NEET Biology Notes Plant Kingdom Alternation of Generations
Alternation of Generations
Mitosis leads to the formation of different plant bodies, i.e. haploid and diploid. The haploid plant body produces gametes by mitosis. This plant body represents a gametophyte. Following fertilisation, the zygote also divides by mitosis to produce a diploid sporophytic plant body. Haploid spores are produced by this plant body by meiosis. Thus, during the life cycle of any sexually reproducing plant, there is an alternation of generation between gametes producing haploid gametophyte and spore producing diploid sporophyte.
- Haplontic life cycle
It is represented by only one-celled zygote. The haploid spores divide mitotically and form the gametophyte. The dominant, photosynthetic phase in such plants is the free-living gametophyte. Many algae such as Volvox, Spirogyra and some species of Chlamydomonas.
- Diplontic life cycle
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. All seed-bearing plants, i.e. gymnosperms and angiosperms follow this pattern.
- Haplo-diplontic life cycle
- These both phases are multicellular. Bryophytes and pteridophytes exhibit, this intermediate condition. They differ in their dominant phase.
A dominant, independent, photosynthetic, thalloid or erect phase is represented by a haploid gametophyte and it alternates with the short lived multicellular sporophyte totally or partially dependent on the gametophyte for its anchorge and nutrition. This pattern is found in all 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. Such a pattern is known as haplo-diplontic life cycle. All pteridophytes exhibit this pattern. One interesting fact is, while most algal genera are haplontic, some of them such as Ectocarpus, Polysiphonia, kelps are haplo-diplontic. Fucus, an alga is diplontic.