Alternative names: Not Applicable

Description:
Herbs or shrubs, often with 4-6-angled branches, usually with cystoliths; leaves decussately arranged, those of opposite pairs frequently joined by a transverse ridge across the node, simple, entire or shortly toothed, exstipulate.

Flowers axillary or terminal, solitary or arranged in spikes, cymes or racemes, usually subtended by a single bract and paired bracteoles, bisexual, but the one plant sometimes producing both showy and inconspicuous flowers; calyx segments 4 or 5; corolla sympetalous, regular or irregular, basally, usually divided into a distinct tube and wider throat, 5-lobed, the lobes imbricate or contorted in bud; stamens inserted on the corolla tube, either 4 fertile and then often in pairs of unequal length or 2 fertile, sometimes with staminodes also present; filaments free or basally connate; anther cells 2, rarely 1, with the cells inserted at the same level or superposed, dehiscing longitudinally, frequently with hairs or basal spurs; disk annular and nectar-producing at the base of the ovary; ovary 2-celled, superior, with 2-10 superposed ovules per cell; style terminal; stigma entire or 2-lobed, one of the lobes sometimes reduced.

Fruit a loculicidal 2-valved capsule with 2-many seeds, usually explosively dehiscent when mature either on drying or the application of water, seed-bearing throughout or the seeds restricted to either the basal or apical half and each seed (in S. Aust.) subtended at its base by a conspicuous hook which persists after the seed has been released; seed compressed, discoid, smooth or tuberculate, sometimes with mucilaginous hairs which expand on wetting.

Distribution:  A world-wide, but predominantly tropical, family consisting of about 250 genera with 2,500 species. The largest subfamily Ruellioideae is represented in S.Aust. by Dipteracanthus (tribe Ruellieae) and Rostellularia (tribe Justiceae), both native, while a third introduced genus, Acanthus, belongs to the subfamily Acanthoideae.

Biology: The Acanthaceae are easily recognisable in the field by the presence of seed-bearing hooks within the capsule. In addition the presence of cystoliths (appearing as white streaks) on the vegetative parts enables easy identification of dried material when seen under a low power microscope.

Uses: The family is predominantly of horticultural interest with a number of exotic species of such genera as Hypoestes (freckle-face), Aphelandra, Thunbergia (black-eyed Susan), Ruellia s.lat., Justicia s.lat, and Beloperone (shrimp plant) being cultivated in South Australian gardens.

Taxonomic notes: The comparatively rare Australian endemic species Sarojusticia kempeana (rock holes of the MacDonnell Ranges, N.T.), Xerothamnella parvifolia and Rhaphidospora bonneyana (both of southwest Qld and north-west N.S.W.) occur just across borders and could possibly be found in northern S.Aust.

Key to Genera:

1. Corolla 2-lipped, white with pink or mauve markings; calyx segments 4, sometimes with a remnant of a fifth lobe; flowers in a dense terminal spike; capsule 2-4-seeded; seeds lacking hairs which expand on wetting
2. Stamens 4; filaments curved, bony; anthers 1-celled, bearded along sutures, appressed against each other, without basal appendages; upper lip of corolla lacking, replaced by the upper calyx segment; calyx segments toothed; leaves very large, arranged in basal rosettes, long-petiolate, the blade deeply divided or toothed, glabrous	ACANTHUS l.
2. Stamens 2; filaments not curved, not bony; anthers 2-celled, glabrous along sutures, not appressed to each other, the lower cell of a pair with a white basal appendage; upper lip present; calyx segments entire; leaves small, arranged decussately along branches, shortly petiolate, the blade entire and pubescent	ROSTELLULARIA 3.
1. Corolla more or less regular, a deep blue to purplish; calyx segments 5; flowers 1-3 in each of the upper axils; capsule 1-16-seeded; seed with a rim of mucilaginous hairs expanding on wetting	DIPTERACANTHUS 2.

Author: Prepared by R. M. Barker