Abstract
The Podostemaceae of Laos are much less understood than those of adjacent Thailand. Comparative morphology using new materials, together with the latest molecular phylogenetic studies, have revealed a high diversity in subfamily Podostemoideae in Laos. This subfamily comprises five genera and 15 species. The genus Hydrobryum Endl. is enlarged, and found to be remarkably diverse in root and shoot morphology. We describe two species, H. subcylindricum sp. nov. and H. taeniatum sp. nov., with subcylindrical and ribbon-like roots, respectively; H. austrolaoticum sp. nov., H. verrucosum sp. nov. and H. subcrustaceum sp. nov. with crustose roots, and H. takakioides sp. nov. with crustose roots and elongate shoots. Diplobryum C. Cusset, comprising four species and characterised solely by the nearly 20-ribbed capsule, is polyphyletic. Two Lao and a Vietnamese species of Diplobryum are transferred to Hydrobryum: H. ramosum (C. Cusset) Koi & M. Kato comb. nov., with floating subcylindrical roots and anchoring disk-like bases, and H. vientianense (M. Kato & Fukuoka) Koi & M. Kato comb. nov. and H. minutale (C. Cusset) Koi & M. Kato comb. nov. with crustose roots. The rootless D. koyamae M. Kato & Fukuoka was recently combined as Hydrodiscus koyamae (M. Kato & Fukuoka) Koi & M. Kato. Cladopus (sect. Griffithella) pierrei (Lecomte) C. Cusset is not segregated even at a sectional rank. We also describe Polypleurum pluricostatum sp. nov., and add Hydrobryum tardhuangense M. Kato, Paracladopus chiangmaiensis M. Kato and Polypleurum schmidtianum Warm. as new records to Laos, and Polypleurum wallichii (R. Br. ex Griff.) Warm. from new localities in Laos.
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