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 branched sporophyte is the dominant phase.

The fern crown group, consisting of the leptosporangiates and eusporangiates, is estimated to have originated in the late Silurian period 423.2 million years ago,[4] but Polypodiales, the group that makes up 80% of living fern diversity, did not appear and diversify until the Cretaceous, 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, as biofertilizer, as ornamental plants, and for remediating contaminated soil.

New leaves typically expand by the unrolling of a tight spiral called a crozier or fiddlehead into fronds.

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.

In stage one, the spores are produced by sporophytes in sporangia, 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 called gametophyte which produce gametes.

The prothallus bears spherical antheridia (s.g. antheridium) which produce antherozoids (male gametophytes) and archegonia (s.g. archegonium) which release a single oosphere.

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.

Ferns were traditionally classified in the class Filices, and later in a Division of the Plant Kingdom named Pteridophyta or Filicophyta.

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:[20] 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.

[22] Grewe et al. (2013) confirmed the inclusion of horsetails within ferns sensu lato, but also suggested that uncertainties remained in their precise placement.

Subsequently, Chase and Reveal considered both lycopods and ferns as subclasses of a class Equisetopsida (Embryophyta) encompassing all land plants.

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: Equisetales Ophioglossales Psilotales Marattiales Osmundales Hymenophyllales Gleicheniales Schizaeales Salviniales Cyatheales Polypodiales Equisetales Ophioglossales Psilotales Marattiales Osmundales Hymenophyllales Gleicheniales Schizaeales Polypodiales Cyatheales Salviniales 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.

Remarkably, the photoreceptor neochrome in the two orders Cyatheales and Polypodiales, integral to their adaptation to low-light conditions, was obtained via horizontal gene transfer from hornworts, a bryophyte lineage.

[39] Ferns appear to have evolved extrafloral nectaries 135 million years ago, nearly simultaneously with the trait's evolution in angiosperms.

Fern species live in a wide variety of habitats, from remote mountain elevations, to dry desert 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 of flowering plants.

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).

Tubers from the "para", Ptisana salicina (king fern) are a traditional food in New Zealand and the South Pacific.

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.

The gene that express the protein Tma12 in an edible fern, Tectaria macrodonta, has been transferred to cotton plants, which became resistant to whitefly infestations.

Pteridomania was a Victorian era craze which involved 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.

A self-similar structure is described by a mathematical function, applied repeatedly at different scales to create a frond pattern.

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.

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 include: Some flowering plants such as palms and members of the carrot family have pinnate leaves that somewhat resemble fern fronds.