Thismia nigricoronata Kumar & S.W.Gale, 2017

Kumar, Pankaj, Gale, Stephan W., Li, Ji-Hong, Bouamanivong, Somsanith & Fischer, Gunter A., 2017, Thismia nigricoronata, a new species of Burmanniaceae (Thismieae, Dioscoreales) from Vang Vieng, Vientiane Province, Laos, and a key to subgeneric classification, Phytotaxa 319 (3), pp. 225-240 : 234-237

Thismia nigricoronata Kumar & S.W.Gale , sp. nov. ( Figs. 4, 5)

Type:— LAOS. Vientiane Province: Vang Vieng District, Nam Pae Village, elevation 544 m, 28 April 2012, Gale, Kumar, Santainsy & Phunthavong HNL-KFBG 0099 (holotype: HNL, spirit).

Thismia nigricoronata is morphologically similar to Thismia taiwanensis but can be differentiated on the basis of its longer vestigial stem leaves (more than 6 mm long in the former versus less than 6 mm in the latter); its reflexed free outer perianth lobes (versus erect and projecting upwards in the latter); the ornamented outer surface of its perianth tube, which is verrucose below and papillose above (versus smooth and glabrous in the latter); and its vibrantly coloured perianth tube (versus translucent white in the latter).

Terrestrial, mycoheterotrophic herbs, erect, ca. 5.5 cm tall. Roots clustered, prostrate, unbranched, pale yellowbrown, vermiform, 3.2–7.5 × 0.8–1.5 mm, tapering towards apex. Stem erect, 10.3–14.5 × 1.2–1.5 mm at the base, ca. 2.5–3.0 mm wide near the apex; unbranched, off-white, somewhat translucent, covered with the bases of sheathing vestigial leaves. Leaves entire, white, translucent, shiny, scattered along the stem, obovate, upper ones 6.0–7.5 × 1.0– 1.5 mm at base, lower ones up to 5.0– 5.3 mm long, acute, margin smooth. Pedicel white, ca. 0.50 × 1.75 mm, finely and irregularly verrucose. Flowers solitary, actinomorphic, 25–40 mm long, 7.0– 7.5 mm wide; tepals fused to form an urceolate perianth tube with a whorl of 3 inner and 3 outer free apical perianth lobes. Perianth tube obovoid, clearly divisible into upper and lower parts by the point of placement of the free perianth lobes; lower part of perianth tube ca. 4.5 mm long, narrower towards the base (ca. 4.5 mm in diameter), wider above (ca. 5.5 mm in diameter), white tinged yellow-green, densely covered with longitudinally arranged irregularly sized verrucae, with 12 depressed green ribs of which 6 alternate ribs run from the base of the tube to the mouth of the annulus on the upper half of the perianth tube, inner wall smooth with a protruding ring-like structure just below the stigma; upper part of perianth tube 3.5–4.0 mm long, wider at the base (ca. 5.5 mm in diameter), narrower at the annular opening (ca. 2.7 mm in diameter), bright yellow-green with 6 dark green ribs terminating at the annulus, covered with densely arranged papillae on the outer surface except on the ribs, inner wall smooth. Free part of outer perianth lobes white, translucent, triangular, 2.5–3.0 mm long, reflexed, ca. 1 mm wide at the base, gradually tapering. Free part of inner perianth lobes distinctly divisible into 2 parts: lower obliquely spathulate bases and upper flagelliform appendages; obliquely spathulate bases 3, black, rough and irregularly carbunculate, ca. 1 mm wide at the point of attachment to the perianth tube, broadening above (ca. 4 mm wide) and then fused with one another in a contorted aestivation to form a crown-like structure arching over the annulus with a concavity in the centre and an aperture directly above the annulus aperture, surrounded by flagellate appendages that arise at the base of the concavity; flagelliform appendages 3, erect, immotile, black and carbunculate towards the base, becoming translucent white and smooth above, 7.2–17.7 mm long, ca. 0.8 mm wide towards the base but abruptly narrowing towards the apex to less than 0.2 mm wide. Stamens 6, 4.8–5.5 mm long, 0.7–0.9 mm wide, quadrangular; filaments partially fused, arranged radially and forming a staminal tube that originates from the opening of the annulus at the base (ca. 1 mm in diameter) and is continuous with the connective towards the apex; suspended above the stigma; connectives laterally connate and forming a narrow opening (0.4–0.5 mm in diameter); anthers ca. 2.5 × 0.5 mm wide, mounted on fused connectives, thecae free, positioned ca. 2.5 mm below the annulus, opening with longitudinal slits facing the inner wall of the perianth tube, covered with sparse glandular trichomes especially along the longitudinal slit; connectives protruding ca. 0.75 mm beyond the anther locules with the apical 1/3 bent at 90° towards the inner wall of the perianth tube, sparsely covered with multicellular cilia. Ovary inferior, unilocular, obconical, outer wall longitudinally verrucose, ca. 3.5 mm long; style cylindrical, ca. 0.75 mm long, ca. 0.70 mm wide; stigmas 3, fused together and forming a dome-like structure, ca. 1 mm long and wide.

Flowering:—April, fruits not seen.

Habitat:— Thismia nigricoronata was discovered on a steep slope of a limestone mountain, growing among leaf litter in clayey soils under a dense evergreen canopy.

Etymology:—The species epithet refers to the black, crown-like structure formed above the annulus by the fusion of the three inner perianth lobes. Hence the taxon may informally be referred to as ‘the black-crowned thismia’.

Specimens examined:— LAOS. Vientiane Province: Vang Vieng District, Nam Pae Village, elevation 544 m, 28 April 2012, Gale, Kumar, Santainsy & Phunthavong HNL-KFBG 0099 (HNL, spirit).

Taxonomic notes:—With diagnostic characters including the free anther thecae, the fused inner perianth lobes that form a mitre with prominent outer perianth lobes and long filiform appendages, there is little doubt that Thismia nigricoronata is presently best placed in section Glaziocharis . However, as noted above, the circumscription of this (and other sections) requires reappraisal, with phylogenetic analysis placing the morphologically distinct T. huangii , a member of section Rodwaya , as sister to our new species. Additionally, it is noteworthy that T. nigricoronata is the only species known so far in section Glaziocharis to have such a vibrantly coloured perianth tube, with that of all other previously described members of the section (namely, T. abei , T. clavarioides , T. taiwanensis and T. tuberculata ) being whitish and somewhat translucent. Further study is required to assess whether or not this pigmentation is related to photosynthetic activity.

Conservation assessment:—Only ca. 10 individuals were observed at the site at a single locality on the lower slopes of a limestone mountain in Nam Pae Village of Vang Vieng District in central Laos. Thismia nigricoronata is an inconspicuous plant not easily observed in the field due to its small size and short flowering period. However, the area continues to be subjected to intense local pressures in the form of forest clearance for agriculture on the plains and selective logging on steeper slopes, cattle grazing and limestone mining for cement factories. Habitat destruction is therefore regarded as a very real threat to the long-term persistence of T. nigricoronata in this area. Although we recommend more surveys to confirm its occurrence in adjacent areas, we assess T. nigricoronata as critically endangered (B1+B2ab(iii); D) ( IUCN 2012). The discovery of this apparently highly restricted endemic in the limestone karst landscape of central northern Laos underscores the need to ensure better protection of this biologically invaluable but highly threatened ecoregion.