- In plants, both haploid and diploid cells can divide by mitosis. This forms haploid and diploid plant bodies.
- Haploid plant body (gametophyte) produces gametes by mitosis.
- After fertilization, the zygote also divides by mitosis to produce a diploid plant body (sporophyte). This produces haploid spores by meiosis.
- Spores divide by mitosis to form a haploid plant body.
- Thus, during the life cycle of any sexually reproducing plant, there is an alternation of generations between gametophyte (n) and sporophyte (2n).
1. Haplontic: In this, sporophytic generation is represented only by the zygote. There are no free-living sporophytes. Zygote undergoes meiosis to form haploid spores. They divide mitotically to form gametophyte. The dominant, photosynthetic phase is the free-living gametophyte. E.g. Algae such as Volvox, Spirogyra and some species of Chlamydomonas.
2. Diplontic: In this, diploid sporophyte is the dominant, photosynthetic, independent phase. Gametophytic phase is represented by the single to few-celled haploid gametophyte. E.g. An alga, Fucus sp., all seed-bearing plants (gymnosperms & angiosperms - the gametophytic phase is few to multi-celled).
3. Haplo-diplontic: It is the intermediate condition between haplontic & diplontic. Both gametophyte & sporophyte are multicellular and often free-living. But they have different dominant phases. E.g. Bryophytes & Pteridophytes.