Gametophyten

Gametophyten, or the gametophyte, is the haploid multicellular phase in the life cycles of plants and many algae that undergo alternation of generations. It develops from a haploid spore produced by meiosis in the diploid sporophyte and, through mitotic divisions, forms a body capable of producing gametes. Fusion of two gametes during fertilization yields a diploid zygote, which grows into a new sporophyte, continuing the cycle.

Across plant groups, the prominence and nature of the gametophyte vary. In bryophytes (mosses, liverworts, hornworts) the gametophyte is the dominant, free-living stage, while the sporophyte is typically smaller and nutritionally dependent on the gametophyte. In ferns and their allies, the gametophyte is usually a small, independent photosynthetic organism called a prothallus, though the conspicuous plant is the sporophyte. In seed plants (gymnosperms and angiosperms), the gametophytes are highly reduced and dependent on the sporophyte: the male gametophyte is the pollen grain, and the female gametophyte develops within the ovule (the embryo sac in flowering plants). These reduced gametophytes produce sperm and eggs by mitosis, enabling fertilization without a free-standing water medium in many cases.

Gametophytes therefore represent one half of the plant life cycle’s alternating generations, a stage in which the genetic material is haploid and the gametes are produced, followed by fertilization that restores diploidy in the resulting sporophyte. The relative size, independence, and visibility of the gametophyte vary widely among plant groups.