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A fern ( Polypodiopsida or Polypodiophyta) is a member of a group of (plants with xylem and phloem) that reproduce via and have neither nor . They differ from by being vascular, i.e., having specialized tissues that conduct water and nutrients and in having life cycles in which the is the dominant phase. Ferns have complex called megaphylls, that are more complex than the of . Most ferns are leptosporangiate ferns. They produce coiled that uncoil and expand into . The group includes about 10,560 known extant . Ferns are defined here in the broad sense, being all of the Polypodiopsida, comprising both the leptosporangiate (Polypodiidae) and eusporangiate ferns, the latter group including or scouring rushes, , , and ophioglossoid ferns.

Ferns first appear in the fossil record about 360 million years ago in the middle period, but many of the current families and species did not appear until roughly 145 million years ago in the early , after flowering plants came to dominate many environments. The fern Osmunda claytoniana is a paramount example of evolutionary stasis; paleontological evidence indicates it has remained unchanged, even at the level of fossilized nuclei and chromosomes, for at least 180 million years.

Ferns are not of major economic importance, but some are used for food, medicine, as , as ornamental plants and for remediating contaminated soil. They have been the subject of research for their ability to remove some chemical pollutants from the atmosphere. Some fern species, such as ( Pteridium aquilinum) and water fern ( Azolla filiculoides) are significant weeds worldwide. Some fern genera, such as , can fix nitrogen and make a significant input to the nitrogen nutrition of rice paddies. They also play certain roles in folklore.


Description
Like the sporophytes of seed plants, those of ferns consist of stems, leaves and roots. Ferns differ from seed plants in reproducing by spores and from bryophytes in that, like seed plants, they are Polysporangiophytes, their sporophytes branching and producing many sporangia. Unlike bryophytes, fern sporophytes are free-living and only briefly dependent on the maternal gametophyte.

: Fern stems are often referred to as , even though they grow underground only in some of the species. Epiphytic species and many of the terrestrial ones have above-ground creeping (e.g., ), and many groups have above-ground erect semi-woody trunks (e.g., ). These can reach up to tall in a few species (e.g., on and Cyathea medullaris in ).

(2020). 9780881926309, Timber Press. .

: The , part of the plant is technically a and in ferns, it is often referred to as a . New leaves typically expand by the unrolling of a tight spiral called a crozier or into . This uncurling of the leaf is termed circinate vernation. Leaves are divided into two types a trophophyll and a sporophyll. A frond is a vegetative leaf analogous to the typical green leaves of seed plants that does not produce spores, instead only producing sugars by photosynthesis. A frond is a fertile leaf that produces spores borne in that are usually clustered to form . In most ferns, fertile leaves are morphologically very similar to the sterile ones, and they photosynthesize in the same way. In some groups, the fertile leaves are much narrower than the sterile leaves, and may even have no green tissue at all (e.g., , ). The anatomy of fern leaves can either be simple or highly divided. In tree ferns, the main stalk that connects the leaf to the stem (known as the stipe), often has multiple leaflets. The leafy structures that grow from the stipe are known as pinnae and are often again divided into smaller pinnules.

: The underground non-photosynthetic structures that take up water and nutrients from . They are always fibrous and structurally are very similar to the roots of seed plants.

Like all other vascular plants, the diploid sporophyte is the dominant phase or generation in the life cycle. The gametophytes of ferns, however, are very different from those of seed plants. They are free-living and resemble , whereas those of seed plants develop within the spore wall and are dependent on the parent sporophyte for their nutrition. A fern gametophyte typically consists of:

  • : A green, photosynthetic structure that is one cell thick, usually heart or kidney shaped, 3–10 mm long and 2–8 mm broad. The prothallus produces gametes by means of:
    • : Small spherical structures that produce sperm.
    • : A flask-shaped structure that produces a single egg at the bottom, reached by the sperm by swimming down the neck.
  • : -like structures (not true roots) that consist of single greatly elongated cells, that absorb water and mineral over the whole structure. Rhizoids anchor the prothallus to the soil.


Taxonomy
(1753) originally recognized 15 genera of ferns and fern allies, classifying them in class in two groups, Filices (e.g. ) and (mosses). By 1806 this had increased to 38 genera, and has progressively increased since ( see Figure 1). Ferns were traditionally classified in the class Filices, and later in a of the Plant Kingdom named or Filicophyta. Pteridophyta is no longer recognised as a valid because it is . The ferns are also referred to as Polypodiophyta or, when treated as a subdivision of (vascular plants), Polypodiopsida, although this name sometimes only refers to leptosporangiate ferns. Traditionally, all of the spore producing were informally denominated the , rendering the term synonymous with ferns and . This can be confusing because members of the division Pteridophyta were also denominated pteridophytes ( sensu stricto).

Traditionally, three discrete groups have been denominated ferns: two groups of eusporangiate ferns, the families (, , and grape ferns) and ; and the leptosporangiate ferns. The Marattiaceae are a primitive group of tropical ferns with large, fleshy rhizomes and are now thought to be a to the leptosporangiate ferns. Several other groups of species were considered fern allies: the , , and quillworts in ; the whisk ferns of ; and the horsetails of . Since this grouping is , the term fern allies should be abandoned, except in a historical context. More recent genetic studies demonstrated that the Lycopodiophyta are more distantly related to other , having radiated evolutionarily at the base of the vascular plant , while both the whisk ferns and horsetails are as closely related to leptosporangiate ferns as the ophioglossoid ferns and . In fact, the whisk ferns and ophioglossoid ferns are demonstrably a , and the and are arguably another clade.


Molecular phylogenetics
Smith et al. (2006) carried out the first higher-level pteridophyte classification published in the molecular phylogenetic era, and considered the ferns as monilophytes, as follows:

Molecular data, which remain poorly constrained for many parts of the plants' phylogeny, have been supplemented by morphological observations supporting the inclusion of Equisetaceae in the ferns, notably relating to the construction of their sperm and peculiarities of their roots. However, there remained differences of opinion about the placement of the genus (see for further discussion). One possible solution was to denominate only the leptosporangiate ferns as "true ferns" while denominating the other three groups as fern allies. In practice, numerous classification schemes have been proposed for ferns and fern allies, and there has been little consensus among them.

The leptosporangiate ferns are sometimes called "true ferns".

(2020). 9780521707725, Cambridge University Press.
This group includes most plants familiarly known as ferns. Modern research supports older ideas based on morphology that the Osmundaceae diverged early in the evolutionary history of the leptosporangiate ferns; in certain ways this family is intermediate between the eusporangiate ferns and the leptosporangiate ferns. Rai and Graham (2010) broadly supported the primary groups, but queried their relationships, concluding that "at present perhaps the best that can be said about all relationships among the major lineages of monilophytes in current studies is that we do not understand them very well"., p. 1450 Grewe et al. (2013) confirmed the inclusion of horsetails within ferns sensu lato, but also suggested that uncertainties remained in their precise placement. Other classifications have raised Ophioglossales to the rank of a fifth class, separating the whisk ferns and ophioglossoid ferns.

One problem with the classification of ferns is that of cryptic species. A cryptic species is a species that is morphologically similar to another species, but differs genetically in ways that prevent fertile interbreeding. A good example of this is the currently designated species Asplenium trichomanes (maidenhair spleenwort). This is actually a species complex that includes distinct diploid and tetraploid races. There are minor but unclear morphological differences between the two groups, which prefer distinctly differing habitats. In many cases such as this, the species complexes have been separated into separate species, thus raising the total number of species of ferns. Possibly many more cryptic species are yet to be discovered and designated.


Phylogeny
The ferns are related to other higher order taxa, as shown in the following cladogram:


Nomenclature and Subdivision
The classification of Smith et al. (2006) treated ferns as four classes:

In addition they defined 11 orders and 37 families. That system was a consensus of a number of studies, and was further refined. The phylogenetic relationships are shown in the following (to the level of orders). This division into four major clades was then confirmed using alone.

Subsequently, Chase and Reveal considered both lycopods and ferns as subclasses of a class Equisetopsida () encompassing all land plants. This is referred to as Equisetopsida to distinguish it from the narrower use to refer to horsetails alone, sensu stricto. They placed the lycopods into subclass Lycopodiidae and the ferns, keeping the term monilophytes, into five subclasses, Equisetidae, Ophioglossidae, Psilotidae, Marattiidae and Polypodiidae, by dividing Smith's Psilotopsida into its two orders and elevating them to subclass (Ophioglossidae and Psilotidae). Christenhusz et al. (2011) followed this use of subclasses but recombined Smith's Psilotopsida as Ophioglossidae, giving four subclasses of ferns again.

and (2014) developed a new classification of ferns and lycopods. They used the term Polypodiophyta for the ferns, subdivided like Smith et al. into four groups (shown with equivalents in the Smith system), with 21 families, approximately 212 genera and 10,535 species;

This was a considerable reduction in the number of families from the 37 in the system of Smith et al., since the approach was more that of rather than splitting. For instance a number of families were reduced to subfamilies. Subsequently, a group was formed, the Pteridophyte Phylogeny Group (PPG), analogous to the Angiosperm Phylogeny Group, publishing their first complete classification in November 2016. They recognise ferns as a class, the Polypodiopsida, with four subclasses as described by Christenhusz and Chase, and which are phylogenetically related as in this cladogram:

In the Pteridophyte Phylogeny Group classification of 2016 (PPG I), the Polypodiopsida consist of four subclasses, 11 orders, 48 families, 319 genera, and an estimated 10,578 species. Thus Polypodiopsida in the broad sense ( sensu lato) as used by the PPG (Polypodiopsida sensu PPG I) needs to be distinguished from the narrower usage ( sensu stricto) of Smith et al. (Polypodiopsida sensu Smith et al.) Classification of ferns remains unresolved and controversial with competing viewpoints (splitting vs lumping) between the systems of the PPG on the one hand and Christenhusz and Chase on the other, respectively. In 2018, Christenhusz and Chase explicitly argued against recognizing as many genera as PPG I.

+ Comparison of fern subdivisions in some classifications
  Class Polypodiopsida
 Subclass
 Subclass
 Subclass
 Subclass Polypodiidae


Evolution and biogeography
Fern-like taxa ( ) first appear in the fossil record in the middle period, ca. 390 Mya. By the , the first evidence of ferns related to several modern families appeared. The great fern radiation occurred in the late , when many modern families of ferns first appeared.


Distribution and habitat
Ferns are widespread in their distribution, with the greatest abundance in the tropics, and least in arctic areas. The greatest diversity occurs in tropical rainforests. New Zealand, for which the fern is a symbol, has about 230 species, distributed throughout the country.


Ecology
Fern species live in a wide variety of , from remote elevations, to dry rock faces, bodies of water or open fields. Ferns in general may be thought of as largely being specialists in marginal habitats, often succeeding in places where various environmental factors limit the success of . Some ferns are among the world's most serious weed species, including the fern growing in the Scottish highlands, or the mosquito fern ( ) growing in tropical lakes, both species forming large aggressively spreading colonies. There are four particular types of habitats that ferns are found in: moist, shady ; crevices in rock faces, especially when sheltered from the full sun; acid wetlands including and ; and tropical , where many species are (something like a quarter to a third of all fern species).

Especially the epiphytic ferns have turned out to be hosts of a huge diversity of invertebrates. It is assumed that bird's-nest ferns alone contain up to half the invertebrate biomass within a hectare of canopy.

Many ferns depend on associations with fungi. Many ferns grow only within specific pH ranges; for instance, the climbing fern ( Lygodium palmatum) of eastern will grow only in moist, intensely soils, while the bulblet bladder fern ( Cystopteris bulbifera), with an overlapping range, is found only on .

The spores are rich in , and , so some vertebrates eat these. The European woodmouse ( Apodemus sylvaticus) has been found to eat the spores of Culcita macrocarpa and the ( Pyrrhula murina) and the New Zealand lesser short-tailed bat ( Mystacina tuberculata) also eat fern spores.


Life cycle
Ferns are differing from by having true (megaphylls), which are often . They differ from ( and ) in reproducing by means of spores and they lack and . Like all , they have a life cycle referred to as alternation of generations, characterized by alternating and phases. The diploid sporophyte has 2 n paired , where n varies from species to species. The haploid gametophyte has n unpaired chromosomes, i.e. half the number of the sporophyte. The gametophyte of ferns is a free-living organism, whereas the gametophyte of the gymnosperms and angiosperms is dependent on the sporophyte.

The life cycle of a typical fern proceeds as follows:

  1. A diploid sporophyte phase produces haploid by (a process of cell division which reduces the number of chromosomes by a half).
  2. A spore grows into a free-living haploid gametophyte by (a process of cell division which maintains the number of chromosomes). The gametophyte typically consists of a photosynthetic .
  3. The gametophyte produces (often both and on the same prothallus) by mitosis.
  4. A mobile, sperm fertilizes an egg that remains attached to the prothallus.
  5. The fertilized egg is now a diploid and grows by mitosis into a diploid sporophyte (the typical fern plant).


Uses
Ferns are not as important economically as seed plants, but have considerable importance in some societies. Some ferns are used for food, including the fiddleheads of Pteridium aquilinum (), Matteuccia struthiopteris (), and Osmundastrum cinnamomeum (). Diplazium esculentum is also used in the tropics (for example in budu pakis, a traditional dish of ) as food. Tubers from the "para", (king fern) are a traditional food in and the . Fern tubers were used for food 30,000 years ago in Europe. "Stone Age humans liked their burgers in a bun", Sonia Van Gilder Cooke, , 23 October 2010, p. 18. "Thirty thousand-year-old evidence of plant food processing" by Anna Revedin et al., , published online 18 October 2010. Fern tubers were used by the to make in the . Ferns are generally not known to be poisonous to humans. were chewed by the natives of the Pacific Northwest for their flavor.

Ferns of the genus Azolla, commonly known as water fern or mosquito ferns are very small, floating plants that do not resemble ferns. The mosquito ferns are used as a biological fertilizer in the rice paddies of southeast Asia, taking advantage of their ability to fix nitrogen from the air into compounds that can then be used by other plants.

Ferns have proved resistant to phytophagous insects. The gene that express the protein Tma12 in an edible fern, Tectaria macrodonta, has been transferred to cotton plants, which became resistant to infestations.

Many ferns are grown in as landscape plants, for and as , especially the Boston fern ( Nephrolepis exaltata) and other members of the genus . The ( Asplenium nidus) is also popular, as are the (genus Platycerium). Perennial (also known as hardy) ferns planted in gardens in the northern hemisphere also have a considerable following.

Several ferns, such as bracken and Azolla species are noxious or . Further examples include Japanese climbing fern ( Lygodium japonicum), ( Onoclea sensibilis) and Giant water fern ( ), one of the world's worst aquatic weeds.

9780881926675
The important fossil fuel consists of the remains of primitive plants, including ferns.

Ferns have been studied and found to be useful in the removal of heavy metals, especially arsenic, from the soil. Other ferns with some economic significance include:

  • Dryopteris filix-mas (male fern), used as a , and formerly in the ; also, this fern accidentally sprouting in a bottle resulted in Nathaniel Bagshaw Ward's 1829 invention of the terrarium or
  • Rumohra adiantiformis (floral fern), extensively used in the trade
  • Microsorum pteropus (Java fern), one of the most popular freshwater aquarium plants.
  • (royal fern) and Osmunda cinnamomea (cinnamon fern), the root fiber being used horticulturally; the fiddleheads of O. cinnamomea are also used as a cooked vegetable
  • Matteuccia struthiopteris (ostrich fern), the fiddleheads used as a cooked vegetable in North America
  • Pteridium aquilinum or Pteridium esculentum (bracken), the fiddleheads used as a cooked vegetable in Japan and are believed to be responsible for the high rate of stomach cancer in Japan. It is also one of the world's most important agricultural weeds, especially in the British highlands, and often poisons cattle and horses.
  • Diplazium esculentum (vegetable fern), a source of food for some societies
  • (brake fern), used to absorb from the soil
  • Polypodium glycyrrhiza (licorice fern), roots chewed for their pleasant flavor
  • , used as building material in some tropical areas
  • (Australian tree fern), an important invasive species in Hawaii
  • richardii, a model plant for teaching and research, often called C-fern


Culture

Pteridologist
The study of ferns and other pteridophytes is called pteridology. A pteridologist is a specialist in the study of pteridophytes in a broader sense that includes the more distantly related .


Pteridomania
is a term for the craze of fern and fern motifs in including , , , , , , and "appearing on everything from presents to and memorials." The fashion for growing ferns indoors led to the development of the , a glazed cabinet that would exclude air pollutants and maintain the necessary humidity.

The dried form of ferns was also used in other arts, being used as a stencil or directly inked for use in a design. The botanical work, The Ferns of Great Britain and Ireland, is a notable example of this type of . The process, patented by the artist and publisher Henry Bradbury, impressed a specimen on to a soft lead plate. The first publication to demonstrate this was 's The Discovery of the Nature Printing-Process.

were popular in America in the 1970s and 80s.


Folklore
Ferns figure in folklore, for example in legends about mythical flowers or seeds. In , ferns are believed to bloom once a year, during the Ivan Kupala night. Although alleged to be exceedingly difficult to find, anyone who sees a is thought to be guaranteed to be happy and rich for the rest of their life. Similarly, tradition holds that one who finds the of a fern in bloom on night will, by possession of it, be guided and be able to travel invisibly to the locations where eternally blazing Will o' the wisps called mark the spot of hidden . These spots are protected by a spell that prevents anyone but the fern-seed holder from ever knowing their locations. In the US, ferns are thought to have magical properties such as a dried fern can be thrown into hot coals of a fire to exorcise evil spirits, or smoke from a burning fern is thought to drive away snakes and such creatures.


Organisms confused with ferns

Misnomers
Several non-fern plants (and even animals) are called ferns and are sometimes confused with ferns. These include:
  • Asparagus fern—This may apply to one of several species of the genus Asparagus, which are flowering plants.
  • Sweetfern—A flowering shrub of the genus Comptonia.
  • —A group of called that are distantly related to and . They are harvested, dried, dyed green, and then sold as a plant that can live on air. While it may look like a fern, it is merely the skeleton of this colonial animal.
  • Fern bush— —a rose family shrub with fern-like leaves.
  • Fern tree— Jacaranda mimosifolia—an of the order .
  • Fern leaf tree— Filicium decipiens—an of the order .


Fern-like flowering plants
Some such as and members of the have leaves that somewhat resemble fern fronds. However, these plants have fully developed seeds contained in fruits, rather than the microscopic spores of ferns.

==Gallery==

's Kunstformen der Natur, 1904]]
in The Ferns of Great Britain and Ireland used fronds to produce the plates]]


See also


Notes

Bibliography

Books and theses


Journal articles

Websites


External links
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