A fern is any one of a group of about 20,000 species of plants classified in the phylum or division Pteridophyta, also known as Filicophyta. The group is also referred to as polypodiophyta, or polypodiopsida when treated as a subdivision of tracheophyta (vascular plants). The study of ferns is called pteridology. The term pteridophytes has traditionally been used to describe all seedless vascular plants so is synonymous with "ferns and fern allies". This can be confusing given that the fern phylum Pteridophyta is also sometimes referred to as pteridophytes.

A fern is a vascular plant that differs from the more primitive lycophytes in having true leaves (megaphylls), and from the more advanced seed plants (gymnosperms and angiosperms) in lacking seeds. Like all vascular plants, it has a life cycle, often referred to as alternation of generations, characterised by a diploid sporophytic and a haploid gametophytic phase. Unlike the gymnosperms and angiosperms, in ferns the gametophyte is a free-living organism. The life cycle of a typical fern is as follows:

1. A sporophyte (diploid) phase produces haploid spores by meiosis;
2. A spore grows by cell division into a gametophyte, which typically consists of a photosynthetic prothallus
3. The gametophyte produces gametes (often both sperm and eggs on the same prothallus) by mitosis
4. A mobile, flagellate sperm fertilizes an egg that remains attached to the prothallus
5. The fertilized egg is now a diploid zygote and grows by mitosis into a sporophyte (the typical "fern" plant).

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