Coleus namuliensis E.Downes & I.Darbysh., 2017

Downes, E. & Darbyshire, I., 2017, Coleus namuliensis and Coleus caudatus (Lamiaceae): a new species and a new combination in the Afromontane flora of Mozambique and Zimbabwe, Blumea 62 (3), pp. 168-173 : 169-172

Coleus namuliensis E.Downes & I.Darbysh. , sp. nov. — Fig. 1

Most likely to be confused with Coleus caudatus (S.Moore) E.Downes & I.Darbysh. (new combination made below),but differs in having:1) conspicu- ously pedunculate spikes (vs lateral spikes sessile in P.caudatus ); 2) smaller bracts subtending the verticils 3.3–5 mm long (vs 6–11 mm long);3) smaller calyces 2.7–4.4 mm (vs 5–7 mm long);4) calyx lobes subequal in length and evenly spaced with the sinus between the anterior pair of lobes markedly longer than the other sinuses, divided almost to the base of the calyx (vs calyx in ‘3+2’ lobe arrangement with the lateral and posterior lobes clearly longer than the anterior pair of lobes, the sinus between the anterior pair of lobes not so markedly longer than the other sinuses); 5) smaller corollas 7.5–10 mm (vs 18–20 mm long); 6) orange-brown arrowhead-shaped nutlets (vs dark brown flattened-ovoid shaped nutlets); 7) usually smaller leaves 27–47 by 17–49 mm (vs 40–80 by 20–80 mm); and 8) leaves less markedly bullate. — Type: Harris 237 (holo K000962083; iso K000962089, LMA (not seen)), Mozambique, Zambezia Province, Mt Namuli, small peak 5 km west of highest peak, S15°22'30.9" E37°02'52.6", fl. fr., 30 May 2007.

Erect semi-succulent perennial herb, with single to several stems arising from a woody base and rootstock, 15–45 cm tall; not reported as aromatic (but inflorescence smells mintlike when soaked); whole plant densely tomentose with long multicellular glandular hairs, with small white or pale yellow gland-tips, also with larger sessile yellow, orange or red glands throughout but these often hidden by the indumentum. Stems fleshy, ascending, convexly-quadrangular; less hairy towards base. Leaves opposite-decussate, shortly petiolate or sessile, petiole up to 4 mm long; blade fleshy, discolorous, grey-green adaxially, paler abaxially due to dense indumentum, sub-

Fig. 1 Coleus namuliensis E.Downes & I.Darbysh. a. Habit; b. medium-sized leaf, abaxial surface; c. apex of inflorescence showing bracts; d. mid-region of inflorescence showing calyces; e. bract, adaxial surface; f. detail of hairs from mid-margin of bract; g. calyx, dorsal view; h. calyx, lateral view; i. detail of hairs from calyx surface; j. sessile glands from calyx surface; k. corolla, stamens and style, lateral view; l. face view of upper lobes of corolla after slitting of corolla and splaying the lobes; m. nutlet, views of opposite faces (a (plant), b: Timberlake et al. 5146; a (rootstock): Harris et al. 237; c–m: Patel 7384; all K). — Scale bars: a = 5 cm; b–d = 1 cm; e, g–h, k–l = 2 mm; f, i, m = 1 mm; j = 250 µm. — Drawn by Andrew Brown.

orbicular to elliptic, 27–47 by 17–49 mm, length: width ratio 0.9–1.95: 1 (mean 1.27: 1), decreasing in size and becoming more elliptic in the distal portion of the plant, base rounded to cuneate, margins crenate, apex rounded to obtuse; venation anastomosing. Inflorescences of dense spikes, arranged singly or up to nine in largely leafless terminal portion of stems, opposite-decussately arranged, individual spikes long-pyramidal, 21–85 by 6–13 mm, each vertical comprised of opposite 3-flowered cymes very compact to neighbouring verticils and adpressed to main axis, without visible rachis between the verticils; peduncle 8–45 mm long; bracts subtending inflorescence branches (spikes) elliptic, 4–15 by 2–9 mm, decreasing in size towards distil portion of plant, base obtuse, apex acute to obtuse; bracts subtending each cyme conspicuous in distal portion of spike, but caducous in proximal portion, sub-elliptic to ovate, 3.3–5 by 2.2–3.6 mm, length: width ratio 1.2–1.9: 1 (mean 1.6: 1), base rounded, sessile, apex acuminate to attenuate, abaxial surface glabrous, red and white hairs recorded by collectors in the field (Harris 237); bracteoles absent. Calyx dark purple towards tips of lobes in dry specimens, flowering calyx 2.7–4.5 by 2.2–3.2 mm, fruiting calyx 4.5–5.8 by 2.4–3.2 mm, with five equal, attenuate lobes, with one longer sinus between the two anterior lobes, this 2.3–3.6 mm long, other four sinuses 1.5–2 mm long. Corolla purple or pale mauve, also recorded as white inside with pink lobes, 7.5–10 mm long, internal surface glabrous, external surface densely tomentose; tube (measured from base to most forward part of upper lip, i.e., up to where tube opens on to lower lip) 4–6.5 mm long, upper lip with four lobes 1.2–2.2 mm long, lower lip navicular, 3.2–4 mm long. Stamens four, adnate to tube at mouth of corolla, filaments free, 2–2.5 mm long, anthers 0.5–0.7 mm long, with patchy concentrations of red/orange sessile glands seen in dry specimens. Pistil with glabrous style 5–9 mm long, easily dislodged; stigma white, shortly bifid. Nutlets shiny, orange-brown, arrowhead-shaped, slightly flattened, 1.2–2 by 0.5–1 mm.

Distribution & Ecology — Coleus namuliensis is known only from Mt Namuli in Zambezia Province of northern Mozambique where it is recorded from montane grassland and in the forestgrassland transition, typically on rock outcrops and rock faces, on sandy loam soil over granite ( Timberlake et al. 2009). It grows together with a variety of herbaceous species, grasses and sedges including Coleochloa setifera (Ridl.) Gilly. It has been recorded at 2050–2060 m elevation but is likely to occur over a somewhat wider altitudinal range.

Additional specimens. MOZAMBIQUE, Zambezia Province, Mt Namuli, small peak 5 km W of highest peak, S15°22'30.6" E37°02'52.5", fl. fr., 30 May 2007, Patel 7384 ( K 4 sheets, LMA (not seen)); Mt Namuli , Muretha Plateau, S15°22'57.3" E37°02'00.7", fl. fr., 31 May 2007, Timberlake 5146 ( K 3 sheets, LMA (not seen)) .

Conservation — The following assessment is derived from a 1-day meeting of the IUCN Southern African Plant Specialist Group held in February 2017 (attended by the second author of this paper) in which this species was assessed. Coleus namuliensis has a highly restricted range, with an Extent of Occurrence (EOO) of less than 23 km 2, this being the maximum extent of montane grassland on the Namuli massif. The Area of Occupancy (AOO) is recorded as 12 km 2 based on application of a standard 2 by 2 km grid cell size (IUCN Standards and Petitions Subcommittee 2017). However, there is currently no subsistence farming or livestock grazing occurring on the upper slopes and plateaus of Mt Namuli where this species occurs. This threat appears to have been overstated in the provisional conservation assessment of Crotalaria namuliensis Polhill & T.Harris ( Harris et al. 2011). Although grasslands of the Namuli massif do get burnt by man, this species is restricted to rocky grassland areas at high altitude that are not impacted by this burning (Legado Initiative, pers. obs.). Historically, there were feral / domestic pigs roaming free on the mountain that potentially damaged the habitat for this species by digging up the thinly vegetated grassy areas over rock ( Timberlake et al. 2009). However, these pigs have since been largely removed as potatoes are now the main crop cultivated on the lower slopes of the mountain and pigs are too damaging to potato farming (Legado Initiative, pers. obs.). They do not appear to have caused lasting damage to the upland grasslands and so are no longer considered to be a plausible threat. Therefore, despite its very small range, there are no current threats to this taxon or its montane grassland and rock habitats and so it is assessed as of Least Concern (LC). In view of its extreme range-restriction, this species and the other Namuli grassland endemic species should be regularly monitored, as any future threats to their habitats such as increased fire frequency or cattle grazing could quickly endanger these species. In the longer term, climate change and resultant changes in montane vegetation assemblages may pose a threat to this species.

Notes — This species is quite easy to separate from other Coleus (Plectranthus) species due to features in the inflorescence; no other species possess the combination of congested long-pyramidal spikes with a pointed apex, subequal calyx lobes except for the deep anterior sinus, and markedly pointed nutlets. In Table 1 we compare Coleus namuliensis with species of Coleus (Plectranthus) that share the following characters: posterior corolla lobe shorter than anterior; crassulaceous leaves; congested spiciform inflorescences; calyx with a deeper sinus between the anterior pair of lobes; pedicels 4 mm long or less, and; dense tomentose hairs covering at least part of the plant. The species that share these characters to a greater or lesser extent are C. caudatus from the Chimanimani Mountains, P. montanus Benth. which is widespread in eastern and southern Africa, Plectranthus crassus N.E.Br. from Mt Mulanje in southern Malawi, Coleus lactiflorus Vatke (= P. lactiflorus (Vatke) Agnew ) from eastern Africa and P. sanguineus Britten from Malawi, Mozambique and Zimbabwe (Table 1). The combinations in Coleus are yet to be made for P. montanus , P. crassus and P. sanguineus .

Coleus namuliensis is found to differ from each of these species in multiple character traits. The shape of the calyx in C. namuliensis is different to all of them, as the lobes are ± equally sized and positioned, whereas the others have a ‘3+2’ arrangement involving more spatial separation between the lateral and median lobes, or ‘1+4’ arrangement with a larger posterior lobe. Plant height and size differences in various organs also occur; in general, C. namuliensis is shorter than the other species and has smaller bracts, calyces and corollas than C. caudatus , P. crassus and C. lactiflorus , but larger than those of P. montanus (see Table 1).

On overall gestalt, it is most likely to be confused with C. caudatus (for which the new combination in Coleus is formalised below) but is easily separated by the diagnostic characters listed in the Recognition section; the differences in, for example, calyx arrangement and nutlet shape are so marked that the similarity may be superficial and we predict that these species may not be phylogenetically close. Plectranthus sanguineus can also appears similar to C. namuliensis due to the dense, slender inflorescence spikes and small flowers and the calyx in that species can approach C. namuliensis in that the posterior lobe is sometimes only slightly larger than the other lobes. However, several characters differentiate it clearly: in P. sanguineus the leaves and inflorescences produce a reddish brown dye when crushed (as the name suggests) and the bright red glands are very conspicuous on, e.g., the bracts, calyces and corollas (in C. namuliensis the glands are largely hidden by the more dense and longer indumentum); the plant is often leafless or with few leaves when in flower; the leaves are obovate or spathulate rather than sub-orbicular to elliptic and the spikes become ± quickly elongated with the verticils separated in at least the proximal portion. Plectranthus sanguineus is also a taller plant, typically with long slender erect stems. This species also occurs on Mt Namuli.

Plectranthus montanus can have similarly pale corollas to Coleus namuliensis but, other characteristics such as the larger corolla length and more congested inflorescences distinguish C. namuliensis from this species.For completeness, C. namuliensis was also compared to P. pseudomarrubioides R.H.Willemse from Ethiopia, Uganda, Kenya and N Tanzania. However, there appear to be more similarities between P. pseudomarrubioides and P. montanus than C. namuliensis , most notably that both P. pseudomarrubioides and P. montanus have smaller leaves when compared to C. namuliensis , both less than 6 by 2.5 cm, and that they both have a noticeably larger posterior lobe of the calyx, giving a ‘1+4’ arrangement of the lobes ( Willemse 1985) rather than subequal lobes of the calyx as in C. namuliensis . For these reasons, P. pseudomarrubioides is omitted from Table 1.