Alternation of generation

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Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
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One very intriguing and significant historical discussion concerns the alternation of generations. For living things that display a remarkably distinct alternation of generations such as between haploid and diploid generations, to what extent are we dealing with distinct biological individualities?
This is called the alternation of generations in plants, as shown in Figure 12-3.
Microgametogenesis, which occurs inside the developed pollen grain, comprises the gametophyte stage, the gamete-producing organism in the alternation of generations. The haploid microspore nucleus divides mitotically to produce two haploid nuclei: the tube nucleus and the generative nucleus.
All plants have alternation of generations between a haploid spore-producing (sporophyte) stage and a gamete-producing (gametophyte) stage.
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.
Although alternation of generations initially referred to the alternation of sexual and asexual forms in animals, the term is now almost exclusively associated with the life cycles of plants, specifically with the alternation of haploid gametophytes and diploid sporophytes.
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?
Since, in evolution, the development of sex surely preceded alternation of generations, the gametophyte generation is considered, necessarily, to be older than the sporophyte generation (cf.

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