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Alternation of generations

Alternation of generations is the term usually reserved for the reproductive cycle of ferns and "fern-allies." The plant usually seen and identified as a fern, or clubmoss or whatever is called the sporophyte[?], or spore-shedding plant. This plant sheds single-celled bodies called spore which then fall to the ground or are blown in the wind (or in some cases, float in the water) to land on new ground. The plant the spore then grows into is not the same as the parent plant, but rather a different type of plant called a gametophyte or prothallus, "gametophyte" meaning "sexually-reproducing plant", and "prothallus" meaning a "before-plant".

The gametophytes of most ferns are green, photosynthesizing bodies, often heart-shaped, and sometimes thread-like. Those of most "fern-allies" are tubular bodies in the soil, subsisting off a symbiotic relationship with soil fungi.

The sporophyte, or parent plant, is diploid. This means it has two full sets of chromosomes, or two of each kind of chromosome (as do humans, who have 46 chromosomes, or two sets of 23 each, although the X/Y is a mismatch). The spore and the gametophyte, however, are haploid. This means they only have one set of chromosomes, or one of each kind (as do human sperm and eggs).

The gametophyte has special bodies within the plant known as archegonia (female cells) and antheridia (male cells). This is where sexual fertilization takes place, and a new diploid sporophyte then grows.

Spermatophytes[?] also have alternation of generations, but the gametophyte is much reduced: the male gametophyte is the three-cell sprouted pollen grain (the unsprouted pollen grain is a spore) and the female one is eight cells in the ovule[?].



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