Xanthostemon F.Muell.

Scientific Description

Common name. Xanthostemon. Family Myrtaceae.

Habit and leaf form. Trees, or ‘arborescent’, or shrubs; evergreen; bearing essential oils. Plants with neither basal nor terminal concentrations of leaves; to 2–40 m high. Leptocaul. Helophytic to xerophytic. Leaves medium-sized to large; alternate, or opposite (when alternate in the juvenile stage only), or whorled (apparently, in X. verticillatus); decussate; (3–)4(–5) per whorl; leathery (or chartaceous); petiolate to sessile; gland-dotted; aromatic; edgewise to the stem, or with ‘normal’ orientation; simple; epulvinate. Leaf blades dorsiventral, or isobilateral, or centric; entire; flat; linear, or lanceolate, or oblong, or ovate; ovate, or obovate, or oblong, or elliptic; pinnately veined, or parallel-veined, or one-veined; cross-venulate. Mature leaf blades adaxially glabrous, or pubescent; abaxially glabrous, or pubescent. Leaves without stipules; without a persistent basal meristem. Leaf anatomy. Hairs present, or 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. Pollination mechanism unspecialized.

Inflorescence and flower features. Flowers solitary (apparently), or aggregated in ‘inflorescences’; in cymes, or in panicles. Inflorescences simple, or compound. The terminal inflorescence unit cymose. Inflorescences axillary; monads, triads, botryoids, metabotryoides, thrysoids or panicles. Flowers pedicellate; (bi) bracteolate, or ebracteolate; small to large; regular; 4 merous, or 5 merous; cyclic. Free hypanthium present (petals ‘inserted on the calyx’); tubular, or campanulate (to saucer-shaped); not extending beyond ovary, or extending beyond ovary; free of the sides of the ovary, with concave depressions opposite the petals in some species. Hypogynous disk present (lining the hypanthium). Perianth with distinct calyx and corolla; 8, or 10; 2 -whorled; isomerous. Calyx present; 4, or 5; 1 -whorled; polysepalous; imbricate, or valvate; persistent. Sepals oblong, or triangular, or orbicular. Corolla present; 4, or 5; 1 -whorled; polypetalous; imbricate; regular; hairy abaxially; plain; white, or cream, or yellow, or red. Petals elliptic, or ovate, or obovate, or orbicular. Androecial members indefinite in number. Androecium 20–100 (ie. to ‘many’). Androecial members branched. Androecial sequence determinable, or not determinable. Androecial members maturing centripetally; free of the perianth; all equal, or markedly unequal; free of one another (although occasionally pairs of filaments are connate, giving the impression of single stamens with 2 anthers), or coherent (at the extreme base); when united, 1 - adelphous (shortly connate into a ring), or 4–5 - adelphous (almost, in that the ring is sometimes more or less interrupted opposite each sepal). The androecial groups if seen as such, opposite the petals. Androecial members 1 -whorled, or 2(–5) -whorled (ie. to ‘several’). Stamens 20–100; attached on the rim of the hypanthium; becoming exserted (exceeding the petals); all more or less similar in shape; polystemonous; alternisepalous and oppositisepalous; both opposite and alternating with the corolla members; erect in bud, or inflexed in bud. Filaments not geniculate; filiform. Anthers all alike; dorsifixed (the point of attachment enclosed by the connective); versatile; dehiscing via longitudinal slits; introrse; tetrasporangiate; appendaged (the connective gland-tipped, often with smaller glands elsewhere in the connective), or unappendaged. Gynoecium 2–6 carpelled. The pistil (2–)3–5(–6) celled. Gynoecium syncarpous; synstylovarious to eu-syncarpous; partly inferior (to almost superior). Ovary plurilocular; 2–6 locular. Ovary summit glabrous, or hairy, the hairs not confined to radiating bands. Epigynous disk absent. Gynoecium stylate. Styles 1; simple; attenuate from the ovary, or from a depression at the top of the ovary; apical; persistent, or deciduous. Stigmas 1. Placentation axile. Ovules 8–50 per locule (i.e. to ‘many’); ascending (the ovules attached to the centre of the axis or to the rod- or bracket-like placenta upright in the basal angle of, or projecting from, the axis horizontally or obliquely into the loculus); non-arillate; hemianatropous.

Fruit and seed features. Fruit non-fleshy (woody or coriaceous); dehiscent; a capsule. Capsules loculicidal. Fruit few-seeded. Seeds non-endospermic; compressed (semi-circular to deltoid); winged (occasionally, at the chalazal end), or wingless. Cotyledons 2. Embryo with broad cotyledons lying face to face; hypocotyl accumbent.

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.

Etymology. From the Greek for "yellow" and "thread, stamen".