Polystichum oligodontum (subg. Haplopolystichum, Dryopteridaceae), a new cave fern from Guangxi, China

<i>Polystichum oligodontum</i> (Dryopteridaceae), a new fern species in P. subg. Haplopolystichum from three karst caves in northwest Guangxi, China, is described and illustrated. This species is unique in having pinnae with a shallow or nearly free lobe on the acroscopic base and coarsely toothed distal margins. It resembles <i>P. membranifolium</i> and <i>P. kwangtungense</i> in its reflexed and oblong pinnae with an obtuse or rounded apex, but can be easily distinguished by its remote pinnae bearing a lobe at the acroscopic base. The new species also somewhat resembles <i>P. leveillei</i> in the nearly free basal lobe, but differs by its toothed upper pinna margins. Molecular phylogenetic analysis of 56 <i>Polystichum</i> species, based on the <i>rbc</i>L gene, supports the placement of <i>P. oligodontum</i> within P. sect. Sphaenopolystichum, with <i>P. alcicorne</i> as its closest relative. According to the IUCN Red List Categories and Criteria, the new species is assigned to the Critically Endangered (CR) category.