3.4 Plant life cycle and alternation of generations:

Life cycleof a plant includes two phases or distinct generations namely sporophyte (diploid : 2n) and gametophyte (haploid : n).

Some special diploid cells of sporophyte divide by meiosis to produce haploid cells.

These haploid cells divide mitotically to give rise to gametophyte.

The gametophyte produces male and female gametes which fuse during fertilization to produce diploid zygote.

It divides by mitosis to form diploid sporophyte. The sporophytic and gametophytic generations generally occur alternately in the life cycle of a plant. This phenomenon is called alternation of generations.

Distinct alternation of these two generations is observed in Bryophytes and Pteridophytes.

In Gymnosperms and Angiosperms, gametophyte is much reduced and exists within sporophyte.

Based upon the nature of dominant phase in life cycle, it is called haplontic, diplontic or haplodiplontic life cycles.

Life cycle patterns : (a) Haplontic (b) Diplontic (c) Haplo-diplontic

In Bryophytes haploid gametophyte is dominant. It is photosynthetic, independent thalloid or erect phase. Sporophyte is short lived, multicellular and depends totally or partially on gametophyte for nutrition and anchorage.

Whereas in Pteridophytes, sporophyte is dominant, independent and vascular plant body. Haploid multicellular gametophyte is generally autotrophic and short lived. It alternates with Sporophyte.

1. What is alternation of generations?

2. Which phase is dominant in the life cycle of Bryophyta and Pteridophyta ?