Rotheca Raf. is a genus of Lamiaceae (Lamiales) established by Steane & Mabberley (1998). Species of Rotheca were formerly included in Clerodendrum subg. Cycloenema (Hochst.) Gürke and Clerodendrum sect. Konocalyx Verdcourt, but placed in a separate clade in the molecular phylogenetic studies using chloroplast DNA restriction site analysis (Steane et al., 1997) and nuclear ribosomal DNA internal transcribed spacers (Steane et al., 1999). Compared to Clerodendrum s.s., Rotheca species have asymmetrical floral buds, anterior corolla lobe much larger than the other four, corolla expanding abruptly on lower side only, anthers usually basifixed, stigma lobes frequently unequal, while Clerodendrum s.s. has symmetrical floral bud, anterior corolla lobe only slightly larger than the others, anthers versatile, stigma lobes equal (Steane and Mabberley, 1998; Harley et al., 2004). Most of the 34 species recognized as Rotheca are distributed throughout sub-Saharan Africa and Madagascar, and only a few species is distributed in SE Asia (Steane and Mabberley, 1998; Fernandes and Verdcourt, 2000; Leeratiwong and Chantaranothai, 2010; POWO, 2025), especially of Thailand, such as R. serrata (L.) Steane & Mabb., R. myricoides (Hochst.) Steane & Mabb., and R. vanprukii (Craib) Leerat. & P. Chantar. (Leeratiwong and Chantaranothai, 2010). However, R. myricoides is introduced from Africa and its cultivar ’Ugandensis’ is widely cultivated in SE Asia for ornamental purpose, and Leeratiwong et al. (2018) synonymized R. vanprukii with R. serrata, while transferring Clerodendrum macrostachyum Turcz. to Rotheca (R. macrostachya [Turcz.] Leerat. & Chantar.), which made only two native Rotheca species, i.e., R. serrata and R. macrostachya, in SE Asia. For Laos, only R. serrata has been reported so far (Newman et al., 2017–present; https://padme.rbge.org.uk/laos/).
Since 2015, we have been conducting field surveys in Laos in collaboration with the Department of Forestry, Ministry of Agriculture and Forestry of the Lao People’s Democratic Republic (PDR). During our fieldwork in the limestone area of Phou Hin Poun National Biodiversity Conservation Area, Khammouane province, we collected a sample with unusual features – a hairy herb with long, elongated inflorescences consisting of opposite single flowers at each node – from a steep limestone wall. These features are comparable to those of Rotheca macrostachya, and further examination suggests a new Rotheca species. Here, we present a description of the new Rotheca species, R. laotica, along with illustrations and photos.
MATERIALS AND METHODS
Overall morphology of the newly collected Rotheca species and relevant Rotheca s p e cies, R. serrata and R. macrostachyum, stored at Daegu University Herbarium (DGU) was compared and examined using stereomicroscopes Leica S8APO and Nikon SMZ645 (Appendix 1). Flower and floral buds were stored in alcohol during the field work and examined. Digitized images of type material and specimens of R. macrostachyum available from the Global Biodiversity Information Facility (GBIF) website (https://www.gbif.org) and the Chinese Virtual Herbarium (https://www.cvh.ac.cn) were also carefully studied.
TAXONOMIC TREATMENT
Rotheca laotica H. Won, sp. nov.—TYPE: Laos. Khammouane, Hin Boun District, at Ban Bong Village, along a stream at the base of limestone mountain slope, south of the village, on limestone cliff, Phou Hin Poun National Biodiversity Conservation Area, 17°48′15.86″N, 104°37′41.51″E, elev. 170 m, 6 Aug 2022, H. Won, J. H. Kim, J. Jang, & V. Saysavanh 17475 (holotype: KB; isotypes: NUL, DGU) (Figs. 1, 2).
Diagnosis: Rotheca laotica is similar to R. macrostachya in perennial herb and having long extending inflorescence, but distinct from it in having much smaller plant size, densely pubescent stems and leaves, inflorescence consisted of single flowers in opposite position by reduction of cyme, glabrous inflorescence axes, pedicels, and bracts, oblanceolate to narrow obovate bracts, much smaller white corolla and stamens not exserted from corolla.
Perennial herbs, ca. 10 cm tall excluding inflorescence. Roots fibrous. Stems quadrangular and pubescent. Leaves decussate, simple, orbicular, deltoid-ovate, ovate, 5–8.5 cm long, 4–5 cm wide; apex acute, base shallowly cordate, margin weakly dentate and lined with 7–9-celled multi-cellular uniseriate hairs, teeth above half of the blade, adaxial surface dark green, pubescent with 0.5–1 mm long hairs, abaxial surface light green, pubescent with 0.5–1 mm long 7–9-celled multi-cellular uniseriate hairs on veins, membranous; veins 3–4 pairs; petiole 2–8 cm long, pubescent with ca. 1 mm long 7–9-celled multi-cellular uniseriate hairs. Inflorescence terminal or axillary, racemose, terminal inflorescence branching once or twice at the base, with opposite flowers at each node formed from the reduction of cyme forming thyrse, 10–20 cm long, glabrous, green, with sparse reddish peltate glandular hairs, internodes 8–18 mm long; bracts sessile, oblanceolate to narrow obovate, 1–1.5 mm long, 0.5–0.7 mm wide, green, glabrous; pedicel slender, 5–8 mm long, glabrous, green. Flowers bisexual, zygomorphic, cucullate at buds; calyx campanulate, lobes 5, truncate to minutely 5-dentate, 1 mm long, 1.2 mm wide, glabrous with sparse brown glandular hairs, green, membranous, persistent; corolla white, 5 mm long, tube 2 mm long, lobes 5, unequal, anterior lobe much larger than lateral four, expanded at the lobe tip, ca. 3.5 mm long, lateral lobes oblong, ca. 2 mm long; stamens 4, didynamous, inserted in the corolla tube; filaments 3–4 mm long, glabrous but basally pubescent at the inserted part; anthers basifixed, two-lobed, each lobe opening by a longitudinal slit; ovary superior, oblongoid, 0.8 mm long, 0.3 mm in diameter, subtended by a gland, shallowly four-lobed, surface verrucose; gland deltoidovoid, 1 mm long, 0.3 mm wide; style terminal, long-exerted, linear, ca. 9 mm long, curved, glabrous; stigma two lobed but one of the lobe shortened, ca. 2.5 mm long, flattened, acute. Fruit ovoid, ca. 1.2 mm long, 1 mm in diam., brown, surface verrucose.
Phenology: Flowering August, fruiting August–September.
Distribution: Laos. Khammouane, Limestone rock surface. So far only found at the Type locality, but a high chance of finding the species in the limestone area of the Annamite Range.
Note: Rotheca laotica is clearly distinguished from the two previously reported Rotheca species from Indochina, R. macrostachya and R. serrata. Overall morphological characters suggest that R. laotica is more closely related to R. macrostachya than R. serrata in plant habit, inflorescence structure, shape of bract, calyx, corolla, and fruit (Figs. 1, 2, Table 1). Further collection and comparative studies will help understand the morphological evolution of the group in SE Asia.
Key to the Rotheca species in Laos
1. Shrub or small tree ······························· R. serrata
1. Perennial herbs.
2. Plants 30–40 cm tall; leaves 8–18 × 6–16 cm, sparsely pubescent with short glandular hairs; stem glabrous; inflorescence axis pubescent, 20–50 cm long; 3-flowered cyme at node; corolla violet ··············································································· R. macrostachya
2. Plants ca. 10 cm tall; leaves 5–8.5 × 4–5 cm, densely pubescent with 7–9-celled multi-cellular uniseriate hairs; stem densely pubescent with 7–9-celled multi-cellular uniseriate hairs; inflorescence axis glabrous, 10–20 cm long; 1-flowered at node; corolla white ··········································································· R. laotica
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