Plant Life Cycle and Alternation of Generations

A plant's life cycle unfolds across two alternating generations: the diploid sporophyte (2n) and the haploid gametophyte (n).

The cycle, step by step:

1. Certain diploid cells within the sporophyte undergo meiosis to produce haploid cells (spores).
2. These haploid cells divide by mitosis to build up the multicellular gametophyte.
3. The gametophyte produces separate male and female gametes, again by mitosis.
4. Two gametes unite at fertilization, producing a diploid zygote.
5. The zygote divides by mitosis to build up the multicellular diploid sporophyte, completing the cycle.

Because the sporophytic and gametophytic generations occur one after another in this way, the phenomenon is called alternation of generations.

How dominant each generation is varies by group. A clearly visible alternation between an independent gametophyte and an independent sporophyte is seen in Bryophytes and Pteridophytes. In Gymnosperms and Angiosperms, by contrast, the gametophyte is greatly reduced and lives out its short existence tucked inside the sporophyte. Among algae, the pattern is described by which phase (haploid or diploid) dominates the life cycle, giving three named patterns:

• Haplontic — mitosis occurs only in haploid cells, building up a single haploid cell or a multicellular haploid organism; gametes are produced by mitosis; after fertilization the resulting zygote is the only diploid cell in the whole life cycle, and it undergoes meiosis immediately (zygotic meiosis) to restore the haploid state. Example: some algae and fungi.
• Diplontic — mitosis occurs only in diploid cells; meiosis is delayed until gametes are formed, so the gametes themselves are the only haploid cells, and the diploid zygote divides by mitosis to build a multicellular diploid organism. Example: animals.
• Haplo-diplontic — mitosis occurs in both diploid and haploid cells, so the organism passes through a genuinely multicellular haploid phase (the gametophyte) and a genuinely multicellular diploid phase (the sporophyte), one alternating with the other. Example: land plants and many algae. …