Alternation of Generation

Every plant's life cycle unfolds across two alternating generations: the diploid sporophyte (2n) and the haploid gametophyte (n), linked to each other by two key events -- meiosis and fertilisation.

The cycle, step by step. Certain diploid cells within the sporophyte undergo meiosis, halving the chromosome number to produce haploid spores. Each spore then divides repeatedly by mitosis to build up the multicellular gametophyte. The gametophyte, in turn, produces haploid gametes by mitosis (not meiosis, since it is already haploid). Two gametes -- typically one male, one female -- fuse at fertilisation, restoring the diploid number and forming a zygote. The zygote then divides by mitosis to rebuild the multicellular diploid sporophyte, completing the cycle. Because the sporophytic and gametophytic phases occur one after another in this way, the phenomenon is called alternation of generations.

Three named patterns, seen mainly among algae. Depending on which phase in the cycle is multicellular and long-lived, three patterns are recognised:

• Haplontic -- mitosis occurs only on the haploid side, building up a multicellular (or single-celled) haploid organism; gametes form by mitosis; after fertilisation, the zygote is the only diploid cell in the entire cycle, and it undergoes meiosis almost immediately (zygotic meiosis) to restore the haploid state. Seen in some algae.
• Diplontic -- mitosis occurs only on the diploid side; meiosis is delayed until gametes are formed, so the gametes are the only haploid cells produced. This pattern, common in animals, is comparatively rare among the plant groups covered in this chapter.
• Haplo-diplontic -- mitosis occurs on both sides, so the organism passes through a genuinely multicellular haploid phase (the gametophyte) and a genuinely multicellular diploid phase (the sporophyte), each alternating with the other. This is the general pattern followed by land plants and by many algae.

How dominance shifts across the four groups of this chapter. While the underlying cycle -- meiosis, gametophyte, gametes, fertilisation, zygote, sporophyte -- stays the same, which of the two generations is the larger, more conspicuous, independent phase changes markedly as one moves from algae to gymnosperms, and this shift is one of the clearest evolutionary threads running through the whole chapter.

• In Algae, the pattern varies by species, spanning all three types above (haplontic, diplontic and haplo-diplontic), without one dominance pattern holding for the whole group.
• In Bryophyta, the gametophyte is dominant: it is the green, photosynthetic, independent plant body we ordinarily notice (whether thalloid, as in a liverwort, or leafy, as in a moss), while the sporophyte is comparatively small, short-lived, and remains physically attached to and nutritionally dependent on the gametophyte throughout its existence.
• In Pteridophyta, the pattern reverses: the sporophyte is dominant, independent, vascular and long-lived -- it is what we recognise as 'the fern plant' -- while the gametophyte (the small prothallus) is a separate, short-lived, non-vascular structure that, although capable of independent photosynthesis, exists only briefly before or alongside the sporophyte generation. …