Fepao

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24.10.2012

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No information given.A fern is a member of a group of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having certain tissue that conducts water and nutrients, and having branched stems. Like other vascular plants, ferns have leaves, and these are "megaphylls", which are more complex than the "microphylls" of clubmosses. Most ferns are leptosporangiate ferns, sometimes termed "true ferns"; they produce what are called "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 itself comprising "ferns" other than those denominated "true ferns": horsetails (including scouring rushes), whisk ferns, marattioid ferns, and ophioglossoid ferns.

Ferns first appear in the fossil record about 360 million years ago in the late Devonian period, but many of the current families and species did not appear until roughly 145 million years ago in the early Cretaceous, after flowering plants came to dominate many environments. The fern Osmunda claytoniana is a paramount example of evolutionary stasis; paleontological evidence indicates it has remained unchanged, even at the level of fossilized nuclei and chromosomes, for at least 180 million years.

Ferns are not of major economic importance, but some are used for food, medicine or as ornamentals, and for remediating contaminated soil. They have been the subject of research for their ability to remove some chemical pollutants from the atmosphere.
Like the sporophytes of seed plants, those of ferns consist of stems, leaves and roots.

Stems: Fern stems are often referred to as "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). 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).No information given.No information given.