Sarcostemma R.Br.

Scientific Description

Common name. Caustic Bushes. Family Asclepiadaceae.

Sometimes included in Apocynaceae. Subfamily Asclepiadoideae, Tribe Asclepiadeae. A broad circumscription of Sarcostemma is adopted here that includes Oxystelma (Forster, 1996).

Habit and leaf form. Shrubs, or lianas; laticiferous (with white latex). ‘Normal’ plants, or switch-plants; with the principal photosynthesizing function transferred to stems. Leaves well developed, or much reduced. Plants with roots (fibrous or tuberous); succulent, or non-succulent; unarmed. Leaves cauline. Self supporting, or climbing; stem twiners, or scrambling. Mesophytic, or xerophytic. Not heterophyllous. Leaves minute, or medium-sized; not fasciculate; opposite; not decurrent on the stems; not imbricate; petiolate (when present); with ‘normal’ orientation; simple; epulvinate. Leaf blades dorsiventral; entire; flat; linear, or ovate; cuneate at the base. Leaf blade margins entire; flat. Leaf anatomy. Extra-floral nectaries present (at base of lamina), or absent.

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

Inflorescence and flower features. Flowers aggregated in ‘inflorescences’. Inflorescence many-flowered. Flowers in umbels. Inflorescences simple; axillary; ascending; umbelliform or racemiform cymes. Flowers pedicellate; small, or medium-sized; regular; 5 merous; cyclic; tetracyclic. Free hypanthium absent. Hypogynous disk absent. Perianth with distinct calyx and corolla; 10; 2 -whorled; isomerous; cream. 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; green, or cream, or pink. 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 connate ring of staminal and interstaminal parts and 5 proximal staminal corona parts basally adnate to this ring); connate into a tube, adnate to stigma. Anthers all alike; appendaged. The anther appendages apical. 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; flat-topped to umbonate. Placentation axile. Ovules 30–50 per locule (‘many’).

Fruit and seed features. Fruit 40–80 mm long; dehiscent; 1 locular (in 2 follicles, with one often aborting). Dispersal unit the seed. Seeds endospermic; compressed (ovate); small, or medium sized; conspicuously hairy (comose at micropylar end); with a tuft of hairs. Cotyledons 2.

Geography, cytology, number of species. World distribution: occurring in the Old and New Worlds. Native of Australia, or adventive. Not endemic to Australia. Australian states and territories: Western Australia, South Australia, Queensland, New South Wales, and Australian Capital Territory. Northern Botanical Province, or Eremaean Botanical Province, or South-West Botanical Province. 3 species native in Australia, 1 endemic. A genus of 60–70 species; 3 species in Western Australia; 0 endemic to Western Australia.

Additional comments. From the Greek sarco (fleshy) and stemma (garland), referring to the fleshy staminal corona.

Additional characters Calyx with colleters (secreting mucilage) (at sinus bases), or without colleters. Annular corona present, or absent. Corolline corona absent. Gynostegial corona present; staminal, or interstaminal. Gynostegial staminal corona without a conspicuous hump; without an adaxial appendage. 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.

Etymology. From the Greek for "flesh" and "anything with which a person is crowned"; refers to the fleshy corona.