Plant Life Cycles & Alternation of Generations (Old NCERT) - NEET PDF Download

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 a haploid plant body.

Different plant groups representing gametophytes and sporophytes differ in the following patterns:

(i) Haplontic

• Sporophytic generation is represented only by the one-celled zygote and free-living sporophytes are absent.
• Haploid spores are formed by meiosis, which divides mitotically to form the gametophyte.
• The dominant, photosynthetic phase in such plants is the free-living gametophyte.
  Examples: Volvox, Spirogyra and some species of Chlamydomonas.
  

(ii) Diplontic

• Here, the diploid sporophyte is the dominant, photosynthetic, independent phase of the plant.
• The gametophytic phase is represented by the single to the few-celled haploid gametophyte.
  Examples: Gymnosperms and Angiosperms.
  Diplontic Life Cycle

(iii) Haplo-diplontic

• 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.
  Haplo-diplontic Life Cycle