Fern

Ferns Temporal range: Late Devonian[1]—Present PreЄ Є O S D C P T J K Pg N
A fern unrolling a young frond
Scientific classification
Kingdom:	Plantae
Subkingdom:	Embryophyta
(unranked):	Tracheophyta
(unranked):	euphyllophytes
Class:	Polypodiopsida
Subclasses[2]
Extant Equisetidae Marattiidae Ophioglossidae Polypodiidae Extinct †Cladoxylopsida †Zygopteridales †Stauropteridales †Rhacophytales
Synonyms
Monilophyta Polypodiophyta Filicophyta Filices

A fern is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissues that conduct water and nutrients, in having branched stems and in having life cycles in which the sporophyte is the dominant phase. Like other vascular plants, ferns have complex leaves called megaphylls, that are more complex than the microphylls of clubmosses. Most ferns are leptosporangiate ferns, sometimes referred to as true ferns. They produce coiled fiddleheads that uncoil and expand into fronds.^[3] The group includes about 10,560 known extant species.^[4]

Ferns are defined here in the broad sense, being all of the Polypodiopsida, comprising both the leptosporangiate (Polypodiidae) and eusporangiate ferns, the latter itself comprising ferns other than those denominated true ferns, including horsetails or scouring rushes, whisk ferns, marattioid ferns, and ophioglossoid ferns.

Ferns first appear in the fossil record about 360 million years ago in the late Devonian period,^[5] but many of the current families and species did not appear until roughly 145 million years ago in the early Cretaceous, 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.^[6]

Ferns are not of major economic importance, but some are used for food, medicine, as biofertilizer, 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 are significant weeds.^{[citation needed]} They also play certain roles in mythology and art.

Description[edit]

Like the sporophytes of seed plants, those of ferns consist of stems, leaves and roots.

Stems: Fern stems are often referred to as rhizomes, even though they grow underground only in some of the species. Epiphytic species and many of the terrestrial ones have above-ground creeping stolons (e.g., Polypodiaceae), and many groups have above-ground erect semi-woody trunks (e.g., Cyatheaceae). These can reach up to 20 meters (66 ft) tall in a few species (e.g., Cyathea brownii on Norfolk Island and Cyathea medullaris in New Zealand).^[7]

Leaf: The green, photosynthetic part of the plant is technically a megaphyll and in ferns, it is often referred to as a frond. New leaves typically expand by the unrolling of a tight spiral called a crozier or fiddlehead fern. This uncurling of the leaf is termed circinate vernation. Leaves are divided into two types a trophophyll and a sporophyll. A trophophyll 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 sporophyll frond is a fertile leaf that produces spores borne in sporangia that are usually clustered to form sori. 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., Blechnaceae, Lomariopsidaceae). 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 have multiple leaflets. The leafy structures that grow from the stipe are known as pinnae and are often again divided into smaller pinnules.^[8]

Roots: The underground non-photosynthetic structures that take up water and nutrients from soil. 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 liverworts, 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:

• Prothallus: 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:
  • Antheridia: Small spherical structures that produce flagellate sperm.
  • Archegonia: A flask-shaped structure that produces a single egg at the bottom, reached by the sperm by swimming down the neck.
• Rhizoids: root-like structures (not true roots) that consist of single greatly elongated cells, that absorb water and mineral salts over the whole structure. Rhizoids anchor the prothallus to the soil.

Taxonomy[edit]

Ferns first appear in the fossil record in the early Carboniferous period. By the Triassic, the first evidence of ferns related to several modern families appeared. The great fern radiation occurred in the late Cretaceous, when many modern families of ferns first appeared.

Ferns were traditionally classified in the class Filices, and later in a Division of the Plant Kingdom named Pteridophyta or Filicophyta. Pteridophyta is no longer recognised as a valid taxon because it is paraphyletic. The ferns are also referred to as Polypodiophyta or, when treated as a subdivision of Tracheophyta (vascular plants), Polypodiopsida, although this name sometimes only refers to leptosporangiate ferns. Traditionally, all of the spore producing vascular plants were informally denominated the pteridophytes, rendering the term synonymous with ferns and fern allies. 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 Ophioglossaceae (adder's tongues, moonworts, and grape ferns) and Marattiaceae; and the leptosporangiate ferns. The Marattiaceae are a primitive group of tropical ferns with large, fleshy rhizomes and are now thought to be a sibling taxon to the leptosporangiate ferns. Several other groups of species were considered fern allies: the clubmosses, spikemosses, and quillworts in Lycopodiophyta; the whisk ferns of Psilotaceae; and the horsetails of Equisetaceae. Since this grouping is polyphyletic, the term fern allies should be abandoned, except in a historical context.^[9] More recent genetic studies demonstrated that the Lycopodiophyta are more distantly related to other vascular plants, having radiated evolutionarily at the base of the vascular plant clade, while both the whisk ferns and horsetails are as much true ferns as the ophioglossoid ferns and Marattiaceae. In fact, the whisk ferns and ophioglossoid ferns are demonstrably a clade, and the horsetails and Marattiaceae are arguably another clade.

Molecular phylogenetics[edit]

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:^[10]

• Division Tracheophyta (tracheophytes) - vascular plants
  • Sub division Euphyllophytina (euphyllophytes)
    • Infradivision Moniliformopses (monilophytes)
    • Infradivision Spermatophyta - seed plants, ~260,000 species
  • Subdivision Lycopodiophyta (lycophytes) - less than 1% of extant vascular plants

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.^[10] However, there remained differences of opinion about the placement of the genus Equisetum (see Equisetopsida for further discussion). One possible solution was to only denominate 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.^[11] 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".^[12] Grewe et al. (2013) confirmed the inclusion of horsetails within ferns sensu lato, but also suggested that uncertainties remained in their precise placement.^[13] Other classifications have raised Ophioglossales to the rank of a fifth class, separating the whisk ferns and ophioglossoid ferns.^[13]

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[edit]

The ferns are related to other higher order taxa, as shown in the following cladogram:^{[9][14][15][2]}

Tracheophyta - vascular plants	Lycopodiophyta (Lycopodiopsida) - lycophytes   Euphyllophyta   Polypodiophyta (Polypodiopsida) - ferns   Spermatophyta - seed plants   Gymnospermae     Angiospermae - flowering plants

Subdivision[edit]

Smith's 2006 classification treated the ferns as four classes:^[16][17]

• Psilotopsida 2 orders (whisk ferns and ophioglossoid ferns) ~92 species
• Equisetopsida (Sphenopsida) 1 order, Equisetales (Horsetails) ~ 15 species
• Marattiopsida 1 order, Marattiales ~ 150 species
• Polypodiopsida (Filicopsida) 7 orders (leptosporangiate ferns) ~ 9,000 species

In addition they defined 11 orders and 37 families.^[16] That system was a consensus of a number of studies, and was further refined.^[13][18] The phylogenetic relationships are shown in the following cladogram (to the level of orders).^[19] This division into four major clades was then confirmed using morphology alone.^[20]

Tracheophyta   Lycopodiophytes (club mosses, spike mosses, quillworts)   Euphyllophytes   Spermatophytes (seed plants)   Ferns   Psilotopsida   Psilotales (whisk ferns)       Ophioglossales (grapeferns etc.)       Equisetopsida   Equisetales (horsetails)           Marattiopsida   Marattiales       Polypodiopsida   Osmundales         Hymenophyllales (filmy ferns)         Gleicheniales         Schizaeales         Salviniales (heterosporous)         Cyatheales (tree ferns)       Polypodiales

Eusporangiate Ferns Leptosporangiate Ferns

Subsequently, Chase and Reveal considered both lycopods and ferns as subclasses of a class Equisetopsida (Embryophyta) encompassing all land plants. This is referred to as Equisetopsida sensu lato to distinguish it from the narrower use to refer to horsetails alone, Equisetopsida 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).^[15] Christenhusz et al.^[a] (2011) followed this use of subclasses but recombined Smith's Psilotopsida as Ophioglossidae, giving four subclasses of ferns again.^[21]

Christenhusz and Chase (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;^[9]

• Equisetidae (=Equisetopsida) - monotypic (Equisetales, Equisetaceae, Equisetum) horsetails ~ 20 species)
• Ophioglossidae (=Psilotopsida) - 2 monotypic orders ~ 92 species
• Marattiidae (=Marattiopsida) - 1 monotypic order (Marattiales, Marattiaceae, 2 subfamilies) ~ 130 species
• Polypodiidae (=Polypodiopsida) - 7 orders

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 lumping rather than splitting. For instance a number of families were reduced to subfamilies. Subsequently, a consensus 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:^[2]

Polypodiopsida	Equisetidae       Ophioglossidae       Marattiidae     Polypodiidae

In the Pteridophyte Phylogeny Group classification 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) needs to be distinguished from the narrower usage (sensu stricto) of Smith et al. (Polypodiopsida sensu Smith et al.)^[2]

Ecology[edit]

The stereotypical image of ferns growing in moist shady woodland nooks is far from a complete picture of the habitats where ferns can be found growing. Fern species live in a wide variety of habitats, from remote mountain elevations, to dry desert rock faces, to bodies of water or in 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 flowering plants. Some ferns are among the world's most serious weed species, including the bracken fern growing in the Scottish highlands, or the mosquito fern (Azolla) 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 forests; crevices in rock faces, especially when sheltered from the full sun; acid wetlands including bogs and swamps; and tropical trees, where many species are epiphytes (something like a quarter to a third of all fern species^[22]).

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 rainforest canopy.^[23]

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

The spores are rich in lipids, protein and calories, so some vertebrates eat these. The European woodmouse (Apodemus sylvaticus) has been found to eat the spores of Culcita macrocarpa and the bullfinch (Pyrrhula murina) and the New Zealand lesser short-tailed bat (Mystacina tuberculata) also eat fern spores.^[24]

Life cycle[edit]

Ferns are vascular plants differing from lycophytes by having true leaves (megaphylls), which are often pinnate. They differ from seed plants (gymnosperms and angiosperms) in reproducing by means of spores and they lack flowers and seeds. Like all land plants, they have a life cycle referred to as alternation of generations, characterized by alternating diploid sporophytic and haploid gametophytic phases. The diploid sporophyte has 2n paired chromosomes, 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 spores by meiosis (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 mitosis (a process of cell division which maintains the number of chromosomes). The gametophyte typically consists of a photosynthetic prothallus.
3. The gametophyte produces gametes (often both sperm and eggs on the same prothallus) by mitosis.
4. A mobile, flagellate sperm fertilizes an egg that remains attached to the prothallus.
5. The fertilized egg is now a diploid zygote and grows by mitosis into a diploid sporophyte (the typical fern plant).

Uses[edit]

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 (bracken), Matteuccia struthiopteris (ostrich fern), and Osmundastrum cinnamomeum (cinnamon fern). Diplazium esculentum is also used by some tropical persons (for example in budu pakis, a traditional dish of Brunei^[25]) as food. Tubers from the "para", Ptisana salicina (king fern) are a traditional food in New Zealand and the South Pacific. Fern tubers were used for food 30,000 years ago in Europe.^[26][27] Fern tubers were used by the Guanches to make gofio in the Canary Islands. Ferns are generally not known to be poisonous to humans.^[28] Licorice fern rhizomes were chewed by the natives of the Pacific Northwest for their flavor.^{[citation needed]}

Ferns of the genus Azolla are very small, floating plants that do not resemble ferns. Called the mosquito fern, they 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.[1]

Many ferns are grown in horticulture as landscape plants, for cut foliage and as houseplants, especially the Boston fern (Nephrolepis exaltata) and other members of the genus Nephrolepis. The bird's nest fern (Asplenium nidus) is also popular, as are the staghorn ferns (genus Platycerium). Perennial (also known as hardy) ferns planted in gardens in the northern hemisphere also have a considerable following.^{[citation needed]}

Several ferns are noxious weeds or invasive species, including Japanese climbing fern (Lygodium japonicum), mosquito fern and sensitive fern (Onoclea sensibilis). Giant water fern (Salvinia molesta) is one of the world's worst aquatic weeds. The important fossil fuel coal consists of the remains of primitive plants, including ferns.^{[citation needed]}

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:^{[citation needed]}

• Dryopteris filix-mas (male fern), used as a vermifuge, and formerly in the US Pharmacopeia; also, this fern accidentally sprouting in a bottle resulted in Nathaniel Bagshaw Ward's 1829 invention of the terrarium or Wardian case
• Rumohra adiantiformis (floral fern), extensively used in the florist trade
• Microsorum pteropus (Java fern), one of the most popular freshwater aquarium plants.
• Osmunda regalis (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^{[citation needed]} 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 native societies
• Pteris vittata (brake fern), used to absorb arsenic from the soil
• Polypodium glycyrrhiza (licorice fern), roots chewed for their pleasant flavor
• Tree ferns, used as building material in some tropical areas
• Cyathea cooperi (Australian tree fern), an important invasive species in Hawaii
• Ceratopteris richardii, a model plant for teaching and research, often called C-fern

Culture[edit]

Pteridologist[edit]

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 lycophytes.

Pteridomania[edit]

Pteridomania is a term for the Victorian era craze of fern collecting and fern motifs in decorative art including pottery, glass, metals, textiles, wood, printed paper, and sculpture "appearing on everything from christening presents to gravestones and memorials." The fashion for growing ferns indoors led to the development of the Wardian case, a glazed cabinet that would exclude air pollutants and maintain the necessary humidity.^[29]

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 nature printing. 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 Alois Auer's The Discovery of the Nature Printing-Process.

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

Folklore[edit]

Ferns figure in folklore, for example in legends about mythical flowers or seeds.^[30] In Slavic folklore, 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 fern flower is thought to be guaranteed to be happy and rich for the rest of their life. Similarly, Finnish tradition holds that one who finds the seed of a fern in bloom on Midsummer 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 aarnivalkea mark the spot of hidden treasure. These spots are protected by a spell that prevents anyone but the fern-seed holder from ever knowing their locations.^[31]

Organisms confused with ferns[edit]

Misnomers[edit]

Several non-fern plants (and even animals) are called ferns and are sometimes confused with true ferns. These include:

• Asparagus fern—This may apply to one of several species of the monocot genus Asparagus, which are flowering plants.
• Sweetfern—A flowering shrub of the genus Comptonia.
• Air fern—A group of animals called hydrozoan that are distantly related to jellyfish and corals. 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—Chamaebatiaria millefolium—a rose family shrub with fern-like leaves.
• Fern tree—Jacaranda mimosifolia—an ornamental tree of the order Lamiales.

Fern-like flowering plants[edit]

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

Gallery[edit]

• 

  Adiantum lunulatum

• 

  Fern leaf, probably Blechnum nudum

• 

  A tree fern unrolling a new frond

• 

  Tree fern, probably Dicksonia antarctica

• 

  Tree ferns, probably Dicksonia antarctica

• 

  "Filicinae" from Ernst Haeckel's Kunstformen der Natur, 1904

• 

  Unidentified tree fern in Oaxaca

• 

  Tree Fern Spores San Diego, CA

• 

  Leaf of fern

• 

  Unidentified fern with spores showing in Rotorua, NZ.

• 

  Ferns in one of many natural Coast Redwood undergrowth settings Santa Cruz, CA.

• 

  Nature prints in The Ferns of Great Britain and Ireland used fronds to produce the plates

• 

  A young, newly formed fern frond

• 

  Fern bed under a forest canopy in woods near Franklin, Virginia

• 

  Pyrrosia piloselloides, Dragon's Scale, in Malaysia

• 

  Fern growing on a wall

• 

  Spores of Dryopteris filix-mas

See also[edit]

• British Pteridological Society
• Chirosia betuleti - Fern gall
• Fern spike
• Fern sports
• Paisley (design)
• Pteridophyte
• Silver fern flag

Notes[edit]

References[edit]

Bibliography[edit]

External links[edit]

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