Question
Classify the plant kingdom from Thallophyta to Angiosperms. Explain the concept of alternation of generations and how it changes across plant groups.
Solution — Step by Step
flowchart TD
A[Plant Kingdom] --> B[Algae - Thallophyta]
A --> C[Bryophytes]
A --> D[Pteridophytes]
A --> E[Gymnosperms]
A --> F[Angiosperms]
B --> B1[Gametophyte dominant]
C --> C1[Gametophyte dominant]
D --> D1[Sporophyte dominant]
E --> E1[Sporophyte dominant]
F --> F1[Sporophyte dominant]All plants alternate between two multicellular phases: the gametophyte (n, haploid — produces gametes by mitosis) and the sporophyte (2n, diploid — produces spores by meiosis). Spores grow into gametophytes; gametes fuse to form sporophytes. The key difference across plant groups is which phase dominates.
In algae (some) and bryophytes (all), the gametophyte is the main plant body. The sporophyte is small and often depends on the gametophyte. In mosses, the green leafy plant is the gametophyte (n); the stalk with capsule is the sporophyte (2n). Bryophytes lack vascular tissue and need water for fertilization.
In ferns, the sporophyte becomes dominant — the large fern plant with fronds is the sporophyte (2n). The gametophyte is a tiny, independent, heart-shaped prothallus. Pteridophytes have vascular tissue but no seeds. They still need water for fertilization (flagellated sperm).
The sporophyte is the massive plant body (tree/shrub). The gametophyte is extremely reduced: male gametophyte = pollen grain, female gametophyte = embryo sac (in angiosperms) or endosperm tissue (in gymnosperms). Seeds evolve in gymnosperms. Pollen eliminates the need for water in fertilization. Angiosperms add flowers, fruits, and double fertilization.
Why This Works
The evolutionary trend is clear: gametophyte reduces, sporophyte expands. This shift correlates with the move from water to land — the diploid sporophyte offers genetic buffering against environmental stress, while seeds and pollen free reproduction from water dependency.
| Group | Dominant Phase | Vascular Tissue | Seeds | Water for Fertilization |
|---|---|---|---|---|
| Algae | Gametophyte (variable) | No | No | Yes |
| Bryophytes | Gametophyte | No | No | Yes |
| Pteridophytes | Sporophyte | Yes | No | Yes |
| Gymnosperms | Sporophyte | Yes | Yes (naked) | No (pollen) |
| Angiosperms | Sporophyte | Yes | Yes (enclosed) | No (pollen) |
Common Mistake
Students say “the fern plant is the gametophyte” because it is green and photosynthetic. Wrong — the fern plant is the sporophyte (2n). The gametophyte is the tiny prothallus that develops from a spore. This is the critical switch that happens at the pteridophyte level and is a very common NEET trap.