| 单词 | alternation of generations |
| 释义 | alternation of generationsnoun Biology.the alternation in an organism's life cycle of dissimilar reproductive forms, especially the alternation of sexual with asexual reproduction. Origin of alternation of generationsFirst recorded in 1855–60 Words nearby alternation of generationsalternating pulse, alternating series, alternating tremor, alternating voltage, alternation, alternation of generations, alternation of heart, alternative, alternative conjunction, alternative curriculum, alternative energy Dictionary.com UnabridgedBased on the Random House Unabridged Dictionary, © Random House, Inc. 2020 British Dictionary definitions for alternation of generationsalternation of generations nounthe production within the life cycle of an organism of alternating asexual and sexual reproductive forms. It occurs in many plants and lower animalsAlso called: metagenesis, heterogenesis, digenesis, xenogenesis Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012 Medical definitions for alternation of generationsalternation of generations n.The regular alternation of forms or of mode of reproduction in the life cycle of an organism, such as between diploid and haploid phases, or between sexual and asexual reproductive cycles.metagenesis xenogenesis The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company. Scientific definitions for alternation of generationsalternation of generations [ ôl′tər-nā′shən ] 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 LookThe 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. The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved. |
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