Leucas R.Br.

Harley, R.M. et al. (2004). Labiatae, in The Families and Genera of Vascular Plants (K. Kubitzki, ed. in chief) VI: 167-275. Springer-Verlag, Berlin, Heidelberg, New York.

Morphology General Habit

Annual or perennial herbs or subshrubs with simple hairs

Morphology Leaves

Leaves usually toothed, petiolate or at least narrowed at base

Morphology Reproductive morphology Inflorescences

Inflorescence thyrsoid or rarely racemoid

Morphology Reproductive morphology Flowers Calyx

Calyx zygomorphic, 2-lipped, or sometimes actinomorphic, 5-10(-12)-lobed, lobes variously unequal or rarely equal, mostly spinose

Morphology Reproductive morphology Flowers Corolla

Corolla strongly 2-lipped, 4-lobed (1/3), white or rarely slightly reddish, posterior lip long or moderately long, hooded, bearded and densely pubescent outside with whitish hairs

Morphology Reproductive morphology Flowers Androecium Stamens

Stamens not or only shortly exserted from corolla, thecae confluent or weakly distinct

Morphology Reproductive morphology Flowers Gynoecium Stigma

Stigma-lobes unequal to sometimes equal

Morphology Reproductive morphology Fruits

Nutlets with apex truncate or rounded, with or without glands, otherwise glabrous or rarely shortly eglandular hairy.

Note

2n = (20), 22, (24), (26), 28 (30).  About 100 species.

Ecology

Mostly dry or disturbed ground

Distribution

Tropical to southern Africa, Arabian Peninsula, Iran to Southern China, Taiwan, Japan and Southeastern Asia, some species, especially L. martinicensis (Jacq.) R. Br., introduced as weeds elsewhere.

Lamiaceae (Labiatae), A.J. Paton, G. Bramley, O. Ryding, R.M. Polhill, Y.B. Harvey, M. Iwarsson, F. Willis, P.B. Phillipson, K. Balkwill, C.W. Lukhoba, D.F. Otieno, & R.M. Harley. Leucas, G. Bramley. Flora of Tropical East Africa. 2009

Morphology General Habit

Annual or perennial herbs, subshrubs or small shrubs

Morphology Stem

Stems erect, ascending or procumbent, usually pubescent with upward or downward-pointing to spreading hairs

Morphology Leaves

Leaves simple, opposite, sessile to distinctly petiolate

Morphology Reproductive morphology Inflorescences

Inflorescence with few to many verticils, congested or laxly spaced; verticils few–many-flowered, usually congested; bracts subtending verticils as leaves, often becoming reduced in size towards inflorescence axis, usually caducous in fruit; bracteoles minute to conspicuous

Morphology Reproductive morphology Flowers Calyx

Calyx tubular, throat symmetric to slightly oblique to asymmetric (lipped); lobes 5–10

Morphology Reproductive morphology Flowers Corolla

Corolla white, posterior lip curved inwards at apex, forming a hood over the style and stamens, densely pubescent on the outer surface, the hairs more plentiful, especially erect and downward-pointing along the margin of the hood, these also present on the inner surface of the lip margin; anterior lip divided into three lobes, the centre lobe longer and with a slight division at the apex

Morphology Reproductive morphology Flowers Androecium Stamens

Stamens 4; filaments with eglandular hairs about halfway up; thecae divaricate, confluent or weakly distinct

Morphology Reproductive morphology Flowers Gynoecium Ovary

Ovary 4-lobed, style gynobasic, unequally lobed

Morphology Reproductive morphology Fruits

Nutlets oblong-trigonous to obovoid with a small basal scar

Note

The first dichotomy in the key should be used with caution – in general the differences in bracteole length between species following the first lead and those following the second are readily apparent, but occasionally species with bracteoles typically 0.5–3 mm long have some up to 5(–6) mm long (hence the slight overlap in measurements); in this case whether the calyx is accrescent or not must be taken into account, and in addition bracteole shape (lead one species always have linear or subulate bracteoles, and in those following lead two tend to be linearlanceolate to ovate). This account is based on Sebald (1980), a revision of the Leucas of the African continent, the Arabian peninsula and Socotra. Sebald divided the genus into 13 sections: these sections are not listed here, although the species order roughly follows that of Sebald, with the addition of three new species, one new variety and change of status of two others.