Question
Describe the lifecycle of a moss showing alternation of generations. Which generation is dominant — the gametophyte or the sporophyte? How does it differ from ferns?
(NEET + CBSE Class 11)
Solution — Step by Step
The green, leafy moss plant you see growing on damp walls is the gametophyte (haploid, n). It is the main, independent, photosynthetic phase.
The gametophyte produces gametes:
- Antheridia (male organs) produce sperm (antherozoids)
- Archegonia (female organs) produce eggs
Sperm swim through water to reach the egg — this is why mosses need moisture for reproduction.
When sperm reaches the egg inside the archegonium, fertilisation occurs:
The zygote develops into the sporophyte — the stalk with a capsule on top that grows attached to the gametophyte. The sporophyte is NOT independent; it depends on the gametophyte for water and nutrients.
Inside the capsule, meiosis occurs to produce haploid spores (n). When the capsule matures and opens, spores are released into the wind. Each spore that lands in a suitable spot germinates to form a protonema (a thread-like structure), which eventually grows into a new gametophyte plant.
Moss Lifecycle Flowchart
flowchart TD
A["Spore (n) — haploid"] -->|"Germinates"| B["Protonema — thread-like"]
B -->|"Grows into"| C["Gametophyte plant (n) — leafy moss"]
C --> D["Antheridia produce sperm (n)"]
C --> E["Archegonia produce egg (n)"]
D -->|"Sperm swims through water"| F["Fertilisation in archegonium"]
E --> F
F -->|"Zygote (2n)"| G["Sporophyte — stalk + capsule"]
G -->|"Meiosis in capsule"| A
G -.->|"Attached to and dependent on"| CWhy This Works
Mosses show a clear alternation of generations — a haploid gametophyte phase alternates with a diploid sporophyte phase. The key feature of bryophytes (mosses, liverworts, hornworts) is that the gametophyte is dominant and independent, while the sporophyte is small and dependent.
This is the opposite of what we see in higher plants (angiosperms), where the sporophyte is the dominant plant body and the gametophyte is reduced.
Common Mistake
Students mix up which generation is dominant in mosses vs ferns. In mosses, the gametophyte (n) is dominant. In ferns, the sporophyte (2n) is dominant. The evolutionary trend in plants is: gametophyte dominance (bryophytes) to sporophyte dominance (pteridophytes, gymnosperms, angiosperms). NEET loves asking: “In which group is the gametophyte the main plant body?” Answer: Bryophytes.