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Demonstrate alternation of generations in ferns.

Alternation of Generations

The Sporophyte Generation

The plant we recognize as a fern is the diploid sporophyte generation.Sori form on the fronds. Each contains many sporangia mounted on stalks.

Within each sporangium, the spore mother cells undergo meiosis producing four haploid spores each.
  • When the humidity drops,
  • The thin-walled lip cells of each sporangium separate.
  • The annulus slowly straightens out.
  • Then the annulus snaps forward expelling the spores.


The Gametophyte Generation
If a spore is blown to a suitable moist locationIt germinates into a filament of cells.This grows into a prothallus withrhizoids, which absorb water and minerals from the soil;
archegonia, which produce a single egg (by mitosis) orantheridia, which form swimming sperm (again, by mitosis) or both.


Fertilization


If moisture is plentiful, the sperm swim to archegonia — usually on another prothallus because the two kinds of sex organs generally do not mature at the same time on a single prothallus.

Another method for promoting cross-fertilization: The first spores to germinate develop into prothallia with archegonia. These prothallia secrete a gibberellin into their surroundings. This is absorbed by younger prothallia and causes them to produce antheridia exclusively.

Fertilization restores the diploid number and begins a new sporophyte generation.

The embryo sporophyte develops a foot that penetrates the tissue of the prothallus and enables the sporophyte to secure nourishment until it becomes self-sufficient.

Although it is tiny, the haploid fern prothallus is a fully-independent, autotrophic plant.

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This alternation of generations is a life cycle that includes both diploid and haploid multicellular stages. Most of the definition is probably unfamiliar, so let's review the terms before talking about what the cycle actually looks like. You may remember some of these terms from genetics. 'Diploid' and 'haploid' both refer to the number of copies of chromosomes. Chromosomes carry genetic information.

'Diploid' means 'two sets of chromosomes.' This is commonly abbreviated as 2n because the n stands for chromosomes and diploid cells have 2 copies. In diploid cells, one copy of the chromosome comes from each parent. For example, in humans, you get one copy of chromosomes from your mom and one copy of chromosomes from your dad. The same idea is found in plants. Each diploid cell contains one copy of chromosomes from the male parent and one copy of chromosomes from the female parent. 'Haploid' means 'one set of chromosomes.' This is commonly abbreviated as n because there's only one copy of the chromosomes.

The last vocab word we need to look at before moving on is 'multicellular.' 'Multicellular' means that it contains more than one cell. This is different from many life cycles in other organisms - such as humans - because our haploid cells are unicellular, meaning 'only one cell.' In plants, part of the life cycle is completed by multicellular haploid cells.

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All plants undergo a life cycle that takes them through both haploid and diploid generations. The multicellular diploid plant structure is called the sporophyte, which produces spores through meiotic (asexual) division. The multicellular haploid plant structure is called the gametophyte, which is formed from the spore and give rise to the haploid gametes. The fluctuation between these diploid and haploid stages that occurs in plants is called the alternation of generations. The way in which the alternation of generations occurs in plants depends on the type of plant. In bryophytes (mosses and liverworts), the dominant generation is haploid, so that the gametophyte comprises what we think of as the main plant. The opposite is true for tracheophytes (vascular plants), in which the diploid generation is dominant and the sporophyte comprises the main plant.
Bryophyte Generations: Bryophytes are nonvascularized plants that are still dependent on a moist environment for survival (see Plant Classification, Bryophytes. Like all plants, the bryo

phyte life cycle goes through both haploid (gametophyte) and diploid (sporophyte) stages. The gametophyte comprises the main plant (the green moss or liverwort), while the diploid sporophyte is much smaller and is attached to the gametophyte. The haploid stage, in which a multicellular haploid gametophyte develops from a spore and produces haploid gametes, is the dominant stage in the bryophyte life cycle. The mature gametophyte produces both male and female gametes, which join to form a diploid zygote. The zygote develops into the diploid sporophyte, which extends from the gametophyte and produces haploid spores through meiosis. Once the spores germinate, they produce new gametophyte plants and the cycle continues.
Tracheophyte Generations: Tracheophytes are plants that contain vascular tissue; two of the major classes of tracheophytes are gymnosperms (conifers) and angiosperms (flowering plants). Tracheophytes, unlike bryophytes, have developed seeds that encase and protect their embryos. The dominant phase in the tracheophyte life cycle is the diploid (sporophyte) stage. The gametophytes are very small and cannot exist independent of the parent plant. The reproductive structures of the sporophyte (cones in gymnosperms and flowers in angiosperms), produce two different kinds of haploid spores: microspores (male) and megaspores (female). This phenomenon of sexually differentiated spores is called heterospory. These spores give rise to similarly sexually differentiated gametophytes, which in turn produce gametes. Fertilization occurs when a male and female gamete join to form a zygote. The resulting embryo, encased in a seed coating, will eventually become a new sporophyte.
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