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Alternation of generations (also known as metagenesis or heterogenesis) [1] is the predominant type of life cycle in plants and algae. In plants both phases are multicellular : the haploid sexual phase – the gametophyte – alternates with a diploid asexual phase – the sporophyte .
The gradual evolution of an independent, sporophyte phase was viewed by Bower as being closely related to the transition from aquatic to terrestrial plant life on Earth. Evidence supporting this theory can be found in the life cycle of modern Bryophytes in which the sporophyte is physiologically dependent on the gametophyte .
Prothallus of the tree fern Dicksonia antarctica (note new moss plants for scale) Spore-bearing plants, like all plants, go through a life-cycle of alternation of generations. The fully grown sporophyte, what is commonly referred to as the fern, produces genetically unique spores in the sori by meiosis.
Diagram showing the alternation of generations between a diploid sporophyte (bottom) and a haploid gametophyte (top) A sporophyte (/ ˈ s p ɔːr. ə ˌ f aɪ t /) is the diploid multicellular stage in the life cycle of a plant or alga which produces asexual spores. This stage alternates with a multicellular haploid gametophyte phase.
For example, plants produce gametes through mitosis in gametophytes. The gametophytes grow from haploid spores after sporic meiosis. The existence of a multicellular, haploid phase in the life cycle between meiosis and gametogenesis is also referred to as alternation of generations.
Primitive land plants such as liverworts and mosses had motile sperm that swam in a thin film of water or were splashed in water droplets from the male reproduction organs onto the female organs. As taller and more complex plants evolved, modifications in the alternation of generations evolved.
Alternation of generations between a haploid (n) gametophyte (top) and a diploid (2n) sporophyte (bottom), in all types of plant. When reproducing sexually, plants have complex lifecycles involving alternation of generations.
Sporophyte – Diploid multicellular stage in the life cycle of a plant or alga; Alternation of generations – Reproductive cycle of plants and algae; Archegonium – Organ of the gametophyte of certain plants, producing and containing the ovum; Antheridium – Part of a plant producing and containing male gametes