Dichotomous Key/Polypodiopsida

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 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.