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alternation of generations

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alternation of generations
n.
The regular alternation of forms or of mode of reproduction in the life cycle of an organism, such as the alternation between diploid and haploid phases, or between sexual and asexual reproductive cycles. Also called metagenesis, xenogenesis.

alternation of generations
n
1. (Biology) the production within the life cycle of an organism of alternating asexual and sexual reproductive forms. It occurs in many plants and lower animals. Also called: metagenesis, heterogenesis, digenesis or xenogenesis

alterna′tion of genera′tions
n.
the alternation in an organism's life cycle of dissimilar reproductive forms, esp. the alternation of sexual with asexual generations.
[1855–60]

alternation of generations  (ôltr-nshn)
The regular alternation of forms or of mode of reproduction in the life cycle of an organism, especially the alternation between sexual and asexual reproductive phases in plants and some invertebrates. In plants, the alternation involves alternating generations of haploid and diploid organisms. Often one of these generations is the dominant form of the organism, and the other generation is nutritionally dependent upon it or just grows as a smaller plant. For example, in mosses and liverworts, the haploid phase is the large, familiar form of the plant. The diploid phase is smaller and grows upon the haploid phase. In angiosperms, however, the diploid phase of the organism is large and independent, while the haploid phase is reduced to the pollen grain and the eight-celled female gametophyte located in the ovule.
A Closer Look The life cycle of fern species provides a good example of the differing roles played by the gametophyte and sporophyte in organisms that display an alternation of generations. The familiar large frond-bearing fern plant is the sporophyte generation of the fern. By meiosis it produces haploid spores that are dispersed and develop into gametophytes. Fern gametophytes are inconspicuous matlike plants that can make their own food by photosynthesis. The gametophytes produce both sperm and eggs. Sperm from another gametophyte reaches one of these eggs and fuses with it to form an embryo, which then grows out of the gametophyte as a new sporophyte plant. In many nonvascular plants, such as the mosses and liverworts, the sporophyte is a relatively small plant that grows in or on top of the gametophyte, which is larger. In gymnosperms and angiosperms, however, the sporophyte is the main plant form, and the gametophyte is dependent on the sporophyte.
Thesaurus Legend:  Synonyms Related Words Antonyms
Noun1.alternation of generations - the alternation of two or more different forms in the life cycle of a plant or animal
organic phenomenon - (biology) a natural phenomenon involving living plants and animals
digenesis, metagenesis - alternation of sexual and asexual generations


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Prelude Steenstrup (1845) defined alternation of generations as "the remarkable, and till now inexplicable phenomenon of an animal producing an offspring, which at no times resembles its parent, but which, on the other hand, itself brings forth a progeny, which returns in its form and nature to the parent animal, so that the maternal animal does not meet with its own resemblance in its own brood, but in its descendants of the second, third, or fourth degree of generation" (p.
In any event, in the context of an improved knowledge of putative land-plant ancestors, we may now appropriately ask again the question, which theory of alternation of generations (and sporophyte origin) in land plants is more plausible, the homologous theory or the antithetic theory?
 
 
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