<P> Life cycles of plants and algae with alternating haploid and diploid multicellular stages are referred to as diplohaplontic (the equivalent terms haplodiplontic, diplobiontic or dibiontic are also in use). Life cycles, such as those of animals, in which there is only a diploid multicellular stage are referred to as diplontic . Life cycles in which there is only a haploid multicellular stage are referred to as haplontic . </P> <P> Alternation of generations is defined as the alternation of multicellular diploid and haploid forms in the organism's life cycle, regardless of whether or not these forms are free - living . 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 isomorphic . 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 . </P> <P> However, in some other groups, 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 modern vascular land plants the gametophytes are strongly reduced, although the fossil evidence indicates that they were derived from isomorphic ancestors . In seed plants, the female gametophyte develops totally within the sporophyte which protects and nurtures it and the embryo 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 "(s) porophyte and gametophyte effectively function as a single organism". The alternative term' alternation of phases' may then be more appropriate . </P> <P> Debates about alternation of generations in the early twentieth century can be confusing because various ways of classifying "generations" co-exist (sexual vs. asexual, gametophyte vs. sporophyte, haploid vs. diploid, etc .). </P>

In terms of alternation of generations the pollen grains