Alternation of generation
- Haploid and diploid cells produce haploid and diploid plant bodies, respectively, by mitosis.
- The haploid plant body produces gametes and hence represents gametophytes.
- The zygote divides by mitosis to produce a diploid sporophytic plant body and this plant body produces haploid spores by meiosis, which again divides by mitosis to produce haploid plant body.
Different plant groups representing gametophytes and sporophytes differ in the following patterns:
- Sporophytic generation is represented only by the one-celled zygote and free-living sporophytes are absent.
- Haploid spores are formed by meiosis, which divide mitotically to form the gametophyte.
- The dominant, photosynthetic phase in such plants is the free-living gametophyte.
Examples-Volvox, Spirogyra and some species of Chlamydomomas.
Fig. haplontic life cycle
- Here, the diploid sporophyte is the dominant, photosynthetic, independent phase of the plant.
- The gametophytic phase is represented by the single to few-celled haploid gametophyte.
For example- gymnosperms and angiosperms.
Fig. diplontic life cycle
- A dominant, independent, photosynthetic, thalloid or erect phase is represented by a haploid gametophyte and it alternates with the short lived multicellular sporophyte. Example- bryophytes
- The diploid sporophyte is represented by a dominant, independent, photosynthetic, vascular plant body. It alternates with multicellular, saprophytic/autotrophic, independent but short-lived haploid gametophyte. Example- pteridophytes.
- Some alga genera such as Ectocarpus, Polysiphonia, kelps are haplo-diplontic.
Fig. haplo-diplontic life cycle