Acanthaceae Juss.

Timothy Utteridge & Gemma Bramley (2020). The Kew Tropical Plant Families Identification Handbook, Second Edition. Kew Publishing Royal Botanic Gardens, Kew

Note

Cystoliths often present. Stems often swollen above the nodes. Leaves opposite. Flowers bisexual, corolla tubular, often zygomorphic; stamens 2 or 4(5); ovary superior. Fruit a bilocular capsule. Seeds often held on hooks (retinacula).

Recognition

Characters of similar families: Linderniaceae or Orobanchanceae: fruits usually many-seeded capsules, lacking retinacula, cystoliths absent, stamens Z-shaped or with appendages (Linderniaceae), holo-/hemiparasites, can dry blue/black (Orobanchanceae). Lamiaceae: fruit comprising 4 nutlets or a (1–)4(–14)-seeded drupe, cystoliths absent. Bignoniaceae: usually trees or lianas, leaves usually compound, cystoliths and retinacula absent, seeds commonly winged. Gesneriaceae: many-seeded capsule or drupe, parietal placentation, cystoliths and retinacula absent.

Morphology General Habit

Herbs, shrubs and climbers, rarely trees

Morphology General Cystoliths

Cystoliths (calcium carbonate concretions) often present in vegetative parts, punctate or rod-shaped

Morphology Leaves Stipules

Stipules absent

Morphology Stem

Stems often 4- or 6-angular, often swollen above the nodes, with an interpetiolar ridge

Morphology Leaves

Leaves opposite, rarely whorled; simple, entire or variously lobed or toothed

Morphology Reproductive morphology Inflorescences

Inflorescences cymose, often compounded into spiciform or paniculate thyrses or reduced to single flowers, often with conspicuous bracts, paired bracteoles usually present

Morphology Reproductive morphology Flowers

Flowers bisexual; calyx fused at least at base, usually 4–5-lobed, more rarely subentire; corolla often showy, tubular, regularly 5-lobed or commonly zygomorphic, often (1–)2-lipped; stamens epipetalous, 2 or 4(5), with or without 1–3 staminodes; anthers dithecal or monothecal; ovary superior, (1)2-locular; style filiform; stigma 1–2-lobed

Morphology Reproductive morphology Fruits

Fruit a capsule, often stipitate and/or beaked, dehiscing explosively; rarely a single-seeded drupe (Mendoncia)

Morphology Reproductive morphology Seeds

Seeds (1)2– 20+, discoid to spheroid, smooth or commonly sculptured or hairy, often held on a hook-like extension from the funicle (retinaculum).

Distribution

Mainly tropical and subtropical, c. 210 genera and c. 4,000 species. The largest group of genera and species (Ruellieae sensu Scotland & Vollesen) with cystoliths and retinacula. APGIII places the mangrove genus Avicennia within Acanthaceae, but it is morphologically different and is not included in the description here.

Description Author

Iain Darbyshire

Timothy M. A. Utteridge and Laura V. S. Jennings (2022). Trees of New Guinea. Kew Publishing. Royal Botanic Gardens, Kew

Distribution

The Acanthaceae is a largely tropical family with 212 genera and c. 3175 species. The majority of genera and species are herbs or small shrubs. In New Guinea, the family is represented by 26 genera of which only three genera have tree species.

Recognition

The genus Avicennia L. was previously placed in its own family, Avicenniaceae, but is included in the Acanthaceae in subfamily Avicennioideae. The family has nodes which are usually swollen in the field (shrinking on drying), opposite leaves with cystoliths (lacking in Avicennia) and a tubular corolla which is usually zygomorphic; the fruits are usually 2-valved capsules with internal hooks.

Morphology General Habit

Herbs or small (climbing) shrubs, rarely small trees, sometimes with spines, nodes often swollen in the field (shrunken in dried material)

Morphology General

Sap absent

Morphology Leaves Stipules

Stipules absent

Morphology Leaves

Leaves simple, opposite, rarely in whorls (not in New Guinea), margin usually entire or nearly so, often covered with cystoliths

Morphology Reproductive morphology Inflorescences

Inflorescences terminal or axillary, solitary to many-flowered, often with large colourful bracteoles

Morphology Reproductive morphology Flowers

Flowers usually bisexual, usually zygomorphic (often bilabiate); calyx 4 or 5(–18)-lobed, campanulate; corolla sympetalous, limb 4- or 5-lobed, contorted or imbricate in bud; stamens (2) or 4(–5), didynamous, epipetalous, anthers with 1–2 thecae, dehiscing longitudinally, sometimes spurred; staminodes 0–3; ovary superior, 2-locular, ovules (1–)2 or many, in 2 rows, placentation axial; style 1, terminal, stigma 2(–3)lobed or capitate

Morphology Reproductive morphology Fruits

Fruit a capsule or rarely a drupe, 2-valved, usually loculicidal, often with hooks internally

Morphology Reproductive morphology Seeds

Seeds 2–many, compressed.

Acanthaceae, H. Heine. Flora of West Tropical Africa 2. 1963

Morphology General Habit

Herbaceous or climbing, rarely somewhat shrubby

Morphology Leaves

Leaves opposite, often with distinct cystoliths; stipules absent

Morphology Reproductive morphology Flowers

Flowers hermaphrodite, zygomorphic, often with conspicuous bracts

Morphology Reproductive morphology Flowers Calyx

Calyx-segments or lobes 4 or 5, imbricate or valvate; rarely the calyx reduced to a ring

Morphology Reproductive morphology Flowers Corolla

Corolla gamopetalous, 2-lipped or sometimes 1-lipped, lobes imbricate or contorted

Morphology Reproductive morphology Flowers Androecium

Stamens 4, didynamous or 2, inserted on the corolla-tube and alternate with its lobes; filaments free amongst themselves or partially connate in pairs; anthers 2-celled or 1-celled by reduction, cells confluent or separated, sometimes one much smaller than the other, opening lengthwise

Morphology Reproductive morphology Flowers Nectaries

Disk present

Morphology Reproductive morphology Flowers Gynoecium

Ovary superior, sessile on the disk, 2-celled

Morphology Reproductive morphology Fruits

Capsule mostly elastically dehiscent from the apex downwards, the valves recurved and leaving the central axis

Morphology Reproductive morphology Seeds

Seeds mostly with indurated funicle; endosperm rarely present; embryo large

Diagnostic

Recognised by the usually herbaceous habit, opposite leaves and the peculiar capsule

George R. Proctor (2012). Flora of the Cayman Isands (Second Edition). Royal Botanic Gardens, Kew

Morphology General Habit

Herbs, shrubs or trees, sometimes vines, the shoots often angled and swollen above the nodes; leaves opposite or rarely in whorls of 3, simple, entire or toothed, the epidermis often with cystoliths.

Morphology Reproductive morphology Flowers

Flowers perfect, zygomorphic or nearly regular; solitary, clustered or cymose in the leaf-axils, or in terminal spikes, racemes, cymes or panicles, often with large bracts

Morphology Reproductive morphology Flowers Calyx

Calyx 4–5-lobed, imbricate or valvate, or reduced to a ring; corolla gamopetalous, 5-lobed and nearly regular or 2-lipped, sometimes 1-lipped.

Morphology Reproductive morphology Flowers Androecium Stamens

Stamens mostly 4, in 2 pairs inserted at different levels of the corolla-tube, sometimes reduced to 2; 1 or more staminodes sometimes present; anthers 1–2-celled, opening lengthwise

Disc

Disc present

Morphology Reproductive morphology Flowers Gynoecium Ovary

Ovary superior, 2-celled, with 2–many ovules in each cavity on axile placentas; style simple, with capitate or lobed stigma

Morphology Reproductive morphology Fruits

Fruit usually a capsule, often elastically dehiscent, the valves rupturing and recurving explosively, flinging the seeds out, the action aided by the hook-like and hardened ovule-stalks; seeds usually flat, with little or no endosperm.

Distribution

A pantropical and subtropical family of about 250 genera and 2,500 species.

J. R. Timberlake, E. S. Martins (2013). Flora Zambesiaca, Vol 8 (5) Acanthaceae Part 1. Royal Botanic Gardens, Kew

Morphology General Habit

Herbs, shrubs, twiners or (rarely) trees; branches often angled or ridged, usually with a distinct transverse ridge at nodes

Morphology Stem

Stems and leaves often with cystoliths (linear intercellular calcium carbonate concretions) looking like small white streaks or rods

Morphology Leaves

Leaves opposite, rarely whorled or in rosettes, without stipules

Morphology Reproductive morphology Inflorescences

Inflorescences of single flowers, dichasia or cymules, often aggregated into axillary and/or terminal paniculate to racemoid cymes; bracts foliaceous to distinctly differentiated or minute; bracteoles usually present

Morphology Reproductive morphology Flowers

Flowers zygomorphic to nearly actinomorphic, bisexual$Calyx 4–5-lobed (in Thunbergioideae a subentire to undulate rim or 10–20 irregular lobes), fused at least at base, with equal to unequal lobes or distinctly 2-lipped

Morphology Reproductive morphology Flowers Corolla

Corolla sympetalous, with a narrow basal tube widening into a ± distinct throat; limb almost regularly 5-lobed to distinctly 2-lipped, more rarely 1-lipped, lobes imbricate or contorted in bud

Morphology Reproductive morphology Flowers Androecium Stamens

Stamens 4, usually in 2 pairs of unequal length (didynamous), or 2 and then with or without 2(3) staminodes, attached at base of throat or in narrow corolla tube, basal part of filaments often connate in two pairs; anthers 1–2-thecous, thecae usually parallel, attached at same height or one above the other, rounded at base or mucronate to apiculate or with distinct appendages or spurs, dehiscing longitudinally (rarely with apical or basal pores)

Morphology Reproductive morphology Flowers Gynoecium Ovary

Ovary superior, bilocular, surrounded at base by an annular disc, with 2 collateral (Thunbergioideae) or with 2–28 superposed ovules per locule; style simple, filiform, with 2 linear (one often reduced) to oblong or ovoid or capitate stigmatic lobes

Morphology Reproductive morphology Fruits

Fruit a bilocular capsule, rarely a drupe, usually explosively dehiscent, placentae sometimes separating from capsule wall on opening

Morphology Reproductive morphology Seeds

Seeds 1 to over 20 per locule, held on thickened curved laterally compressed hook-shaped outgrowths (retinaculae) from the funicle (retinaculae papilliform in Nelsonioideae and absent in Thunbergioideae), discoid to sphaeroid, reniform or cordiform to subglobose, surface often covered in hygroscopic hairs or glabrous and then smooth or variously sculptured.

Distribution

A family of about 250 genera and 2500–3000 species widely distributed in all tropical and subtropical regions, extending sparingly into warm temperate zones in Europe and Asia and as far north as Canada in temperate North America. The largest diversity at both genus and species level is in the dry woodland, bushland and grassland regions of tropical and southern Africa, Madagascar, India and Central and South America; 43 genera and c.350 species in the Flora area.

Avicenniaceae, F.N. Hepper. Flora of West Tropical Africa 2. 1963

Morphology General Habit

Shrubs or trees, branches terete

Morphology Leaves

Leaves opposite and decussate, simple, entire; stipules absent

Morphology Reproductive morphology Inflorescences

Inflorescences axillary or terminal; flowers sessile, hermaphrodite, actinomorphic

Morphology Reproductive morphology Flowers Calyx

Calyx very deeply 5-lobed, lobes imbricate, subtended by a pseudo-involucre composed of a scale-like bractlet and 2 alternate scale-like prophylla

Morphology Reproductive morphology Flowers Corolla

Corolla gamopetalous, campanulate-rotate, actinomorphic, deeply 4-lobed

Morphology Reproductive morphology Flowers Androecium

Stamens 4, inserted in throat of the corolla-tube, equal or subdidynamous

Morphology Reproductive morphology Flowers Gynoecium

Ovules 4, pendulous Style shortly divided Ovary superior, with 2 united carpels, placenta free, central

Morphology Reproductive morphology Fruits

Fruit dehiscent by 2 valves, usually only 1 seed develops

Morphology Reproductive morphology Seeds

Seed with 2 cotyledons, embryo viviparous

Flora Zambesiaca. Vol 8, Pt 7. Avicenniaceae, R. Fernandes. Nesogenaceae, M.A. Diniz. Verbenaceae, R. Fernandes. Lamiaceae, R. Fernandes. 2005.

Morphology General

Glabrous (or canescent) evergreen small trees of mangrove swamps, with extensive widely-spreading root systems bearing pneumatophores

Morphology Reproductive morphology Inflorescences

Inflorescences terminal or axillary, of contracted cymes at the end of short peduncles Inflorescences terminal or axillary, of contracted cymes at the end of short peduncles

Morphology Reproductive morphology Flowers

Flowers actinomorphic, hermaphrodite, sessile, subtended by an involucre formed by a bract and two bracteoles Flowers actinomorphic, hermaphrodite, sessile, subtended by an involucre formed by a bract and two bracteoles

Morphology Reproductive morphology Flowers Calyx

Calyx gamosepalous, deeply 5-lobed with lobes imbricate Calyx gamosepalous, deeply 5-lobed with lobes imbricate

Morphology Reproductive morphology Flowers Corolla

Corolla gamopetalous, campanulate or campanulate-rotate; tube short; limb 4(5)-lobed, spreading, with subequal lobes or the posterior lobe slightly broader Corolla gamopetalous, campanulate or campanulate-rotate; tube short; limb 4(5)-lobed, spreading, with subequal lobes or the posterior lobe slightly broader

Morphology Reproductive morphology Flowers Androecium Stamens

Stamens 4, inserted in the corolla throat and alternating with the lobes, equal or sub-didynamous; filaments ± short, filiform; anthers ovoid or suborbicular, subexserted

Morphology Reproductive morphology Flowers Gynoecium Ovary

Ovary superior, 2-carpellate, but appearing 4-locular due to ovary wall intrusions (false septa); placenta free, axile; style 1 short, bifid or bilobed, with apical stigmas; ovules 4, with 1 each apparent locule, pendent, orthotropous

Morphology Reproductive morphology Seeds

Seed without a testa; embryo with large longitudinally folded cotyledons and a villous radicle, viviparous; endosperm fleshy. Seed without a testa; embryo with large longitudinally folded cotyledons and a villous radicle, viviparous; endosperm fleshy

Note

Usually treated as a subfamily of the Verbenaceae , it is here recognized as a separate family readily distinguished by the involucre of 1 bract and 2 bracteoles which subtend each flower, the free-central placentation with a c. 4-winged placenta, the orthotropous ovules and by the viviparous embryo and fleshy endosperm.  Molecular evidence now places it near the more basal Acanthaceae , with which it shares few essential morphological characters.  [A.E. Schwarzbach & L.A. Mc Dade.  Phylogenetic relationships of the mangrove family Avicenniaceae based on chloroplast and nuclear chromosomal DNA sequences.  Systematic Botany 27 : 84–98 (2002)].

Distribution

A monogeneric family of the Old and New World tropics and subtropics.

Morphology General Habit

Glabrous (or canescent) evergreen small trees of mangrove swamps, with extensive widely-spreading root systems bearing pneumatophores

Morphology Leaves

Leaves simple, decussate, petiolate, without stipules; lamina entire, pulverulent (powdery)-puberulent or felted-tomentellous on lower surface, ± coriaceous Leaves simple, decussate, petiolate, without stipules; lamina entire, pulverulent (powdery)-puberulent or felted-tomentellous on lower surface, ± coriaceous

Morphology Reproductive morphology Flowers Androecium

Stamens 4, inserted in the corolla throat and alternating with the lobes, equal or sub-didynamous; filaments ± short, filiform; anthers ovoid or suborbicular, subexserted

Morphology Reproductive morphology Flowers Gynoecium

Ovary superior, 2-carpellate, but appearing 4-locular due to ovary wall intrusions (false septa); placenta free, axile; style 1 short, bifid or bilobed, with apical stigmas; ovules 4, with 1 in each apparent locule, pendent, orthotropous

Morphology Reproductive morphology Fruits

Fruit a capsule, l-seeded (by abortion of 3 ovules), usually asymmetric, compressed, opening by 2(4) thickened valves Fruit a capsule, l-seeded (by abortion of 3 ovules), usually asymmetric, compressed, opening by 2(4) thickened valves

George R. Proctor (2012). Flora of the Cayman Isands (Second Edition). Royal Botanic Gardens, Kew

Morphology General Habit

Shrubs or small trees of mangrove swamps, the roots sending up erect pencil-like aerial pneumatophores; leaves opposite and simple; stipules absent

Morphology Reproductive morphology Flowers

Flowers perfect, sessile in the leaf-axils or in terminal spikes or heads, each flower subtended by 3 imbricate bracteoles

Morphology Reproductive morphology Flowers Calyx

Calyx of 5 imbricate sepals, almost free, persistent

Morphology Reproductive morphology Flowers Corolla

Corolla gamopetalous, with short tube and 4 almost equal lobes

Morphology Reproductive morphology Flowers Androecium Stamens

Stamens 4, inserted near the base of the corolla-tube, shortly exserted; anthers 2-celled, opening lengthwise inwardly.

Morphology Reproductive morphology Flowers Gynoecium Ovary

Ovary superior, incompletely 4-celled, with a free-based placenta having 4 pendulous ovules attached to the apex; style terminal, with 2 stigmatic lobes.

Morphology Reproductive morphology Fruits

Fruit a compressed, leathery, asymmetric, 1-seeded capsule, eventually splitting by 2 valves; seed with folded cotyledons, germinating in the fruit.

Distribution

A single genus with about 11 species, widely distributed on tropical and subtropical seashores.