Question
Classify the plant kingdom from Thallophyta to Angiosperms. Explain the alternation of generations and how the dominant phase changes across plant groups.
(NEET, CBSE Class 11 — Plant Kingdom)
Solution — Step by Step
Plant body is a thallus — no true roots, stems, or leaves. Mostly aquatic. Include green algae (Chlorophyceae), brown algae (Phaeophyceae), and red algae (Rhodophyceae). Reproduction by fragmentation, zoospores, or gametes. The gametophytic phase is dominant.
First land plants. Have stem-like and leaf-like structures but no true vascular tissue. Called “amphibians of the plant kingdom.” Gametophyte is dominant. Sporophyte is dependent on gametophyte. Examples: Funaria (moss), Marchantia (liverwort), Anthoceros (hornwort).
Have true roots, stems, leaves, and vascular tissue (xylem and phloem). Still need water for fertilization (motile sperms). Sporophyte is dominant for the first time. Gametophyte is a small, independent prothallus. Examples: Selaginella, Dryopteris (fern), Equisetum (horsetail).
Seeds appear for the first time — a major evolutionary advance (no need for water in fertilization). Seeds are naked (not enclosed in fruit). Sporophyte is dominant. Examples: Pinus, Cycas, Ginkgo. Pollination is by wind.
Seeds enclosed in fruits (ovary matures into fruit). Have flowers as reproductive organs. Double fertilization is unique to angiosperms. Sporophyte is dominant. Divided into monocots and dicots. Examples: rice, wheat (monocots), mango, pea (dicots).
graph TD
A["Plant Kingdom"] --> B["Thallophyta (Algae)<br/>No vascular tissue<br/>Gametophyte dominant"]
A --> C["Bryophyta<br/>No vascular tissue<br/>Gametophyte dominant"]
A --> D["Pteridophyta<br/>Vascular tissue<br/>Sporophyte dominant"]
A --> E["Gymnospermae<br/>Naked seeds<br/>Sporophyte dominant"]
A --> F["Angiospermae<br/>Seeds in fruit<br/>Sporophyte dominant"]Why This Works
The classification reflects evolutionary progression: from water to land, from gametophyte-dominant to sporophyte-dominant, from spore-based to seed-based reproduction.
Alternation of generations means the life cycle alternates between a haploid gametophyte (produces gametes) and a diploid sporophyte (produces spores). In algae and bryophytes, the gametophyte is the main plant body. From pteridophytes onwards, the sporophyte becomes dominant — this makes sense because the diploid phase offers genetic redundancy, which helps survive on land.
Alternative Method — Tracking Key Innovations
Each group introduced a major innovation: Bryophytes = land colonisation, Pteridophytes = vascular tissue, Gymnosperms = seeds, Angiosperms = flowers and fruits. Track these four milestones and the classification makes sense.
NEET frequently asks: “In which group does the sporophyte first become dominant?” Answer: Pteridophyta. This is the transition point. Bryophytes and below = gametophyte dominant. Pteridophytes and above = sporophyte dominant.
Common Mistake
Students confuse bryophytes with pteridophytes. The key difference: bryophytes lack vascular tissue and their gametophyte is dominant. Pteridophytes have vascular tissue and their sporophyte is dominant. If you remember “vascular tissue = pteridophytes onwards,” you will not mix them up.