Alternation of generations is common in plants, particularly in the lower plants, including mosses, ferns, and gymnosperms. Although sexual and asexual phases are part of it, what really alternates is the haploid and diploid condition. For instance, let's consider a fern life cycle:
Here the fern plant that you usually see is the diploid phase, meaning that each nucleus has 2 sets of DNA. The fern creates haploid spores, which contain only one set of DNA. Each spore grows into a prothallus, which is a small haploid fern plant. The prothallus develops sexual structures which contain gametes. The sperm swims to the egg, and a diploid zygote is created in the female structure of the prothallus. This diploid zygote then grows up into a new diploid fern plant and the cycle begins again.
By this method alternation of generations creates two separate, free-living entities, one haploid and one diploid, which are both part of the life cycle of a single plant.
Alternation of generation used to describe the life cycle of plants.a sporophyte which is diploid(2n) alternates with gametophyte which is haploid(n).A mature sporophyte produces spores by meiosis which germinate and grow into a gametophyte.Gametophyte produces gametes by mitosis.when two gametes fuse to produce a zygote which develop into a diploid sporophyte.this cycle is the way in which all land plants,many algae,fern and gymnosperm are involved.the all process undergo sexual reproduction.