The families of flowering plants

L. Watson and M.J. Dallwitz

Thunbergiaceae Van Tiegh.

~ Acanthaceae p.p.

Including Meyeniaceae Sreemadhavan

Habit and leaf form. Shrubs, or lianas, or herbs. Self supporting, or climbing; the climbers stem twiners (commonly with articulated stems); Thunbergia twining anticlockwise. Mesophytic. Leaves opposite; petiolate; simple. Lamina entire; pinnately veined, or palmately veined (?); cross-venulate; hastate, or attenuate to the base, or rounded at the base. Leaves exstipulate; leaf development not ‘graminaceous’.

Leaf anatomy. The mesophyll without sclerenchymatous idioblasts. Minor leaf veins with phloem transfer cells, or without phloem transfer cells (variably present, in Thunbergia).

Axial (stem, wood) anatomy. Cork cambium present; initially superficial. Internal phloem present, or absent (?). Secondary thickening anomalous (at least in Thunbergia, see illustration), or developing from a conventional cambial ring. The anomalous secondary thickening from a single cambial ring.

‘Included’ phloem present (Thunbergia, see illustration), or absent (in other genera?).

Reproductive type, pollination. Plants hermaphrodite.

Inflorescence, floral, fruit and seed morphology. Flowers solitary, or aggregated in ‘inflorescences’; when solitary, axillary; when aggregated, in racemes. The ultimate inflorescence units racemose. Inflorescences terminal; racemes. Flowers bracteate; (bi-) bracteolate (the bracteoles large and spathaceous, by contast with the reduced calyx); regular, or somewhat irregular; cyclic; tetracyclic. Free hypanthium absent. Hypogynous disk present (large); intrastaminal; annular (more strongly developed adaxially).

Perianth with distinct calyx and corolla, or petaline (the calyx always reduced, sometimes obsolete, its functions assumed by the large bracteoles); 10–21; 1 whorled, or 2 whorled; isomerous, or anisomerous. Calyx when ascertainable, 5–16 (sometimes ringlike); 1 whorled; gamosepalous; entire, or lobulate, or blunt-lobed, or toothed; unequal but not bilabiate, or regular. Corolla 5; 1 whorled; gamopetalous; contorted (left-contorted in Thunbergia); hypocrateriform; unequal but not bilabiate, or regular.

Androecium 4, or 5. Androecial members adnate (to the corolla); markedly unequal; free of one another; 1 whorled. Androecium exclusively of fertile stamens, or including staminodes. Staminodes when present, 1 (minute); in the same series as the fertile stamens; representing the posterior median member; non-petaloid (the posterior androecial member reduced or missing). Fertile stamens representing the posterior-lateral pair and the anterior-lateral pair, or the posterior median member, the posterior-lateral pair, and the anterior-lateral pair. Stamens 4; inserted near the base of the corolla tube; didynamous; reduced in number relative to the adjacent perianth; oppositisepalous; alternating with the corolla members. Anthers dorsifixed; dehiscing via longitudinal slits; tetrasporangiate; appendaged, or unappendaged. The anther appendages when present, basal (in the form of awns or bristles). Anther epidermis persistent. Microsporogenesis simultaneous. Anther wall initially with more than one middle layer (2 to 3); of the ‘dicot’ type. Pollen grains aperturate; spiraperturate; 2-celled (Thunbergia).

Gynoecium 2 carpelled. Carpels reduced in number relative to the perianth. The pistil 2 celled. Gynoecium syncarpous; eu-syncarpous; superior. Ovary 2 locular. Gynoecium median; stylate. Styles 1; attenuate from the ovary; apical. Stigmas 1; large, funnel-shaped or bilobed; wet type; papillate; Group III type. Placentation axile. Ovules 2 per locule; collateral; hemianatropous, or anatropous, or campylotropous; unitegmic; tenuinucellate. Endothelium not differentiated. Endosperm formation cellular.

Fruit non-fleshy; dehiscent; a capsule (with a large ensiform beak). Capsules loculicidal. Seeds non-endospermic; with amyloid. Embryo well differentiated. Embryo curved.

Physiology, phytochemistry. C3. Anatomy non-C4 type (Thunbergia). Not cyanogenic. Iridoids detected (in Thunbergia); ‘Route II’ type (+decarb., including stilbericoside). Saponins/sapogenins absent. Proanthocyanidins absent. Ellagic acid absent (Thunbergia).

Special distinguishing feature. The funicles not as in Acanthaceae (retinacula absent or much reduced).

Geography, cytology. Paleotropical, Neotropical, Cape, and Australian. Tropical. Pantropical.

Taxonomy. Subclass Dicotyledonae; Tenuinucelli. Dahlgren’s Superorder Lamiiflorae; Scrophulariales. Cronquist’s Subclass Asteridae; Scrophulariales. APG III core angiosperms; core eudicot; Superorder Asteranae; lamiid. APG IV Order Lamiales (as a synonym of Acanthaceae).

Species 205. Genera 4; Thunbergia, Pseudocalyx, Meyenia, Whitfieldia (Pounguia).

General remarks. Additional to the single stigma and conventional ovular funicles, the compiled data have these genera differing from Acanthaceae sensu stricto in assorted, potentially interesting ‘esoteric characters’ relying on limited sampling; viz., leaf laminae without cystoliths, details of the anther wall, pollen morphology, and the single, wet, papillate stigma.

Illustrations. • Thunbergia grandiflora: Bot. Reg. 6 (1820). • Thunbergia laurifolia: Bot. Mag. 83 (1857). • Whitfieldia lateritia: Bot. Mag. 71 (1845). • Thunbergia species (Chittenden). • Glandular leaf hair of Thunbergia, with foliar cystoliths of Acanthaceae (Solereder, 1908).. • TS stem of Thunbergia coccinea, with anomalous secondary thickening (Solereder, 1908).

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 5th March 2018.’.