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 and in having life cycles in which the sporophyte is the dominant phase. 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. The group includes about 10,560 known extant species.
Ferns are defined here in the broad sense, being all of the Polypodiopsida, comprising both the leptosporangiate (Polypodiidae) andeusporangiate 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, 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.
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 world wide. Some fern genera, such as Azolla
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).
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 has multiple leaflets. The leafy structures that grow from the stipe are known as pinnae and are often again divided into smaller pinnules.
Like all other vascular plants, the diploid sporophyte is the dominant phase or generation in the life cycle.
- Prothallus: A green, photosynthetic structure that is one cell thick, usually heart or kidney shaped, 3–10 mm long and 2–8 mm broad.
- 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.
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. 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.
- 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. However, there remained differences of opinion about the placement of the genus Equisetum (see Equisetopsida 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. 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". 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
- 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. That system was a consensus of a number of studies, and was further refined. The phylogenetic relationships are shown in the following cladogram (to the level of orders). This division into four major clades was then confirmed using morphology alone.
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). Christenhusz et al. (2011) followed this use of subclasses but recombined Smith's Psilotopsida as Ophioglossidae, giving four subclasses of ferns again.
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;
- Equisetidae (=Equisetopsida) - monotypic (Equisetales, Equisetaceae, Equisetum
- 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:
In the Pteridophyte Phylogeny Group classification the Polypodiopsida consist of four subclasses, 11 orders, 48 families, 319 genera, and an estimated 10,578 species.
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.
Especially the epiphytic ferns have turned out to be hosts of a huge diversity of invertebrates.
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 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.
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:
Ferns are not as important economically as seed plants, but have considerable importance in some societies.
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.Azolla | Feedipedia 
Ferns have proved resistant to phytophagous insects.
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.
Several ferns, such as bracken and Azolla  species are noxious weeds or invasive species. Further examples include Japanese climbing fern (Lygodium japonicum), sensitive fern (Onoclea sensibilis) and Giant water fern (Salvinia molesta), one of the world's worst aquatic weeds. The important fossil fuel coal 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.
- 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
- Pteridium aquilinum or Pteridium esculentum
- Diplazium esculentum
- 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
- Ceratopteris]] richardii, a model plant for teaching and research, often called C-fern
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 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.
The dried form of ferns was also used in other arts, being used as a stencil or directly inked for use in a design.
Fern bars were popular in America in the 1970s and 80s.
Ferns figure in folklore, for example in legends about mythical flowers or seeds. 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.
Organisms confused with ferns
Several non-fern plants (and even animals) are called ferns and are sometimes confused with true ferns.
- Asparagus fern—This may apply to one of several species of the monocot genus Asparagus
- 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.
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.