Alternation of generations is a type of life cycle in which a diploid sporophytic spore producing phase is alternating with a haploid gametophytic gamete producing generation.
Lets
take the example of fern life cycle for better understanding.
Ferns
(Pteris: most common fern) belongs to
pteridophytes
Lets
us explain in simplified steps (Figure)
- Here the fern plant that we see is the sporophytic diploid plant; the dominant phase in the life cycle.
- This sporophytic plant produces haploid spores by meiosis.
- These spores germinate to form minute heart shaped haploid gametophyte (also called as prothallus)
- Gametophytic plant produces male (antheridium) and female sex organs (archegonium) finally forming male gamete and female gamete (egg) by mitosis
- The haploid gametes fuse to form diploid zygote (2n). Zygote germinate to form the sporophyte again. Thus the gametophytic phase is alternating with the sporophytic phase in the life cycle; therefore called as alternation of generation.
This
type of life cycle occurs in plants and some algae
Now let us see the difference between
gametophyte and sporophyte.
Sporophyte (2n) |
Gametophyte (1n) |
Diploid plant with two sets of chromosome (2n) |
Haploid plant with a single set of chromosome in the cells(1n) |
Reproduce asexually or asexual generation that produces asexual spores |
Reproduce sexually or sexual generation as it bears the sex organs |
Sporophyte forms
spores inside sporangia |
Gametophyte forms
gametes inside gametangium or sex organs |
Spore mother cell;
a diploid cell undergo meiosis forming haploid spores |
Gametes (1n) are produced by mitosis inside sex organs (1n) |
Spores produced
germinate to form gametophyte |
Gametes fuses to
form zygote which germinates to form the sporophyte |
Sporophytic phase
is dominant in higher vascular
plants like pteridophytes, gymnosperms and angiosperms |
Gametophytic phase
is dominant in bryophytes and some
algae |
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