Abstract
Podostemaceae, the river-weeds, are characterized by remarkable differences between species and genera, which resulted from saltational evolution. This paper presents additional cases of such two genera, which are described here from the Phou Khao Khouay National Protected Area in northern Central Laos. Molecular phylogenetic data show that Ctenobryum mangkonense (gen. & sp. nov.) is sister to Hydrodiscus koyamae, while Laosia ramosa (gen. & sp. nov.) is isolated from all Asian genera of subfamily Podostemoideae. Ctenobryum mangkonense is distinct from Hydrodiscus koyamae in the crustose roots (versus rootless in the latter), scattered flowers on the root (versus alternate on the shoot), and pectinate bracts (versus simple, sheath-like). Laosia ramosa is distinct from all the genera in the columnar, endogenously branched axes and single style-stigma complex. The axis is an enigmatic organ with combined characteristics of root, stem and leaf, pending further study. Laos, together with Thailand, is a center of diversity of the Southeast and East Asian Podostemoideae. The three monotypic genera, i.e., Ctenobryum, Hydrodiscus and Laosia, occur in neighboring and closely similar aquatic habitats within the Area. The new taxa are formally described.
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