This article is about a group of plants.
For other uses, see Fern (disambiguation).
"Ferns" redirects here.
For the city in Ireland, see Ferns, County Wexford.
For other uses, see Ferns (disambiguation).
|Scientific classification Polypodiopsida|
Cronquist, Takht. & W.Zimm.
A fern (Polypodiopsida or Polypodiophyta /ˌpɒliˌpɒdiˈɒfɪtə, -oʊfaɪtə/) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers.
Most ferns are leptosporangiate ferns.
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) and eusporangiate ferns, the latter group 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 middle 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.
They have been the subject of research for their ability to remove some chemical pollutants from the atmosphere.
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.
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.
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.
In most ferns, fertile leaves are morphologically very similar to the sterile ones, and they photosynthesize in the same way.
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.
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.
By 1806 this had increased to 38 genera, and has progressively increased since (see Figure 1).
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.
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.
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 closely related to leptosporangiate ferns as the ophioglossoid ferns and Marattiaceae.
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:
- Division Tracheophyta (tracheophytes) - 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.
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 (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.
The ferns are related to other higher order taxa, as shown in the following cladogram:
Nomenclature and subdivision
See also: List of fern families
The classification of Smith et al.
(2006) treated ferns as four classes:
- Equisetopsida (Sphenopsida) 1 order, Equisetales (Horsetails) ~ 15 species
- Psilotopsida 2 orders (whisk ferns and ophioglossoid ferns) ~92 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.
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.
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) 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.
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.
|Smith et al. (2006)||Chase & Reveal (2009)||Christenhusz et al. (2011)||Christenhusz & Chase (2014, 2018)||PPG I (2016)|
|Class Equisetopsida||Subclass Equisetidae||Subclass Equisetidae||Subclass Equisetidae||Subclass Equisetidae|
|Class Psilotopsida||Subclass Ophioglossidae
|Subclass Ophioglossidae||Subclass Ophioglossidae||Subclass Ophioglossidae|
|Class Marattiopsida||Subclass Marattiidae||Subclass Marattiidae||Subclass Marattiidae||Subclass Marattiidae|
|Class Polypodiopsida||Subclass Polypodiidae||Subclass Polypodiidae||Subclass Polypodiidae||Subclass Polypodiidae|
Evolution and biogeography
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.
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.
New Zealand, for which the fern is a symbol, has about 230 species, distributed throughout the country.
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).
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 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.
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:
- A diploid sporophyte phase produces haploid spores by meiosis (a process of cell division which reduces the number of chromosomes by a half).
- 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.
- The gametophyte produces gametes (often both sperm and eggs 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 diploid zygote and grows by mitosis into a diploid sporophyte (the typical fern plant).
Ferns are not as important economically as seed plants, but have considerable importance in some societies.
Fern tubers were used for food 30,000 years ago in Europe.
Ferns are generally not known to be poisonous to humans.
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.
Perennial (also known as hardy) ferns planted in gardens in the northern hemisphere also have a considerable following.
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.
Other ferns with some economic significance include:
- 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 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
- 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
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.
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.
Ferns figure in folklore, for example in legends about mythical flowers or seeds.
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.
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
Several non-fern plants (and even animals) are called ferns and are sometimes confused with ferns.
- 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 leaf tree—Filicium decipiens—an ornamental tree of the order Sapindales.
Fern-like flowering plants
However, these plants have fully developed seeds contained in fruits, rather than the microscopic spores of ferns.
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Fern.