20. Leucas Burm. ex R. Br., Prodr. Fl. Nov. Holland. 504. 1810. Benth. in DC., Prodr. 12: 523. 1848; Hook. f., Fl. Brit. Ind. 4: 680. 1885; Briquet in Engler & Prantl, Nat. Pflanzenfam. ed. 1, 4, 3A: 250. 1896; Mukerjee in Rec. Bot. Surv. Ind. 14, 1: 162. 1940; Sebald, in Stuttg. Beitr. Naturk. ser. A, 341. 1980; Rech. f. Fl. Iran. 150: 318-320. 1982.
Annual or perennial herbs with a pilose to villous eglandular indumentum of simple hairs. Leaves undivided, entire to crenate-serrate, petiolate at least on lower leaves, usually gland-dotted. Verticillasters many-flowered, distant from each other, borne in axils of upper leaves. Bracts numerous, subulate to ovate-lanceolate, shorter than to as long as calyces. Flowers white, sometimes with cream, rose or green markings. Calyx with 10 often ribbed veins, tubular, straight or curved; mouth truncate to prominently oblique, pilose to vinous at throat; teeth 8-10, equal or unequal. Corolla rather small, bilabiate; tube included in calyx, internally oblique-annulate or not; upper lip hooded or ± straight, densely white-bearded; lower lip with a median lobe clearly larger than laterals, usually much longer than upper lip. Stamens 4, didynamous, included within corolla upper lip; thecae 2-locular, eventually confluent. Style shortly 2-lobed. Nutlets 4, oblong, trigonous, obtuse to truncate-rounded at apex.
A palaeotropical genus of about 80 species with two main centres: one in Africa, especially east tropical Africa; the other in the Indian subcontinent. Most of our species are quite clear-cut.