Chapter 6: Classification of Plants
Comprehensive chapter summary with detailed explanations and examples.
Grade 9 Learning: Chapter 6: Classification of Plants
Introduction to Plant Classification
The plant kingdom is incredibly diverse, with millions of species exhibiting a wide range of forms, habitats, and life cycles. To study this vast diversity systematically, biologists classify plants into different groups based on their similarities and differences. This process helps us understand evolutionary relationships and makes the study of plants more organized.
Hierarchy of Classification
Living organisms are classified into a hierarchical system, from broad categories to more specific ones. For plants, the main ranks are:
- Kingdom: Plantae (the broadest category for plants)
- Division: (Equivalent to Phylum in animals)
- Class:
- Order:
- Family:
- Genus:
- Species: (the most specific category)
This hierarchy helps in identifying and naming organisms precisely.
Criteria for Plant Classification
The classification of plants is primarily based on the following criteria:
- Presence or absence of organs: Whether the plant body is differentiated into distinct roots, stems, and leaves.
- Presence or absence of separate conducting tissues: Whether specialized vascular tissues (xylem for water transport and phloem for food transport) are present.
- Presence or absence of seeds: Whether the plant reproduces by forming seeds or by spores.
- Seeds enclosed in fruit or not: If seeds are present, whether they are enclosed within a fruit (flowering plants) or are naked.
- Number of cotyledons in seeds: For flowering plants, whether the seed has one or two cotyledons (seed leaves).
Kingdom Plantae: Subkingdoms
Based on the presence or absence of flowers, fruits, and seeds, the Kingdom Plantae is broadly divided into two subkingdoms:
- Cryptogams: Non-flowering plants (hidden reproductive organs).
- Phanerogams: Flowering plants (visible reproductive organs, produce seeds).
Subkingdom Cryptogams (Non-flowering plants)
These plants have hidden reproductive organs and reproduce by spores. They do not produce flowers, fruits, or seeds.
1. Division Thallophyta (Algae)
- Body structure: Plant body is not differentiated into roots, stems, or leaves. It is a simple, undifferentiated thallus.
- Habitat: Mostly aquatic (freshwater and marine).
- Chlorophyll: Contain chlorophyll and are autotrophic.
- Vascular tissues: Lack specialized vascular tissues.
- Examples: Spirogyra, Ulothrix, Ulva, Sargassum, Fucus, Chara. (Note: Fungi, though thalloid, are not included here as they lack chlorophyll).
Example: Spirogyra
Spirogyra is a common freshwater green alga. It forms slimy green mats in ponds. Its chloroplasts are spiral-shaped, which is a distinctive feature.
2. Division Bryophyta (Amphibians of the Plant Kingdom)
- Habitat: Grow in moist, shady places. They are called 'amphibians' because they need water for reproduction, even though they live on land.
- Body structure: Plant body is flat, ribbon-like, or prostate. It is differentiated into root-like rhizoids, stem-like, and leaf-like parts, but no true roots, stems, or leaves.
- Vascular tissues: Lack specialized vascular tissues (xylem and phloem).
- Reproduction: Reproduce by spore formation.
- Examples: Moss (Funaria), Marchantia, Riccia.
Example: Funaria (Moss)
Funaria is a typical moss. It has a main plant body (gametophyte) that is green and photosynthetic, and a sporophyte (spore-producing capsule) that grows out of it.
3. Division Pteridophyta (Ferns and their relatives)
- Body structure: These are the first plants to have true roots, stems, and leaves.
- Vascular tissues: Possess well-developed vascular tissues (xylem and phloem) for conduction of water and food throughout the plant body.
- Reproduction: Reproduce by spores, which are formed in sori (clusters) on the undersurface of their leaves.
- Flowers/Seeds: Do not bear flowers, fruits, or seeds.
- Examples: Ferns (Nephrolepis), Marsilea, Pteris, Adiantum, Equisetum.
Example: Nephrolepis (Fern)
Ferns are popular ornamental plants. Their leaves (fronds) are often large and compound, and you can often see small brown spots (sori) on their undersides, which contain the spores.
Subkingdom Phanerogams (Flowering plants)
These plants have special structures for reproduction, produce flowers, and bear seeds. The seeds contain an embryo and stored food for its initial growth.
1. Division Gymnosperms (Naked-seeded plants)
- Characteristics: Mostly perennial, evergreen, and woody plants.
- Stem: Stems can be unbranched (like Cycas) or branched (like Pinus, Deodar).
- Leaves: Leaves often form a crown.
- Seeds: Their seeds are 'naked,' meaning they are not enclosed within a fruit. They are typically exposed on cone scales.
- Examples: Cycas, Pinus (Pine), Thuja (Christmas tree), Deodar.
Example: Cycas
Cycas resembles a palm tree but is a gymnosperm. It produces large cones instead of flowers, and its seeds are visible on these cones.
2. Division Angiosperms (Flowering plants, seeds enclosed in fruit)
- Characteristics: These are the most dominant and diverse group of plants on Earth.
- Seeds: Their seeds are enclosed within a fruit, which develops from the ovary after fertilization.
- Reproduction: Reproduce through flowers, which are specialized reproductive structures.
Angiosperms are further classified based on the number of cotyledons in their seeds:
Class Dicotyledons
- Cotyledons: Seeds have two cotyledons.
- Root system: Possess a tap root system (a main root with smaller lateral roots).
- Stem: Strong, often branched stems.
- Leaves: Leaves show reticulate (net-like) venation.
- Flowers: Flowers are typically tetramerous (parts in multiples of 4) or pentamerous (parts in multiples of 5).
- Examples: Pea, Bean, Mango, Banyan, Sunflower, Rose.
Class Monocotyledons
- Cotyledons: Seeds have a single cotyledon.
- Root system: Possess a fibrous root system (a cluster of thin, equal-sized roots).
- Stem: Often hollow or unbranched stems.
- Leaves: Leaves show parallel venation.
- Flowers: Flowers are typically trimerous (parts in multiples of 3).
- Examples: Maize, Wheat, Rice, Sugarcane, Onion, Grasses.
Comparison of Dicotyledons and Monocotyledons
| Feature | Dicotyledons | Monocotyledons |
|---|---|---|
| Cotyledons | Two | One |
| Root System | Tap root | Fibrous root |
| Stem | Strong, branched | Often hollow, unbranched |
| Leaf Venation | Reticulate (net-like) | Parallel |
| Flower Parts | Tetramerous or Pentamerous (multiples of 4 or 5) | Trimerous (multiples of 3) |
| Examples | Pea, Bean, Mango, Sunflower | Maize, Wheat, Rice, Sugarcane |
Conclusion
The classification of plants helps us to organize and understand the vast diversity of the plant kingdom. From simple thalloids to complex flowering plants, each group exhibits unique characteristics and adaptations. Understanding this classification provides a fundamental framework for studying plant biology and ecology.
References
- Maharashtra State Board Science and Technology Standard Nine Textbook (Latest Edition) - Chapter 6: Classification of Plants.
- Maharashtra State Board 9th Standard Science Syllabus.
- Balbharati Science and Technology Textbook Part 1.