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In biology, a life cycle is a series of changes in form that an organism undergoes, returning to the starting state. "The concept is closely related to those of the life history, development and ontogeny, but differs from them in stressing renewal." Transitions of form may involve growth, asexual reproduction, and/or sexual reproduction. In some organisms, different "generations" of the species succeed each other during the life cycle. For plants and many algae, there are two multicellular stages, and the life cycle is referred to as alternation of generations. The term life history is often used, particularly for organisms such as the red algae which have three multicellular stages (or more), rather than two.〔Dixon, P.S. 1973. ''Biology of the Rhodophyta.'' Oliver & Boyd. ISBN 0 05 002485 X〕 Life cycles that include sexual reproduction involve alternating haploid (''n'') and diploid (2''n'') stages, i.e., a change of ploidy is involved. To return from a diploid stage to a haploid stage, meiosis must occur. In regard to changes of ploidy, there are 3 types of cycles: *''haplontic'' life cycle — the haploid stage is multicellular and the diploid stage is a single cell, meiosis is "zygotic". *''diplontic'' life cycle — the diploid stage is multicellular and haploid gametes are formed, meiosis is "gametic". *''haplodiplontic'' life cycle (also referred to as ''diplohaplontic'', ''diplobiontic'', or ''dibiontic'' life cycle) — multicellular diploid and haploid stages occur, meiosis is "sporic". The cycles differ in when mitosis (growth) occurs. Zygotic meiosis and gametic meiosis have one mitotic stage: mitosis occurs during the ''n'' phase in zygotic meiosis and during the 2''n'' phase in gametic meiosis. Therefore, zygotic and gametic meiosis are collectively termed haplobiontic (single mitotic phase, not to be confused with haplontic). Sporic meiosis, on the other hand, has mitosis in two stages, both the diploid and haploid stages, termed diplobiontic (not to be confused with diplontic). ==Discovery== The study of reproduction and development in organisms was carried out by many botanists and zoologists. Wilhelm Hofmeister demonstrated that alternation of generations is a feature that unites plants, and published this result in 1851 (see plant sexuality). Some terms (haplobiont and diplobiont) used for the description of life cycles were proposed initially for algae by Nils Svedelius,〔Svedelius, N. 1931. (Nuclear Phases and Alternation in the Rhodophyceae. ) In: ''Beihefte zum Botanischen Centralblatt.'' Band 48/1: 38-59.〕 and then became used for other organisms. Other terms (gametogamy, autogamy and gamontogamy) used in protist life cycles were introduced by Karl Gottlieb Grell.〔Margulis, L. (1996): (Archaeal-eubacterial mergers in the origin of Eukarya: Phylogenetic classification of life ).〕 The description of the complex life cycles of various organisms contributed to the disproof of the ideas of spontaneous generation in the 1840s and 1850s.〔Moselio Schaechter (2009). ''Encyclopedia of Microbiology''. Academic Press. Volume 4, p. 85. ().〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Biological life cycle」の詳細全文を読む スポンサード リンク
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