The regular alternation of forms or of mode of reproduction in the life cycle of an organism, especially the alternation between sexual and asexual reproductive phases in plants and some invertebrates. In plants, the alternation involves alternating generations of haploid and diploid organisms. Often one of these generations is the dominant form of the organism, and the other generation is nutritionally dependent upon it or just grows as a smaller plant. For example, in mosses and liverworts, the haploid phase is the large, familiar form of the plant. The diploid phase is smaller and grows upon the haploid phase. In angiosperms, however, the diploid phase of the organism is large and independent, while the haploid phase is reduced to the pollen grain and the eight-celled female gametophyte located in the ovule.
A Closer Look The life cycle of fern species provides a good example of the differing roles played by the gametophyte and sporophyte in organisms that display an alternation of generations. The familiar large frond-bearing fern plant is the sporophyte generation of the fern. By meiosis it produces haploid spores that are dispersed and develop into gametophytes. Fern gametophytes are inconspicuous matlike plants that can make their own food by photosynthesis. The gametophytes produce both sperm and eggs. Sperm from another gametophyte reaches one of these eggs and fuses with it to form an embryo, which then grows out of the gametophyte as a new sporophyte plant. In many nonvascular plants, such as the mosses and liverworts, the sporophyte is a relatively small plant that grows in or on top of the gametophyte, which is larger. In gymnosperms and angiosperms, however, the sporophyte is the main plant form, and the gametophyte is dependent on the sporophyte.