Akanksha Soni, Academic Content Writer at Edumarz
Alternation of generation is a type of life cycle in which the next generations of plants alternate between diploid and haploid organisms. Mitosis allows both haploid and diploid cells to divide. After mitosis, the haploid plant body produces gametes. This plant body represents a gametophyte. After fertilization, the zygote undergoes mitosis to produce a diploid sporophytic plant body. Meiosis is the process by which this plant body produces haploid spores. These, in turn, divide by mitosis to form a haploid plant body. As a result, during the life cycle of any sexually reproducing plant, generations alternate between gamete-producing haploid gametophyte and spore-producing diploid sporophyte.
The stages of alternation of generation are as follows:-
Haplontic stage:- The sporophytic generation is represented only by a single-celled zygote. There are no free-living sporophytes. Meiosis results in the formation of haploid spores in the zygote. The haploid spores undergo mitosis and form the gametophyte. The dominant, photosynthetic stage in such plants is the free-living gametophyte. This type of life cycle is known as haplontic. For example, many algae such as Volvox, Spirogyra, and some species of Chlamydomonas follow this pattern.
Diplontic stage:– In this type, the diploid sporophyte is the dominant, photosynthetic, independent stage of the plant. The gametophytic stage is represented by a single to the few-celled haploid gametophyte. Such a life cycle is known as diplontic. For example, algae, Fucus species, represents this pattern. Furthermore, all seed-bearing plants i.e., gymnosperms and angiosperms, follow this pattern with some variations, with the gametophytic stage being less than multi-celled.
Haplo-diplontic stage:- Surprisingly, bryophytes and pteridophytes exhibit an intermediate condition (Haplo-diplontic); both stages are multicellular. Their dominant stages, however, differ. A haploid gametophyte represents a dominant, independent, photosynthetic, thalloid, or erect stage, which is a short-lived multicellular sporophyte that is completely or partially dependent on the gametophyte for anchorage and nutrition. This pattern is shared by all bryophytes. A dominant, independent, photosynthetic, vascular plant body characterises the diploid sporophyte. It is an independent but short-lived haploid gametophyte that alternates with multicellular, saprophytic, or autotrophic gametophytes. The haplo-diplontic life cycle describes such a pattern. This pattern is found in all pteridophytes. While the majority of algae genera are haplontic, some, such as Ectocarpus, Polysiphonia, and kelps, are haplo-diplontic. Fucus is a diplontic alga.