Q.What is meant by 'alternation of generation'? Describe the haplontic, diplontic and haplo-diplontic types of life cycles with one example of each, and explain how the relative dominance of the gametophyte and sporophyte differs across Bryophyta, Pteridophyta and Gymnosperms.

What alternation of generation means. Every plant life cycle alternates a diploid sporophyte (2n) generation with a haploid gametophyte (n) generation. Diploid cells in the sporophyte undergo meiosis to form haploid spores; spores divide by mitosis to build the gametophyte; the gametophyte produces gametes by mitosis; two gametes fuse at fertilisation to form a diploid zygote; and the zygote divides by mitosis to rebuild the sporophyte, completing the cycle.

Haplontic. Mitosis occurs only on the haploid side. The zygote, formed at fertilisation, is the sole diploid cell in the whole cycle, and it undergoes meiosis almost immediately (zygotic meiosis) to restore the haploid state. Example: some algae.

Diplontic. Mitosis occurs only on the diploid side; meiosis is delayed until gamete formation, so only the gametes are haploid, and the diploid organism itself is multicellular and long-lived. Example: animals -- this pattern 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 gametophyte and a genuinely multicellular diploid sporophyte, one alternating with the other. Example: land plants generally, and many algae.

Dominance in Bryophyta. The gametophyte is dominant: it is the green, independent, photosynthetic plant body (thalloid in liverworts, leafy in mosses) that we ordinarily notice. The sporophyte is small, short-lived, and stays physically attached to and nutritionally dependent on the gametophyte throughout its existence.

Dominance in Pteridophyta. The pattern reverses: the sporophyte is dominant, independent, vascular and structurally complex -- the fern plant itself -- while the gametophyte (a small prothallus) is a separate, short-lived, independent but non-vascular structure. …

First define the generic cycle (meiosis/mitosis/fertilisation), then describe the three named life-cycle patterns with one example each, and finally trace gametophyte …

• Confusing haplontic and diplontic -- remember haplontic means only the zygote is diploid, not the reverse. …