Plant Kingdom Class 11 Notes

Classification of Plants and Alternation of Generations

Plants are classified into four groups based on key structural features: Bryophytes (no vascular tissue, no seeds), Pteridophytes (vascular tissue present, no seeds), Gymnosperms (vascular tissue and naked seeds) and Angiosperms (vascular tissue, seeds enclosed in fruit/ovary). A unifying concept across all plant groups is the alternation of generations.

• Haplontic — the gametophyte (n) is dominant; the sporophyte (2n) is short-lived and dependent on the gametophyte. Example: fern prothallus.
• Diplontic — the sporophyte (2n) is dominant; the gametophyte (n) is highly reduced. Examples: gymnosperms and angiosperms (seed plants).
• Haplodiplontic — both gametophyte and sporophyte are independent and photosynthetic. Examples: Bryophytes and Pteridophytes.

Bryophytes — The Amphibians of the Plant Kingdom

Bryophytes are non-vascular plants (no xylem or phloem). They are called 'amphibians of the plant kingdom' because they live on land but require water for reproduction (sperm must swim to the egg). The gametophyte is the dominant, photosynthetic and independent phase.

• Liverworts (e.g. Marchantia) — the plant body is thalloid (flat, ribbon-like) with dichotomous branching. They reproduce by fragmentation or gemmae (cup-like structures on the thallus that produce new plants).
• Mosses (e.g. Funaria, Sphagnum) — have a leafy gametophyte with a central axis (seta) and simple leaf-like structures. The sporophyte consists of a foot, seta and capsule. Spores are released from the capsule.
• Bryophytes lack true roots, stems and leaves — they have rhizoids (root-like), stems and leaf-like structures instead.
• Sphagnum (peat moss) is commercially important: it is used as packing material for transshipping living plants and in peat formation.

Pteridophytes — The First Vascular Plants

Pteridophytes are seedless vascular plants — they have true vascular tissue (xylem and phloem) but reproduce by spores rather than seeds. The sporophyte is the dominant phase, but the gametophyte is still independent and photosynthetic (prothallus).

• Examples: ferns (Dryopteris, Pteris), horsetails (Equisetum) and club mosses (Lycopodium, Selaginella).
• The sporophyte has true roots, stems and leaves — the first plant group to evolve these structures.
• Spores are produced in sporangia, often grouped in clusters called sori (singular: sorus) on the underside of leaves.
• The gametophyte (prothallus) is small, heart-shaped and free-living. It produces antheridia (male) and archegonia (female).
• Water is needed for fertilisation — sperm swim from antheridia to archegonia.

Gymnosperms — Naked Seeds

Gymnosperms are vascular plants that produce 'naked' seeds — seeds that are not enclosed within an ovary or fruit. The seed is borne on the surface of scales or leaves, often arranged in cones (strobili). The sporophyte is the dominant phase.

• Examples: Cycas (cycad), Pinus (pine), Ginkgo (maidenhair tree), Ephedra.
• They are woody, perennial plants — most gymnosperms are trees or shrubs.
• The ovule is not enclosed in an ovary; it lies naked on the megasporophyll (modified leaf).
• Pinus shows a unique association: its seeds lack endosperm at maturity. Nutrition is provided by mycorrhizal fungi that colonise the roots.
• The wood of gymnosperms is softwood (no vessels in xylem of Pinus; Cycas has no vessels at all).

Angiosperms — Flowering Plants

Angiosperms are the most diverse and dominant group of land plants. They are called flowering plants because their reproductive organs are flowers. The seeds are enclosed within the ovary, which develops into a fruit after fertilisation.

• Angiosperms are divided into two groups: Monocotyledons (monocots) and Dicotyledons (dicots).
• Monocots: one cotyledon in seed, parallel leaf venation, fibrous root system, scattered vascular bundles, trimerous flowers (parts in 3s). Examples: grass, wheat, maize, rice, banana.
• Dicots: two cotyledons in seed, reticulate venation, tap root system, ring-arranged vascular bundles, tetramerous or pentamerous flowers (parts in 4s or 5s). Examples: mango, neem, pea, rose.
• Angiosperms show remarkable diversity: from tiny Wolffia (a few mm) to tall trees like Eucalyptus, from herbs to shrubs.
• The double fertilisation unique to angiosperms involves one sperm fertilising the egg (forming zygote) and the other fertilising the polar nuclei (forming endosperm — triploid).

Economic Importance and Summary

Plants are indispensable for human survival: they provide food (cereals, fruits, vegetables), medicine, timber, fibres, rubber, dyes and ornamental products. A summary of plant groups and their key features is useful for quick revision.

• Bryophytes — non-vascular, no seeds, gametophyte dominant, need water for reproduction.
• Pteridophytes — vascular, no seeds, sporophyte dominant, gametophyte (prothallus) free-living.
• Gymnosperms — vascular, naked seeds, sporophyte dominant, seeds not in fruit.
• Angiosperms — vascular, seeds enclosed in fruit, sporophyte dominant, flowers for reproduction.

Solved Examples

Example: Why are bryophytes called 'amphibians of the plant kingdom'?

Solution: Bryophytes live on land (terrestrial habitat) like reptiles and amphibians, but they require water for reproduction — their flagellated sperm must swim through water to reach the egg for fertilisation. This dependence on water for sexual reproduction, despite being terrestrial, earns them the title 'amphibians of the plant kingdom'.

Example: Distinguish between homosporous and heterosporous pteridophytes. Give one example of each.

Solution: Homosporous pteridophytes produce only one type of spore, which germinates into a bisexual (bisexual) gametophyte. Example: Pteris, Lycopodium. Heterosporous pteridophytes produce two types of spores — megaspores (female) and microspores (male) — which develop into unisexual gametophytes. This is an important evolutionary step towards the seed habit. Example: Selaginella, Salvinia.