The Alternation of Generations in Gametophytes and Sporophytes

Alternation of generations is a reproductive cycle of certain plants, fungi, and protists. The term is a bit confusing for people familiar only with the life cycle of a typical animal. A more understandable name would be "alternation of phases of a single generation" because we usually consider a generation of a species to encompass one complete life cycle. The life cycle of organisms with "alternation of generations" is characterized by each phase consisting of one of two separate, free-living organisms: a gametophyte (thallus (tissue) or plant), which is genetically haploid, and a sporophyte (thallus or plant), which is genetically diploid. A haploid plant of the gametophyte generation produces gametes by mitosis. Two gametes (originating from different organisms of the same species or from the same organism) combine to produce a zygote, which develops into a diploid plant of the sporophyte generation. This sporophyte produces spores by meiosis, which germinate and develop into a gametophyte of the next generation. This cycle, from gametophyte to gametophyte, is the way in which plants and many algae undergo sexual reproduction. Nontracheophyte plants including the liverworts, hornworts and mosses undergo an alternation of generations; the gametophyte generation is the most common. The haploid gametophyte produces haploid gametes in multicellular gametangia. Female gametangia are called archegonium and produce eggs, while male structures called antheridium produce sperm. Water is required so that the sperm can swim to the archegonium, where the eggs are fertilized to form the diploid zygote. The zygote develops into a sporophyte that is dependent on the parent gametophyte. Mature sporophytes produce haploid spores by meiosis in sporangia. When a spore germinates, it grows into another gametophyte.

Ferns and their allies, including clubmoss and horsetails, reproduce via an alternation of generations. The conspicuous plant observed in the field is the diploid sporophyte. This plant creates by meiosis single-celled haploid spores which are shed and dispersed by the wind (or in some cases, by floating on water). If conditions are right, a spore will germinate and grow into a rather inconspicuous plant body called a prothallus. The haploid prothallus does not resemble the sporophyte, and as such ferns and their allies have a heteromorphic alternation of generations. The prothallus is short-lived, but carries out sexual reproduction,