During the alternation of generations, gametophytes are the sexual phase of the life cycle. They are typically haploid, meaning they only contain half the number of chromosomes as the sporophyte generation, which is diploid. The main function of a gametophyte is to produce gametes, which are reproductive cells.
In many plants, the gametophytes are small, inconspicuous, and photosynthetically inactive. However, in some plants, the gametophytes are larger, free-living, and independent, even dominant, and carry out photosynthesis for the synthesis of their own food requirements, just like sporophytes. These autotrophic gametophytes occur in certain bryophytes, pteridophytes, and gymnosperms.
In the bryophyte life cycle, for example, the gametophyte is the dominant phase. The sporophyte, which develops from the zygote produced by the gametophytes, is parasitic on the gametophyte. This is in contrast to the life cycles of most other plants, in which the sporophyte generation is dominant.