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Ferns

General characteristics

Natural history

Life cycle

Gametophyte

Sporophyte

Polyploidy

Evolution

Modern ferns

Hybridization

Psilotum and Tmesipteris

Importance to humans

Resources

Ferns are plants in the Filicinophyta phylum, also called the Pteridophyta phylum. They are intermediate in complexity between the more primitive (i.e., evolu-tionarily ancient) bryophytes (mosses, liverworts, and hornworts) and the more advanced (or recent) seed plants. Like bryophytes, ferns reproduce sexually by making spores rather than seeds. Most ferns produce spores on the underside or margin of their leaves. Like seed plants, ferns have stems with a vascular system for efficient transport of water and food. Ferns also have leaves, known technically as megaphylls, with a complex system of branched veins. There are about 11,000 species of ferns, most of them indigenous to tropical and subtropical regions.

General characteristics

A fern plant generally consists of one or more fronds attached to a rhizome. A frond is simply the leaf of the fern. A rhizome is a specialized, rootlike stem. In most temperate-zone species of ferns, the rhizome is subterranean and has true roots attached to it. Fronds are generally connected to the rhizome by a stalk, known technically as the stipe. The structures of the frond, rhizome, and stipe are important characteristics for species identification.

The sizes of ferns and their fronds vary considerably among the different species. Tree ferns of the Cyatheaceae family are the largest. They are tropical plants that can grow 60 feet (18 m) or more in height and have fronds 15 feet (5 m) or more in length. In contrast, species in the genus Azolla, a group of free-floating aquatic ferns, have very simple fronds which are less than 0.2 inches (0.5 cm) in diameter.

The fern frond develops from a leaf bud referred to as a crozier. The crozier is coiled up in most species, with the frond apex at the middle of the coil. This pattern of coiled leaf arrangement in a bud is called

circinate vernation. Circinate vernation is found in a few other seed plants, but not in any other free-sporing plants. During growth of a bud with circinate vernation, the cells on one side of the coil grow more rapidly than those on the other, so the frond slowly uncoils as it develops into a full-grown leaf.

The horsetails (phylum Sphenophyta) and club mosses (phylum Lycodophyta) are known colloquially as fern allies. The fern allies also reproduce sexually by making spores and have stems with vascular systems. However, there are two principal differences between ferns and fern allies. First, unlike the ferns, the leaves of fern allies, known technically as microphylls, are small scale-like structures with a single midvein. Second, fern allies make their spores at the bases of their leaves or on specialized branches. There are about 1,500 species of fern allies in the world.

The reproductive cells of ferns are microscopic spores that are often clustered together in the brown spots visible on the fronds undersides. Since fern spores are microscopic, fern reproduction was not well understood until the mid-1800s. This led some people to attribute mystical powers to the ferns. According to folklore, ferns made invisible seeds and a person who held these would also become invisible. Even Shakespeare drew upon this folklore and wrote in Henry IV, we have the receipt of fern seed; we walk invisible. Nowadays, anyone with a simple microscope can tease apart the brown spots on the underside of a fern frond and see the tiny spores.

Natural history

There are about 11,000 species of ferns in the world. Ferns are found throughout the world, from the tropics to the subarctic region. The greatest species diversity is in the tropical and subtropical region from southern Mexico to northern South America.

In temperate North America, most ferns are terrestrial plants and grow in woodlands. However, in the tropics, many ferns grow as epiphytes. Epiphytes are plants which rely upon other plants, such as trees, for physical support, while obtaining their nutrition from organic debris and rain water that falls through the forest canopy.

Ferns can be found in very different habitats throughout the world. Some species are free-floating aquatic plants, some species grow in moist woodlands, and a few species grow in arid or semiarid regions. Most species require some rainfall because their sperm cells must swim through a fluid to reach the egg cells.

Interestingly, sperm cells of the resurrection fern (Polypodium polypodioides ) swim through a fluid exuded by the fern itself to reach the females egg. This species is widely distributed in semiarid and arid regions of the world, such as central Australia, central Mexico, and central Africa. The resurrection fern is sold in some garden shops as a brown, dried-out, and curled-up ball of fronds. When this dried-out fern is soaked in water, it rapidly expands and becomes green in a day or so, attesting to the remarkable desiccation tolerance of this species.

At the other extreme are water ferns of the genus Azolla, which grow free-floating on fresh water. Azolla is particularly interesting because it has special pockets in its leaves which apparently have evolved to accommodate symbiotic cyanobacteria of the genus Anabaena. These cyanobacteria transform atmospheric nitrogen (N2) to ammonia (NH3), a chemical form useful to plants. This process is called nitrogen fixation. Many Asian farmers encourage the growth of Azolla and its associated Anabaena in their rice paddies to increase the amount of nitrogen available to their rice plants.

Many species of ferns can act as alternate hosts for species of rust fungi that are pathogenic to firs, economically important timber trees. The rust fungi are a large and diverse group of fungi, which have very complex life cycles, often with four or five different reproductive stages. In the species of rust fungi that attack ferns, part of the life cycle must be completed on the fern, and part on the fir tree. These parasitic fungi can usually be eradicated by simply eliminating one of the hosts, in this case, either the fern or the fir tree.

Life cycle

Like all plants, the life cycle of ferns is characterized as having an alternation of a gametophyte phase and a sporophyte phase. A typical fern sporophyte is the large, familiar plant seen in nature. Its cells have the unreduced number of chromosomes, usually two sets. Most fern gametophytes are not seen in nature. A typical gametophyte is about 0.4 inches (1 cm) in diameter, multicellular, flat, heart-shaped, and green. Its cells have the reduced number of chromosomes, usually one set.

Interestingly, the gametophyte and sporophyte are about equally dominant in the life cycle of ferns. In contrast, the gametophyte is dominant in the more evolutionarily primitive bryophytes (mosses, liverworts, and hornworts), whereas the sporophyte is dominant in the more evolutionarily advanced seed plants.

Gametophyte

The gametophyte phase of the fern life cycle begins with a spore. A fern spore is a haploid reproductive cell, which unlike the seeds of higher plants, does not contain an embryo. Fern spores are often dispersed by the wind. Upon germination, a spore gives rise to a green, threadlike tissue, called a protonema. The protonema develops into a prothallus, a small, green, multicellular tissue that is rarely seen in nature. The prothallus has numerous subterranean rhizoids to anchor it to the substrate and absorb nutrients.

Light and other environmental factors control the development of fern gametophytes. In many species, gametophytes kept in darkness do not develop beyond the thread-like protonemal stage. However, illumination with blue or ultraviolet radiation causes the protonema to develop into a heart-shaped prothallus. This is an example of photomorphogenesis, the control of development by light.

Male and female reproductive structures form on the prothallus, and these are referred to as antheridia and archegonia, respectively. Each antheridium produces many flagellated sperm cells which swim toward the archegonia. The sperm cells of some ferns have up to several hundred flagella each. Each archegonium produces a single egg which is fertilized by a sperm cell.

Sporophyte

Fusion of the egg and sperm nuclei during fertilization leads to the formation of a zygote, with the unreduced number of chromosomes, usually two sets. The zygote develops into a sporophyte, the most familiar stage of the fern life cycle. As the sporophyte grows, the prothallus to which it is attached eventually decays. Most fern sporophytes in temperate North America are green and terrestrial.

As the sporophyte continues to grow, it eventually develops numerous structures with spores inside, referred to as sporangia. The sporangia form on the underside of fronds or on specialized fertile fronds, depending on the species. In many species, the sporangia develop in clusters referred to as sori (singular, sorus). The size, shape, and position of the sori are frequently used in species identification. As development proceeds, the sporangium dries out, releasing the many spores inside for dispersal into the environment.

Most ferns are homosporous, in that all their spores are identical and all spores develop into a game-tophyte with antheridia and archegonia. However, some water ferns are heterosporous. In these species, separate male and female spores develop on the spor-ophyte. The smaller and more numerous male spores germinate and develop into male gametophytes with antheridia. The female spores germinate and develop into female gametophytes with archegonia.

Polyploidy

In many species of ferns, the sporophyte phase is diploid (two sets of chromosomes) and the gameto-phyte phase is haploid (one set of chromosomes). However, many other ferns are considered polyploid, in that their sporophyte contains three or more sets of chromosomes. In polyploid ferns, the gametophyte and sporophyte phases are said to have the reduced and the unreduced number of chromosomes, respectively.

Apospory and apogamy are special types of asexual reproduction that have important roles in the generation and proliferation of polyploidy. In apospory, the gametophyte develops directly from special cells on the sporophyte, so that the gametophyte and sporophyte both have the unreduced number of chromosomes. The sperm and egg cells produced by such a gametophyte have twice the original number of chromosomes. In apogamy, the sporophyte develops directly from special cells on the gametophyte, so that the sporophyte and gametophyte have the same reduced number of chromosomes. Apogamy typically occurs in gametophytes which themselves have arisen by apospory.

Evolution

Most botanists believe that the ferns and fern allies are descendants of the Rhyniopsida, an extinct group of free-sporing plants that originated in the Silurian period (about 430 million years ago) and went extinct in the mid-Devonian period (about 370 million years ago). The Rhyniopsida were primitive vascular plants which were photosynthetic, had branched stems, and produced sporangia at their stem tips, but had no leaves or roots.

The Cladoxylales is a group of plants known colloquially as the preferns. They also evolved from the Rhyniopsida, but went extinct in the lower Carboniferous period (about 340 million years ago). Some botanists previously considered these as ancestors of the ferns, because they had leaves somewhat similar to fern fronds. However, most botanists now believe the evolutionary line which led to the Cladoxylales went extinct, and that the modern ferns evolved from a separate lineage of the Rhyniopsida.

As a group, the ferns were the first plants to have megaphyllsa leaf with a complex system of branched veins. Many botanists believe that ferns evolved megaphylls by developing a flattened and webbed version of the simple, three-dimensional branching system of the Rhyniopsida. The evolution of the megaphyll was a major event in plant evolution, and nearly all ecologically dominant plants in the modern world have megaphylls.

Modern ferns

There are two evolutionarily distinct groups of modern ferns: the leptosporangiates and the eusporan-giates. In the leptosporangiates, the sporangium develops from one cell and is usually only one cell thick. In the eusporangiates, the sporangium develops from several cells and is usually several cells thick. Leptosporangiate and eusporangiate ferns probably separated evolutionarily in the lower Carboniferous (about 340 million years ago) or earlier. Modern leptosporangiate ferns are often placed into the Filicales class, and eusporangiate ferns into the Marattiales or Ophioglossales classes.

While there is general agreement about the natural division between the leptosporangiate and eusporangiate ferns, there is considerable uncertainty about other relationships among the modern ferns. Thus, there have been many proposed classification schemes. The widespread occurrence of polyploidy (see above) and hybridization (see below) in ferns has complicated the determination of evolutionary relationships.

Hybridization

Many species of ferns form hybrids in nature and hybridization is believed to have had a major role in fern evolution. A hybrid species is the offspring of a sexual union between two different species. Most hybrids cannot engage in sexual reproduction because they lack homologous (corresponding) chromosomes, which typically pair off during production of sperm and egg cells. However, since many fern species can engage in apogamy and apospory, fern hybrids can often reproduce and proliferate.

A hybrid species is often identified by the number of chromosomes in its cells and by the presence of

KEY TERMS

Apogamy Development of a sporophyte directly from the gametophyte without fusion of sex cells.

Apospory Development of a gametophyte directly from the sporophyte without sex cell production.

Epiphyte A plant which relies upon another plant, such as a tree, for physical support, but does not harm the host plant.

Flagellum Thread-like appendage of certain cells, such as sperm cells, which controls their locomotion.

Megaphyll Leaf with a complex system of branched veins, typical of ferns and seed plants.

Microphyll Scale-like leaf with a single midvein, typical of fern allies.

Prothallus Gametophyte phase of ferns and fern allies, rarely seen in nature.

Rhizome This is a modified stem that grows horizontally in the soil and from which roots and upward-growing shoots develop at the stem nodes.

Sorus Group of many sporangia, which often appear as a brown spot on the margin or underside of a fern frond.

Sporangium Structure that produces spores.

Spore Small reproductive cell that develops into a gametophyte.

Symbiosis A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.

aborted spores. Chromosome number is often used to infer evolutionary relationships of hybrid ferns. The ferns, as a group, tend to have very high chromosome numbers due to the widespread occurrence of poly-ploidy. One fern species, Ophioglossum reticulatum, has 631 chromosomes, the largest number of any organism.

Psilotum and Tmesipteris

Lastly, the evolutionary status of two additional genera of modern plants must be considered: Psilotum and Tmesipteris. These free-sporing tropical and subtropical plants have very simple morphologies. In particular, species in the genus Psilotum superficially resemble plants of the Rhyniopsida in that their spor-ophytes consist of three-dimensional branched stems, with tiny scale-like appendages believed to be leaf derivatives. Moreover, like the Rhyniopsida, Psilotum does not have true roots. Thus, some botanists have suggested that Psilotum is a direct descendant of the Rhyniopsida. Others reject this hypothesis and point to the lack of a fossil record connecting these two groups. They suggest that Psilotum and Tmesipteris may have evolved by evolutionary simplification of an ancestor of the modern fern genus, Stromatopteris. Clearly, further research is needed to resolve the relationships of these fascinating fernlike plants.

Importance to humans

In general, ferns are of minor economic importance to humans. However, ferns are popular horticultural plants and many species are grown in ornamental gardens or indoors.

Most people can recognize ferns as understory or groundcover plants in woodland habitats. However, several hundred million years ago ferns and fern allies were the dominant terrestrial plants. Thus, the fossils of these plants have contributed greatly to the formation of our fossil fuelscoal, oil and natural gas.

Various cultures have used the starch-rich rhizomes and stems of some fern species as a food, and those who frequent restaurants known for their haute cuisine will occasionally find croziers or fiddleheads (unfurled fern leaves, see above) of the ostrich fern (Matteuccia struthiopteris ) served in salads, around a bowl of ice cream, or as a steamed vegetable.

Herbalists have advocated some fern species for treatment of ulcers, rheumatism, intestinal infections, and various other ailments. Although many modern pharmaceuticals are derived from chemicals produced by plants, there is little scientific evidence that ferns are useful as treatments for these or other ailments.

See also Maidenhair fern; Seed ferns.

Resources

BOOKS

Jones, D. Encyclopedia of Ferns. vol. 1. Portland, OR: Timber Press, 1987.

Margulis, L., and K.V. Schwartz. Five Kingdoms. New York: W. H. Freeman and Company, 1988.

McHugh, A. The Cultivation of Ferns. North Pomfret, VT: Trafalgar Press, 1992.

OTHER

American Fern Society. A Brief Introduction to Ferns <http://amerfernsoc.org/> (accessed November 24, 2006).

University of California Museum of Paleontology. Introduction to the Pteridopsida: The Ferns <http://www.ucmp.berkeley.edu/plants/pterophyta/pteridopsida.html> (accessed November 24, 2006).

Peter A. Ensminger

views updated

Ferns

Ferns are plants in the Filicinophyta phylum, also called the Pteridophyta phylum. They are intermediate in complexity between the more primitive (i.e., evolutionarily ancient) bryophytes (mosses, liverworts, and hornworts) and the more advanced (or recent) seed plants. Like bryophytes, ferns reproduce sexually by making spores rather than seeds . Most ferns produce spores on the underside or margin of their leaves. Like seed plants, ferns have stems with a vascular system for efficient transport of water and food. Ferns also have leaves, known technically as megaphylls, with a complex system of branched veins . There are about 11,000 species of ferns, most of them indigenous to tropical and subtropical regions.


General characteristics

A fern plant generally consists of one or more fronds attached to a rhizome . A frond is simply the leaf of the fern. A rhizome is a specialized, root-like stem. In most temperate-zone species of ferns, the rhizome is subterranean and has true roots attached to it. Fronds are generally connected to the rhizome by a stalk, known technically as the stipe. The structures of the frond, rhizome, and stipe are important characteristics for species identification.

The sizes of ferns and their fronds vary considerably among the different species. Tree ferns of the Cyatheaceae family are the largest ferns. They are tropical plants which can grow 60 ft (18 m) or more in height and have fronds 15 ft (5 m) or more in length. In contrast, species in the genus Azolla, a group of free-floating aquatic ferns, have very simple fronds which are less than 0.2 in (0.5 cm) in diameter.

The fern frond develops from a leaf bud referred to as a crozier. The crozier is coiled up in most species, with the frond apex at the middle of the coil. This pattern of coiled leaf arrangement in a bud is called circinate vernation. Circinate vernation is found in a few other seed plants, but not in any other free-sporing plants. During growth of a bud with circinate vernation, the cells on one side of the coil grow more rapidly than those on the other, so the frond slowly uncoils as it develops into a full-grown leaf.

The horsetails (phylum Sphenophyta) and club mosses (phylum Lycodophyta) are known colloquially as fern allies. The fern allies also reproduce sexually by making spores and have stems with vascular systems. However, there are two principal differences between ferns and fern allies. First, unlike the ferns, the leaves of fern allies, known technically as microphylls, are small, scale-like structures with a single mid-vein. Second, fern allies make their spores at the bases of their leaves or on specialized branches. There are about 1,500 species of fern allies in the world.

The reproductive cells of ferns are microscopic spores which are often clustered together in the brown spots visible on the fronds' undersides. Since fern spores are microscopic, fern reproduction was not well understood until the mid-1800s. This led some people to attribute mystical powers to the ferns. According to folklore, ferns made invisible seeds and a person who held these would also become invisible. Even Shakespeare drew upon this folklore and wrote in Henry IV; "we have the receipt of fern seed; we walk invisible." Nowadays, anyone with a simple microscope can tease apart the brown spots on the underside of a fern frond and see the tiny spores.

Natural history

There are about 11,000 species of ferns in the world. Ferns are found throughout the world, from the tropics to the subarctic region. The greatest species diversity is in the tropical and subtropical region from southern Mexico to northern South America .

In temperate North America , most ferns are terrestrial plants and grow in woodlands. However, in the tropics, many ferns grow as epiphytes. Epiphytes are plants which rely upon other plants, such as trees, for physical support, while obtaining their nutrition from organic debris and rain water that falls through the forest canopy.

Ferns can be found in very different habitats throughout the world. Some species are free-floating aquatic plants, some species grow in moist woodlands, and a few species grow in arid or semiarid regions. Most species require some rainfall because their sperm cells must swim through a fluid to reach the egg cells.

Interestingly, sperm cells of the resurrection fern (Polypodium polypodioides) swim through a fluid exuded by the fern itself to reach the female's egg. This species is widely distributed in semi-arid and arid regions of the world, such as central Australia , central Mexico, and central Africa . The resurrection fern is sold in some garden shops as a brown, dried-out, and curledup ball of fronds. When this dried-out fern is soaked in water, it rapidly expands and becomes green in a day or so, attesting to the remarkable desiccation tolerance of this species.

At the other extreme are water ferns of the genus Azolla, which grow free-floating on fresh water. Azolla is particularly interesting because it has special pockets in its leaves which apparently have evolved to accommodate symbiotic cyanobacteria of the genus Anabaena. These cyanobacteria transform atmospheric nitrogen (N2) to ammonia (NH3), a chemical form useful to plants. This process is called nitrogen fixation . Many Asian farmers encourage the growth of Azolla and its associated Anabaena in their rice paddies to increase the amount of nitrogen available to their rice plants.

Many species of ferns can act as alternate hosts for species of rust fungi that are pathogenic to firs , economically important timber trees. The rust fungi are a large and diverse group of fungi, which have very complex life cycles, often with four or five different reproductive stages. In the species of rust fungi that attack ferns, part of the life cycle must be completed on the fern, and part on the fir tree. These parasitic fungi can usually be eradicated by simply eliminating one of the hosts, in this case, either the fern or the fir tree.


Life cycle

Like all plants, the life cycle of ferns is characterized as having an alternation of a gametophyte phase and a sporophyte phase. A typical fern sporophyte is the large, familiar plant seen in nature. Its cells have the unreduced number of chromosomes, usually two sets. Most fern gametophytes are not seen in nature. A typical gametophyte is about 0.4 in (1 cm) in diameter, multicellular, flat, heart-shaped, and green. Its cells have the reduced number of chromosomes, usually one set.

Interestingly, the gametophyte and sporophyte are about equally dominant in the life cycle of ferns. In contrast, the gametophyte is dominant in the more evolutionarily primitive bryophytes (mosses, liverworts, and hornworts), whereas the sporophyte is dominant in the more evolutionarily advanced seed plants.


Gametophyte

The gametophyte phase of the fern life cycle begins with a spore . A fern spore is a haploid reproductive cell , which unlike the seeds of higher plants, does not contain an embryo. Fern spores are often dispersed by the wind . Upon germination , a spore gives rise to a green, thread-like tissue , called a protonema. The protonema develops into a prothallus, a small, green, multicellular tissue that is rarely seen in nature. The prothallus has numerous subterranean rhizoids to anchor it to the substrate and absorb nutrients .

Light and other environmental factors control the development of fern gametophytes. In many species, gametophytes kept in darkness do not develop beyond the thread-like protonemal stage. However, illumination with blue or ultraviolet radiation causes the protonema to develop into a heart-shaped prothallus. This is an example of photomorphogenesis, the control of development by light.

Male and female reproductive structures form on the prothallus, and these are referred to as antheridia and archegonia, respectively. Each antheridium produces many flagellated sperm cells which swim toward the archegonia. The sperm cells of some ferns have up to
several hundred flagella each. Each archegonium produces a single egg which is fertilized by a sperm cell.


Sporophyte

Fusion of the egg and sperm nuclei during fertilization leads to the formation of a zygote, with the unreduced number of chromosomes, usually two sets. The zygote develops into a sporophyte, the most familiar stage of the fern life cycle. As the sporophyte grows, the prothallus to which it is attached eventually decays. Most fern sporophytes in temperate North America are green and terrestrial.

As the sporophyte continues to grow, it eventually develops numerous structures with spores inside, referred to as sporangia. The sporangia form on the underside of fronds or on specialized fertile fronds, depending on the species. In many species, the sporangia develop in clusters referred to as sori (singular, sorus). The size, shape, and position of the sori are frequently used in species identification. As development proceeds, the sporangium dries out, releasing the many spores inside for dispersal into the environment.

Most ferns are homosporous, in that all their spores are identical and all spores develop into a gametophyte with antheridia and archegonia. However, some water ferns are heterosporous. In these species, separate male and female spores develop on the sporophyte. The smaller and more numerous male spores germinate and develop into male gametophytes with antheridia. The female spores germinate and develop into female gametophytes with archegonia.


Polyploidy

In many species of ferns, the sporophyte phase is diploid (two sets of chromosomes) and the gametophyte phase is haploid (one set of chromosomes). However, many other ferns are considered polyploid, in that their sporophyte contains three or more sets of chromosomes. In polyploid ferns, the gametophyte and sporophyte phases are said to have the "reduced" and the "unreduced" number of chromosomes, respectively.

Apospory and apogamy are special types of asexual reproduction which have important roles in the generation and proliferation of polyploidy. In apospory, the gametophyte develops directly from special cells on the sporophyte, so that the gametophyte and sporophyte both have the unreduced number of chromosomes. The sperm and egg cells produced by such a gametophyte have twice the original number of chromosomes. In apogamy, the sporophyte develops directly from special cells on the gametophyte, so that the sporophyte and gametophyte have the same reduced number of chromosomes. Apogamy typically occurs in gametophytes which themselves have arisen by apospory.


Evolution

Most botanists believe that the ferns and fern allies are descendants of the Rhyniopsida, an extinct group of free-sporing plants which originated in the Silurian period (about 430 million years ago) and went extinct in the mid-Devonian period (about 370 million years ago). The Rhyniopsida were primitive vascular plants which were photosynthetic, had branched stems, and produced sporangia at their stem tips, but had no leaves or roots.

The Cladoxylales is a group of plants known colloquially as the "pre-ferns." They also evolved from the Rhyniopsida, but went extinct in the lower Carboniferous period (about 340 million years ago). Some botanists previously considered these as ancestors of the ferns, because they had leaves somewhat similar to fern fronds. However, most botanists now believe the evolutionary line which led to the Cladoxylales went extinct, and that the modern ferns evolved from a separate lineage of the Rhyniopsida.

As a group, the ferns were the first plants to have megaphylls. A megaphyll is a leaf with a complex system of branched veins. Many botanists believe that the ferns evolved megaphylls by developing a flattened and webbed version of the simple, three-dimensional branching system of the Rhyniopsida. The evolution of the megaphyll was a major event in plant evolution, and nearly all ecologically dominant plants in the modern world have megaphylls.


Modern ferns

There are two evolutionarily distinct groups of modern ferns, the leptosporangiates and the eusporangiates.

In the leptosporangiates, the sporangium develops from one cell and is usually only one cell thick. In the eusporangiates, the sporangium develops from several cells and is usually several cells thick. Most botanists believe that the leptosporangiate and eusporangiate ferns separated evolutionarily in the lower Carboniferous (about 340 million years ago) or earlier. Modern leptosporangiate ferns are often placed into the Filicales class, and eusporangiate ferns into the Marattiales or Ophioglossales classes.

While there is general agreement about the natural division between the leptosporangiate and eusporangiate ferns, there is considerable uncertainty about other relationships among the modern ferns. Thus, there have been many proposed classification schemes. The widespread occurrence of polyploidy (see above) and hybridization (see below) in ferns has complicated the determination of evolutionary relationships.


Hybridization

Many species of ferns form hybrids in nature and hybridization is believed to have had a major role in fern evolution. A hybrid species is the offspring of a sexual union between two different species. Most hybrids cannot engage in sexual reproduction because they lack homologous (corresponding) chromosomes, which typically pair off during production of sperm and egg cells. However, since many fern species can engage in apogamy and apospory, fern hybrids can often reproduce and proliferate.

A hybrid species is often identified by the number of chromosomes in its cells and by the presence of aborted spores. Chromosome number is often used to infer evolutionary relationships of hybrid ferns. The ferns, as a group, tend to have very high chromosome numbers due to the widespread occurrence of polyploidy. One fern species, Ophioglossum reticulatum, has 631 chromosomes, the largest number of any organism .


Psilotum and Tmesipteris

Lastly, the evolutionary status of two additional genera of modern plants must be considered: Psilotum and Tmesipteris. These free-sporing tropical and subtropical plants have very simple morphologies. In particular, species in the genus Psilotum superficially resemble plants of the Rhyniopsida in that their sporophytes consist of three-dimensional branched stems, with tiny scale-like appendages believed to be leaf derivatives. Moreover, like the Rhyniopsida, Psilotum does not have true roots. Thus, some botanists have suggested that Psilotum is a direct descendant of the Rhyniopsida. Others reject this hypothesis and point to the lack of a fossil record connecting these two groups. They suggest that Psilotum and Tmesipteris may have evolved by evolutionary simplification of an ancestor of the modern fern genus, Stromatopteris. Clearly, further research is needed to resolve the relationships of these fascinating, fern-like plants.


Importance to humans

In general, ferns are of minor economic importance to humans. However, ferns are popular horticultural plants and many species are grown in ornamental gardens or indoors.

Most people can recognize ferns as understory or groundcover plants in woodland habitats. However, several hundred million years ago ferns and fern allies were the dominant terrestrial plants. Thus, the fossils of these plants have contributed greatly to the formation of our fossil fuels—coal, oil and natural gas .

Various non-western cultures have used the starchrich rhizome and stems of some fern species as a food. Westerners generally eschew ferns as a food. However, those who frequent restaurants known for their haut cuisine will occasionally find croziers or fiddleheads (unfurled fern leaves, see above) of the ostrich fern (Matteuccia struthiopteris) served in salads, around a bowl of ice cream, or as a steamed vegetable.

Herbalists have advocated some fern species for treatment of ulcers , rheumatism, intestinal infections, and various other ailments. Although many modern pharmaceuticals are derived from chemicals produced by plants, there is little scientific evidence that ferns are useful as treatments for these or other ailments.

See also Maidenhair fern; Seed ferns.


Resources

books

Cobb, B. A Field Guide to Ferns and Their Related Families: Northeastern and Central North America. New York: Houghton Mifflin Company, 1975.

Jones, D. Encyclopedia of Ferns. vol. 1. Portland, OR: Timber Press, 1987.

Margulis, L., and K.V. Schwartz. Five Kingdoms. New York: W. H. Freeman and Company, 1988.

McHugh, A. The Cultivation of Ferns. North Pomfret, VT: Trafalgar Press, 1992.


Peter A. Ensminger

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Apogamy

—Development of a sporophyte directly from the gametophyte without fusion of sex cells.

Apospory

—Development of a gametophyte directly from the sporophyte without sex cell production.

Epiphyte

—A plant which relies upon another plant, such as a tree, for physical support, but does not harm the host plant.

Flagellum

—Thread-like appendage of certain cells, such as sperm cells, which controls their locomotion.

Megaphyll

—Leaf with a complex system of branched veins, typical of ferns and seed plants.

Microphyll

—Scale-like leaf with a single midvein, typical of fern allies.

Prothallus

—Gametophyte phase of ferns and fern allies, rarely seen in nature.

Rhizome

—This is a modified stem that grows horizontally in the soil and from which roots and upward-growing shoots develop at the stem nodes.

Sorus

—Group of many sporangia, which often appear as a brown spot on the margin or underside of a fern frond.

Sporangium

—Structure that produces spores.

Spore

—Small reproductive cell that develops into a gametophyte.

Symbiosis

—A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.

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Ferns

Ferns, like the more familiar seed plants, have stems, roots, and large, highly veined leaves. Ferns do not reproduce by seeds, however, and have several other distinctive features. The leaf of a fern is called a frond and, in many species, the green blade is divided into segments called pinnae. The leaves of most ferns have a distinctive juvenile stage called a fiddlehead, where all the segments are curled in a manner resembling the end of a violin's neck. Most ferns have underground stems called rhizomes and the only parts of the fern plant visible above ground are the leaves. Some tropical ferns, called tree ferns, have erect, unbranched stems up to 20 meters tall with all of the fronds arising from the tip. Ferns are perennial plants and some may grow for many years, but, as they lack annual growth rings, their age is not easily determined. However, in 1993, researchers using molecular genetic markers found some individual bracken fern (Pteridium aquilinum ) plants more than 1 kilometer across. Researchers estimated that these ferns took more than 1,180 years to grow to this size, possibly putting them among the oldest living plants on Earth.

Ferns and seed plants are similar in having two kinds of plants present in their reproductive life cycle, but overall ferns reproduce very differently than seed plants. The familiar fern plant, described in the preceding paragraph, is the sporophyte (spore-bearing phase). Fern fronds bear organs known as sporangia. Inside each sporangium certain cells undergo reduction division, or meiosis, which yields haploid spores that have one set of genes for the fern. All of the cells in the sporophyte fern plant itself are diploid, having two sets of genes. The sporangia of most ferns are very small, scalelike, and contain only sixty-four spores, but some ferns have large sporangia containing hundreds of spores. A typical fern sporophyte plant may produce up to one billion spores per year. When the sporangia open, the spores are shed into the air and dispersed. While most fern spores land within one hundred meters of the fern producing them, some may be spread very far. Fern spores have been recovered from the upper atmosphere in samples collected by airplanes and weather balloons.

When the single-celled fern spore lands on a suitable substrate , it may undergo mitotic cell division and develop into a very different kind of plant. The plant that grows from a fern spore, called the prothallus, is barely visible to the naked eye. It resembles a tiny, heart-shaped ribbon and lacks any stems, roots, leaves, or internal food- or water-conducting tissues. Reproductively, this small, independent fern plant is critical because it bears the sex organs. Although the basics of sexual reproduction in ferns were discovered in the nineteenth century, many crucial details are still being clarified. A single fern gametophyte may produce both sperm-bearing sex organs, called antheridia, and egg-bearing sex organs, known as archegonia, but frequently an individual gametophyte has only one type of sex organ. Fertilization occurs when a sperm swims to unite with an egg to form a diploid zygote, which then develops into the sporophyte.

Whether the individual gametophyte plants in a population are bisexual or unisexual is very important because it is basic to determining the degree of genetic variation possible in the sporophyte generation produced. A single bisexual gametophyte can fertilize its own eggs and produce a new sporophyte plant, but such a sporophyte would be highly inbred because both the sperm and egg producing it would be genetically identical. Most ferns control the sexual expression of the individual plants in a gametophyte population so that each plant is either male or female. Thus, fertilization usually requires two gametophytes that are close enough for sperm to swim in water between them. Receptive archegonia secrete a sperm attractant to help the sperm find its way. When genetic material from two different gametophytes is mixed in the zygote, the sporophyte that develops has more genetic variation than one arising from a single gametophyte. If the entire fern sporophyte population is reproduced this way, it may be more likely to survive because some of its members may have inherited the traits needed to endure unforeseen changes in its environment. On the other hand, a distinct survival advantage arises when a single fern spore, dispersed a long distance, can produce a sporophyte from one gametophyte, because this permits rapid colonization of distant, favorable habitats.

Although factors regulating fern spore germination and development are fairly well known from laboratory studies, relatively little is known about how ferns actually reproduce in their environments. Most fern spores germinate readily on moist soil. Germination often requires red light that is absorbed by a pigment in the spore. Calcium ions are important to germination, and red and blue light control the pattern of gametophyte development. Fern spores are known to persist in the soil, forming spore banks. These factors and many more interact in complex ways in the field. Ecologically, most fern species are found in habitats where moisture is readily available, permitting gametophytes to grow and sperm to swim to eggs in water. Those concerned with preserving a rare fern species at a site must understand that if the locality does not provide safe sites for the independent gametophytes, with their distinct ecological requirements, the species cannot reproduce and the sporophytes will eventually die off.

Moist, tropical mountain forest communities contain the largest number of fern species. Of the approximately 12,000 fern species worldwide, about 75 percent are tropical. The flora of North America, north of Mexico, contains about 350 fern species, whereas southern Mexico and Central America have about 900. A few ferns are occasionally eaten. However, some, like bracken fern, contain poisons or carcinogens. Ferns are present in most plant communities but dominant in few.

see also Epiphytes; Seedless Vascular Plants.

James C. Parks

Bibliography

Burnie, David. How Nature Works. Pleasantville, NY: Reader's Digest, 1991.

Jones, David. Encyclopaedia of Ferns. Melbourne: Lothian Publishing Co., Ltd., 1987.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.

Tryon, Rolla, and Alice Tryon. Ferns and Allied Plants. New York: Springer-Verlag,1982.

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fern Non-flowering plant. Ferns grow mainly in warm, moist areas; there are c.10,000 species. The best-known genus Pteridium (bracken) grows on moorland and in open woodland. Ferns are characterized by two generations: the conspicuous sporophyte, which possesses leafy fronds, stems, rhizomes and roots, and reproduces by minute spores usually clustered on the leaves; and the inconspicuous gametophyte, which resembles moss and produces sperm and ova. Phylum Filicinophyta. See also alternation of generations

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fern / fərn/ • n. (pl. same or ferns ) a flowerless plant (class Filicopsida, division Pteridophyta) that has feathery or leafy fronds and reproduces by spores released from the undersides of the fronds. DERIVATIVES: fern·y adj.

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Fern

Many occult beliefs have adhered to the common fern. In ancient times the fern was thought not to have seed. Later on, people thought that the seed was invisible, and if a man could find this invisible seed, it would confer the power of invisibility upon him. The fern was also believed to flower at midnight on St. John's Eve, one of the more magical days of the year in medieval Europe. Legend said that anyone who gained possession of the flower would be protected from all evil influences and would obtain a revelation of hidden treasure.

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fernadjourn, astern, Berne, burn, churn, concern, discern, earn, fern, fohn, kern, learn, Lucerne, quern, Sauternes, spurn, stern, Sterne, tern, terne, Traherne, turn, urn, Verne, yearn •Bayern • Blackburn • heartburn •Hepburn • Raeburn • Swinburne •Gisborne, Lisburn •sideburn • sunburn • Bannockburn •lady-fern • Vättern • extern •cittern, gittern •Comintern • taciturn •nocturn, nocturne •U-turn • upturn

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fern OE. fearn = MDu. væren (Du. varen), OHG. (G.) farn :- WGmc. *farna :- IE. *porno-, whence Skr. parṇá- wing, feather, leaf; rel. further to Lith. papártis, Russ. páporotnik, (O)Ir. raith (:- *pratis). The prim. meaning is doubtless ‘feathery leaf’; cf. also Gr. pterón feather, pteris fern.

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ferns See PTEROPSIDA.

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ferns See Filicinophyta.

NEARBY TERMS

fern