Editing Alternation of generations (section)

==Life cycles of different plant groups==

{{further|Marchantiophyta#Life cycle|Moss#Life cycle|Hornwort#Life cycle|Fern#Life cycle}}

The term "plants" is taken here to mean the [[Archaeplastida]], i.e. the [[glaucophyte]]s, [[red algae|red]] and [[green algae]] and [[land plants]].

Alternation of generations occurs in almost all multicellular red and green algae, both freshwater forms (such as ''[[Cladophora]]'') and seaweeds (such as ''[[Ulva (genus)|Ulva]]''). In most, the generations are homomorphic (isomorphic) and free-living. Some species of red algae have a complex triphasic alternation of generations, in which there is a gametophyte phase and two distinct sporophyte phases. For further information, see [[Red algae#Reproduction|Red algae: Reproduction]].

Land plants all have heteromorphic (anisomorphic) alternation of generations, in which the sporophyte and gametophyte are distinctly different. All [[bryophyte]]s, i.e. [[Marchantiophyta|liverwort]]s, [[moss]]es and [[hornwort]]s, have the gametophyte generation as the most conspicuous. As an illustration, consider a monoicous moss. Antheridia and archegonia develop on the mature plant (the gametophyte). In the presence of water, the biflagellate sperm from the antheridia swim to the archegonia and fertilisation occurs, leading to the production of a diploid sporophyte. The sporophyte grows up from the archegonium. Its body comprises a long stalk topped by a capsule within which spore-producing cells undergo meiosis to form haploid spores. Most mosses rely on the wind to disperse these spores, although ''[[Splachnum sphaericum]]'' is [[entomophilous]], recruiting insects to disperse its spores.

<gallery class=center mode=nolines widths=220 heights=220>
File:Liverwort life cycle.jpg|Alternation of generations in liverworts
File:Lifecycle moss svg diagram.svg|Moss life cycle
File:Hornwort life cicle svg diagram.svg|Hornwort life cycle
</gallery>

The [[Fern#Life cycle|life cycle of ferns]] and their allies, including [[clubmoss]]es and [[horsetail]]s, the conspicuous plant observed in the field is the diploid sporophyte. The haploid spores develop in [[sorus|sori]] on the underside of the fronds and are 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 called a [[prothallus]]. The haploid prothallus does not resemble the sporophyte, and as such ferns and their allies have a heteromorphic alternation of generations. The prothallus is short-lived, but carries out sexual reproduction, producing the diploid [[zygote]] that then grows out of the prothallus as the sporophyte.

<gallery class=center mode=nolines widths=220 heights=220>
File:Alternation of generations in ferns.svg|{{center|Alternation of generations in ferns}}
File:Dixonia prothallus.jpg|{{center|A gametophyte (prothallus) of ''Dicksonia ''}}
File:Dicksonia antarctica Cultivated GardenEngland.jpg|{{center|A sporophyte of ''Dicksonia antarctica''}}
File:SoriDicksonia.jpg|{{center|''Dicksonia antarctica'' frond with spore-producing structures}}
</gallery>

In the [[spermatophyte]]s, the seed plants, the sporophyte is the dominant multicellular phase; the gametophytes are strongly reduced in size and very different in morphology. The entire gametophyte generation, with the sole exception of pollen grains (microgametophytes), is contained within the sporophyte. The life cycle of a dioecious flowering plant (angiosperm), the willow, has been outlined in some detail in an earlier section ([[#A complex life cycle|A complex life cycle]]). The life cycle of a [[gymnosperm]] is similar. However, flowering plants have in addition a phenomenon called '[[double fertilization]]'. In the process of [[double fertilization]], two sperm nuclei from a pollen grain (the microgametophyte), rather than a single sperm, enter the archegonium of the megagametophyte; one fuses with the egg nucleus to form the zygote, the other fuses with two other nuclei of the gametophyte to form '[[endosperm]]', which nourishes the developing embryo.