Gametophyte

A gametophyte is a stage in the life cycle of plants and algae that undergo alternation of generations. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the sexual phase in the life cycle of plants and algae. It develops sex organs that produce gametes, haploid sex cells that participate in fertilization to form a diploid zygote in which each cell has two sets of chromosomes. Cell division of the zygote results in a new diploid multicellular organism, the second stage in the life cycle known as the sporophyte, the function of which is to produce haploid spores by meiosis.

In some multicellular green algae (Ulva lactuca is one example), red algae and brown algae sporophytes and gametophytes may be externally indistinguishable (isomorphic). In Ulva the gametes are isogamous, all of one size, shape and general morphology.

In land plants, anisogamy is universal. As in animals, female and male gametes are called, respectively, eggs and sperm. Either the sporophyte or the gametophyte may be reduced (heteromorphic).

In bryophytes (mosses, liverworts, and hornworts), the gametophyte is the most visible stage of the life cycle. The bryophyte gametophyte is longer lived, nutritionally independent, and the sporophytes are typically attached to the gametophytes and dependent on them. When a moss spore germinates it grows to produce a filament of cells (called the protonema). The mature gametophyte of mosses develops into leafy shoots that produce sex organs (gametangia) that produce gametes. Eggs develop in archegonia and sperm in antheridia.

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