Question
Explain the life cycle of fern with reference to alternation of generations. Which generation is dominant in ferns?
Solution — Step by Step
Alternation of generations is the life cycle pattern where a plant alternates between two distinct multicellular forms:
- Sporophyte (2n): Diploid generation — produces spores by meiosis
- Gametophyte (n): Haploid generation — produces gametes (egg and sperm) by mitosis
These two generations take turns — sporophyte produces gametophyte, gametophyte produces sporophyte. The cycle continues indefinitely.
In ferns, the sporophyte is dominant — it’s the large, familiar fern plant you see. The gametophyte is tiny and short-lived.
The sporophyte (2n) is the familiar fern with:
- Rhizome (underground stem)
- Fronds (leaves/pinnae)
- Sori (clusters of sporangia on the underside of fronds) — these are the brown dots visible on fern leaves
Inside the sori, spore mother cells (2n) undergo meiosis to produce spores (n) — haploid cells with a tough wall. Each sorus is covered by a flap called an indusium.
The spores are dispersed by wind when they are mature.
When a spore lands in suitable conditions (moist soil), it germinates into a prothallus — the tiny, heart-shaped gametophyte (n).
The prothallus is:
- 1–2 cm wide, green, heart-shaped
- Anchored by rhizoids (root-like structures)
- Bears archegonia (flask-shaped, produce egg cells) and antheridia (produce motile sperm)
Fertilisation: Sperm swim through water film from antheridium to archegonium (this is why ferns need water for reproduction). Sperm fuses with egg → zygote (2n).
The zygote (2n) develops into a new sporophyte — first a small embryo attached to the prothallus, then an independent fern plant. The prothallus withers and dies once the sporophyte is established.
Complete cycle summary:
Dominant generation: Sporophyte — the large visible fern plant. The gametophyte (prothallus) is small and short-lived.
Why This Works
Alternation of generations is nature’s way of maintaining genetic diversity while using both sexual (gamete fusion) and asexual (spore) modes of reproduction. The sporophyte provides structural complexity and protection; the gametophyte enables genetic recombination through fertilisation.
Ferns represent a transitional point in plant evolution: unlike mosses (where gametophyte dominates), ferns have shifted dominance to the sporophyte — the pattern that continues in gymnosperms and angiosperms. The dependence on water for sperm swimming is why ferns are found in moist environments.
Alternative Method — Compare Across Plant Groups
| Plant Group | Dominant Generation | Example |
|---|---|---|
| Bryophytes (mosses) | Gametophyte | Funaria |
| Pteridophytes (ferns) | Sporophyte | Dryopteris |
| Gymnosperms | Sporophyte | Pinus |
| Angiosperms | Sporophyte | Oryza (rice) |
The evolutionary trend: gametophyte becomes more and more reduced; sporophyte becomes dominant.
Common Mistake
Saying the fern “prothallus” is the dominant generation. The prothallus is the gametophyte — it is tiny, temporary, and dependent on moist conditions. The large, leafy fern plant is the sporophyte, which is dominant. In bryophytes it’s the other way around (gametophyte dominant), and students often confuse the two groups.