Alternation of Generations

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  • Topic: Fern, Spore, Alternation of generations
  • Pages : 3 (833 words )
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  • Published : February 5, 2011
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The alternation of generations (also known as alternation of phases[1] or metagenesis) describes the life cycle of plants, fungi and protists. A multicellular diploid phase alternates with a multicellular haploid phase. The term can be 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 distinct 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 all land plants and many algae undergo sexual reproduction.

The ferns alternation of generation life cycle goes from Sporophyte to Gametophyte. It starts out as a spore. The spore goes through meiosis, and germinates in to a mature gametophyte. After this has happened the antheridium and archegonium perform sexual reproduction creating fertilizing an egg. The egg grows in to a sporophyte which grows clusters of sporengia on the underside of its leaves. The sporengia burst sending the spores cells into the surroundings and the process duplicates itself. (Purves 559) Moss

The lifecycle of moss is very similar to that of the fern. Its gametophyte stage is however longer than that of the ferns. After the spore has landed in the soil it...
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