A fern is any one or more of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem (making them vascular plants). They have stems, leaves, and roots like other vascular plants. Ferns reproduce via spores and have neither seeds nor flowers.
By far the largest group of ferns is the leptosporangiate ferns, but ferns as defined here (also called monilophytes) include horsetails, whisk ferns, marattioid ferns, and ophioglossoid ferns. The term pteridophyte also refers to ferns and a few other seedless vascular plants (see classification section below).
Ferns first appear in the fossil record 360 million years ago in the Carboniferous 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).
Ferns are not of major economic importance, but some are grown or gathered for food, as ornamental plants, for remediating contaminated soils, and have been the subject of research for their ability to remove some chemical pollutants from the air. Some are significant weeds. They also play a role in mythology, medicine, and art.
Ferns are vascular plants differing from lycophytes by having true leaves (megaphylls), which are often pinnate. They differ from seed plants (gymnosperms and angiosperms) in their mode of reproduction—lacking flowers and seeds. Like all other vascular plants, they have a life cycle referred to as alternation of generations, characterized by alternating diploid sporophytic and haploid gametophytic phases. The diploid sporophyte has 2n paired chromosomes, where n varies from species to species. The haploid gametophyte has n unpaired chromosomes, i.e. half the number of the sporophyte. The gametophyte of ferns is a free-living organism, whereas the gametophyte of the gymnosperms and angiosperms is dependent on the sporophyte.