Editing Alternation of generations (section)

==Definition==
Alternation of generations is defined as the alternation of multicellular diploid and haploid forms in the organism's life cycle, regardless of whether these forms are free-living.<ref name="Taylor2005">{{Harvnb|Taylor|Kerp|Hass|2005}}</ref> In some species, such as the alga ''[[Ulva lactuca]]'', the diploid and haploid forms are indeed both free-living independent organisms, essentially identical in appearance and therefore said to be [[Glossary of botanical terms#isomorphic|isomorphic]]. In many algae, the free-swimming, haploid gametes form a diploid zygote which germinates into a multicellular diploid sporophyte. The sporophyte produces free-swimming haploid spores by [[meiosis]] that germinate into haploid gametophytes.<ref>"{{cite web |url= http://www.plantscience4u.com/2014/05/diplohaplontic-life-cycle-in-algae.html#.V3vOW6s7o3R |title= Plant Science 4 U |access-date= 5 July 2016 |archive-date= 18 August 2016 |archive-url= https://web.archive.org/web/20160818104509/http://www.plantscience4u.com/2014/05/diplohaplontic-life-cycle-in-algae.html#.V3vOW6s7o3R |url-status= live }}</ref>

However, in [[Embryophyte|land plant]]s, either the sporophyte or the gametophyte is very much reduced and is incapable of free living. For example, in all [[bryophytes]] the gametophyte generation is dominant and the sporophyte is dependent on it. By contrast, in all [[seed plant]]s the gametophytes are strongly reduced, although the fossil evidence indicates that they were derived from isomorphic ancestors.<ref name="Kerp-2003"/> In [[seed plant]]s, the female gametophyte develops totally within the sporophyte, which protects and nurtures it and the embryonic sporophyte that it produces. The pollen grains, which are the male gametophytes, are reduced to only a few cells (just three cells in many cases). Here the notion of two generations is less obvious; as Bateman & Dimichele say "sporophyte and gametophyte effectively function as a single organism".<ref name="Bateman-1994-2">{{Harvnb|Bateman|Dimichele|1994|p=403}}</ref> The alternative term 'alternation of phases' may then be more appropriate.<ref name="StewartRothwell-1993">{{Harvnb|Stewart|Rothwell|1993}}</ref>