Question

What actually is the definition for haplontic, diplontic and haplodiplontic life cycle.

Answer 1

1) In haplonts the mitoses only occur in haploid cells. This results in the formation of single haploid cells or a multicellular haploid organism. The haplontic life forms produce the gametes through mitosis. After fertilization a zygote is formed: this cell is the only diploid cell in the entire life cycle. It is thus that same zygotic cell that later undergoes meiosis. This is called the haplontic life cycle. or
Haplontic life cycle is that when sporophytic generation is represented by only one celled zygote and gametophyte is dominant. Example : volvox,spirogyra,chlamydomonas

2) Diplontic life cycle is that when the gametophytic phase is represented by single to few celled haploid gametophyte and sporophyte is dominant.
or
In gametic meiosis, the zygote divides mitotically to produce a multicellular diploid individual or a group of more unicellular diploid cells. Cells from the diploid individuals then undergo meiosis to produce haploid cells or gametes. Haploid cells may divide again (by mitosis) to form more haploid cells.The individuals or cells as a result of mitosis are haplonts, this life cycle is called Diplontic life cycle.
Example : gymnosperms, angiosperms,fucus

3) The sporophyte creates spores via meiosis which also then divide mitotically producing haploid individuals called "gametophytes". Gametophytes now produce gametes via mitosis. In many plants the gametophyte is not only small-sized but also short-lived. So, this life cycle is called haplo-diplontic life cycle.
or
An intermediate condition between haplontic and diplontic lifecycle where both sporophyte and gametophyte are free living and multicellular but have different dominant phases is called the haplodiplontic life cycle.
Example : Ectocarpus , polysiphonia , kelps