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A gametophyte is the haploid multicellular stage in the alternation of generations during the life cycle of land plants and algae. It is produced from mitotic cell division of spores, which are produced by meiosis in sporophytes. Gametophytes produce haploid male or female gametes (or both), by mitosis.
The female and male gametes are also called, respectively, egg cells and sperm cells. The fusion of male and female gametes produces a diploid zygote, which develops by repeated mitotic cell divisions into a multicellular sporophyte. Because sporophytes are the product of the fusion of two haploid gametes, these sporophyte cells are normally diploid, containing two sets of chromosomes. The mature sporophyte produces spores by a process of cell division called meiosis, or reduction division, in which the chromosome pairs are separated once again to form single sets. The spores are therefore once again haploid and develop into haploid gametophytes.
In bryophytes (mosses, liverworts and hornworts), the gametophyte is the dominant form and thus the most familiar phase of the life cycle. The sporophytes are attached to and dependent on the long-lived gametophytes. Moss gametophytes originate from the germination of a spore. The initial phase of growth leads to a filament of cells (called the protonema). The mature gametophyte of mosses produces the gamete-producing structures, archegonia and antheridia. In gametophyte dominant organisms, such as bryophytes, specialized structures or even whole individual plants that produce gametes are sometimes called gametophores.
In other living land plants (the vascular plants), an interesting pattern exists with regard to gametophytes. Homosporous vascular plants have exosporic gametophytes—that is, the gametophyte develops outside of the spore wall. In addition, these exosporic gametophytes are normally bisexual (produce both sperm and eggs). In heterosporous vascular plants, the gametophyte develops endosporically—inside the wall of the spore. These gametophytes are unisex, producing either sperm or eggs but not both. All vascular plants are sporophyte dominant, and a trend toward smaller and shorter-lived gametophytes is evident as land plants became increasingly adapted to a terrestrial environment.
In modern clubmosses the gametophyte is subterranean and mycotrophic, deriving its nutrients from symbiotic fungi. In lycophytes which are heterosporous,[which?] the megagametophyte develops inside the spore, which cracks open to allow access to internal archegonia.
In many ferns the gametophyte is a photosynthetic free living organism called a prothallus. However in some groups, notably Ophioglossaceae and Psilotum, the gametophytes are subterranean and subsist by forming mycotrophic relationships with fungi. In leptosporangiate ferns such as Dryopteris the gametophyte is a free-living, autotrophic prothallus that maintains the sporophyte during its early multicellular development. By contrast, in seed plants (gymnosperms and angiosperms) the gametophytes are nurtured by the sporophyte during development and become multicellular while still enclosed within the sporangium.
In heterosporous plants (water ferns, some lycophytes, gymnosperms and angiosperms) the egg producing gametophyte is known as a megagametophyte and the sperm producing gametophyte as a microgametophyte. In the seed plants, the microgametophyte is called pollen. Seed plant microgametophytes consists of two or three cells when the pollen grains exit the sporangium. The megagametophyte develops within the megaspore of extant seedless vascular plants and within the megasporangium in a cone or flower in seed plants. In seed plants, the microgametophyte (pollen grain) travels to the vicinity of the egg cell (carried by a physical or animal vector), and produces two sperm by mitosis.
In gymnosperms the megagametophyte consists of several thousand cells and produces one to several archegonia, each with a single egg cell. The gametophyte becomes a food storage tissue in the seed.
In angiosperms, the megagametophyte is reduced to only a few nuclei and cells, and is sometimes called the embryo sac. A typical flowering plant ovule contains seven cells and eight nuclei, one of which is the egg cell. Two nuclei fuse with a sperm nucleus form endosperm, which becomes the food storage tissue in the seed.
In some multicellular green algae, red algae, or brown algae (Ulva lactuca is one example) sporophytes and gametophytes may be externally indistinguishable (isomorphic) or either the sporophyte or the gametophyte may be reduced (heteromorphic).
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