Theferns (Polypodiopsida orPolypodiophyta) are a group ofvascular plants (plants withxylem andphloem) that reproduce viaspores and have neitherseeds norflowers. They differ frommosses by being vascular, i.e., having specialized tissues that conduct water and nutrients, and in having life cycles in which the branchedsporophyte is the dominant phase.[3]
The ferncrown group, consisting of the leptosporangiates and eusporangiates, is estimated to have originated in the lateSilurian period 423.2 million years ago,[4] butPolypodiales, the group that makes up 80% of living fern diversity, did not appear and diversify until theCretaceous, 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, asbiofertilizer, 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 asAzolla, canfix nitrogen and make a significant input to the nitrogen nutrition ofrice paddies. They also play certain roles in folklore.
Extant ferns are herbaceousperennials and most lackwoody growth.[5] When woody growth is present, it is found in the stem.[6] Their foliage may bedeciduous orevergreen,[7] and some are semi-evergreen depending on the climate.[8] Like the sporophytes of seed plants, those of ferns consist of stems, leaves and roots. Ferns differ fromspermatophytes in that they reproduce by spores rather than having flowers and producing seeds.[6] However, they also differ from spore-producingbryophytes in that, like seed plants, they arepolysporangiophytes, theirsporophytes branching and producing many sporangia. Also unlike bryophytes, fern sporophytes are free-living and only briefly dependent on the maternalgametophyte.
Thegreen,photosynthetic part of the plant is technically amegaphyll and in ferns, it is often called afrond. New leaves typically expand by the unrolling of a tight spiral called a crozier orfiddlehead intofronds.[9] This uncurling of the leaf is termedcircinate vernation. Leaves are divided into two types: sporophylls and tropophylls.Sporophylls produce spores;tropophylls do not. Fern spores are borne insporangia which are usually clustered to formsori. The sporangia may be covered with a protective coating called anindusium. The arrangement of the sporangia is important in classification.[6]
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 aremorphologically distinct.[10] The fertile leaves are much narrower than the sterile leaves, and may have no green tissue at all, as in theBlechnaceae andLomariopsidaceae.
Croziers, fronds, andrhizomes of bracken. In this species the stems grow underground, allowing the plant to spread horizontally.
The anatomy of fern leaves can be anywhere from simple to highly divided, or evenindeterminate (e.g.Gleicheniaceae,Lygodiaceae). The divided forms arepinnate, 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.[11][12] 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.[13]
Fern stems are often loosely calledrhizomes, even though they grow underground only in some of the species. Epiphytic species and many of the terrestrial ones have above-ground creepingstolons (e.g.,Polypodiaceae), and many groups have above-ground erect semi-woody trunks (e.g.,Cyatheaceae, the scaly tree ferns). These can reach up to 20 meters (66 ft) tall in a few species (e.g.,Cyathea brownii onNorfolk Island andCyathea medullaris inNew Zealand).[14]
Roots are underground non-photosynthetic structures that take up water and nutrients fromsoil. They are alwaysfibrous and are structurally very similar to the roots of seed plants.
As in allvascular plants, the sporophyte is the dominant phase orgeneration in the life cycle. Thegametophytes of ferns, however, are very different from those of seed plants. They are free-living and resembleliverworts, 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:[3]
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:
Archegonia: A flask-shaped structure that produces a single egg at the bottom, reached by the male gametophyte by swimming down the neck.[3]
Rhizoids:root-like structures (not true roots) that consist of single greatly elongated cells, that absorb water and mineralsalts over the whole structure. Rhizoids anchor the prothallus to the soil.[3]
The lifecycle of a fern involves two stages, as inclub mosses andhorsetails. In stage one, the spores are produced bysporophytes insporangia, which are clustered together in sori (s.g. sorus), developing on the underside of fertile fronds. In stage two, the spores germinate into a short-lived structure anchored to the ground by rhizoids calledgametophyte which produce gametes. When a mature fertile frond bears sori, and spores are released, the spores will settle on the soil and send outrhizoids, while it develops into aprothallus. The prothallus bears sphericalantheridia (s.g. antheridium) which produce antherozoids (male gametophytes) andarchegonia (s.g. archegonium) which release a singleoosphere. The antherozoid swims up the archegonium and fertilize the oosphere, resulting in a zygote, which will grow into a separate sporophyte, while the gametophyte shortly persists as a free-living plant.[3]
Carl Linnaeus (1753) originally recognized 15 genera of ferns and fern allies, classifying them in classCryptogamia in two groups, Filices (e.g.Polypodium) andMusci (mosses).[15][16][17] By 1806 this had increased to 38 genera,[18] and has progressively increased since (seeSchuettpelz et al (2018)). Ferns were traditionally classified in theclass Filices, and later in aDivision of the Plant Kingdom namedPteridophyta or Filicophyta. Pteridophyta is no longer recognised as a validtaxon because it isparaphyletic. The ferns are also referred to as Polypodiophyta or, when treated as a subdivision ofTracheophyta (vascular plants), Polypodiopsida, although this name sometimes only refers to leptosporangiate ferns. Traditionally, all of the spore producingvascular plants were informally denominated thepteridophytes, rendering the term synonymous with ferns andfern 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 familiesOphioglossaceae (adder's tongues,moonworts, and grape ferns) andMarattiaceae; and the leptosporangiate ferns. The Marattiaceae are a primitive group of tropical ferns with large, fleshy rhizomes and are now thought to be asibling taxon to the leptosporangiate ferns. Several other groups of species were considered fern allies: theclubmosses,spikemosses, andquillworts inLycopodiophyta; the whisk ferns ofPsilotaceae; and the horsetails ofEquisetaceae. Since this grouping ispolyphyletic, the term fern allies should be abandoned, except in a historical context.[19] More recent genetic studies demonstrated that the Lycopodiophyta are more distantly related to othervascular plants, having radiated evolutionarily at the base of the vascular plantclade, while both the whisk ferns and horsetails are as closely related to leptosporangiate ferns as theophioglossoid ferns andMarattiaceae. In fact, the whisk ferns and ophioglossoid ferns are demonstrably aclade, and thehorsetails andMarattiaceae are arguably another clade.
Smith et al. (2006) carried out the first higher-level pteridophyte classification published in themolecular phylogenetic era, and considered the ferns as monilophytes, as follows:[20]
InfradivisionSpermatophyta – seed plants, ~260,000 species
SubdivisionLycopodiophyta (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.[20]
The leptosporangiate ferns are sometimes called "true ferns".[21] 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".[22] Grewe et al. (2013) confirmed the inclusion of horsetails within fernssensu lato, but also suggested that uncertainties remained in their precise placement.[23] Other classifications have raised Ophioglossales to the rank of a fifth class, separating the whisk ferns and ophioglossoid ferns.[23]
In addition they defined 11 orders and 37 families.[20] That system was a consensus of a number of studies, and was further refined.[23][27] The phylogenetic relationships are shown in the followingcladogram (to the level of orders).[20][28][23] This division into four major clades was then confirmed usingmorphology alone.[29]
Subsequently,Chase andReveal considered both lycopods and ferns as subclasses of a class Equisetopsida (Embryophyta) encompassing all land plants. This is referred to asEquisetopsidasensu lato to distinguish it from the narrower use to refer to horsetails alone,Equisetopsidasensu 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).[25] Christenhusz et al.[a] (2011) followed this use of subclasses but recombined Smith's Psilotopsida as Ophioglossidae, giving four subclasses of ferns again.[30]
Christenhusz andChase (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;[19]
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 oflumping rather than splitting. For instance a number of families were reduced to subfamilies. Subsequently, aconsensus group was formed, thePteridophyte Phylogeny Group (PPG), analogous to theAngiosperm 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.[32] Thus Polypodiopsida in the broad sense (sensu lato) as used by the PPG (Polypodiopsidasensu PPG I) needs to be distinguished from the narrower usage (sensu stricto) of Smith et al. (Polypodiopsidasensu Smith et al.)[2] 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.[17][33]
Comparison of fern subdivisions in some classifications
Fern-like taxa (Wattieza) first appear in the fossil record in the middleDevonian period, ca. 390 Mya. By theTriassic, the first evidence of ferns related to several modern families appeared. The great fern radiation occurred in the lateCretaceous, when many modern families of ferns first appeared.[35][1][36][37] Ferns evolved to cope with low-light conditions present under the canopy of angiosperms.
Due to the very large genome seen in most ferns, it was suspected they might have gone throughwhole genome duplications, butDNA sequencing has shown that their genome size is caused by the accumulation of mobile DNA liketransposons and other genetic elements that infect genomes and get copied over and over again.[39]
Ferns appear to have evolvedextrafloral nectaries 135 million years ago, nearly simultaneously with the trait's evolution in angiosperms. However, nectary-associated diversifications in ferns did not hit their stride until nearly 100 million years later, in theCenozoic. There is weak support for the rise of fern-feeding arthropods driving this diversification.[40]
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.[41] New Zealand, for which the fern is a symbol, has about 230 species, distributed throughout the country.[42] It is a common plant inEuropean forests.
Fern species live in a wide variety ofhabitats, from remotemountain elevations, to drydesert 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 offlowering plants. Some ferns are among the world's most serious weed species, including thebracken 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, shadyforests; crevices in rock faces, especially when sheltered from the full sun; acid wetlands includingbogs andswamps; and tropicaltrees, where many species areepiphytes (something like a quarter to a third of all fern species).[43]
Especially the epiphytic ferns have turned out to be hosts of a huge diversity of invertebrates. It is assumed thatbird's-nest ferns alone contain up to half the invertebrate biomass within a hectare ofrainforest canopy.[44]
Many ferns depend on associations withmycorrhizal fungi. Many ferns grow only within specific pH ranges; for instance, the climbing fern (Lygodium palmatum) of easternNorth America will grow only in moist, intenselyacid soils, while the bulblet bladder fern (Cystopteris bulbifera), with an overlapping range, is found only onlimestone.
Ferns arevascular plants differing fromlycophytes by having trueleaves (megaphylls), which are oftenpinnate. They differ fromseed plants (gymnosperms andangiosperms) in reproducing by means of spores and lackingflowers andseeds. Like allland plants, they have alife cycle referred to asalternation of generations, characterized by alternatingdiploidsporophytic andhaploidgametophytic phases. The diploid sporophyte has 2n pairedchromosomes, wheren varies from species to species. The haploid gametophyte hasn 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:
A diploid sporophyte phase produces haploidspores bymeiosis (a process of cell division which reduces the number of chromosomes by a half).
A spore grows into a free-living haploid gametophyte bymitosis (a process of cell division which maintains the number of chromosomes). The gametophyte typically consists of a photosyntheticprothallus.
The gametophyte producesgametes (often bothsperm andeggs on the same prothallus) by mitosis.
A mobile,flagellate sperm fertilizes an egg that remains attached to the prothallus.
The fertilized egg is now a diploidzygote 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.[46]
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 ofPteridium aquilinum (bracken),Matteuccia struthiopteris (ostrich fern), andOsmundastrum cinnamomeum (cinnamon fern).Diplazium esculentum is also used in the tropics (for example inbudu pakis, a traditional dish ofBrunei[47]) as food. Tubers from the "para",Ptisana salicina (king fern) are a traditional food inNew Zealand and theSouth Pacific. Fern tubers were used for food 30,000 years ago in Europe.[48][49] Fern tubers were used by theGuanches to makegofio in theCanary Islands. Ferns are generally not known to be poisonous to humans.[50]Licorice fernrhizomes were chewed by the natives of thePacific Northwest for their flavor.[51] Some species of ferns arecarcinogenic, and the British Royal Horticultural Society has advised not to consume any species for health reasons of both humans and livestock.[52]
Ferns of the genusAzolla, 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 tofix 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 towhitefly infestations.[53]
Many ferns are grown inhorticulture as landscape plants, forcut foliage and ashouseplants, especially the Boston fern (Nephrolepis exaltata) and other members of the genusNephrolepis. Thebird's nest fern (Asplenium nidus) is also popular, as are thestaghorn ferns (genusPlatycerium). Perennial (also known as hardy) ferns planted in gardens in the northern hemisphere also have a considerable following.[54]
Several ferns, such as bracken[55] andAzolla[56] species are noxiousweeds orinvasive 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.[57][58] The important fossil fuelcoal consists of the remains of primitive plants, including ferns.[59]
The study of ferns and other pteridophytes is calledpteridology. Apteridologist is a specialist in the study of pteridophytes in a broader sense that includes the more distantly relatedlycophytes.
TheBarnsley fern is afractal named after the BritishmathematicianMichael Barnsley who first described it in his bookFractals Everywhere. Aself-similar structure is described by a mathematical function, applied repeatedly at different scales to create a frond pattern.[61]
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 ofnature 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 wasAlois Auer'sThe Discovery of the Nature Printing-Process.
Fern bars were popular in America in the 1970s and 80s.
Ferns figure in folklore, for example in legends about mythical flowers or seeds.[62] InSlavic folklore, ferns are believed to bloom once a year, during theIvan Kupala night. Although alleged to be exceedingly difficult to find, anyone who sees afern 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 theseed of a fern in bloom onMidsummer night will, by possession of it, be guided and be able to travel invisibly to the locations where eternally blazingWill o' the wisps calledaarnivalkea mark the spot of hiddentreasure. These spots are protected by a spell that prevents anyone but the fern-seed holder from ever knowing their locations.[63] In Wicca, 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.[64]
Ferns are the national emblem of New Zealand and feature on its passport and in the design of its national airline,Air New Zealand, and of its rugby team, theAll Blacks.
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 themonocot genusAsparagus, which are flowering plants.
Sweetfern—A flowering shrub of the genusComptonia.
Air fern—A group ofanimals calledhydrozoans that are distantly related tojellyfish andcorals. 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 thiscolonial animal.
Someflowering plants such aspalms and members of thecarrot family havepinnate leaves that somewhat resemble fern fronds. However, these plants have fully developed seeds contained in fruits, rather than the microscopic spores of ferns.
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