A fern is a member of a group of about 10,560 known extant species of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular (i.e. having water-conducting vessels). They have branched stems and leaves, like additional vascular plants. These are megaphylls, more complex than the simple microphylls of clubmosses. Most ferns have what're called fiddleheads that expand into fronds, which are each delicately divided.
Leptosporangiate ferns (sometimes called true ferns) are by far the largest group, but ferns as defined here (ferns sensu lato) include horsetails, whisk ferns, marattioid ferns, and ophioglossoid ferns. This group might be referred to as monilophytes. The term pteridophyte traditionally refers to ferns plus a few additional seedless vascular plants (see the classification section below), although a few recent authors have used the term to refer strictly to the monilophytes.
Ferns first appear in the fossil record 360 million years ago in the late Devonian period but a large number of of the current families and species didn't appear until roughly 145 million years ago in the early Cretaceous, after flowering plants came to dominate a large number of environments. The fern Osmunda claytoniana is a paramount example of evolutionary stasis. Paleontological evidence indicates it has remained unchanged, even at the level of fossilised nuclei and chromosomes, for at least 180 million years.
Ferns aren't of major economic importance, but a few are grown or gathered for food, as ornamental plants, for remediating contaminated soils, and have been the subject of research for their ability to remove a few chemical pollutants from the air. Some are significant weeds. They additionally play a role in mythology, medicine, and art.
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 their mode of reproduction—lacking flowers and seeds. Like all land plants, they have a life cycle referred to as alternation of generations, characterised 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.
Life cycle of a typical fern:
- 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 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 fertilises an egg that remains attached to the prothallus.
- The fertilised egg is now a diploid zygote and grows by mitosis into a diploid sporophyte (the typical "fern" plant).
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 a few of the species. Epiphytic species and a large number of of the terrestrial ones have above-ground creeping stolons (e.g., Polypodiaceae), and a large number of groups have above-ground erect semi-woody trunks (e.g., Cyatheaceae). These can reach up to 20 metres (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 doesn't 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 quite similar to the sterile ones, and they photosynthesize in the same way. In a few groups, the fertile leaves are much narrower than the sterile leaves, and might 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 leafy. The leafy structures that grow from the stipe are known as "pinnae" and are often again divided into smaller pinnules.
The gametophytes of ferns, however, are quite different from those of seed plants. Instead, they resemble liverworts. A fern gametophyte typically consists of:
- Prothallus: A green, photosynthetic structure that's one cell thick, usually heart or kidney shaped, 3–10 mm long and 2–8 mm broad. The prothallus produces gametes by means of:
- Rhizoids: root-like structures (not true roots) that consist of single greatly elongated cells, water and mineral salts are absorbed over the whole structure. Rhizoids anchor the prothallus to the soil.
Systematics and evolution
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 a large number of modern families of ferns first appeared.
Ferns are classified together as phylum (or division) Pteridophyta. They additionally belong to an informal group called "pteridophytes" that has traditionally been used to describe all seedless vascular plants, and thus is synonymous with "ferns and fern allies". This terminology can be confusing at times after members of the fern phylum Pteridophyta are additionally at times referred to as pteridophytes.
Traditionally, three discrete groups of plants have been considered ferns: two groups of eusporangiate ferns—families Ophioglossaceae (adders-tongues, moonworts, and grape-ferns) and Marattiaceae—and the leptosporangiate ferns. The Marattiaceae are a primitive group of tropical ferns with a large, fleshy rhizome, and are now thought to be a sibling taxon to the main group of ferns, the leptosporangiate ferns. Several additional groups of plants were considered "fern allies": the clubmosses, spikemosses, and quillworts in the Lycopodiophyta, the whisk ferns in Psilotaceae, and the horsetails in the Equisetaceae. More recent genetic studies have shown that the Lycopodiophyta are more distantly related to additional 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 are the Ophioglossoids and Marattiaceae. In fact, the whisk ferns and Ophioglossoids are demonstrably a clade, and the horsetails and Marattiaceae are arguably another clade. Molecular data—which remain poorly constrained for a large number of parts of the plants' phylogeny — have been supplemented by recent morphological observations supporting the inclusion of Equisetaceae within the ferns, notably relating to the construction of their sperm, and peculiarities of their roots. Notwithstanding there are still differences of opinion about the placement of the Equisetum species (see Equisetopsida for further discussion). One possible means of treating this situation is to consider only the leptosporangiate ferns as "true" ferns, while considering the additional 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 at times 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. Research by Rai and Graham (2010) after this 2006 classification broadly supports the main groups, but queries 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 don't understand them quite well". Grewe et al. (2013) confirmed the inclusion of horsetails within ferns sensu lato, but additionally suggested that uncertainties remained in their precise placement.
For the most recent classification of ferns and lycopods, see: Christenhusz & Chase (2014)
A 2006 phylogeny of living fern groups by Smith et al. is based on recent molecular systematic studies, in addition to morphological data. Their phylogeny is a consensus of a number of studies, and has been refined in more recent years, and is shown below (to the level of orders).
Ferns have traditionally been grouped in the Class Filices, but modern classifications assign them their own phylum or division in the plant kingdom, called Pteridophyta, additionally known as Filicophyta. The group is additionally referred to as Polypodiophyta, (or Polypodiopsida when treated as a subdivision of tracheophyta (vascular plants), although Polypodiopsida at times refers to only the leptosporangiate ferns). The term "pteridophyte" has traditionally been used to describe all seedless vascular plants, making it synonymous with "ferns and fern allies". This can be confusing after members of the fern phylum Pteridophyta are additionally at times referred to as pteridophytes.
The 2006 paper by Smith et al. proposed a classification, based on their phylogeny, which divides extant ferns into four classes:
- Psilotopsida (whisk ferns and ophioglossoid ferns), about 92 species
- Equisetopsida (horsetails), about 15 species
- Marattiopsida, about 150 species
- Polypodiopsida (leptosporangiate ferns), over 9000 species
Other recent classifications have raised the Ophioglossales to the rank of a fifth class, separating the whisk ferns and ophioglossoid ferns.
One problem with fern classification is the problem of cryptic species. A cryptic species is a species that's 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, the 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 a large number of cases such as this, the species complexes have been separated into separate species, thus raising the number of overall fern species. Possibly a large number of more cryptic species are yet to be discovered and designated.
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 might 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 a large number of 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. It is assumed that bird's-nest ferns alone contain up to half the invertebrate biomass within a hectare of rainforest canopy.
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 a few 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) additionally eat fern spores.
Ferns aren't as important economically as seed plants but have considerable importance in a few societies. Some ferns are used for food, including the fiddleheads of bracken, Pteridium aquilinum, ostrich fern, Matteuccia struthiopteris, and cinnamon fern, Osmundastrum cinnamomeum. Diplazium esculentum is additionally used by a few tropical people as food. Tubers from the King Fern or para (Ptisana salicina) are a traditional food in New Zealand and the South Pacific. Fern tubers were used for food 30,000 years ago in Europe. Fern tubers were used by the Guanches to make gofio in the Canary Islands. Ferns are generally not known to be poisonous to humans. Licorice fern rhizomes were chewed by the natives of the Pacific Northwest for their flavor.
Ferns of the genus Azolla are quite small, floating plants that don't resemble ferns. Called mosquito fern, they're used as a biological fertiliser 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 additional plants.
Many ferns are grown in horticulture as landscape plants, for cut foliage and as houseplants, especially the Boston fern (Nephrolepis exaltata) and additional members of the genus Nephrolepis. The Bird's Nest Fern (Asplenium nidus) is additionally popular, as are the staghorn ferns (genus Platycerium). Perennial (also known as hardy) ferns planted in gardens in the northern hemisphere additionally have a considerable following.
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.
Ferns have been studied and found to be useful in the removal of heavy metals, especially arsenic, from the soil. Other ferns with a few 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 fibre being used horticulturally; the fiddleheads of O. cinnamomea are additionally 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 additionally 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 a few 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 a few 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 additional 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 additionally used in additional 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.
Ferns figure in folklore, for example in legends about mythical flowers or seeds. In Slavic folklore, ferns are believed to bloom once a year, throughout 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 at times confused with true ferns. These include:
- "Asparagus fern"—This might 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 might 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-like flowering plants
Some flowering plants such as palms and members of the carrot family have pinnate leaves that somewhat resemble fern fronds. Notwithstanding these plants have fully developed seeds contained in fruits, rather than the microscopic spores of ferns.