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A pteridophyte sensu lato refers to a sub group of Seedless Vascular Plants. Pteridopyhtes reproduce via spores rather than seeds, and typically represent "ferns and related plants". Pteridophyte was a member of the former and now invalid taxon denominated Pteridophyta; the denomination sensu stricto presently only has informal use to denominate a monilophyte (fern) or lycophyte, or simply and only a monilophyte. Because pteridophytes produce neither flowers or seeds, they are also denominated "cryptogams". The pteridophytes include the ferns, horsetails, and the lycophytes (clubmosses, spikemosses, and quillworts). These are not a monophyletic group because ferns and horsetails are more closely related to seed plants than to the lycophytes. Therefore, "Pteridophyta" is now an invalid taxon. Similar to many plants, pteridophytes are know to form Mycorrhiza relationships with fungi on their root structures. Multiple historical studies have identified that development of Mychorrhizol relationships in currently extinct pteridophytes played a key role in the development of fungal and plant symbioses during periods of plant land invasion 460-480 million years ago.

Classification

Pteridophytes consist of two separate classes:

• Lycopodiopsida
  • Lycopodiidae (clubmosses)
  • Selaginellidae (spikemosses and quillworts)
• Polypodiopsida
  • Psilotidae: Psilotales (whisk ferns and Ophioglossales, e. g., grape ferns)
  • Equisetidae (horsetails)
  • Marattiidae (marattioid ferns)
  • Polypodiidae (leptosporangiate ferns, the group with the most species)

In addition to these living groups, several groups that are now extinct and known only from fossils are considered pteridophytes. These groups include: Early fern-like plants such as Rhacopytales, Stauropteridales, and Zygopteridales, True ferns including eusporangiates and leptosporangiates, and a similar extant group referred to as progymnosperms.

Modern studies of the land plants agree that all pteridophytes share a common ancestor, which is also the ancestor of seed plants. Therefore, pteridophytes do not form a clade but a paraphyletic group.

Morphology

Ferns

Pteridophytes grow in a large array of sizes, from the smallest floating ferns to giant ferns the size of trees. The largest fern, Cyathea brownii, known as the Norfolk tree fern, can grow to be 20 meters or more in height.

Cyathea brownii

The smallest pteridophyte is Azolla, commonly known as mosquito fern, and floats on top of water forming a mat-like appearance.

The leaves of pteridophytes include a transition betweens simple and complex leaves. Some groups have simple microphyllous leaves, which have a single unbranched vein. Larger ferns have large, complex macrophyllus leaves that use extensive veins.

Horsetails

Horsetails, another genus under pteridophyte, are herbaceous perennials, which consist of upright aerial stems that connect to an extensive underground rhizome. Horsetails have an external appearance that is similar to bamboo, though these stems are anatomically unique among plants. These aerial stems have one source of growth and exhibit a monopodial branching pattern, meaning they have one main stem. The leaves of horsetails alternate with branches at each node.

Lycophytes

Club Mosses

Club Moss

The name club mosses refers to the fact that these lycophytes look similar to true mosses. These plants are evergreen herbs with needlelike leaves and clusters of small leaves with a spore at its base. Club mosses are differentiated into root, stem, and leaves. The photosynthetic organs resemble true leaves but are actually microphylls. The spores are produced on either fertile single leaves or club-like tightly bound modified leaves.

Spike Mosses

Spike mosses are mostly forest plants, with some growing on trees, and others thrive in dry areas. They have scale-like leaves that are either in spirals or groups of four. These spore-bearing leaves are organized in spikes, or strobili.

Quillworts

Quillworts contain grassy, spiked leaves that are spirally arranged. These leaves are divided into vertical rows that are connected to one central strand.

Distribution

Pteridophytes are a large and ancient species found all over the world, excepting Antartica and most islands. They favor moist and temperate climates, yet they can be found most other places that are not too hot or dry. The greatest diversity of pteridophyte species is found in tropics because of the wet conditions present. There are about eleven thousand species of pteridophytes found in the world, and only around six hundred species that live in more temperate climates. Due to this large number of pteridophytes species, they are the second most diverse group of land plants outnumbered only by the variety of flowering plant species.

Life cycle

Pteridophyte life cycle

Just as with seed plants and mosses, the life cycle of pteridophytes involves alternation of generations. The dominant, diploid sporophyte generation produces spores through meiosis, while the haploid and independent gametophyte, or prothallus generation forms gametes via mitosis.

Pteridophytes differ from mosses and seed plants in that both generations are independent and free-living, although the sporophyte is generally much larger and more conspicuous, and what is usually thought of as the 'plant'. The sexuality of pteridophyte gametophytes can be classified as follows:

• Dioicous: each individual gametophyte is either male (producing antheridia and hence sperm) or female (producing archegonia and hence egg cells).
• Monoicous: each individual gametophyte produces both antheridia and archegonia and can function both as a male and as a female.

  Protandrous: the antheridia mature before the archegonia (male first, then female).

  Protogynous: the archegonia mature before the antheridia (female first, then male).

These terms are not the same as monoecious and dioecious, which refer to whether a seed plant's sporophyte bears both male and female gametophytes, i. e., produces both pollen and seeds, or just one of the sexes.

Uses

While there are no pteridophytes cultivated as crop plants for use, there are some other uses commercially. Fern leaves can be used for decoration, such as in the floral arrangement industry as well as some cultures using tree fern stems to make bowls.

Historically, photographers used masses of club moss spores as flash powder, since it could be quickly and easily ignited. Club moss spores were also used to coat rubber gloves and prophylactics.

See also

• Embryophyte
• Fern ally
• Plant sexuality

References

• Gifford, Ernest M. & Foster, Adriance S. (1988). Morphology and Evolution of Vascular Plants, (3rd ed.). New York: W. H. Freeman and Company. ISBN 0-7167-1946-0.