Polystichum is a genus of ferns in the family Dryopteridaceae, subfamily Dryopteridoideae, according to the Pteridophyte Phylogeny Group classification of 2016 (PPG I). The genus has about 500 species and has a cosmopolitan distribution. The highest diversity is in eastern Asia, with about 208 species in China alone; the region from Mexico to Brazil has at least 100 additional species; Africa (at least 17 species), North America (at least 18 species), and Europe (at least 5 species) have much lower diversity. Polystichum species are terrestrial or rock-dwelling ferns of warm-temperate and montane-tropical regions (a few species grow in alpine regions). They are often found in disturbed habitats such as road cuts, talus slopes, and stream banks. Description Many ferns of this genus have stout, slowly creeping rootstocks that form a crown, with a vase-like ring of evergreen fronds long. The sori are round, with a circular indusium, except in South American species which lack an indusium. The stipes have prominent scales with often have hair-like cilia, but lack any true hairs. The genus differs from the well-known and allied fern genus Dryopteris in the indusium being circular, not reniform, and in having the leaf segments with auricles—asymmetrical blades where one side of the segment is much longer than the other at the base. Apomixis Apomixis, the development of an embryo without the occurrence of fertilization, is particularly common among ferns. Apomixis evolved several times independently in three different clades of polystichoid ferns. Taxonomy Polystichum is one of the 10 largest fern genera and is grouped within the Dryopteridaceae. Polystichum s.l. is well defined as its own monophyletic group, including species from the genera Cyrtomidictyum, Cyrtogonellum, Cyrtomium, and Phanerophlebia. Research concerning taxonomy within Polystichum s.s. is ongoing, with high levels of hybridization, allopolyploidy, and apomixis making distinctions difficult. Based on genetic analysis Little & Barrington (2003) originally defined a monophyletic Polystichum s.s. by removing Cyrtomium as its own genus. It was further separated by Li et al. (2008) into a separate clade along with Phanerophlebia. Selected species The genus has a large number of species. The PPG I classification suggested that there were about 500 species; , the Checklist of Ferns and Lycophytes of the World listed 397 species and 58 hybrids, noting that "many undescribed species remain". The genus Polystichum includes, but is not limited to, the following species. In this list, a species name preceded by (=) is considered