Fern

Ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (land plants with vascular tissues such as xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from non-vascular plants (mosses, hornworts and liverworts) by having specialized transport bundles that conduct water and nutrients from and to the roots, as well as life cycles in which the branched sporophyte is the dominant phase.^{[3] [4]}

Ferns have leaves called megaphylls that are more complex than the microphylls of clubmosses. Most ferns are leptosporangiate ferns that produce coiled fiddleheads that uncoil and expand into fronds. 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, whisk ferns, marattioid ferns and ophioglossoid ferns.

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 during the rapid radiation of land plants,^[5] 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. 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 as Azolla , can fix nitrogen and make a significant input to the nitrogen nutrition of rice paddies. They also play certain roles in folklore.

Extant ferns are herbaceous perennials and most lack woody growth.^[6] When woody growth is present, it is found in the stem.^[7] Their foliage may be deciduous or evergreen,^[8] and some are semi-evergreen depending on the climate.^[9] Like the sporophytes of seed plants, those of ferns consist of stems, leaves and roots. Ferns differ from spermatophytes in that they reproduce by spores rather than by seeds.^[7] However, they also differ from spore-producing bryophytes in that, like seed plants, they are polysporangiophytes, their sporophytes branching and producing many sporangia. Unlike those of bryophytes, fern sporophytes are free-living and only briefly dependent on the maternal gametophyte.

The green, photosynthetic part of the plant is technically a megaphyll and in ferns, it is often called a frond . In leptosporangiate ferns new leaves typically expand by the unrolling of a tight spiral called a crozier or fiddlehead into fronds.^[10] This uncurling of the leaf is termed circinate vernation. In some families, such as the Blechnaceae, the leaves are divided into two types, sporophylls or fertile fronds that produce spores and trophophylls or sterile fronds that do not.^{[11] : 32} Fern spores are borne in sporangia which are usually clustered to form sori. The sporangia may be covered with a protective coating called an indusium. The arrangement of the sporangia is important in classification.^[7]

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 are morphologically distinct.^[12] The fertile leaves are much narrower than the sterile leaves, and may have no green tissue at all, as in the Blechnaceae and Lomariopsidaceae.

The anatomy of fern leaves can be anywhere from simple to highly divided, or even indeterminate (e.g. Gleicheniaceae, Lygodiaceae). The divided forms are pinnate, 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.^{[13] [14]} 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.^[15]

Fern stems are often loosely called 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, the scaly tree ferns). These can reach up to 20 meters (66 ft) tall in a few species (e.g., Cyathea brownii on Norfolk Island and Cyathea medullaris in New Zealand).^[16]

Roots are underground non-photosynthetic structures that take up water and nutrients from soil. They are always fibrous and are structurally very similar to the roots of seed plants.^{[citation needed ]}

As in all vascular plants, the 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.^[17] A fern gametophyte typically consists of:^[3]

• Prothallus: A green, photosynthetic structure, whose initial growth is planar in one cell layer,^[18] usually heart or kidney shaped, 3–10 mm long and 2–8 mm broad.^{[citation needed ]} The prothallus produces gametes by means of:
  • Antheridia: Small spherical structures that produce flagellate antherozoids.^[19]
  • 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^[20] greatly elongated cells, that absorb water and mineral salts over the whole structure. Rhizoids anchor the prothallus to the soil.^[3]

The lifecycle of a fern involves two stages, as in club mosses and horsetails. 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 short-lived gamete-producing structures called gametophytes anchored to the ground by rhizoids. When a mature fertile frond bears sori, and spores are released, the spores will settle on the soil and germinate to form initial rhizoids and protonemata that develop into the gametophyte's prothallus.^[21] 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 fertilizes the oosphere, resulting in a zygote, which will grow into a separate sporophyte, while the gametophyte shortly persists as a free-living plant.^{[3] [22]}

Ferns are vascular plants differing from lycophytes by having true leaves (megaphylls), which are often pinnate.^{[citation needed ]} They differ from seed plants (gymnosperms and angiosperms) in reproducing by means of spores and 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.^{[citation needed ]}

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 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.
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).

Sometimes a gametophyte can give rise to sporophyte traits like roots or sporangia without the rest of the sporophyte.^[23]

Carl Linnaeus (1753) originally recognized 15 genera of ferns and fern allies, classifying them in class Cryptogamia in two groups, Filices (e.g. Polypodium ) and Musci (mosses).^{[24] [25] [26]} By 1806 this had increased to 38 genera,^[27] and has progressively increased since (see Schuettpelz et al (2018) ). Ferns were traditionally classified in the class Filices, and later in a Division of the Plant Kingdom named Pteridophyta or Filicophyta. Pteridophyta is no longer recognised as a valid taxon because it is paraphyletic. 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. 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 division Pteridophyta were also denominated pteridophytes (sensu stricto).^{[citation needed ]}

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. Since this grouping is polyphyletic, the term fern allies should be abandoned, except in a historical context.^[28] 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. In fact, the whisk ferns and ophioglossoid ferns are demonstrably a clade, and the horsetails and Marattiaceae are arguably another clade.^{[citation needed ]}

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:^[29]

• Division Tracheophyta (tracheophytes) – vascular plants
  • Sub division Euphyllophytina (euphyllophytes)
    • Infradivision Moniliformopses (monilophytes)
    • Infradivision Spermatophyta – seed plants, ~260,000 species
  • Subdivision Lycopodiophyta (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.^[29]

The leptosporangiate ferns are sometimes called "true ferns".^[30] 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".^[31] Grewe et al. (2013) confirmed the inclusion of horsetails within ferns sensu lato, but also suggested that uncertainties remained in their precise placement.^[32] Other classifications have raised Ophioglossales to the rank of a fifth class, separating the whisk ferns and ophioglossoid ferns.^[32]

The ferns are related to other groups as shown in the following cladogram:^{[28] [33] [34] [2]}