Polystichum

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 to be a synonym of the accepted species name above it.
Hybrids.
Hybridisation is frequent in the genus, with several named hybrids, including:-
Former species.
Species that were at one time considered part of the genus "Polystichum", but are now categorized elsewhere, include:
Distribution.
With species in six continents and many islands, "Polystichum" is remarkable for its global spread. "Polystichum" ferns first emerged in Asia during the late Eocene, around 49 million years ago. During this period there were high temperatures across the globe, which may have contributed to the diversification of flora.
"Polystichum's" spread to the New World took place during the late Eocene to early Oligocene. The most likely dispersal method was across a paratropical forest on a Pacific Ocean land bridge, such as the Bering Land Bridge. For a period before the height of the Ice Age temperatures froze enough seawater to lower the sea level but still allowed a forest to grow in Northern latitudes. From there "Polystichum" was able to spread through North American and into Central America.
Original theories described "Polystichum" spreading further into South America from Central America, but recent research has shown that South American "Polystichum" instead spread through long-distance dispersal from Australia. Genetic study has revealed close evolutionary relationships between "Polystichum" species in these two areas. Both Austral and Austral South American species lack and indusium. Austral "Polystichum" lack cilia, while South American species have marginal cilia.
Hawaiian "Polystichum" also spread through long-distance dispersal, with two separate dispersal events leading to the three "Polystichum" now observed in Hawaii.
Ecology.
"Polystichum" species are used as food plants by the larvae of some Lepidoptera species including "Pharmacis fusconebulosa". Specimens of some of these can be found at the Royal Botanic Gardens, Sydney.
Cultivation.
Several species are grown as ornamental plants in gardens, notably "P. setiferum". One species, "P. tsus-simense" of eastern Asia, is commonly offered as a houseplant.