Hoya R.Br.
Prodr. 459 (1810)

Name Status: Current
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Scientific Description
B. Richardson, Friday 3 October 2008

Common name. Hoyas. Family Asclepiadaceae.

Sometimes included in Apocynaceae. Subfamily Asclepiadoideae, Tribe Marsdenieae. The infrageneric classification of Hoya requires revision and future studies may reveal that some taxa are better placed in segregate genera (Forster, 1996).

Habit and leaf form. Shrubs, or lianas; laticiferous (with white latex). ‘Normal’ plants. Plants with roots (fibrous). Leaves cauline. Epiphytic, or self supporting. Mesophytic. Leaves medium-sized; not fasciculate; opposite; not decurrent on the stems; not imbricate; petiolate. Petioles wingless. Leaves with ‘normal’ orientation; simple; epulvinate. Leaf blades dorsiventral; entire; flat; ovate, or obovate, or oblong, or elliptic, or orbicular (or rhomboidal); usually obscurely sub- parallel-veined, or pinnately veined (rarely); usually without cross-venules, or cross-venulate (rarely); cordate (or sharply truncate), or cuneate at the base, or rounded at the base. Mature leaf blades adaxially glabrous, or pubescent; abaxially glabrous, or pubescent. Leaf blade margins entire; flat. Leaf anatomy. Hairs absent, or present (occasionally).

Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Plants homostylous.

Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in umbels, or in racemes. Inflorescences simple; axillary. Flowers pedicellate; small, or medium-sized, or large; regular; 5 merous; cyclic; tetracyclic. Free hypanthium absent. Hypogynous disk absent. Perianth with distinct calyx and corolla; 10; 2 -whorled; isomerous. Calyx present; 5; 1 -whorled; gamosepalous; lobed. Corolla present; 5; 1 -whorled; appendiculate; gamopetalous; lobed; lobulate. Corolla lobes markedly longer than the tube. Corolla valvate; rotate, or campanulate; regular; variously coloured; fleshy, or not fleshy (waxy). Corolla members entire. Androecium present. Androecial members definite in number. Androecium 5. Androecial members adnate (epipetalous); united with the gynoecium (as a gynostegium); all equal; coherent (connate); 1 -whorled. Stamens 5; all more or less similar in shape (not markedly capitate); isomerous with the perianth. Filaments appendiculate (in a staminal gynostegial corona composed of 5 fleshy horizontally-spreading lobes with parallel inrolled keels attached to staminal column); connate into a tube, adnate to stigma. Anthers all alike. Pollen shed in aggregates; in tetrads. Gynoecium 2 carpelled. The pistil 2 celled. Carpels reduced in number relative to the perianth. Gynoecium syncarpous; synstylous; superior. Ovary plurilocular; 2 locular. Gynoecium stylate. Styles 2; partially joined (at the stylehead); apical. Stigmas 1; conical. Placentation axile. Ovules 30–50 per locule (‘many’).

Fruit and seed features. Fruit 80–250 mm long; dehiscent; 1 locular (in 2 follicles, with one often aborting). Dispersal unit the seed. Seeds endospermic; compressed (ovate to oblong); conspicuously hairy (comose at micropylar end); with a tuft of hairs. Cotyledons 2.

Geography, cytology, number of species. Native of Australia, or adventive. Endemic to Australia, or not endemic to Australia. Australian states and territories: Western Australia, Northern Territory, Queensland, and New South Wales. Northern Botanical Province. A genus of 7 species; 1 species in Western Australia; H. australis (R.Br.) ex Traill.; 0 endemic to Western Australia.

Additional comments. Named after Thomas Hoy (1788–1809), gardener for the Duke of Northumberland.

Additional characters Calyx with colleters (secreting mucilage) (at sinus bases). Annular corona absent. Corolline corona absent. Gynostegial corona present; consisting of lobes; staminal. Gynostegial staminal corona without a conspicuous hump; without an adaxial appendage. Corpusculum oblong. Caudicles winged, or not winged; not geniculate. Pollinia in the anther locule erect; pellucid germinating mouth of pollinia present; pellucid germinating mouth of pollinia located on outer margin. Pollen tetrads linear. Mature leaf blades bearing colleters (at lamina base), or lacking colleters.

Etymology. After Thomas Hoy (1788–1809), gardener for the Duke of Northumberland.

Taxonomic Literature

Wheeler, J. R.; Rye, B. L.; Koch, B. L.; Wilson, A. J. G.; Western Australian Herbarium (1992). Flora of the Kimberley region. Western Australian Herbarium. Como, W.A.

Forster, Paul I.; Liddle, David J. (1990). Hoya R. Br. (Asclepiadaceae) in Australia : an alternative classification.