a persistent basal meristem. Leaf anatomy. Hairs present; glandular hairs present; complex hairs present. Branched hairs absent. Complex hairs stellate. 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 axillary; a long interrupted spike of many flowered, globular verticillasters, forming dense clusters in each leaf axil. Flowers sessile; bracteate; bracteolate; 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; toothed (spreading, subulate, each ending in a glabrous, uncinate spine); prominently 5–10 veined; lobes spreading; imbricate, or open in bud; exceeded by the corolla; tubular; regular; non-fleshy; persistent; with the median member posterior. Calyx lobes narrow triangular. Corolla present; supposedly 5; 1 -whorled; gamopetalous; blunt-lobed; imbricate; bilabiate (the upper lip straight and usually 2-lobed, the lower lip spreading and 3-lobed); hairy abaxially; hairy adaxially; plain; white. Corolla lobes narrow oblong to oblong. Corolla members entire. Androecium present. Fertile stamens present. Androecial members definite in number. Androecium 4. Androecial members adnate; all equal; free of one another; 1 -whorled. Androecium exclusively of fertile stamens. Stamens 4. Staminal insertion midway down the corolla tube. Stamens remaining included; 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 connivent, or separate from one another; dorsifixed; versatile, or non-versatile; dehiscing via longitudinal slits; introrse; bilocular (cells divergent); 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. Styles 1; from a depression at the top of the ovary (the ovary deeply lobed); ‘gynobasic’. 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. 5–25 flowers subtended by each floral leaf (‘many’). Calyx limb 5–10 lobed. Corolla tube exceeding the calyx. The upper lip of the corolla incorporating 2 members, the lower 3; (posterior, adaxial) lip of the corolla bilobed (straight). Lower (abaxial) lip of the corolla 3 lobed (spreading).
Geography, cytology, number of species. Adventive. Not endemic to Australia. Australian states and territories: Western Australia, South Australia, Queensland, New South Wales, Victoria, Australian Capital Territory, and Tasmania. A genus of 40 species; 1 species in Western Australia; 0 endemic to Western Australia.
Etymology. From the Latin marrubium; name used by Pliny for M. vulgare and M. peregrinum, said to be from the Hebrew for "a bitter juice".
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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/