🔁 Introduction

Alternation of generations refers to the cyclic change between haploid (n) and diploid (2n) stages in the life cycle of plants. Each generation gives rise to the other — ensuring both gamete formation and spore formation occur in an organized sequence.

🌱 Basic Concept

  • Gametophyte (n): Haploid plant producing gametes (sex cells) by mitosis.
  • Sporophyte (2n): Diploid plant producing spores by meiosis.
  • Fertilization: Fusion of male and female gametes → Zygote (2n) → Sporophyte.
  • Meiosis: In sporangia → Spores (n) → Gametophyte.

🧬 Alternation of Generations

The life cycle of any sexually reproducing plant involves an alternation of generations between the haploid gamete-producing gametophyte and the diploid spore-producing sporophyte. However, different plant groups, as well as individuals representing them, differ in the following patterns:

  1. Haplontic Life Cycle:

    • The dominant, photosynthetic phase is the free-living gametophyte (n).
    • The sporophyte (2n) generation is represented only by the one-celled zygote.
    • There are no free-living sporophytes.
    • Meiosis in the zygote results in the formation of haploid spores.
    • The haploid spores divide mitotically to form the gametophyte.
    • Examples: Many algae such as Volvox, Spirogyra, and some species of Chlamydomonas represent this pattern.

  2. Diplontic Life Cycle:

    • The diploid sporophyte (2n) is the dominant, photosynthetic, independent phase.
    • The gametophytic phase (n) is represented by the single to few-celled haploid gametophyte.
    • Gametes are formed directly from the diploid sporophyte via meiosis (gametic meiosis, though typically meiosis produces spores which then form gametophytes).
    • The gametophyte is highly reduced and dependent.
    • Examples: All seed-bearing plants i.e., gymnosperms and angiosperms, follow this pattern. The alga Fucus is also diplontic.

  3. Haplo-diplontic Life Cycle:

    • Both the gametophyte (n) and sporophyte (2n) phases are multicellular and often free-living, though they differ in their dominant phases.
    • This is an intermediate condition between haplontic and diplontic.
    • Bryophytes: The dominant, independent, photosynthetic phase is the haploid gametophyte. The multicellular sporophyte is short-lived and totally or partially dependent on the gametophyte for anchorage and nutrition.
    • Pteridophytes: The dominant, independent, photosynthetic, vascular plant body is the diploid sporophyte. The multicellular gametophyte (prothallus) is also independent but short-lived.
    • Examples: All bryophytes and pteridophytes exhibit this pattern. Some algae like Ectocarpus, Polysiphonia, and kelps are also haplo-diplontic.

🌿 Examples

  1. Haplontic Life Cycle
  • Dominant gametophyte; sporophyte is short-lived and dependent.
  • Found in most algae (Chlamydomonas, Spirogyra).
  1. Diplontic Life Cycle
  • Dominant sporophyte; gametophyte reduced (in pollen & embryo sac).
  • Seen in Pinus, Mango, Wheat.
  1. Haplo-diplontic Life Cycle
  • Alternating prominent gametophyte and sporophyte.
  • Found in Bryophytes (dominant gametophyte) and Pteridophytes (dominant sporophyte).

🔬 Key Differences Between Gametophyte and Sporophyte

Feature Gametophyte Sporophyte
Ploidy Haploid (n) Diploid (2n)
Function Produces gametes Produces spores
Division Mitosis Meiosis
Phase Sexual generation Asexual generation
Example Prothallus in fern Frond-bearing plant in fern

🧠 Summary

  • Alternation of generations ensures genetic diversity through meiosis and fertilization.
  • Life cycles evolved from haplontic → haplo-diplontic → diplontic forms.
  • Reflects increasing dominance of the sporophyte in plant evolution.

💡 Solved Examples

Q1. What is the main difference between haplontic and diplontic life cycles?
A1. In haplontic cycles, the dominant phase is gametophyte; in diplontic cycles, the dominant phase is sporophyte.

Q2. Give one example of a haplo-diplontic plant.
A2. Pteris (fern) — both gametophyte and sporophyte are independent.

Q3. Why is alternation of generations important?
A3. It ensures alternation between meiosis (spore formation) and fertilization (zygote formation) maintaining chromosome number.

Q4. Name one plant with haplontic life cycle.
A4. Chlamydomonas or Spirogyra.

Q5. In which group of plants is sporophyte dependent on gametophyte?
A5. In bryophytes, sporophyte depends on gametophyte for nutrition.