From Academic Kids
A gametophyte is the haploid structure or phase of life of a sexually reproducing plant. Each cell of a gametophyte contains one complete set of chromosomes. The gametophyte can be the dominant part of the plant's life cycle as in mosses, or very reduced as in ferns and flowering plants (angiosperms), where the female form (ovule) is known as a megagametophyte and the male form (pollen) is called a microgametophyte.
An early developmental stage in the gametophyte of both mosses and ferns (immediately following the meiospore) is called the Protonema.