Didymoplexiella Garay (1954: 33)

Kumar, Pankaj, Bouamanivong, Somsanith, Fischer, Gunter A. & Gale, Stephan W., 2020, Didymoplexiella laosensis (Orchidaceae: Epidendroideae: Gastrodieae), a new holomycotrophic orchid from Lao PDR, Phytotaxa 430 (1), pp. 1-8 : 1-2

Didymoplexiella Garay (1954: 33) .

Type of the genus:— Didymoplexiella ornata (Ridley) Garay (1954: 33) . Basionym: Leucolena ornata Ridley (1891: 43) .

Didymoplexiella is a genus of seven holomycotrophic species distributed from Thailand and Indochina to West Malesia and southern Japan (Govaerts et al. 2019). Of these, four are distributed in IBBH and one is known from Laos ( D. siamensis ; Gale et al. 2018). All species grow in the understorey of tropical evergreen hill forest, with plants emerging aboveground from an elongate, fusiform tuber only for the brief flowering period ( Hu et al. 2014). Individual flowers typically last just one day.

The genus was originally described as Leucolena by Ridley (1891: 340) with L. ornata Ridley (1891 : t. 43) nominated as the type species. However, Smith (1920) synonymised the genus under Didymoplexis Griffith (1844: 383) , reducing Leucolaena to the status of a section. This approach was followed by Holttum (1953). Garay (1954) considered Ridley’s Leucolena to be a latter homonym of Leucolaena Bentham (1837: 55) , and hence established a new genus, Didymoplexiella Garay (1954: 33) , to accommodate four of its component species, namely, Didymoplexiella borneensis (Schltr.) Garay (≡ Leucolena borneensis Schlechter 1911: 428 ), D. forcipata (J.J.Sm.) Garay (≡ Didymoplexis forcipata Smith 1927: 87 ), D. ornata (Ridl.) Garay (≡ Leucolena ornata Ridley 1891: 341 ) and D. trichechus (J.J.Sm.) Garay (≡ Didymoplexis trichechus Smith 1920: 19 ). In doing so, he argued that Didymoplexiella should be regarded as distinct from Didymoplexis on the grounds that it lacks a column-foot, that its column bears two filiform stelidia at its apex and that the pollinarium bears a long, linear, movable filament by which it is attached to the column. More recently, Seidenfaden (1997) described a third related genus, Didymoplexiopsis , stating that its morphology is intermediate between Didymoplexis and Didymoplexiella but that it can be differentiated from both on account of its lateral sepals being adnate to the base of the column and by the presence of a substantial column-foot that is significantly longer than that of Didymoplexis . Seidenfaden’s genus remains monotypic, comprising only Didymoplexiopsis khiriwongensis Seidenfaden (1997: 13) . This treatment of the three genera has since been followed by most authors ( Seidenfaden 1978; Seidenfaden & Wood 1992; Pridgeon et al. 2006; Averyanov et al. 2011; Hu et al. 2014; Pedersen et al. 2014; Suetsugu et al. 2018; Govaerts et al. 2019), although Didymoplexiopsis was inexplicably omitted from recent familylevel classifications by Szlachetko (1995), Pridgeon et al. (2006) and Chase et al. (2015).

Suetsugu et al. (2018) recently reported Didymoplexiella siamensis from Kagoshima Prefecture in southwest Japan. Later, in recognising this Japanese entity as a distinct variety, Didymoplexis siamensis var. amamiana Suetsugu (2019: 177) , Suetsugu & Hsu (2019) merged both Didymoplexiella and Didymoplexiopsis Seidenfaden (1997: 13) with Didymoplexis , stating that differences in the morphology of the stelidia among these three genera are minor, as evidenced by the fact that their new variety lacks stelidia but matches Didymoplexiella siamensis (Rolfe ex Downie, 1925: 416) Seidenfaden (1972: 99) in all other respects. However, the present authors contend that Didymoplexis siamensis var. amamiana is no more than a mutant form of Didymoplexiella siamensis with reduced stelidia but lacking the column-foot characteristic of Didymoplexis and that there are, therefore, inadequate grounds for combining the three genera. The typical form of D. siamensis was previously known from other parts of southwestern Japan anyway ( Maekawa 1977, Nakajima 2012), and in merging the three genera, Suetsugu & Hsu (2019) did not undertake a conspectus of all component species or review important generic characters. Pending compelling morphological and molecular phylogenetic evidence to prove otherwise, we feel that the three are better maintained as separate taxonomic units. Central to this is the understanding that Didymoplexis , Didymoplexiella and Didymoplexiopsis can be readily differentiated on the basis of clear morphological differences, as shown in the following key to all six genera currently accepted in tribe Gastrodieae .