Editing Alternation of generations (section)

===Fundamental elements===

The diagram above shows the fundamental elements of the alternation of generations in plants. There are many variations in different groups of plants. The processes involved are as follows:<ref name="Foster-1974">{{Harvnb|Foster|Gifford|1974}},{{pn|date=May 2023}} {{Harvnb|Sporne|1974a}}{{pn|date=May 2023}} and {{Harvnb|Sporne|1974b}}. {{pn|date=May 2023}} </ref>
* Two single-celled haploid gametes, each containing ''n'' unpaired chromosomes, fuse to form a single-celled diploid zygote, which now contains ''n'' pairs of chromosomes, i.e. 2''n'' chromosomes in total.<ref name="Foster-1974"/>
* The single-celled diploid zygote germinates, dividing by the normal process ([[mitosis]]), which maintains the number of chromosomes at 2''n''. The result is a multi-cellular diploid organism, called the ''sporophyte'' (because at maturity it produces spores).<ref name="Foster-1974"/>
* When it reaches maturity, the sporophyte produces one or more '''sporangia''' (singular: sporangium) which are the organs that produce diploid spore mother cells (sporocytes). These divide by a special process ([[meiosis]]) that reduces the number of chromosomes by a half. This initially results in four single-celled haploid spores, each containing ''n'' unpaired chromosomes.<ref name="Foster-1974"/>
* The single-celled haploid spore germinates, dividing by the normal process (mitosis), which maintains the number of chromosomes at ''n''. The result is a multi-cellular haploid organism, called the ''gametophyte'' (because it produces gametes at maturity).<ref name="Foster-1974"/>
* When it reaches maturity, the gametophyte produces one or more '''gametangia''' (singular: gametangium) which are the organs that produce haploid gametes. At least one kind of gamete possesses some mechanism for reaching another gamete in order to fuse with it.<ref name="Foster-1974"/>
The 'alternation of generations' in the life cycle is thus between a diploid (2''n'') generation of multicellular sporophytes and a haploid (''n'') generation of multicellular gametophytes.<ref name="Foster-1974"/>

[[File:Onoclea sensibilis 4 crop.jpg|thumb|Gametophyte of the fern ''[[Onoclea sensibilis]]'' (flat thallus, bottom) with a descendant sporophyte beginning to grow from it (small frond, top) ]]

The situation is quite different from that in animals, where the fundamental process is that a multicellular diploid (2''n'') individual {{em|directly}} produces haploid (''n'') gametes by meiosis. In animals, spores (i.e. haploid cells which are able to undergo mitosis) are not produced, so there is no asexual multicellular generation. Some insects have haploid males that develop from unfertilized eggs, but the females are all diploid.<ref name="Foster-1974"/>