Polypodiophyta
| Ferns Temporal range: Late Devonian—Recent |
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| A fern unrolling a young frond | |
| Scientific classification | |
| Kingdom: | Plantae |
| Subkingdom: | Embryophyta |
| (unranked): | Monilophytes or Pteridophytes |
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A fern is a member of a group of about 10,560 known extant species 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. They have branched stems and leaves like other vascular plants. These are "megaphylls", which are more complex than the simple "microphylls" of clubmosses. Most ferns are Leptosporangiate ferns, sometimes denominated the "true ferns": they produce what are called "fiddleheads" that uncoil and expand into fronds.
"Ferns" as defined herein are the "ferns sensu lato", being all of the "Monilophytes". The Monilophytes comprise both the "Leptosporangiate Ferns" 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. The Pteridophytes traditionally denominate all seedless vascular plants, of which the Monilophytes predominate (see "Classification and Evolution" below), although some recent authors have used it to refer strictly to the Monilophytes alone.
Ferns first appear in the fossil record 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.
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