Asclepias L.

Scientific Description

Common name. Cottonbushes. Family Asclepiadaceae.

Sometimes included in Apocynaceae. Subfamily Asclepiadoideae, Tribe Asclepiadeae.

Habit and leaf form. Shrubs, or herbs; laticiferous (with white latex). ‘Normal’ plants. Plants with roots (fibrous). Annual, or perennial. Leaves cauline. Young stems cylindrical. To 1.5 m high (in A. curassavica in Australia). Mesophytic. Leaves small; not fasciculate; opposite; not decurrent on the stems; ‘herbaceous’; not imbricate; petiolate. Petioles wingless. Leaves simple; epulvinate. Leaf blades dorsiventral; entire; flat; ovate; pinnately veined; cross-venulate; attenuate at the base to cuneate 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’; usually in umbels. Inflorescences simple; axillary. Flowers pedicellate; 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. Corolla present; 5; 1 -whorled; appendiculate; gamopetalous; lobed; lobulate. Corolla lobes markedly longer than the tube. Corolla imbricate, or valvate (occasionally); rotate; regular; orange, or red. 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 distinct erect or ascending hood-like lobes); connate into a tube, adnate to stigma. Anthers all alike; unappendaged (terminally). 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-globose. Placentation axile. Ovules 30–50 per locule.

Fruit and seed features. Fruit 50–60 mm long; dehiscent; 1 locular (in 2 follicles, with one often aborting). Dispersal unit the seed. Seeds endospermic; compressed; conspicuously hairy (comose at micropylar end); with a tuft of hairs. Cotyledons 2.

Geography, cytology, number of species. World distribution: occurring in the Americas. Adventive. Australian states and territories: Western Australia, South Australia, Queensland, and New South Wales. Naturalised in Australia via private gardens. A genus of 100 species; 1 species in Western Australia; A. curassavica L. Red Headed Cottonbush; 0 endemic to Western Australia.

Additional comments. Named for Asklepios, the Greek God of healing.

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

Etymology. Name used by Pliny for Vincetoxicum officinale. From the Greek Asclepios, in Homer, the name of a Thessalian prince famous as a physician; in later literature, the name of the son of Apollo and Coronis, the tutelary god of medicine. The plant was believed by Pliny to be an antidote to colic and snake bite.