Habit and leaf form. Herbs (with cystoliths, often with long thin tuberous roots). Annual. Young stems tetragonal. Hydrophytic, or helophytic, or mesophytic, or xerophytic. Leaves opposite (pairs connected by a transverse ridge); petiolate; gland-dotted, or not gland-dotted; simple. Leaf blades dissected, or entire; flat; pinnately veined, or palmately veined; cross-venulate. Leaves without stipules. Leaf blade margins entire. Stem anatomy. Nodes unilacunar. Secondary thickening developing from a conventional cambial ring. Roots. Aerial roots present, or absent.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Entomophilous. Pollination mechanism conspicuously specialized, or unspecialized.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in a long, peduncled dichasium. The terminal inflorescence unit cymose, or racemose. Inflorescences axillary. Flowers bracteate; bracteolate. Bracteoles shorter than calyx, except in Ruellia sp. A Kimberley Flora (K.F. Kenneally 9007), which has bracteoles usually longer than calyx. Flowers large and showy (chasmogamous), or cleistogamous; regular, or somewhat irregular; zygomorphic. The floral asymmetry involving the androecium. Flowers 5 merous; tetracyclic. Free hypanthium absent. Hypogynous disk present; annular. Perianth with distinct calyx and corolla; 10; 2 -whorled; isomerous. Calyx 5; 1 -whorled; gamosepalous; lobed; regular (equal); with the median member posterior. Calyx lobes long- linear. Corolla 5; 1 -whorled; gamopetalous; contorted; with narrow tube expanding into funnel-shaped, ventricose throat at angle to tube; regular. Androecium 4. Androecial members adnate; markedly unequal; coherent; 2 - adelphous (connected at base by membrane); 1 -whorled. Stamens 4. Staminal insertion in the throat of the corolla tube. Stamens remaining included (pressed against adaxial surface together with style); didynamous; reduced in number relative to the adjacent perianth; oppositisepalous; all alternating with the corolla members. Anthers dorsifixed; dehiscing via longitudinal slits; bilocular (cells linear); tetrasporangiate; appendaged (? sometimes mucronate at base), or unappendaged. Pollen shed as single grains. Gynoecium 2 carpelled. The pistil 2 celled. Gynoecium syncarpous; synstylovarious to eu-syncarpous; superior. Ovary plurilocular; 2 locular. Gynoecium median. Ovary sessile. Gynoecium stylate, or non-stylate (the stigma sessile in cleistogamous flowers of R. tuberosa). Styles 1; attenuate from the ovary; apical; much longer than the ovary at anthesis. Stigmas 1; 2 - lobed (one lobe flattened, much larger than the rudimentary second lobe); dry type; non-papillate; Group II type. Placentation axile. Ovules 10–13 per locule; superposed; non-arillate, or arillate; hemianatropous, or anatropous, or campylotropous.
Fruit and seed features. Fruit non-fleshy; dehiscent; a capsule (erect, fusiform, seed-bearing throughout but for the very base). Capsules loculicidal. Fruit elastically dehiscent (exploding on contact with water). Dispersal unit the seed. Fruit c. 20 seeded. Seeds discoid; non-endospermic; conspicuously hairy (when wet). Embryo well differentiated. Cotyledons 2. Embryo achlorophyllous; curved; large. Seedling. Germination phanerocotylar.
Physiology, biochemistry. Aluminium accumulation not found.
Special features. The seeds on elongated, indurated, hook-shaped funicles (‘retinacula’).
Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, Northern Territory, and Queensland. Northern Botanical Province.