Unit 5: Gymnosperms

Syllabus Reference: Classification, structure and reproduction of Cycas, Pinus, Ginkgo and Gnetum. Economic importance. [cite_start]Fossils, Geological Time Scale, fossil flora of India [cite: 486-487].

1. Distinguishing Features of Gymnosperms

• Name Meaning: Gymnos = Naked, Sperma = Seed. Ovules/seeds are not enclosed inside an ovary (fruit).
• Habitat: Mostly Xerophytic (adapted to dry conditions).
• Vascular Tissue: Xylem has tracheids (Vessels usually absent, except in Gnetales). Phloem has sieve cells (Companion cells absent).
• Reproduction: Heterosporous. Pollination is direct (pollen lands on ovule) and anemophilous (wind).

2. Study of Genera

A. Cycas (Cycadales)

• Habit: Palm-like tree. Unbranched stem with a crown of pinnately compound leaves.
• Root: Coralloid roots present (associated with Cyanobacteria for N₂ fixation).
• Reproduction: Dioecious (Male and Female plants separate).
  • Male Cone: Large, compact.
  • Female: No true cone. Megasporophylls are arranged loosely.
• Sperm: Largest in plant kingdom, multiciliate and motile (Swimming sperm in a seed plant is a primitive trait).

B. Pinus (Coniferales)

• Habit: Tall, pyramidal tree. Branched.
• Leaves: Needle-like (acicular) to reduce transpiration.
• Roots: Mycorrhizal association (fungus helps in absorption).
• Reproduction: Monoecious (Male and Female cones on the same tree).
  • Pollen: Winged pollen grains (yellow dust).

C. Ginkgo (Ginkgoales)

• Living Fossil: Ginkgo biloba is the only surviving species.
• Leaves: Fan-shaped with dichotomous venation.
• Significance: Resistant to pollution. Swimming sperm (like Cycas).

D. Gnetum (Gnetales)

• Advanced Gymnosperm: Shows affinities with Angiosperms.
• Angiosperm-like characters:
  • Presence of Vessels in Xylem.
  • Leaves are broad with reticulate venation.
  • Flower-like arrangement of sporophylls.

3. Economic Importance

• Timber: Pinus, Cedrus (Deodar) provide soft wood for furniture and construction.
• Resin/Turpentine: Obtained from Pinus species. Used in paints and varnishes.
• Food: Seeds of Pinus gerardiana (Chilgoza) are edible. Sago (starch) is obtained from Cycas stem.
• Medicine: Ephedra yields Ephedrine (for asthma). Taxus yields Taxol (anti-cancer).

4. Paleobotany & Fossils

Types of Fossils (Modes of Preservation)

• Compression: Organic matter flattened by pressure (e.g., leaves in clay).
• Petrifaction: Mineral solutions (silicates/carbonates) infiltrate tissues, preserving cellular details (e.g., Petrified wood).
• Impression: Only the imprint remains; organic matter is lost.
• Amber: Preservation in fossilized resin (insects/spores).

Geological Time Scale

History of Earth divided into Eras and Periods.
Paleozoic Era: Age of Pteridophytes.
Mesozoic Era: Age of Gymnosperms (Jurassic period).
Cenozoic Era: Age of Angiosperms (Current).

Fossil Flora of India

Birbal Sahni is the Father of Indian Paleobotany (Institute in Lucknow). Major fossil sites in India include the Rajmahal Hills (Jurassic flora). Notable Indian fossils: Williamsonia (Gymnosperm), Pentoxylon.