Sunday, July 26, 2020

Plant Kingdom - Notes | Class 11 | Part 6: Plant Life cycles and alternation of generations



-    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).

Patterns of Plant life cycles

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.

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