Editing Alternation of generations (section)

=== A complex life cycle ===

[[File:Alternation of generations complex.svg|upright=2.5|thumb|Alternation of generations in a species which is heteromorphic, sporophytic, oogametic, dioicous, heterosporic and dioecious]]

Plant life cycles can be complex. Alternation of generations can take place in plants which are at once heteromorphic, sporophytic, oogametic, dioicous, heterosporic and dioecious, such as in a [[willow]] tree (as most species of the genus ''Salix'' are dioecious).<ref>{{cite EB1911|wstitle=Willow|volume=28|pages=688–689}}</ref> The processes involved are:
* An immobile egg, contained in the archegonium, fuses with a mobile sperm, released from an antheridium. The resulting zygote is either male or female.
** A male zygote develops by mitosis into a microsporophyte, which at maturity produces one or more microsporangia. Microspores develop within the microsporangium by meiosis.<br />In a willow (like all seed plants) the zygote first develops into an embryo microsporophyte within the ovule (a megasporangium enclosed in one or more protective layers of tissue known as integument). At maturity, these structures become the [[seed]]. Later the seed is shed, germinates and grows into a mature tree. A male willow tree (a microsporophyte) produces flowers with only stamens, the anthers of which are the microsporangia.
** Microspores germinate producing microgametophytes; at maturity one or more antheridia are produced. Sperm develop within the antheridia.<br />In a willow, microspores are not liberated from the anther (the microsporangium), but develop into pollen grains (microgametophytes) within it. The whole pollen grain is moved (e.g. by an insect or by the wind) to an ovule (megagametophyte), where a sperm is produced which moves down a pollen tube to reach the egg.
** A female zygote develops by mitosis into a megasporophyte, which at maturity produces one or more megasporangia. Megaspores develop within the megasporangium; typically one of the four spores produced by meiosis gains bulk at the expense of the remaining three, which disappear.<br />Female willow trees (megasporophytes) produce flowers with only carpels (modified leaves that bear the megasporangia).
** Megaspores germinate producing megagametophytes; at maturity one or more archegonia are produced. Eggs develop within the archegonia.<br /> The carpels of a willow produce ovules, megasporangia enclosed in integuments. Within each ovule, a megaspore develops by mitosis into a megagametophyte. An archegonium develops within the megagametophyte and produces an egg. The whole of the gametophytic generation remains within the protection of the sporophyte except for pollen grains (which have been reduced to just three cells contained within the microspore wall).{{-}}