alternation of generations

American  

noun

Biology.

1. the alternation in an organism's life cycle of dissimilar reproductive forms, especially the alternation of sexual with asexual reproduction.

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alternation of generations British  

noun

1. Also called: metagenesis.   heterogenesis.   digenesis.   xenogenesis.  the production within the life cycle of an organism of alternating asexual and sexual reproductive forms. It occurs in many plants and lower animals

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alternation of generations Scientific  

/ ôl′tər-nā′shən /

1. 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.

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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.

Etymology

Origin of alternation of generations

First recorded in 1855–60

Example Sentences

See Examples For:

alternation of generations

The lifecycle of bryophytes and pterophytes is characterized by the alternation of generations, like gymnosperms and angiosperms; what sets bryophytes and pterophytes apart from gymnosperms and angiosperms is their reproductive requirement for water.

From Textbooks ● Jan. 1, 2015

The moss lifecycle follows the pattern of alternation of generations as shown in Figure 6.

From Textbooks ● Jan. 1, 2015

Four major adaptations are found in all terrestrial plants: the alternation of generations, a sporangium in which the spores are formed, a gametangium that produces haploid cells, and apical meristem tissue in roots and shoots.

From Textbooks ● Jan. 1, 2015

The red algae life cycle is an alternation of generations.

From Textbooks ● Jan. 1, 2015

Kölliker, in 1872, compared the evolution of species with the processes which we can observe in the individual life in cases of alternation of generations.

From Evolution in Modern Thought by Weismann, August