Coleus monostachyus (P.Beauv.) A.J.Paton, Phytokeys 129 (2019): 76.

Kiew, Ruth & Kamin, Imin, 2021, Coleus (Lamiaceae) in Peninsular Malaysia including two new species, PhytoKeys 186, pp. 93-110 : 93

3. Coleus monostachyus (P.Beauv.) A.J.Paton, Phytokeys 129 (2019): 76.

Plectranthus monostachyus Homotypic synonym: Plectranthus monostachyus (P.Beauv.) B.J. Pollard, Kew Bull. 56: 980 (2001); Solenostemon monostachyus (P.Beauv.) Briq. in H.G.A.Engler & K.A.E.Prantl, Nat. Pflanzenfam. 4(3a): 359. 1897. Basionym: Ocimum monostachyum P.Beauv., Fl. Oware 2: 60, t. 95 (1818). Type: Africa, Benin, Palisot de Beauvois s.n. (holotype G, n.v.).

Plectranthus monostachyus Subsp. Plectranthus monostachyus monostachyus . Chung et al., Nature in Singapore 8: 1 (2015); Kiew, Conservation Malaysia 23: 6 (2016).

Note.

The description is based on specimens from Peninsular Malaysia.

Description.

Erect herb, almost shrubby, not aromatic, without tubers. Stem quadrangular, densely hairy on the angles, hairs descending ca. 0. 5 mm long, green, woody but brittle, to 30 cm tall, ca. 4 mm diameter, at first unbranched, flowering at ca. 20 cm tall, soon branching near the base, internodes (2.5-)4.5-6.5 cm, branches ascending, to 29 cm long. Leaves held horizontally; petiole 2-5.6 cm long, narrowly winged in the upper third, grooved above, densely hairy on the angles; lamina broadly ovate, (3.5-)6-13.5 × (2.5-)4.5-11.5 cm, base rounded or slightly truncate, shortly decurrent into the petiole, margin shallowly crenate, apex acute, tip rounded, membranous, glabrous and completely dull green above, pale beneath, lateral veins ca. 4 on either side of the midrib, impressed above, beneath prominent and finely short hairy. Inflorescence terminal on stem and branches, spicate, ca. 14 cm long in unbranched plants, in branched plants 21-38 cm, often with a subsidiary pairs at the base ca. 16.5-23 cm long, peduncle short 3-5 cm, 8-8.5 cm in inflorescences from the lower branches; peduncle and rachis quadrangular, finely pubescent on the angles; verticils 1-2 cm apart at the base, ca. 0.5 cm apart near the apex, each verticil with two sessile, condensed cymes each with 3-5 flowers. Bracts broadly ovate apex caudate, keeled, ca. 4 × 2.5 mm, pale green, deflexed and appressed to rachis. Pedicels reddish on the upper side, slightly excentrically attached behind the posterior lip, minutely pubescent, 1.5-2 mm long. Flower with calyx funnel-shaped, pale green, densely pubescence 2-3.5 mm long, in fruit 5-5.5 mm long, upper lip curved upwards, oval, minutely punctate at the apex, ca. 4-4.5 mm long, lower lip broadly oval with two fine apical teeth, curved upwards and closing the throat; corolla 8.5-10 mm long, minutely pubescent outside, tube abruptly decurved above the calyx, dilating to the mouth, white except for the upper lip and lateral lobes outlined in deep purple and the deep purple lower lip 3-4 mm long; stamens fused at base, filament white, glabrous, anthers, ca. 0.25-0.3 mm long, deep purple; stigma ca. 0.4 mm long, positioned above the anthers. Nutlets 4, plain brown, broadly ovoid, almost 1 mm long, producing mucilage when wet.

Distribution.

Native in tropical West Africa, this is a recent introduction into Peninsular Malaysia and Singapore, probably an escape from the horticultural trade ( Kiew 2016). First collected in Peninsular Malaysia in 2003, it is now naturalised and since about 2010 has rapidly become widespread. Apparently, it has not yet been recorded from Southeast Asia ( Suddee et al. 2004).

Ecology.

In Peninsular Malaysia, it is found in light shade to fully exposed conditions in almost all lowland habitats associated with disturbance, e.g. roadsides, plantations, farms and gardens. It begins to flower at 20 cm tall, and its many-flowered spikes produce fruits that shatter at the slightest touch, scattering hundreds of seeds. This makes it a weed that is extremely difficult to eradicate. The seeds are sticky and may be dispersed by animals or water but long distance dispersal is probably effected by soil on vehicles or in planting material. It therefore threatens to become a noxious weed in nurseries and gardens where it cannot be exterminated by herbicides.

Etymology.

Latin, mono - = one or single; stachys = ear of corn or spike, referring to the inflorescence.

Additional Peninsular Malaysian specimens examined.

Johor: Senterre et al. s.n. 30 Sept 2003 (KEP!) . Kelantan: Felda Chiku, Nazrul et al. FRI 83177 31 August 2015 (KEP!) . Selangor: Bukit Nanas, Norzielawati et al. FRI 83050 (KEP!), Kepong , Forest Research Institute Malaysia, Kiew FRI 81947 13 Jan 2016 (KEP!); Rasa, Kiew FRI 655542 17 April 2010 (KEP!); Subang Rafidah et al., FRI 75694 4 April 2013 (KEP!) .