the alternation in an organism’s life cycle of dissimilar reproductive forms, especially the alternation of sexual with asexual reproduction.
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, xenogenesis
alternation 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. Also called metagenesis, xenogenesis.
alternation of generations
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.
Our Living Language : 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.
a choice limited to one of two or more possibilities, as of things, propositions, or courses of action, the selection of which precludes any other possibility: You have the alternative of riding or walking. one of the things, propositions, or courses of action that can be chosen: The alternative to riding is walking. a possible […]
- Alternative pathway
the activation of complement by contact with polysaccharides on bacteria, protozoa, or yeast cells: a nonspecific immune response. Compare .
- Alternative conjunction
a conjunction, as or, that expresses an alternative relationship between the words, phrases, or clauses it connects.
- Alternative energy
energy, as solar, wind, or nuclear energy, that can replace or supplement traditional fossil-fuel sources, as coal, oil, and natural gas. Contemporary Examples Long before it was trendy, Carter preached the gospel of alternative energy. Carter in Oscarland: The Rehabilitation of the 39th President Douglas Brinkley February 23, 2013 One of those is alternative energy, […]