Give an account of plant life Cycles and alternation of Generations.

Explanation

In plant, both haploid and diploid cells can divide by mitosis which leads to the formation of different plant bodies, haploid and diploid. The haploid plant body produces gametes by mitosis and is called gametophyte. It is followed by fertilization, which results in the formation of zygote which also divides by mitosis to form diploid sporophytic plant body. In this Meiosis occurs, results in the formation of spores. These spores again divide by mitosis to form a Haploid plant body. Thus during life cycle there is a alternation of generations between gamete producing haploid gametophyte and spore producing diploid sporophyte.
Different plants show different life cycles. For Ex :
1) Many Algae such as Volvox, Spirogyra and some species of Chlamydomomas shows Haplontic life cycle. In this, zygote represents the sporophytic stage which divides by meiosis results in the formation of Haploid spores. These spores divide mitotically and form the gametophyte.
2) In some species, the diploid sporophyte is the dominant photosynthetic. Independent phase of the plant. The haploid phase is represented by gametes only. So this lifecycle is called diplontic type. In some pteridophytes, the gametophyte is represented by few celled stage so called diplo-haplontic type. Other examples are polysiphonia.
3) In Bryophytes, both phases are multicellular with dominant gametophytic phases and dependent sporophytic phage. So this life cycle is called haplo-diplontic type. Other examples for this are Ectocarpus, Laminaria.