Alternation of generations is the universal feature of the Plant Kingdom — and arguably the single most important conceptual framework for understanding plant evolution as tested in NEET.
The Core Mechanism: Every plant alternates between two phases in its life cycle:
- Gametophyte (haploid, n): Produces gametes (sex cells) by mitosis. Two gametes fuse to form the zygote.
- Sporophyte (diploid, 2n): Develops from the zygote. Produces spores by meiosis. Spores develop into gametophytes.
The cycle is: Gametophyte → (mitosis) → Gametes → (fusion) → Zygote → Sporophyte → (meiosis) → Spores → Gametophyte.
The Evolutionary Shift in Dominant Phase:
| Group | Dominant Phase | Nature of Gametophyte | Nature of Sporophyte |
|---|---|---|---|
| Algae | Variable | Variable | Variable |
| Bryophytes | Gametophyte | Large, independent, photosynthetic (the visible plant) | Small, dependent (foot+seta+capsule) |
| Pteridophytes | Sporophyte | Small, independent (prothallus) | Large, independent (fern plant) |
| Gymnosperms | Sporophyte | Microscopic, completely dependent on sporophyte | Massive (the tree) |
| Angiosperms | Sporophyte | Maximally reduced (3-cell pollen grain; 7-cell embryo sac) | The entire visible plant |
The Progressive Reduction of Gametophyte: From bryophytes to angiosperms, the gametophyte undergoes progressive reduction from the entire visible plant (bryophytes) to a few cells enclosed within sporophyte tissue (angiosperms). This trend parallels increasing independence from water for fertilization.
Key NEET Questions from This Concept:
- Which phase is dominant in bryophytes? Answer: Gametophyte
- What is the gametophyte of pteridophytes called? Answer: Prothallus
- In angiosperms, name the male and female gametophytes: Answer: Pollen grain (3-celled) and Embryo sac (7-celled)
- What is the ploidy of the endosperm in angiosperms? Answer: Triploid (3n)