Acanthus L.

Scientific Description

Family Acanthaceae.

Habit and leaf form. Erect or decumbent shrubs, or herbs (without cystoliths). Plants spiny, or unarmed. The herbs perennial; plants with a basal concentration of leaves, or with neither basal nor terminal concentrations of leaves. Young stems cylindrical. Hydrophytic, or helophytic, or mesophytic, or xerophytic. Leaves opposite (the pairs not connected by transverse ridges); petiolate; gland-dotted, or not gland-dotted; simple. Leaf blades entire, or dissected (lobed); flat; pinnately veined, or palmately veined; cross-venulate. Leaves without stipules. Leaf blade margins entire, or dentate (sometimes spine-tipped). 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.

Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in spikes. The terminal inflorescence unit cymose, or racemose. Inflorescences terminal. Flowers uni- bracteate (not intergrading with leaves); bi- bracteolate, or ebracteolate; very irregular; zygomorphic. The floral asymmetry involving the perianth and involving the androecium. Flowers 3 merous, or 4 merous; tetracyclic. Free hypanthium absent. Hypogynous disk present. Perianth with distinct calyx and corolla; 7; 2 -whorled; anisomerous. Calyx 4; 1 -whorled; polysepalous; unequal but not bilabiate (the two larger (abaxial and adaxial) lobes outermost, the two inner (lateral) lobes smaller; in the introduced Acanthus species the adaxial calyx lobe functions as the upper lip of corolla). Corolla 3; 1 -whorled; gamopetalous; with open aestivation; tube very short; unequal but not bilabiate. Androecium 4. Androecial members adnate; all equal; free of one another, or coherent; 1 -whorled. Stamens 4. Staminal insertion in the throat of the corolla tube. Stamens becoming exserted; oppositisepalous; all alternating with the corolla members. Filaments thick, bony. Anthers connivent (pressed together due to the slight twisting of the abaxial pair); usually bearded along suture and abaxial surface; dorsifixed; dehiscing via longitudinal slits; unilocular; tetrasporangiate; appendaged, 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. Styles 1; attenuate from the ovary; apical; much longer than the ovary at anthesis. Stigmas 1; 2 - lobed (sometimes obscurely so); dry type; non-papillate; Group II type. Placentation axile. Ovules 2 per locule; superposed; non-arillate, or arillate; hemianatropous, or anatropous, or campylotropous.

Fruit and seed features. Fruit non-fleshy; dehiscent; a capsule (ovoid-oblong, shiny, coriaceous). Capsules loculicidal. Fruit elastically dehiscent. Dispersal unit the seed. Fruit 2 seeded, or 4 seeded. Seeds more or less discoid; non-endospermic; small to medium sized; not conspicuously hairy. Embryo well differentiated. Cotyledons 2. Embryo achlorophyllous; curved. Testa smooth, or with tubercles. Seedling. Germination phanerocotylar.

Physiology, biochemistry. Aluminium accumulation not found.

Special features. The seeds on elongated, indurated, hook-shaped funicles (‘retinacula’). Mangroves, or non-mangrove species. The upper (posterior, adaxial) lip of the corolla suppressed. Lower (abaxial) lip of the corolla 3 lobed; large.

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.

Additional characters Fruit erostrate. Pollen grains colpate.