Skip to main content
Select Source:

Alternation of Generations

Alternation of Generations

For sexually reproducing multicellular organisms such as plants and animals, the life cycle requires that diploid cells divide by meiosis to create haploid cells. Haploid cells then fuse to recreate the diploid number and a new organism. Alternation of generations refers to the occurrence in the plant life cycle of both a multicellular diploid organism and a multicellular haploid organism, each giving rise to the other. This is in contrast to animals, in which the only multicellular phase is the diploid organism (such as the human man or woman), whereas the haploid phase is a single egg or sperm cell.

Alternation of generations is easiest to understand by considering the fern. The large, leafy fern is the diploid organism. On the undersurface of its fronds or leaves, its cells undergo meiosis to create haploid cells. However, these cells do not immediately unite with others to recreate the diploid state. Instead, they are shed as spores and germinate into small haploid organisms. Because the diploid organism creates spores, it is called the sporophyte generation of the life cycle. Upon reaching maturity, the haploid organism creates haploid egg and sperm cells (gametes) by mitosis . Because the haploid organism creates gametes , it is called the gametophyte generation of the life cycle. The male gametes (sperm) are then released and swim to the female egg. Fusion of the gametes creates the new diploid sporophyte, completing the life cycle.

Whereas the fern gametophyte and sporophyte generations are completely independent, in some types of plants one generation lives on or in the other and depends on it for nutrition. In mosses, the familiar lush carpet of moss is the gametophyte, and its gametes require a moist environment for short-distance swimming before fusing. The sporophyte lives as a thin stalk attached to the gametophyte. Spores are released into the air and can travel on the slightest breezes to other habitats.

In contrast, in flowering plants (angiosperms), the sporophyte is the dominant form. The male gametophyte has been reduced to just three cells, two of which are sperm. These together form the pollen grain, which is formed from the anther, part of the sporophyte. Similarly, the female gametophyte has been reduced to just seven cells, one of which is the egg cell. These are retained inside the ovule , which is part of the sporophyte. In angiosperms, two fertilization events take place: one sperm fertilizes the egg to form the diploid zygote of the new individual, and the other sperm fertilizes the so-called polar nuclei to form the triploid endosperm , a nutritive tissue. Together with maternal sporophyte tissue, these make up the seed.

see also Angiosperms; Bryophytes; Life Cycles; Meiosis; Pollination and Fertilization; Pteridophytes; Reproduction in Plants

Richard Robinson

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Alternation of Generations." Biology. . Encyclopedia.com. 20 Aug. 2017 <http://www.encyclopedia.com>.

"Alternation of Generations." Biology. . Encyclopedia.com. (August 20, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/alternation-generations

"Alternation of Generations." Biology. . Retrieved August 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/alternation-generations

alternation of generations

alternation of generations The alternate development of two types of individual in the life cycle of an organism. Usually one type reproduces asexually and the other sexually (the term is often restricted to organisms which have a haploid generation alternating with a diploid generation). No instances of such alternation are known among multicellular animals; where two types of individual occur, both capable of reproduction, they are invariably diploid and the process is better described as metagenesis. In many parasitic protozoa (e.g. the malaria parasite Plasmodium) the sexual phase inhabits one host and the asexual phase another. In mosses, vascular plants, many algae, and some fungi, a haploid phase, during which gametes are produced by mitosis (gametophyte phase), alternates with a spore-producing diploid (sporophyte) phase. In an isomorphic alternation of generations (found in some algae, for example) the sporophyte and gametophyte are morphologically similar or identical; in a heteromorphic alternation of generations they are dissimilar (e.g. in mosses the gametophyte is the dominant and conspicuous generation, whereas in higher plants it is the sporophyte that forms the conspicuous plant).

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"alternation of generations." A Dictionary of Ecology. . Encyclopedia.com. 20 Aug. 2017 <http://www.encyclopedia.com>.

"alternation of generations." A Dictionary of Ecology. . Encyclopedia.com. (August 20, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/alternation-generations

"alternation of generations." A Dictionary of Ecology. . Retrieved August 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/alternation-generations

alternation of generations

alternation of generations The occurrence within the life cycle of an organism of two or more distinct forms (generations), which differ from each other in appearance, habit, and method of reproduction. The phenomenon occurs in some protoctists, certain lower animals (e.g. cnidarians and parasitic flatworms), and in plants. The malaria parasite (Plasmodium), for example, has a complex life cycle involving the alternation of sexually and asexually reproducing generations. In plants the generation with sexual reproduction is called the gametophyte and the asexual generation is the sporophyte, either of which may dominate the life cycle, and there is also alternation of the haploid and diploid states. Thus in vascular plants the dominant plant is the diploid sporophyte; it produces spores that germinate into small haploid gametophytes. In mosses the gametophyte is the dominant plant and the sporophyte is the spore-bearing capsule. See interpolation hypothesis; transformation hypothesis.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"alternation of generations." A Dictionary of Biology. . Encyclopedia.com. 20 Aug. 2017 <http://www.encyclopedia.com>.

"alternation of generations." A Dictionary of Biology. . Encyclopedia.com. (August 20, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/alternation-generations-2

"alternation of generations." A Dictionary of Biology. . Retrieved August 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/alternation-generations-2

alternation of generations

alternation of generations An alternate development of two types of individual in the life cycle of an organism. Usually one type reproduces asexually and the other sexually. (The term is often restricted to organisms that have a haploid generation alternating with a diploid generation.) In mosses, vascular plants, many algae, and some fungi, for example, a haploid phase, during which gametes are produced by mitosis (gametophyte phase), alternates with a spore-producing diploid (sporophyte) phase. In an isomorphic alternation of generations (found in some algae, for example) the sporophyte and gametophyte are morphologically similar or identical; in a heteromorphic alternation of generations they are dissimilar (e.g. in mosses the gametophyte is the dominant and conspicuous generation, whereas in higher plants it is the sporophyte that forms the conspicuous plant).

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"alternation of generations." A Dictionary of Plant Sciences. . Encyclopedia.com. 20 Aug. 2017 <http://www.encyclopedia.com>.

"alternation of generations." A Dictionary of Plant Sciences. . Encyclopedia.com. (August 20, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/alternation-generations-0

"alternation of generations." A Dictionary of Plant Sciences. . Retrieved August 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/alternation-generations-0

alternation of generations

alternation of generations The alternate development of two types of individual in the life cycle of an organism, one diploid and asexual, the other haploid and sexual, both types being capable of reproduction, implying alternating phases of meiosis and mitosis. No instances of such alternation are known among animals; where two types of individual occur, both capable of reproduction, they are invariably diploid and the process is better described as metagenesis. In many parasitic protozoa (e.g. the malaria parasite Plasmodium) the sexual phase is in one host and the asexual phase in another.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"alternation of generations." A Dictionary of Zoology. . Encyclopedia.com. 20 Aug. 2017 <http://www.encyclopedia.com>.

"alternation of generations." A Dictionary of Zoology. . Encyclopedia.com. (August 20, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/alternation-generations-1

"alternation of generations." A Dictionary of Zoology. . Retrieved August 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/alternation-generations-1

alternation of generations

alternation of generations Two-generation cycle by which plants and some algae reproduce. The asexual diploid sporophyte form with two sets of chromosomes produces haploid spores with one set of chromosomes that, in turn, grow into the sexual (gametophyte) form. The gametophyte produces the egg cell that is fertilized by a male gamete to produce a diploid zygote that grows into another sporophyte.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"alternation of generations." World Encyclopedia. . Encyclopedia.com. 20 Aug. 2017 <http://www.encyclopedia.com>.

"alternation of generations." World Encyclopedia. . Encyclopedia.com. (August 20, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/alternation-generations

"alternation of generations." World Encyclopedia. . Retrieved August 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/alternation-generations

alternation of generations

alternation of generations: see gametophyte; reproduction.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"alternation of generations." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 20 Aug. 2017 <http://www.encyclopedia.com>.

"alternation of generations." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (August 20, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/alternation-generations

"alternation of generations." The Columbia Encyclopedia, 6th ed.. . Retrieved August 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/alternation-generations