Sometimes included in Apocynaceae. Subfamily Secamonoideae, Tribe Secamoneae. The genus is broadly circmscribed here to include Toxacarpus. Generic limits between Toxocarpus and Secamone require critical study (Forster, 1996).
Habit and leaf form. Lianas; laticiferous (with white latex). ‘Normal’ plants. Plants with roots (fibrous). Perennial. Leaves cauline. Young stems cylindrical. Climbing. Mesophytic. Leaves medium-sized; not fasciculate; opposite; not decurrent on the stems; ‘herbaceous’, or leathery (S. elliptica); not imbricate; petiolate. Petioles wingless. Leaves simple; epulvinate. Leaf blades dorsiventral; entire; flat; ovate, or obovate, or elliptic (to narrowly so); pinnately veined; cross-venulate; attenuate at the base, or rounded at the base. Mature leaf blades adaxially glabrous; abaxially glabrous. Leaf blade margins entire; flat.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Plants homostylous.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in cymes (in 1-many fascicles). Inflorescences simple; axillary. Flowers pedicellate; minute, or small; 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; polysepalous; erect. Corolla present; 5; 1 -whorled; appendiculate (with a corolline corona); gamopetalous; lobed; lobulate. Corolla lobes markedly longer than the tube. Corolla campanulate, or rotate; regular; green, or cream, or yellow. Androecium present. Androecial members definite in number. Androecium 5. Androecial members adnate (epipetalous); united with the gynoecium; all equal; coherent (connate); 1 -whorled. Stamens 5; all more or less similar in shape; isomerous with the perianth. Filaments appendiculate; connate into a tube, adnate to stigma. Anthers all alike; four locular; appendaged. The anther appendages apical. Pollen shed in aggregates; in tetrads. Gynoecium 2 carpelled. The pistil 2 celled. Gynoecium syncarpous; synstylous; superior. Ovary plurilocular; 2 locular. Gynoecium stylate. Styles partially joined (at the stylehead); apical. Stigmas 1; capitate (or obtuse). Placentation axile. Ovules 30–50 per locule (‘many’).
Fruit and seed features. Fruit 40–100 mm long; dehiscent; 1 locular (in 2 follicles, with one often aborting). Dispersal unit the seed. Seeds endospermic. Endosperm oily. Seeds compressed; conspicuously hairy (comose at micropylar end); with a tuft of hairs. Cotyledons 2.
Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, Northern Territory, Queensland, and New South Wales. 4 species native in Australia, none endemic. A genus of about 20 species; 0 endemic to Western Australia.
Additional comments. From squamouna, the Arabic name for one of the species.
Additional characters Calyx with colleters (secreting mucilage) (at sinus bases), or without colleters. Annular corona absent. Corolline corona present, or absent; of longitudinal ridges; free from gynostegium. Gynostegial corona present; consisting of lobes; staminal. Gynostegial staminal corona without a conspicuous hump. Translator with a non-sticky end. Pollinia in the anther locule erect; pellucid germinating mouth of pollinia absent. Pollen tetrads rectangular. Mature leaf blades bearing colleters (at lamina base), or lacking colleters (S. elliptica).
Etymology. From squamouna, the Arabic name for one of the species.