3. PLANT KINGDOM
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.
