Alternation of generations is a process that allows each generation to give rise to the other generation. Alternation of generation typically occurs in algae that are closely related to the land plants.
Alternation of generation is characterized by a cycle that includes both multicellular haploid and diploid organisms. The haploid organism is known as gametophytes, or gamete producing plant. Gametophytes are produced through the mitosis of eggs and sperm (haploid gametes). The gametes would undergo fertilization, and form diploid zygotes (2n). Through mitotic division of the zygote, a diploid known as sporophytes, or spore producing plants, is formed. Through meiosis, the sporophyte would product spores, which act as the sporophytes' reproductive cells that would develop into a new haploid organism. Through mitotic division of one of the spores, a gametophyte is formed and the cycle would resume.
Mitosis and meiosis are not used for the same thing. Meiosis in plants are used to produce spores to initiate the gametophyte generation while mitosis is to turn the gametes into sporophytes.
http://kvhs.nbed.nb.ca/gallant/biology/alternation_generations.jpg
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