Alternation of generations is a term usually applied to the reproductive cycle of ferns and fern allies, although it is more correct to say "alternation of phases of a single generation" (see Sporic meiosis lifecycle). This life cycle is characterized by each generation consisting of two separate, free_living plant bodies: the gametophyte (or haploid) plant and the sporophyte (or diploid) plant.

Ferns and fern allies

The conspicuous plant observed in the field as, for example, a fern or clubmoss, is the sporophyte or spore_shedding phase. This plant creates by meiosis single-celled bodies called spores (haploid) which are shed and dispersed by the wind (or in some cases, by floating on water). If conditions are right, a spore will germinate and grow into a rather inconspicuous plant body or prothallus (meaning "before plant_body") that does not at all resemble the sporophyte plant. This is a gametophyte, meaning "sexually_reproducing plant". The prothallus is short_lived, but carries out sexual reproduction, producing the diploid zygote that then grows out of the prothallus as the sporophyte.

It can be seen that there is not actually an alternation of generations here, simply a variation on the theme of sexual reproduction that we are more accustomed to: that in which the large individual (the diploid, plant or animal) carries the haploid phase as germ cells within its body or thallus, dispersing an embryo (in higher plants, a seed). Thus, all organisms that have, as a part of their life, a haploid phase and a diploid phase, undergo alternation of phases of a single generation (see Biological life cycles). In the ferns and fern allies, dispersion is by a haploid spore, reasonably considered a more primitive process compared with dispersion by seed, and it is at this spore stage that the individuals (or generations) are separated.

Thus, the life cycle of a typical fern or fern ally can be summarized as follows, starting with a spore as the new individual or next generation:

1. Free spore germinates and grows into a small, haploid prothallus (gametophyte)
   1. Prothallus produces sexual, haploid gametes by budding
   2. Gametes combine, to produce diploid, embryonic sporophyte
2. Sporophyte grows into a diplod, macrothallic plant
   1. produces asexual, haploid spores
   2. spores disperse

The small gametophytes of most ferns are green, photosynthesizing bodies, often heart-shaped, and sometimes thread-like. Those of most fern-allies are tubular bodies that live in the soil, subsisting off a symbiotic relationship with soil fungi. The gametophyte has special bodies within the thallus known as archegonia (female cell containing structure) and antheridia (male cell or sperm containing structure). The sperm cells in an antheridium are motile, and reach an archegonium by swimming when water is present. Their recombination is sexual fertilization, with a new diploid zygote formed.

For bryophytes (mosses), the dominant form (macroscopically visible, structurally complex, and nutritionally independent) is the gametophyte.

Algae

The situation is somewhat different in the seaweeds, because the two phases can be very independent generations. Species of algae in which the two phases have significantly different macroscopic appearances are called heteromorphic. If they are the same, as in most red algae, many green algae, a few brown algae), they are isomorphic.

There is alternation of generations in some animals (especially parasites) that needs amplification here