Dipteracanthus Nees

Scientific Description

Family Acanthaceae.

Habit and leaf form. Shrubs, or herbs (with cystoliths). The herbs annual to perennial. Young stems tetragonal. Hydrophytic, or helophytic, or mesophytic, or xerophytic. Leaves opposite (the pairs connected by a transverse ridge across the node); petiolate; gland-dotted, or not gland-dotted; simple. Leaf blades 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 solitary, or aggregated in ‘inflorescences’; when solitary axillary. Inflorescence few-flowered (2(-3) flowered). Flowers in racemes, or in spikes. The terminal inflorescence unit cymose, or racemose. Inflorescences terminal. Flowers bracteate. Bracts intergrading with the leaves. Flowers bi- bracteolate. Bracteoles leafy, exceeding the calyx. Flowers large and showy or inconspicuous and cleistogamous; regular; 5 merous; tetracyclic. Free hypanthium absent. Hypogynous disk present; annular (with an irregular rim). Perianth with distinct calyx and corolla; 10; 2 -whorled; isomerous. Calyx 5; 1 -whorled; gamosepalous; lobed; with the median member posterior. Calyx lobes linear. Corolla 5; 1 -whorled; gamopetalous; contorted; tubular at the base and widening into the throat; regular. Corolla lobes equal, basally with longitudinal folds forming palate-like projections. Androecium 4(–5). Androecial members adnate; markedly unequal; coherent; 2 - adelphous (the filaments connected in pairs by a basal membrane); 1 -whorled. Androecium exclusively of fertile stamens, or including staminodes (sometimes). Staminodes when present, 1. Stamens 4. Staminal insertion in the throat of the corolla tube. Stamens all inserted at the same level; usually remaining included (and held against the upper surface of the corolla), or becoming exserted; didynamous; reduced in number relative to the adjacent perianth; oppositisepalous; all alternating with the corolla members. Filaments hairy, or glabrous. Anthers dorsifixed; dehiscing via longitudinal slits; bilocular; tetrasporangiate; 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. Styles 1; attenuate from the ovary; apical; much longer than the ovary at anthesis. Stigmas 1; 1 - lobed, or 2 - lobed (then 1 lobe flattened and much larger than the other); dry type; non-papillate; Group II type. Placentation axile. Ovules 4–8 per locule; non-arillate, or arillate; hemianatropous, or anatropous, or campylotropous.

Fruit and seed features. Fruit non-fleshy; dehiscent; a capsule (narrow and seedless at the base, expanded and seed-bearing apically). Capsules loculicidal. Fruit elastically dehiscent (mature fruits exploding violently on exposure to water). Dispersal unit the seed. Fruit 1–16 seeded. Seeds non-endospermic; with only the thickened rim possessing mucilaginous hairs that expand on wetting. Embryo well differentiated. Cotyledons 2. Embryo achlorophyllous; curved. 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, South Australia, Northern Territory, Queensland, and New South Wales. Northern Botanical Province.

Additional characters Corolla lobes spreading.