Fern

Life cycle

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, 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:

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 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 gametes (often both sperm and 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).

Morphology

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 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 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 have multiple leafy. The leafy structures that grow from the stipe are known as "pinnae" and are often again divided into smaller pinnules.

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

The gametophytes of ferns, however, are very different from those of seed plants: they resemble liverworts. A fern gametophyte typically consists of:

Classification 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 many modern families of ferns first appeared.

"Ferns" were traditionally classified in the class "Filices", but modern classifications assign them their own phylum, i. e., division of the Plant Kingdom, denominated "Pteridophyta" or "Filicophyta". The taxon is 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 phylum "Pteridophyta" are 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. 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. Molecular data, which remain poorly constrained for many parts of the plants' phylogeny, have been supplemented by recent 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 are still differences of opinion about the placement of the Equisetum genus (see Equisetopsida for further discussion). One possible solution is to only denominate 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. Research by Rai and Graham (2010) since this 2006 classification broadly supports the primary 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 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.

For the most recent classification of ferns and the Lycopods of Lycopodiophyta, see: Christenhusz and Chase (2014).

The 2006 paper by Smith et al. proposed a classification, based on phylogeny, that divided extant ferns into four classes:

Other recent 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.

Phylogeny

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).

Ecology

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 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. 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 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.

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 (Bracken Fern), Matteuccia struthiopteris (Ostrich Fern), and Osmundastrum cinnamomeum (Cinnamon Fern). Diplazium esculentum is also used by some tropical persons (for example in budu pakis, a traditional dish of Brunei) as food. Tubers from the "para", Ptisana salicina (King Fern) 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 very small, floating plants that do not resemble ferns. Called the mosquito fern, they 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.[1]

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 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 some economic significance include:

Pteridologist

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

"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 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.

Folklore

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.

Misunderstood names

Several non-fern plants (and even animals) are called "ferns" and are sometimes confused with true ferns. These include:

Fern-like flowering plants

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.