We here describe Machaerina ascendens R.L.Barrett & K.L.Wilson as a new species from swamps within forests in the far south-west of Western Australia. The new species is readily identifiable by its scrambling habit and compressed, multi-noded culms, features which separate it from all other Australian species. The clearly distichous glumes, few hypogynous scales fused at the base into a ring and prominently stipitate nutlets are also unusual features in Machaerina Vahl. Machaerina ascendens is highly range-restricted by its specific habitat and appears to qualify for listing as Endangered. Study of all names applicable to the current concept of Machaerina revealed an earlier name for the eastern Australian species Machaerina nuda (Steud.) J.Kern, and a new combination is provided here for Schoenus abbreviatus Nees as Machaerina abbreviata (Nees) R.L.Barrett & K.L.Wilson.
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