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| image = Athyrium filix-femina.jpg | image_width = 240px | image_caption = A fern unrolling a young frond | regnum = Plantae | subregnum = Embryophyta | unranked_divisio = Monilophytes or pteridophytes | subdivision_ranks = Classes〔 | subdivision = * †Cladoxylopsida * Psilotopsida * Equisetopsida (''alias'' Sphenopsida) * Marattiopsida * Polypodiopsida (''alias'' Pteridopsida, Filicopsida) * †Zygopteridales * †Stauropteridales * †Rhacophytales | synonyms = *Monilophyta *Polypodiophyta *Filices *Filicophyta }} A fern is a member of a group of approximately 12,000 species of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular (i.e. having water-conducting vessels). They have stems and leaves, like other vascular plants. Most ferns have what are called fiddleheads that expand into fronds, which are each delicately divided. Leptosporangiate ferns (sometimes called true ferns) are by far the largest group, but ferns as defined here (ferns ''sensu lato'') include horsetails, whisk ferns, marattioid ferns, and ophioglossoid ferns. This group may be referred to as monilophytes. The term pteridophyte traditionally refers to ferns plus a few other seedless vascular plants (see the classification section below), although some recent authors have used the term to refer strictly to the monilophytes. Ferns first appear in the fossil record 360 million years ago in the late Devonian period〔(【引用サイトリンク】title=Pteridopsida: Fossil Record )〕 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 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. ==Description== ===Life cycle=== 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 2''n'' 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. Life cycle of a typical fern: ### # A diploid sporophyte phase produces haploid spores by meiosis (a process of cell division which reduces the number of chromosomes by a half). # A spore grows into a haploid gametophyte by mitosis (a process of cell division which maintains the number of chromosomes). The gametophyte typically consists of a photosynthetic prothallus. # The gametophyte produces gametes (often both sperm and eggs on the same prothallus) by mitosis. # A mobile, flagellate sperm fertilizes an egg that remains attached to the prothallus. # The fertilized egg is now a diploid zygote and grows by mitosis into a diploid sporophyte (the typical "fern" plant). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fern」の詳細全文を読む スポンサード リンク
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