A gametophyte is a haploid multicellular adult stage in the alternation of generations during the life cycle of land plants and algae. It produces haploid gametes. It is produced from mitotic cell division of spores, which are produced by meiosis in sporophytes.
Gametophytes produce 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 called meiosis, 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, the megagametophyte develops inside the spore, which cracks open to allow access to internal archegonia.