Thismia sumatrana differs from its close relative T. clavigera, in having a much larger flower (ca. 8 cm vs. ca. 2.8 cm long).

INDONESIA. West Sumatra: Padang Pariaman, Sipisang, ca. 300 m alt., 0°33'S, 100°21'E, 27 Feb 1994, Okada et al. 112 (holotype TNS!, dried plant on a herbarium sheet (TNS-01051838) and liquid-preserved material in a bottle, labelled as the same specimen).

Terrestrial, mycoheterotrophic herb. Roots not seen. Stems erect, unbranched, 5–7.5 cm long. Leaves ca. 10, scale-like, appressed, triangular-ovate to lanceolate, 3–9 mm long, apex acute or slightly acuminate. Flowers solitary, sessile, terminal. Floral bracts ovate-lanceolate, 10–13 mm long, apex acute to acuminate, slightly curved. Flowers bisexual, ca. 8 cm long (including appendages); perianth tube, campanulate, ca. 2.7 by 1.3 cm, narrowest just above the ovary, widest at the top, with 12 longitudinal ribs, transverse bars inside present; outer perianth lobes 3, broadly triangular, ca 2.5 mm long; inner perianth lobes 3, incurved, apically adnate to form a dome-shaped mitre with three lateral holes, dome-shaped mitre ca. 9 mm in diam., bearing three eaves-like to hood-like accessory lobes at the tip and three slender claviform appendages at the top, appendages ca. 3.8 cm long; stamens 6, borne on the thickened margin of the perianth tube; filaments short, ribbon-shaped, free; connective broad, connate to form a tube with a quadrangular lateral appendage, apex acute, hairy; individual connective bearing four thecae; theca oblong, 1.2–1.4 mm long on the uppermost part of connective; interstaminal gland rectangular, 1.2–1.4 mm long on the line of fusion between each connective; style short, ca. 0.8 mm long; stigmas elliptic-oblong, ca. 2.5 mm long, 3-lobed; apex of lobes truncate; ovary ca. 5 mm long, cup-shaped. Mature fruit and seeds not seen.

Figure 1. 

Thismia sumatrana from West Sumatra, Indonesia (from the holotype). A Flower B Dome-shaped mitre bearing the eaves-like to hood-like accessory lobes. The accessory lobes are indicated using the arrows C Upper part of perianth tube. Outer perianth lobes are indicated by arrows D Inner view of two pendulous stamens. Thecae and glands are indicated by upper and lower arrows, respectively E Outer view of a pendulous stamen. Thecae and glands are indicated by upper and lower arrows, respectively. One grid is equal to 1 mm².

It is known from only a single collection comprising of one flowering and one fruiting individual.

Thismia sumatrana was collected from a forest floor beside a rheophytic zone along Anak Air Ganggu (Ganggu Stream), 0°33'S, 100°21'E, at Sipisang Village, Padang Pariaman, West Sumatra, Indonesia. The area was covered by mixed primary and secondary forest along a stream, where relatively natural conditions remained. For example, there were many individuals of a rare and large herbaceous plant, Amorphophallus titanum (Becc.) Becc. (Araceae). In addition, many individuals of a rheophytic plant, Furtadoa sumatrensis M.Hotta (Araceae), grew on small rocks both in the stream and on the stream bank (Mori and Okada 2001).

Figure 2. 

Thismia sumatrana from West Sumatra, Indonesia (Drawn from the holotype). A Flowering plant B Flower C Inner view of two pendulous stamens D Outer view of two pendulous stamens E Ovary with style and stigmas. Drawn by Kumi Hamasaki. Scale bars: 1 cm (A–B), 1 mm (C–E).

The distinctive characteristics of Thismia sumatrana include 1) minute outer tepals, 2) stamens with acute distal parts and 3) large flower. The combination of the first two characteristics, which have also been reported for Thismia clavigera and T. kelantanensis, but not for the other Thismia species (Stone 1980, Chantanaorrapint and Chantanaorrapint 2009, Tsukaya and Okada 2012, Yunoh 2018), suggests that T. kelantanensis, T. sumatrana, and T. clavigera are closely related. However, T. kelantanensis can be easily distinguished from the other two species by the six-partite hood on its mitre (Yunoh 2018).

Thismia clavigera was originally described as a member of the genus Geomitra Becc., based on collections from Sarawak in the early 1860s (Beccari 1878). Unfortunately, Beccari (1878) did not describe the inner floral characteristics, such as the structure of the connectives that are crucial in the identification of Thismia species and, instead, provided detailed drawings of the taxon’s external appearance. Nonetheless, comparing our material to Beccari’s original description, illustration and holotype specimen of T. clavigera revealed that T. sumatrana can be easily distinguished from T. clavigera by its much larger flowers (ca. 8 cm vs. ca. 2.8 cm long), whereas stems of T. sumatrana and T. clavigera are similar in length (ca. 5–7.5 cm long vs. 6–9 cm long; Table 1). As noted above, the flower of the T. sumatrana specimen may be shorter than those in nature, owing to desiccation and shrinkage. Nevertheless, the flowers are much larger than those of T. clavigera.

It should be noted that T. clavigera has been reported not only in type collections but also from different localities. Stone (1980) reported the rediscovery of T. clavigera from Pulau Langkawi, in the western part of the Malay Peninsula and Aceh, in northern Sumatra in 1979. Chantanaorrapint and Chantanaorrapint (2009) also reported that T. clavigera occurs on Tarutao Island, southern Thailand, which is close to Langkawi. In addition, one specimen seems to have been collected in Sarawak by Caddick (Caddick et al. 1998) and subjected to DNA sequencing by other authors (e.g. Merckx et al. 2006), although neither the precise locality nor the description is known, and no such specimen was found in K from where Caddick et al. (1998) reported on the deposited voucher specimen (Alison Moore, Curator of K, personal communication). However, we found that there are notable morphological variations amongst specimens recorded as T. clavigera.

The shape of the mitre, for example, varies amongst the specimens recorded as T. clavigera from different localities (Table 1). While Beccari (1877) depicted T. clavigera with small eave-like projections, such projections being very poorly developed in T. clavigera specimens from Langkawi and Tarutao (Stone 1980, Chantanaorrapint and Chantanaorrapint 2009) and the slender claviform appendages of the mitriform inner tepals are much shorter in T. clavigera from Aceh (ca. 5 mm) than in T. clavigera from other localities. Moreover, Stone (1980) also noted that T. clavigera from Aceh exhibits unique purple claviform appendages. However, these differences may only represent intraspecific variation, especially considering that Sochor et al. (2018b) noted that variation amongst mitre morphology is not necessarily taxonomically informative, at least for some Thismia species.

Yet, variations amongst T. clavigera from different localities have also been reported for other morphological characters. For example, T. clavigera specimens from Thailand exhibit orbicular interstaminal glands (Chantanaorrapint and Chantanaorrapint 2009), whereas the Langkawi specimens exhibit long, rectangular glands (Stone 1980) and the thecae of T. clavigera specimens from Thailand are situated slightly above the middle of the connective (Chantanaorrapint and Chantanaorrapint 2009), whereas those of Langkawi specimens are located in the uppermost part of the connective (Stone 1980). Therefore, considering that inner floral morphology is considered important for Thismia classification, differences in T. clavigera plants from Langkawi and Tarutao strongly suggest that at least two interspecific groups exist. Here, taxonomic treatments of specimens recorded as T. clavigera from other localities were not conducted because the inner floral characteristics of T. clavigera from the type locality are unavailable. Further investigation, based on specimens from the type locality of T. clavigera, is critical for elucidating the true taxonomic identities of specimens recorded as T. clavigera from other localities.