Habit and leaf form. Herbs. Plants succulent. Annual, or perennial. Young stems tetragonal. Helophytic, or mesophytic, or xerophytic. Leaves opposite; somewhat fleshy; petiolate to sessile; connate (via the stipules), or not connate; gland-dotted, or not gland-dotted; simple; epulvinate. Leaf blades entire; one-veined, or pinnately veined; cross-venulate. Leaves with stipules. Stipules interpetiolar (hyaline, adnate to the base of the leaf to form a sheath around the node); with colleters (secreting mucilage), or without colleters. Leaf blade margins entire, or serrate. Leaves without a persistent basal meristem. Domatia recorded, or not recorded; represented by pits, or pockets, or hair tufts. Leaf anatomy. Hairs present, or absent. Stem anatomy. Nodes unilacunar, or tri-lacunar. Secondary thickening developing from a conventional cambial ring.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Plants homostylous, or heterostylous. Entomophilous. Pollination mechanism conspicuously specialized (with passive pollen presentation involving stylar modification), or unspecialized.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’, or solitary. Inflorescence few-flowered. Flowers in cymes (in sessile cymules with 1–2(-5) flowers per node along all branches). The terminal inflorescence unit cymose. Inflorescences axillary, or terminal (at first). Flowers minute to small; regular; 4 merous; cyclic; tetracyclic. Free hypanthium absent. Perianth with distinct calyx and corolla; 8; 2 -whorled; the two whorls isomerous. Calyx 4; 1 -whorled; gamosepalous; lobed; open in bud; regular; persistent. Corolla 4; 1 -whorled; gamopetalous; valvate; rotate (tube very short); regular; hairy adaxially (lobes more or less hairy particularly towards the apices); white; persistent. Androecium 4. Androecial members adnate (to the corolla); united with the gynoecium (filaments attached to ovary); free of one another; 1 -whorled. Androecium exclusively of fertile stamens. Stamens 4. Staminal insertion near the base of the corolla tube. Stamens becoming exserted; isomerous with the perianth; oppositisepalous. Anthers dorsifixed; dehiscing via longitudinal slits; introrse; tetrasporangiate; unappendaged. Pollen shed in aggregates, or shed as single grains; if aggregated, in tetrads. Gynoecium 2 carpelled. The pistil 2 celled. Carpels reduced in number relative to the perianth. Gynoecium syncarpous; synstylovarious to eu-syncarpous; partly inferior. Ovary plurilocular; 2 locular. Gynoecium transverse. Epigynous disk present. Gynoecium stylate. Styles 1; attenuate from the ovary, or from a depression at the top of the ovary; apical; shorter than the ovary at anthesis to much longer than the ovary at anthesis; deciduous. Stigmas 1; 2 - lobed (lobes globose or linear); wet type, or dry type; papillate, or non-papillate; Group II type and Group IV type. Placentation axile. Ovules 5–50 per locule; pendulous, or horizontal, or ascending; anatropous, or hemianatropous.
Fruit and seed features. Fruit non-fleshy; dehiscent; a capsule. Capsules loculicidal. Fruit 8–16 seeded (or ‘numerous’). Seeds depressed obconic or depressed ovoid; endospermic, or non-endospermic. Endosperm ruminate, or not ruminate; when present, oily. Cotyledons 2. Embryo straight, or curved. Seedling. Germination phanerocotylar, or cryptocotylar.
Physiology, biochemistry. Aluminium accumulation demonstrated, or not found.
Geography, cytology, number of species. Native of Australia. Endemic to Australia. Australian states and territories: Western Australia, South Australia, Northern Territory, Queensland, and New South Wales.
Additional comments. Synaptantha may be distinguished from other genera of the Hedyotis/Oldenlandia complex by its scarcely perceptibly connate corolla lobes; by its staminal filaments being firmly attached to the ovary as well as to the corolla; and by its 1/2 to 3/4 inferior ovaries (Halford 1992).
Additional characters Fruit rostrate (beak rounded).
Etymology. From the Greek for "joined together" and "flower"; the filaments adhere both to the ovary and to the base of the corolla.
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Western Australian Herbarium (1998–). FloraBase—the Western Australian Flora. Department of Biodiversity, Conservation and Attractions. https://florabase.dpaw.wa.gov.au/