Fern

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Ferns Temporal range: Middle Devonian[1]—Present PreꞒ Ꞓ O S D C P T J K Pg N
Hymenophyllales Gleicheniales Cyatheales Polypodiales
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Division:	Polypodiophyta
Class:	Polypodiopsida Cronquist, Takht. & W.Zimm.
Subclasses[2]
†Stauropterididae †Zygopterididae Equisetidae Marattiidae Ophioglossidae Polypodiidae
Synonyms
Filicatae Kubitski 1990 Filices Filicophyta Endlicher 1836 Monilophyta Cantino & Donoghue 2007 Pteridopsida Ritgen 1828

The ferns (Polypodiopsida or Polypodiophyta) are 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 and in having life cycles in which the branched sporophyte is the dominant phase.

Ferns have complex

Ferns first appear in the fossil record about 360 million years ago in the late Devonian period, but Polypodiales, the group that makes up 80% of living fern diversity, did not appear and diversify until the Cretaceous, contemporaneous with the rise of flowering plants that came to dominate the world's flora.

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, such as bracken (Pteridium aquilinum) and water fern (Azolla filiculoides), are significant weeds worldwide. Some fern genera, such as Azolla, can fix nitrogen and make a significant input to the nitrogen nutrition of rice paddies. They also play certain roles in folklore.

Description

Sporophyte

Extant ferns are herbaceous

In monomorphic ferns, the fertile and sterile leaves look morphologically the same, and both are able to photosynthesize. In hemidimorphic ferns, just a portion of the fertile leaf is different from the sterile leaves. In dimorphic (holomorphic) ferns, the two types of leaves are morphologically distinct.^[8] The fertile leaves are much narrower than the sterile leaves, and may have no green tissue at all, as in the Blechnaceae and Lomariopsidaceae.

The anatomy of fern leaves can be anywhere from simple to highly divided, or even indeterminate (e.g. Gleicheniaceae, Lygodiaceae). The divided forms are pinnate, where the leaf segments are completely separated from one other, or pinnatifid (partially pinnate), where the leaf segments are still partially connected. When the fronds are branched more than once, it can also be a combination of the pinnatifid are pinnate shapes. If the leaf blades are divided twice, the plant has bipinnate fronds, and tripinnate fronds if they branch three times, and all the way to tetra- and pentapinnate fronds.^[9][10] 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.^[11]

Fern stems are often loosely called

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

Gametophyte

As in all

• 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.
• salts
  over the whole structure. Rhizoids anchor the prothallus to the soil.

Taxonomy

Traditionally, three discrete groups have been denominated ferns: two groups of eusporangiate ferns, the families

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.[18]

The leptosporangiate ferns are sometimes called "true ferns".^[19] 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".^[20] Grewe et al. (2013) confirmed the inclusion of horsetails within ferns sensu lato, but also suggested that uncertainties remained in their precise placement.^[21] Other classifications have raised Ophioglossales to the rank of a fifth class, separating the whisk ferns and ophioglossoid ferns.^[21]

Phylogeny

The ferns are related to other groups as shown in the following cladogram:^{[17][22][23][2]}

Tracheophyta	Lycophytes Euphyllophyta Ferns Spermatophyta Gymnosperms Angiosperms (seed plants)
(vascular plants)

Nomenclature and subdivision

The classification of Smith et al. in 2006 treated ferns as four classes:^[18][24]

• Horsetails
  ) ~ 15 species
• ophioglossoid ferns
  ) ~92 species
• Marattiales
  ~ 150 species
• Polypodiopsida (Filicopsida) 7 orders (leptosporangiate ferns
  ) ~ 9,000 species

In addition they defined 11 orders and 37 families.[18] That system was a consensus of a number of studies, and was further refined.^[21][25] The phylogenetic relationships are shown in the following cladogram (to the level of orders).^[18][26][21] This division into four major clades was then confirmed using morphology alone.^[27]

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

Eusporangiate Ferns Leptosporangiate Ferns

Subsequently,

• 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

Ferns evolved to cope with low-light conditions present under the canopy of angiosperms.

Remarkably, the photoreceptor neochrome in the two orders Cyatheales and Polypodiales, integral to their adaptation to low-light conditions, was obtained via horizontal gene transfer from hornworts, a bryophyte lineage.^[37]

Due to the very large genome seen in most ferns, it was suspected they might have gone through whole genome duplications, but DNA sequencing has shown that their genome size is caused by the accumulation of mobile DNA like transposons and other genetic elements that infect genomes and get copied over and over again.^[38]

Distribution and habitat

Ferns are widespread in their distribution, with the greatest richness in the tropics and least in arctic areas. The greatest diversity occurs in tropical rainforests.^[39] New Zealand, for which the fern is a symbol, has about 230 species, distributed throughout the country.^[40] It is a common plant in European forests.

Ecology

Fern species live in a wide variety of habitats, from remote mountain elevations, to dry desert 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 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).^[41]

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.^[42]

Many ferns depend on associations with

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

.

The spores are rich in

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.^[43]

• 
  In undergrowth below coast redwoods, California
• 
  Fern bed under a forest canopy, Virginia
• 
  On a wall
• 
  Asplenium hart's tongue fern in a gryke in limestone pavement
• 
  Epiphytic ferns in India
• 
  Azolla duckweed fern covering
  the Canning River, Western Australia

Life cycle

Ferns are

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
   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).

Sometimes a gametophyte can give rise to sporophyte traits like roots or sporangia without the rest of the sporophyte.^[44]

• 
  monarch fern, with sporangium
• 
  Gametophyte (thallus) and sporophyte (ascendant frond) of Onoclea sensibilis

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 (

carcinogenic, and the British Royal Horticultural Society has advised not to consume any species for health reasons of both humans and livestock.^[50]

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 whitefly infestations.^[51]

Many ferns are grown in

staghorn ferns (genus Platycerium). Perennial (also known as hardy) ferns planted in gardens in the northern hemisphere also have a considerable following.^[52]

Several ferns, such as bracken

sensitive fern (Onoclea sensibilis) and Giant water fern (Salvinia molesta), one of the world's worst aquatic weeds.^[55][56] The important fossil fuel coal consists of the remains of primitive plants, including ferns.^[57]

Culture

Pteridology

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

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.^[58]

Other applications

The Barnsley fern is a fractal named after the British mathematician Michael Barnsley who first described it in his book Fractals Everywhere. A self-similar structure is described by a mathematical function, applied repeatedly at different scales to create a frond pattern.^[59]

The dried form of ferns was used in other arts, such 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

Ferns figure in folklore, for example in legends about mythical flowers or seeds.

• 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 hydrozoans 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—
  ornamental tree of the order Lamiales
  .
• Fern leaf tree—
  ornamental tree of the order Sapindales
  .