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 30 to 200 cm 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.

(=) (=) Polystichum garhwalicum (=) Polystichum michelii (=) Polystichum minusculum (=) Polystichum molliculum (=) Polystichum tsuchuense (=) Polystichum tialooshanense (=) Polystichum rarum (=) Polystichum kiusiuense (=) Polystichum woodsioides (=) Polystichum solitarium (=) Polystichum moritzianum (=) Polystichum assurgens (=) Polystichum yigongense (=) Polystichum atroviridissimum (=) Polystichum carvifolium (=) Polystichum bicolor (=) Polystichum aculeatum var. japonicum (=) Polystichum setosum (=) Polystichum erinaceum (=) Polystichum rectipinnum (=) Polystichum brunneum (=) Polystichum glingense (=) Polystichum virescens (=) Polystichum lucidum (=) Polystichum tumbatzense (=) Polystichum aculeatum var. retrosopaleaceum (=) Polystichum rhomboideum (=) Polystichum platychlamys (=) Polystichum mohrioides var. scopulinum (=) Polystichum nyalamense (=) Polystichum angulare (=) Polystichum lichiangense (=) Polystichum atuntzeense (=) Polystichum austrotibeticum (=) Polystichum decorum (=) Polystichum ellipticum (=) Polystichum fukuyamae (=) Polystichum lhasaense (=) Polystichum parasinense (=) Polystichum wilsoni (=) Polystichum apicisterile (=) Polystichum integripinnulum var. conaense (=) Polystichum conaense (=) Polystichum ilicifolium (=) Polystichum paradoxum (=) Polystichum heteropaleaceum (=) Polystichum kodamae (=) Polystichum falcilobum (=) Polystichum monotis (=) Polystichum praelongum (=) Polystichum gyirongense (=) Polystichum jizhushanense
 * – Christmas fern
 * – Hard shield fern
 * – Hard shield fern
 * – Aleutian shield fern
 * – Anderson's hollyfern
 * Polystichum atkinsonii
 * Polystichum australiense
 * Polystichum bakerianum
 * Polystichum biaristatum
 * Polystichum bomiense
 * Polystichum bonapartii
 * Polystichum bonseyi – Bonsey's Hollyfern
 * Polystichum brachypterum – Rusty swordfern
 * Polystichum braunii – Braun's shield fern, Braun's hollyfern
 * Polystichum brachypterum
 * Polystichum brachypterum
 * Polystichum bulbiferum
 * Polystichum calderonense – Monte Guilarte hollyfern
 * Polystichum californicum – California sword fern
 * Polystichum capillipes
 * Polystichum castaneum
 * Polystichum chilense
 * Polystichum christii
 * Polystichum chunii
 * Polystichum craspedosorum
 * Polystichum cyclolobum
 * Polystichum cystostegia – Alpine shield fern
 * Polystichum deltodon
 * Polystichum dielsii
 * Polystichum discretum
 * Polystichum drepanum
 * Polystichum dudleyi – Dudley's sword fern
 * Polystichum duthiei
 * Polystichum echinatum – Rusty swordfern
 * Polystichum erosum
 * Polystichum excellens
 * Polystichum eximium
 * Polystichum falcatipinnum
 * Polystichum falcinellum
 * Polystichum fallax
 * Polystichum formosanum
 * Polystichum glandulosum
 * Polystichum gongboense
 * Polystichum grandifrons
 * Polystichum gymnocarpium
 * Polystichum haleakalense – Island swordfern
 * Polystichum hancockii
 * Polystichum hecatopteron
 * Polystichum herbaceum
 * Polystichum imbricans – Narrowleaf sword fern
 * Polystichum incongruum
 * Polystichum kruckebergii – Kruckeberg's sword fern/holly fern
 * Polystichum kwakiutlii
 * Polystichum lachenense
 * Polystichum lanceolatum
 * Polystichum lemmonii – Lemmon's holly fern, Shasta fern
 * Polystichum lentum
 * Polystichum lonchitis – Holly fern
 * Polystichum longidens
 * Polystichum longipaleatum
 * Polystichum longipes
 * Polystichum luctuosum – Mourning shield fern
 * Polystichum macleae
 * Polystichum macrochlaenum
 * Polystichum makinoi
 * Polystichum martini
 * Polystichum mayebarae
 * Polystichum mediocre
 * Polystichum medogense
 * Polystichum microchlamys
 * Polystichum mohrioides
 * Polystichum mollissimum
 * Polystichum monticola – Mountain shield fern
 * Polystichum moorei
 * Polystichum morii
 * Polystichum moupinense
 * Polystichum munitum – Western sword fern
 * Polystichum muricatum
 * Polystichum nakenense
 * Polystichum neolobatum
 * Polystichum nepalense
 * Polystichum ningshenense
 * Polystichum obliquum
 * Polystichum omeiense
 * Polystichum ordinatum
 * Polystichum orientalitibeticum
 * Polystichum paramoupinense
 * Polystichum parvipinnulum
 * Polystichum piceopaleaceum
 * Polystichum polyblepharum – Tassel fern
 * Polystichum prescottianum
 * Polystichum prionolepis
 * Polystichum proliferum – Mother shield fern
 * Polystichum pseudocastaneum
 * Polystichum pseudomakinoi
 * Polystichum punctiferum
 * Polystichum pungens – Forest shield fern
 * Polystichum qamdoense
 * Polystichum retrosopaleaceum
 * Polystichum rhombiforme
 * Polystichum rhomboidea
 * Polystichum richardii – Common shield fern
 * Polystichum rigens
 * Polystichum rotundilobum
 * Polystichum scopulinum – Mountain holly fern, rock sword fern
 * Polystichum semifertile
 * Polystichum setiferum – Soft shield fern
 * Polystichum setigerum – Alaska hollyfern
 * Polystichum shensiense
 * Polystichum silvaticum
 * Polystichum simplicipinnum
 * Polystichum sinense
 * Polystichum squarrosum
 * Polystichum stenophyllum
 * Polystichum stimulans
 * Polystichum submite
 * Polystichum tacticopterum
 * Polystichum thomsoni
 * Polystichum tibeticum
 * Polystichum transvaalense – Stemmed shield fern
 * Polystichum tripteron
 * Polystichum tsus-simense – Korean rock fern
 * Polystichum vestitum – Prickly shield fern
 * Polystichum wattii
 * Polystichum whiteleggei
 * Polystichum xiphophyllum
 * Polystichum yadongense
 * Polystichum yunnanense

Hybrids
Hybridisation is frequent in the genus, with several named hybrids, including:-


 * P. × bicknellii (P. aculeatum × P. setiferum)
 * P. × illyricum (P. aculeatum × P. lonchitis)
 * P. × lonchitiforme (P. lonchitis × P. setiferum)
 * P. × lesliei (P. setiferum × P. munitum) first found in Surrey in 1995 and a second plant found in Cornwall in 2001.

Former species
Species that were at one time considered part of the genus Polystichum, but are now categorized elsewhere, include:
 * Polystichum auriculatum = Dryopteris auriculata
 * Polystichum falcatum = Cyrtomium falcatum  – Japanese Holly Fern
 * Polystichum lepidocaulon = Cyrtomidictyum lepidocaulon

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.