Hygrophila R.Br.

Scientific Description

Family Acanthaceae.

Habit and leaf form. Erect herbs (with cystoliths). Annual to perennial. Young stems tetragonal. Hydrophytic, or helophytic. Leaves opposite (the pairs connected by transverse ridges); petiolate to sessile; gland-dotted, or not gland-dotted; simple. Leaf blades dissected, or entire; flat; when dissected, pinnatifid; pinnately veined, or palmately veined; cross-venulate. Leaves without stipules. Leaf blade margins entire; flat, or revolute, or involute. 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’. Inflorescence few-flowered to many-flowered. Flowers sessile in axillary whorls resembling a leafy spike or pedicellate and the inflorescence racemose. The terminal inflorescence unit cymose, or racemose. Inflorescences axillary. Flowers bracteate; bi- bracteolate. Bracteoles small (in Australia), usually shorter than calyx. Flowers very irregular; zygomorphic. The floral asymmetry involving the perianth and involving the androecium. Flowers 5 merous; tetracyclic. Free hypanthium absent. Hypogynous disk present. Perianth with distinct calyx and corolla; 10; 2 -whorled; isomerous. Calyx 5; 1 -whorled; polysepalous, or gamosepalous (at base); lobed; regular (equal); with the median member posterior. Corolla 5; 1 -whorled; gamopetalous; imbricate, or contorted, or with open aestivation; tubular at base, widening into throat; bilabiate; hairy adaxially (palate hairy), or glabrous adaxially. Androecium 4. Androecial members adnate; markedly unequal; coherent (each long and short filament connected by a basal membrane); 2 - adelphous; 1 -whorled. Stamens 4; all inserted at the same level; becoming exserted; didynamous; reduced in number relative to the adjacent perianth; oppositisepalous; all alternating with the corolla members. 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 lobes unequal); dry type; non-papillate; Group II type. Placentation axile. Ovules 4–20 per locule (or more); non-arillate, or arillate; hemianatropous, or anatropous, or campylotropous.

Fruit and seed features. Fruit non-fleshy; dehiscent; a capsule (fusiform, flattened longitudinally, seed-bearing throughout). Capsules loculicidal. Fruit elastically dehiscent. Dispersal unit the seed. Fruit 8–24 seeded. Seeds non-endospermic; conspicuously hairy (covered with long mucous hairs, appressed when dry, spreading 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’) (the hooks prominent or minute). The upper (posterior, adaxial) lip of the corolla bilobed (notched and porrect). Lower (abaxial) lip of the corolla 3 lobed.

Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, Northern Territory, Queensland, and New South Wales.