Life Cycle Patterns in Plants

Alternation of generation is the defining rhythm of every plant life cycle: a haploid (n), gamete-producing gametophyte phase alternates with a diploid (2n), spore-producing sporophyte phase. The two phases are linked by two key events — meiosis, which converts diploid tissue back to haploid spores, and syngamy (fertilization), which fuses two haploid gametes back into a diploid zygote. Depending on which phase is long-lived, independent and photosynthetic, three broad patterns are recognised.

In a haplontic life cycle the gametophyte (n) is the dominant, independent, photosynthetic phase; the sporophyte is reduced to nothing more than the zygote itself. The zygote undergoes meiosis immediately (this is called zygotic meiosis) to restore the haploid condition. Volvox and Spirogyra follow this pattern.

In a diplontic life cycle the sporophyte (2n) is the dominant, independent, photosynthetic phase, and the gametophyte is cut down to just a handful of cells that produce gametes directly (gametic meiosis happens in the sporophyte, producing spores/gametes rather than a free-living haploid generation). Fucus, and — importantly for the rest of this chapter — gymnosperms and angiosperms, are diplontic. …