Definition
The discussion of 'alternation of generations' above treats the alternation of a multicellular diploid form with a multicellular haploid form as the defining characteristic, regardless of whether these forms are free-living or not. 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. The free-swimming gametes form a zygote which germinates into a diploid sporophyte; the free-swimming spores germinate into a haploid gametophyte.
However, in other species, either the sporophyte or the gametophyte is very much reduced and is incapable of free living. For example, in seed plants, the gametophyte generation develops totally within the sporophyte which protects and nurtures it, with the sole exception of pollen grains, which are the male gametophytes, but which have been reduced to only three cells. Here the notion of two generations is less obvious; as Bateman & Dimichele say "porophyte and gametophyte effectively function as a single organism". The alternative term 'alternation of phases' may then be more appropriate.
Read more about this topic: Alternation Of Generations
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