Ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants that reproduce via spores and have neither seeds nor flowers. They reproduce in two phases, a short-lived gametophyte that reproduces sexually to develop a spore-bearing leaf structure. Some ferns produce coiled fiddleheads, which expand into fronds.
The group includes over 10,500 extant species. The Polypodiopsida consist of both the leptosporangiate (Polypodiidae) and eusporangiate ferns, the latter group including horsetails, whisk ferns, marattioid ferns and ophioglossoid ferns. The fern crown group is estimated to have originated million years ago (mya), during the late Silurian period and the rapid radiation of land plants, but Polypodiales, the group that makes up 80% of living fern diversity, did not appear and diversify until the Cretaceous ( to 66 mya), contemporaneous with the heightened diversification of flowering plants.
Some fern species, such as bracken (Pteridium aquilinum) and water fern (Azolla filiculoides), are significant weeds worldwide, and brackens are carcinogenic. Ferns are not of major economic importance, but some are used as food, medicine, and ornamental plants. They also play a role in human culture.
Description
The life cycle of a fern occurs in two stages (as in clubmosses and horsetails). Firstly, each active spore germinates into a short-lived gamete-producing gametophyte, known as the prothallus, which is anchored to the ground by rhizoids. Secondly, a sporophyte, which is free-living and only briefly dependent on the maternal gametophyte, develops spores on fertile fronds. As in all vascular plants, the sporophyte is the dominant phase of the alternation of generations expressed by all land plants. Ferns differ from seed plants in their reproduction by spore dispersal and lack of flowers and seeds. Ferns differ from spore-bearing lycophytes by having true leaves, which are often pinnate.
In more detail, after spores settle on the soil, they germinate to form initial rhizoids and protonemata, which develop into a free-living haploid gametophyte by mitosis (a process of cell division which maintains the number of chromosomes). Using mitosis, the gametophyte (the prothallus) produces spherical antheridia containing male gametophytes (i.e. sperm or antherozoids) and archegonia, which release a single oosphere (egg cell). The flagellate sperm swims into the archegonium and fertilizes the egg, which remains attached to the prothallus as it grows by mitosis into a diploid zygote that develops into the sporophyte, while the separate prothallus persists briefly. Finally, the sporophyte undergoes a diploid phase in which it produces haploid spores by meiosis (a process of cell division which reduces the number of chromosomes by a half), and a mature frond releases spores from the sori (clusters of spore-enclosing sporangia) on its underside.
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. Sometimes a gametophyte can give rise to sporophyte traits like roots or sporangia independently from the sporophyte.The Ferns (Filicales): Volume 1, Analytical Examination of the Criteria of Comparison: Treated Comparatively with a View to their Natural Classification
Gametophyte The gametophytes of ferns are very different from those of seed plants (gymnosperms and angiosperms). 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, the initial growth of which is planar in one cell layer. It resembles a heart or kidney in shape and measures by 2–8 mm. The prothallus produces gametes by means of: Antheridia: Small spherical structures that produce flagellate antherozoids. Archegonia: A flask-shaped structure that produces a single egg at one end, which the male gametophyte reaches 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.
Sporophyte
Extant ferns are herbaceous perennial plants and most lack woody growth. When woody growth is present, it is found in the stem. Their foliage may be deciduous or evergreen, and some are semi-evergreen depending on the climate. 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. 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 (more complex than the microphylls of clubmosses) 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. 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. 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.
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.Understanding the contribution of LFY and PEBP flowering genes to fern leaf dimorphism – Botany 2019. 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.Fern Structure – Forest Service, Auckland, New Zealand. 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.
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 growing from the rhizome (e.g., Polypodiaceae), and many groups have dense, erect above-ground trunks (e.g., Cyatheaceae, the scaly tree ferns). These can reach up to tall in a few species (e.g., Cyathea brownii on Norfolk Island and Cyathea medullaris in New Zealand).
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.
Similar species
Many unrelated species are called ferns and sometimes confused with them, including: Asparagus fern – any of several species of flowering plants in the monocot genus Asparagus. Sweetfern – a flowering shrub of the genus Comptonia Fern bush (Chamaebatiaria millefolium) – a rose-family shrub with fernlike leaves Fern tree (Jacaranda mimosifolia) – an ornamental tree of the order Lamiales Fern leaf tree (Filicium decipiens) – an ornamental tree of the order Sapindales Air fern – a group hydrozoans distantly related to jellyfish and corals that are harvested and dried for sale as an artificial plant Additionally, 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. Cycads, which are gymnosperms, also have pinnate leaves that somewhat resemble fern fronds.
Taxonomy
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). By 1806 this had increased to 38 genera, and has progressively increased since. 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).
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. 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.
Molecular phylogenetics
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 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.