The Acanthaceae are mostly herbs or shrubs comprising about 250 genera and 2,500 species, including twining forms. The leaves are simple, opposite and decussate; stipules are lacking. The flowers are bisexual, zygomorphic, and usually are associated with conspicuous, often brightly colored bracts. The calyx is usually deeply 4-5 lobed or sometimes is highly reduced with more numerous minute teeth. The corolla is sympetalous, usually 5-merous, mostly zygomorphic, and commonly 2 lipped. The androecium usually consists of 4 didynamous stamens or only 2 stamens adnate to the corolla tube or epigynous zone, alternate with the lobes. The gynoecium consists of a single compound pistil of 2 carpels, a single style, and a superior ovary with 2 locules, each with usually 2-10 axile ovules in one or two collateral vertical tiers. An annular nectary disk is usually found around the base of the ovary. The fruit is commonly an elastically dehiscent loculicidal capsule. The seed stalk or funiculus of each seed is modified into a hook shaped jaculator or retinaculum that functions in flinging out the seeds during dehiscence.

Each "thumbnail" image below is linked to a larger photograph.

Adhatoda cydoniaefolia, Brazilian bower plant.
Asystasia gangetica, Chinese violet. The small bracts in this species are not typical for the family. However, the very deeply 5-lobed calyx is. The photo in the middle shows half of an immature fruit with one of the seeds remove and the other still resting on the bracket-like jaculator.  The photo on the right shows the whole fruit after dehiscence. Notice the persistent calyx and the well developed jaculators upon which the seeds rested before the explosive separation of the carpels hurled the seeds out of the capsule.
Graptophyllum pictum, caricature plant. The zygomorphic, strongly 2-lipped flowers of this species are much more conspicuous than the floral bracts.
Hemigraphis alternata, metal leaf. This common ground cover has purplish bracts that are about as conspicuous as the flowers.
Justicia betonica, white shrimp plant. Note the conspicuous bracts, the strongly two-lipped corollas, and the didynamous stamens.
Justicia californica, chuparosa, S CA, Mar, 1973.
Justicia brandegeana, red shrimp plant. The bracts in this species are quite showy and are at least as conspicuous as the 2-lipped corollas.
Odontonema tubiforme, ornamental shrub from Central America grown for the bright red, tubular flowers.
Pachystachys spicata
Pseuderanthemum atropurpureum, purple false eranthemum. Smooth shrub from S. Polynesia, with rose-purple or white and purple flowers; leaves variegated with purple or green and white.
Sanchezia speciosa. This species has narrowly tubular flowers that are only weakly zygomorphic. The bracts are reddish orange like the calyx and are therefore not particularly conspicuous.
Thunbergia battescombei, ornamental from tropical Africa.  UH Campus, Honolulu, April, 2004.
Thunbergia erectaThunbergia typically has two bracts closely subtending each flower.  These bracts hide the calyx, which is commonly reduced to a short, toothed rim at the base of the corolla.  The calyx is visible in the fourth image where one of the bracts has been folded back, away from the corolla. U of HI Manoa Campus, Honolulu, HI, 2005.
Thunbergia grandiflora, Bengal trumpet, ornamental liana from India.  UH Campus, Honolulu, April, 2004.
Thunbergia laurifolia, Acanthaceae, purple allamanda. Ornamental vine from India with Showy, violet, trumpet shaped flowers up to three inches across. 1-2, UH Campus, Honolulu, April, 2004.
Thunbergia mysorensis, Lyon Arboretum, O'ahu.
Unknown sp.

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