The Polystichum HomepageWelcome to the website for the hollyferns, members of the fern genus Polystichum and allies.
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Portrait of the Genus |
The genus Polystichum, a diverse and worldwide genusPolystichum is one of the ten largest genera of ferns—there are about 260 species worldwide. Polystichum is one of a cluster of diverse genera that have explored terrestrial habitats in tropical and temperate regions, including Dryopteris, Arachniodes, and Ctenitis. Its closest allies are the genera Phanerophlebia (of the America tropics) and Cyrtomium (of eastern Asia and Hawaii), a well-known greenhouse fern. The greatest diversity in the genus Polystichum is in Eastern Asia, where the landscape from Japan to the Himalayas may harbor more than 100 species. Twelve geographic centers, each limited to a floristic region (sensu Takhtajan, 1986) harbor almost 90% of this diversity. The diversity in leaf design in Polystichum is impressive, especially in Asia, but there is a common theme throughout the genus. Almost all species have their leaf segments with auricles, that is the base of the segment is unequal, more expanded on one side (the side twoards the tip of the leaf or pinna) than the other. No species of Polystichum have hairs, either unicellular or multicellular, though some may have small stalked glands. However, virtually all polystichums have hairlike extensions on their scales, especially the petiole scales, called cilia: these are absent from the scales of most Polystichum allies. When Polystichum has an indusium (most do, outside of South America), it is peltate (shaped like an umbrella covering the sporangia). With the exception of one African species, all polystichums have leaves densely packed on the rhizomes, so that the plants in nature have their leaves in a circle. Polystichum is a fern of mountains. Near the equator it is never found below about 1000 m, and in the temperate and boreal zones, the species are usually found in montane regions. Species in the genus often grow among or on rocks, and they seem to have a particular love for talus. Disturbed woodlands are great favorites, and one of the most Polystichum-rich places in the world is under the trees in Japanese forest plantations. Polystichums also show up in few unusual places. In several different parts of the world, including New Guinea, the Himalayas, the Andes, and Mexico, hollyferns have evolved alpine species. The four different lineages all look more or less the same, with narrow leaves made of strongly revolute (rolled-under) leaf edges and covered with lots of straw or orange-colored scales. Many of these alpine species have lost the characteristic peltate (umvrella-shaped) indusium of the genus. Although Polystichum is a terrestrial fern, one species from Mexico is good at growing on tree fern stems. Hybridization, allopolyploidy, and apogamy are all known to occur in Polystichum. More than 82 different hybrid combinations between Polystichum species have been recognized. As expected, most of these hybrids are totally sterile. However, swarms of fertile hybrids between the western North American Polystichum imbricans and P. munitum have been reported where the two species come into close contact in habitats that are transitional between mesic and xeric by Mike Mesler and coworkers. Of the 81 species of Polystichum that have been investigated cytologically, over one-third are polyploid; a number of these are documented to be derived from interspecific hybrids. Apogamous reproduction (reproduction without fertilization and with altered meiosis) has also been reported within Polystichum: the best known greenhouse species, Polystichum tsus-simense (known in the wild from Japan to India), is one of the apogamous species. A second apogamous species is Polystichum neolobatum, known in the wild from throughout eastern Asia and becoming popular as a garden plant in warm-temperate regions. |
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Please be in touch with me with your questions about and insights into the holly ferns - Dave Barrington, Proprietor |