Family Acanthaceae Juss.

Adaxial hypodermis present (rarely), or absent. Lamina dorsiventral, or isobilateral (rarely); without secretory cavities. Cystoliths very commonly present (showing as streaks in the lamina). The mesophyll containing calcium oxalate crystals, or without calcium oxalate crystals. The mesophyll crystals raphides (rarely), or solitary-prismatic. Main veins embedded. Minor leaf veins with phloem transfer cells (Ruellia), or without phloem transfer cells (9 genera).

Stem anatomy. Cork cambium present; initially deep-seated (sometimes), or superficial. Nodes unilacunar. Primary vascular tissue bicollateral, or centrifugal. Cortical bundles absent. Medullary bundles present (rarely), or absent. Internal phloem present, or absent. Secondary thickening developing from a conventional cambial ring. ‘Included’ phloem absent. Xylem with fibre tracheids; with libriform fibres; with vessels. Vessel end-walls simple. Vessels without vestured pits. Primary medullary rays narrow. Wood diffuse porous; not storied; parenchyma paratracheal.

Reproductive type, pollination. Plants hermaphrodite. Pollination entomophilous; mechanism conspicuously specialized (commonly exhibiting a loose-pollen mechanism, cf. Scophulariaceae etc. — e.g. the large bee-flowers of Acanthus), or unspecialized.

Inflorescence, floral, fruit and seed morphology. Flowers solitary, or aggregated in ‘inflorescences’; in cymes, in racemes, and in verticils. The ultimate inflorescence unit cymose (in about 75%), or racemose. Inflorescences commonly dichasial cymes, becoming monochasial in the ultimate branches, and frequently condensed in the leaf axils, cf. Labiatae; pseudanthial, or not pseudanthial. Flowers bracteate; bracteolate (the bracts and bracteoles often showy); somewhat irregular to very irregular (in about 75% of the genera), or regular; usually more or less zygomorphic. The floral irregularity involving the perianth and involving the androecium. Flowers 4 merous, or 5 merous; tetracyclic. Free hypanthium absent. Hypogynous disk present.

Perianth with distinct calyx and corolla; (6–)8, or 10; 2 whorled; isomerous, or anisomerous. Calyx (3–)4, or 5; 1 whorled; gamosepalous; variously entire, or lobulate, or blunt-lobed. Calyx lobes markedly shorter than the tube to markedly longer than the tube. Degree of gamosepaly (maximum length joined/total calyx length) 0.5–0.9. Calyx imbricate, or valvate, or contorted, or open in bud; when K5, with the median member posterior. Corolla 4, or 5, or 3 (when the upper lip is suppressed); 1 whorled; gamopetalous (at least basally). Corolla tube not noticeably adaxially split, or adaxially deeply split (in Acanthus and relatives, where the upper lip of the corolla is cut away almost to the base of the tube). Corolla lobes markedly shorter than the tube to markedly longer than the tube. Degree of gamopetaly 0.5–0.75. Corolla imbricate (ascending cochlear or quincuncial), or contorted (left or right), or with open aestivation (only in Acanthus?); bilabiate, or unequal but not bilabiate (the upper lip sometimes suppressed).

Androecium 2, or 4(–5). Androecial members adnate (usually exserted, the filaments inserted on the corolla tube); all equal, or markedly unequal; free of one another, or coherent; when coherent, 2 adelphous (partially connate, in pairs); 1 whorled. Androecium exclusively of fertile stamens (rarely, e.g. Pentstemonacanthus), or including staminodes. Staminodes when present, 1–3; in the same series as the fertile stamens; representing the posterior median member, or the posterior median member and the posterior-lateral pair, or the anterior-lateral pair. Fertile stamens representing the anterior-lateral pair, or the posterior-lateral pair and the anterior-lateral pair (commonly), or the posterior-lateral pair (Brillantaisia only). Stamens 4(–5), or 2; inserted near the base of the corolla tube, or midway down the corolla tube, or in the throat of the corolla tube; didynamous (in about 75% of the species), or not didynamous, not tetradynamous; reduced in number relative to the adjacent perianth (mostly), or isomerous with the perianth; oppositisepalous; alternating with the corolla members. Anthers separate from one another, or connivent; dorsifixed (often with one lobe reduced or abortive); dehiscing via longitudinal slits; unilocular to bilocular; tetrasporangiate; appendaged (the connective often long, cf. Salvia), or unappendaged. Endothecium developing fibrous thickenings (usually), or not developing fibrous thickenings (e.g. Barleria, Justicia, Ruellia). Anther epidermis persistent. Microsporogenesis simultaneous. Anther wall initially with one middle layer; of the ‘dicot’ type. Tapetum glandular. Pollen monosiphonous; shed as single grains. Pollen grains aperturate, or nonaperturate (rarely); 2–8 aperturate; colpate, or porate, or colporate, or foraminate; 2-celled (recorded in 11 genera), or 3-celled (in Barleria and Ruellia).

Gynoecium 2 carpelled. The pistil 2 celled. Gynoecium syncarpous; synstylovarious to eu-syncarpous; superior. Ovary 2 locular. Gynoecium median. Ovary sessile. Styles 1; attenuate from the ovary; apical; much longer than the ovary (usually). Stigmas 2 (the posterior often smaller); dry type; non-papillate; Group II type. Placentation axile. Ovules 2–50 per locule (i.e., 2 to many); non-arillate, or arillate (occasionally exhibiting what may be a funicular aril - cf. Corner); anatropous to campylotropous; unitegmic. Endothelium not differentiated. Embryo-sac development Polygonum-type. Polar nuclei fusing only after one has been fertilized. Antipodal cells formed; when not proliferated, 3; proliferating (occasionally, to 4–18 cells), or not proliferating; ephemeral (usually), or persistent. Synergids pear-shaped. Endosperm formation cellular. Endosperm haustoria present; chalazal and micropylar (‘terminal’, the latter usually the more aggressive). Embryogeny onagrad, or solanad.

Fruit non-fleshy; dehiscent; a capsule. Capsules loculicidal. Fruit elastically dehiscent. Dispersal unit the seed. Seeds non-endospermic; borne on minute, hook-like outgrowths (‘retinacula’); conspicuously hairy, or not conspicuously hairy; with amyloid, or without amyloid. Embryo well differentiated. Cotyledons 2; large, planoconvex or crumpled. Embryo achlorophyllous (3/3); large. Testa sometimes covered with hairs or scales which become sticky or slimy when wet. Polyembryony recorded (and common).

Seedling. Germination phanerocotylar.

Physiology, biochemistry. Cyanogenic (rarely), or not cyanogenic. Alkaloids present, or absent. Iridoids detected (in subfamilies Acanthoideae and Ruellioideae); ‘Route II’ type (normal and decarb.). Arthroquinones detected (Barleria); derived from shikimic acid. Verbascosides detected (5 genera). Proanthocyanidins absent. Flavonols present (rarely), or absent; when present, kaempferol and quercetin (traces). Ellagic acid absent (8 species, 7 genera). Saponins/sapogenins present (rarely), or absent. Aluminium accumulation not found. C3, or C4. C3 physiology recorded directly in Adhatoda, Barleria, Beleropone, Fittonia, Graptophyllum, Justicia, Lepidogathis. C4 physiology recorded directly in Blepharis. Anatomy non-C4 type (Acanthopale, Acanthus, Adhatoda, Asystasia, Barleria, Crabbea, Crossandra, Dyschoriste, Echolium, Eremomastax, Hypoestes, Isoglossa, Justicia, Lepidagathis, Monechma, Monothecium, Peristrophe, Phaulopsis, Rhinacanthus, Ruellia, Ruttya, Whitfieldia), or C4 type (Blepharis).

Peculiar feature. The seeds on elongated, indurated, hook-shaped funicles (‘retinacula’).

Geography, cytology. Holarctic, Paleotropical, Neotropical, Cape, and Australian. Temperate to tropical (mainly tropical). Centred on Indomalaysia, Africa, Brazil and central America. X = 7–21.

Taxonomy. Subclass Dicotyledonae; Tenuinucelli. Dahlgren’s Superorder Lamiiflorae; Scrophulariales. Cronquist’s Subclass Asteridae; Scrophulariales. APG (1998) Eudicot; core Eudicot; Asterid; Euasterid I. APG 3 (2009) Order: Lamiales.

Species about 2400. Genera about 250:

Acanthopale, Acanthopsis, Acanthostelma, Acanthura, Acanthus, Achyrocalyx, Adhatoda, Afrofittonia, Ambongia, Ancistranthus, Ancistrostylis, Andrographis, Angkalanthus, Anisacanthus, Anisosepalum, Anisostachya, Anisotes, Apassalus, Aphanosperma, Aphelandra, Aphelandrella, Ascotheca, Asystasia, Asystasiella, Ballochia, Barleria, Barleriola, Beloperone, Benoicanthus, Blechum, Blepharis, Borneacanthus, Boutonia, Brachystephanus, Bravaisia, Brillantaisia, Buceragenia, Calacanthus, Calophanoides, Calycacanthus, Camarotea, Carlowrightia, Celerina, Cephalacanthus, Chaetacanthus, Chalarothyrsus, Chameranthemum, Championella, Chileranthemum, Chlamydocardia, Chlamydostachya, Chroesthes, Clinacanthus, Clistax, Codonacanthus, Conocalyx, Corymbostachys, Cosmianthemum, Crabbea, Crossandra, Crossandrella, Cyclacanthus, Cylindrosolenium, Cyphacanthus, Dactylostegium, Danguya, Dasytropis, Dichazothece, Dicladanthera, Dicliptera, Didyplosandra, Dipteracanthus, Dischistocalyx, Dolichostachys, Drejera, Drejerella, Duosperma, Dyschoriste, Ecbolium, Echinacanthus, Encephalosphaera, Epiclastopelma, Eranthemum, Eremomastax, Eusiphon, Filetia, Fittonia, Forcipella, Forsythiopsis, Gastranthus, Geissomeria, Glossocheilus, Golaea, Graphandra, Graptophyllum, Gymnophragma, Gymnostachyum, Gynocraterium, Gypsacanthus, Habracanthus, Hansteinia, Haplanthodes, Harpochilus, Henrya, Herpetacanthus, Heteradelphia, Holographis, Hoverdenia, Hulemacanthus, Hygrophila, Hypoestes, Ichthyostoma, Indoneesiella, Ionacanthus, Isoglossa, Isotheca, Jadunia, Juruasia, Justicia, Kalbreyeracanthus, Kalbreyeriella, Kosmosiphon, Kudoacanthus, Lankesteria, Lasiocladus, Leandriella, Lepidagathis, Leptostachya, Liberatia, Linariantha, Lindauea, Lophostachys, Louteridium, Lychniothyrsus, Mackaya, Marcania, Megalochlamys, Megalostoma, Megaskepasma, Melittacanthus, Mellera, Metarungia, Mexacanthus, Mimulopsis, Mirandea, Monothecium, Morsacanthus, Neohallia, Neriacanthus, Neuracanthus, Odontonema, Odontonemella, Ophiorrhiziphyllon, Oplonia, Oreacanthus, Orophochilus, Pachystachys, Pelecostemon, Pentstemonacanthus, Perenideboles, Pericalypta, Periestes, Peristrophe, Petalidium, Phaulopsis, Phialacanthus, Phidiasia, Phlogacanthus, Physacanthus, Podorungia, Poikilacanthus, Polylychnis, Pranceacanthus, Pseuderanthemum, Pseudodicliptera, Pseudoruellia, Psilanthele, Ptyssiglottis, Pulchranthus, Pupilla, Razisea, Rhinacanthus, Rhombochlamys, Ritonia, Rostellularia, Ruellia, Ruelliopsis, Rungia, Ruspolia, Ruttya, Salpinctium, Salpixantha, Samuelssonia, Sanchezia, Santapaua, Sapphoa, Satanocrater, Sautiera, Schaueria, Schwabea, Sciaphyllum, Sclerochiton, Sebastiano-schaueria, Siphonoglossa, Spathacanthus, Sphacanthus, Sphinctacanthus, Spirostigma, Standleyacanthus, Steirosanchezia, Stenandriopsis, Stenandrium, Stenostephanus, Streblacanthus, Streptosiphon, Strobilanthes, Strobilanthopsis, Styasasia, Suessenguthia, Synchoriste, Taeniandra, Tarphochlamys, Teliostachya, Tessmanniacanthus, Tetramerium, Theileamea, Thomandersia, Thyanostigma, Tremacanthus, Triaenanthus, Trichanthera, Trichocalyx, Ulleria, Vavara, Vindasia, Warpuria, Xantheranthemum, Xerothamnella, Yeatesia, Zygoruellia.

Economic uses, etc. A few cultivated ornamentals: Acanthus, Aphelandra, Fittonia, Beloperone, Justicia, etc.

Illustrations.
• Technical details - Adhatoda (~Justicia), Ruellia.
• Technical details - Justicia (Thonner).
• Acanthus (inflorescence).
• Beloperone oblongata: Bot. Reg. 1657, 1835.
• Justicia peruviana: Bot. Mag. 430, 1799.
• Justicia quadrangularis: Bot. Reg. 1340, 1830.
• Justicia venusta: Bot. Reg. 1380, 1830.
• Pachystachys coccinea: Bot. Mag. 432, 1799.
• Strobilanthes sabinianus: Bot. Reg. 1238, 1829.
• Strobilanthes scaber: as S. scabra, Bot. Reg. 32, 1841.