Alteration of generation is the phenomenom by which plant exhibits
two phases in its life cycle, each phase performs a specific function. The two phases are gametophyte and sporophyte; gametophyte is haploid (n) and sporophyte diploid (2n). The two phases alternate.
Seen clearly in e.g dryopteris, nephrolepsis, lycopodium, selaginella.
Dominant sporophyte consist of underground stem (rhizome), vertically emergent fronds arise from this. Fronds (sporophylls) bear groups of sporangia (which is covered by indusium and bear spores formed by mitosis).
Mature spores are released by mechanical action of annular and stomal cells of sporangia. Under favorable conditions spores germinate into heart shaped prothallus, prothallus is gametophyte adapted to moist/damp environment.
Environment is necessary to avoid dessication as well as to facilitate transfer of antherizoids. Flagellated antherizoids are released from antheridia and swam towards archegonia. The archegonia rupture and allow entrance of antherizoids.
Fertilization of egg cells take in the base of archegonia where zygote is formed, which grows into sporophyte. Cycle alteration.
SIGNIFICANCE OF ALTERATION OF GENERATION
-permits separation of two phases in time. This allows continuous perpetuation of species, each phase adapted to a different set of environmental condition.
-species are better able to survive varying/changing environments.
-Allows haploidization (by meiosis) in the sporophyte and dipoidization in the gametophyte to occur separately.
-Sporophyte is adapted to sexual reproduction during dry condition.
-Gametophyte is adapted to sexual reproduction on moist environment.