alternation of generations

noun

, Biology.

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

alternation of generations

noun

1. 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 calledmetagenesisheterogenesisdigenesisxenogenesis

alternation of generations

/ 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.

Word History and Origins

Origin of alternation of generations¹

First recorded in 1855–60

A 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.

Example Sentences

A new, diploid sporophyte grows from this fertilised egg, and the cycle begins again … This process is known as alternation of generations, and has, ironically, confused generations of biology undergraduates.

From The Guardian

All plants maintain this dual life cycle, called “the alternation of generations“.

From Scientific American

Nevertheless insects offer, in their metamorphoses, a phenomenon not altogether dissimilar, and give a clue to the manner in which alternation of generations may have originated.

From Project Gutenberg

The alternating activity of these two produces the alternation of generations.

From Project Gutenberg

Life-history always very simple, no well-marked alternation of generations; basidium borne directly on the mycelium.

From Project Gutenberg