In plants and algae that undergo alternation of generations, a gametophyte is the multicellular structure, or phase, that is haploid, containing a single set of chromosomes:
The gametophyte produces male or female gametes (or both), by a process of cell division called mitosis. The fusion of male and female gametes produces a diploid zygote which develops by repeated mitotic cell divisions into a multicellular sporophyte. Because the sporophyte is the product of the fusion of two haploid gametes, its cells are diploid, containing two sets of chromosomes. The mature sporophyte produces spores by a process called meiosis, sometimes referred to as "reduction division" because the chromosome pairs are separated once again to form single sets. The spores are therefore once again haploid and develop into a haploid gametophyte.
In mosses, liverworts and hornworts (bryophytes), the gametophyte is the commonly known phase of the plant. An early developmental stage in the gametophyte of mosses (immediately following germination of the meiospore) is called the protonema.
In most other land plants the gametophyte is very small (as in ferns and their relatives) or even reduced as in flowering plants (angiosperms), where the female gametophyte (ovule) is known as a megagametophyte and the male gametophyte (pollen) is called a microgametophyte.
In some multicellular green algae, red algae, or brown algae (Ulva is one example), the sporophytes and gametophytes are often isomorphic, but in some species the gametophyte may be reduced.