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Family Acanthaceae

Synonyms: Mendonciaceae APNI*
Nelsoniaceae APNI*
Thunbergiaceae APNI*
Avicenniaceae APNI*
Leptophyllum APNI*

Description: Herbs or shrubs, rarely climbers or small trees; branches often longitudinally grooved or angled, usually with transverse ridges across the nodes; mostly with cystoliths (cystoliths can be detected with a hand lens if the leaf is held against a strong light, where they appear as white streaks).

Leaves simple, opposite and decussate, margins entire or shortly toothed, or rarely spiny and deeply incised.

Inflorescences axillary or terminal, often producing both chasmogamous and cleistogamous flowers; flowers solitary or in spikes, cymes or racemes, often clustered, frequently in axil of a bract and with 2 leafy bracteoles. Flowers bisexual, usually zygomorphic, 4- or 5-merous. Calyx segments usually 5, equal and fused only at base, rarely 4 or 8–20-lobed or unequal and fused to a greater degree. Corolla tubular; lobes 5, ± equal or 2-lipped. Stamens epipetalous, 4 or 2 fertile, sometimes with 2 staminodes; filaments free or basally fused in pairs; anthers 2-locular, dehiscing by longitudinal slits. Ovary superior, 2-locular, 2–12 ovules per loculus; style terminal.

Fruit a loculicidal capsule, often opens explosively after drying or on contact with water, usually with seed-bearing hooks (retinacula) that persist after the seeds have been flung out; seeds 2–many, usually compressed and discoid, with or without mucilaginous hairs.

Distribution and occurrence: World: c. 250 gen., c. 2500 spp., chiefly trop. regions. Aust.: 28 gen., 57 spp., all mainland States.

External links:
Angiosperm Phylogeny Website (Family: Acanthaceae, Order: Lamiales)

A number of species are cultivated as ornamentals for their showy flowers and/or bracteoles.

Taxa not yet included in identification key
Acanthus,    Asystasia,    Barleria,    Harnieria,    Justicia,    Ruellia,    Stephanophysum,    Strobilanthes

 Key to the genera 
1Shrubby mangroves with pneumatophoresAvicennia
Not mangroves2
2Flowers on peduncles more than 30 mm long; calyx concealed by 2 erect sagittate bracteoles; twiners or climbersThunbergia
Flowers sessile or peduncles less than 15 mm; large bracteoles if present not sagittate: herbs or shrubs, rarely climbing
                       Back to 1
3Leaves linear to lanceolate, more than 20 mm long, attenuate at base; capsules with a swelling on either side of mucronate apexHygrophila
Leaves either ovate to elliptic or if linear or lanceolate then distinctly petiolate or less than 20 mm long; capsules without apical swellings
                       Back to 2
4Branches with dense linear cystoliths parallel to branch axis (although sometimes partly obscured by indumentum), thickened at the nodes, bent and woody5
Branches without cystoliths or with cystoliths randomly arranged, branches constricted or thickened above the nodes, not bent
                       Back to 3
5Flowers purple, leaves mostly less than 25 mm longDipteracanthus
Flowers red, leaves mostly greater than 25 mm, sometimes up to 18 cm long
                       Back to 4
6Corolla more or less actinomorphic, the 5 lobes equal; seeds hairy or pitted7
Corolla 2-lipped, the 5 lobes very unequal; seeds variously tuberculate but not hairy
                       Back to 4
7Stamens 4; seeds covered with hairsBrunoniella
Stamens 2 with 2 small staminodes; seeds pitted, glabrous
                       Back to 6
8Corolla tube twisted 180°, enclosed within a cylindrical involucre of fused bracts and bracteolesHypoestes
Corolla tube neither twisted 180° nor enclosed within a cylindrical involucre
                       Back to 6
9Corolla with upper lip 4-lobed and lower lip entireXerothamnella
Corolla with upper lip notched or 3-lobed
                       Back to 8
10Calyx segments 4, bracts usually with white marginsRostellularia
Calyx segments 5, bracts without white margins
                       Back to 9
11Bracts conspicuous, 5–9 mm long, obcordateCalophanoides
Bracts inconspicuous, 0.5–2 mm long, triangular or linear-lanceolate
                       Back to 10
12Leaves sessile, branchlets spinescent; seeds with conical protuberancesRhaphidospora
Leaves petiolate, branchlets not spinescent; seeds with concentric ridges
                       Back to 11

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