Question
Describe the lifecycle of a fern. Which generation is dominant? How does the fern lifecycle compare to that of a moss?
(NEET + CBSE Class 11)
Solution — Step by Step
The fern plant you see with large fronds (leaves) is the sporophyte (diploid, 2n). Unlike mosses, the sporophyte is the dominant, independent, photosynthetic generation in ferns.
On the underside of mature fronds, you can see clusters of sporangia called sori (singular: sorus). Each sorus is covered by a protective flap called an indusium.
Inside sporangia, meiosis produces haploid spores (n). When ripe, the sporangium bursts open (due to the annulus mechanism) and spores are scattered by wind.
Each spore germinates on moist soil to form a small, heart-shaped structure called the prothallus — this is the gametophyte generation. The prothallus is tiny (a few mm), green, and independent but short-lived.
The prothallus (gametophyte) produces:
- Antheridia on the lower surface (produce sperm)
- Archegonia near the notch (produce eggs)
Sperm swim through a film of water to reach the archegonium. After fertilisation:
The zygote grows into a new sporophyte plant, and the prothallus withers away.
Fern Lifecycle Flowchart
flowchart TD
A["Sporophyte plant (2n) — the fern with fronds"] --> B["Sori on underside of fronds"]
B --> C["Meiosis in sporangia"]
C --> D["Spores (n) — released by wind"]
D -->|"Germinates on moist soil"| E["Prothallus (n) — heart-shaped gametophyte"]
E --> F["Antheridia produce sperm (n)"]
E --> G["Archegonia produce egg (n)"]
F -->|"Sperm swims through water"| H["Fertilisation"]
G --> H
H -->|"Zygote (2n)"| I["Young sporophyte grows from prothallus"]
I --> A
E -.->|"Prothallus withers after sporophyte establishes"| IWhy This Works
Ferns represent an evolutionary advancement over mosses. The sporophyte has become dominant and independent (it has roots, stems, and leaves), while the gametophyte (prothallus) is reduced to a tiny structure. However, ferns still need water for fertilisation — sperm must swim to the egg — which is why ferns are found in moist habitats.
This dependence on water for reproduction is the key limitation that seed plants (gymnosperms and angiosperms) overcame through pollen.
Common Mistake
Students forget that the prothallus is the gametophyte, not a young fern. The prothallus is haploid, heart-shaped, and photosynthetic — it is a completely separate generation from the sporophyte. Also, the sporangia produce spores by meiosis (not mitosis). Spores are haploid. If the question says “what division occurs in the sporangium?” — the answer is always meiosis.