UNIT 1: PLANT DIVERSITY – AN INTRODUCTION

1.0 Introduction

Plant diversity refers to the variety and variability of plant life on Earth. It includes diversity at genetic, species, and ecosystem levels. Diversity is highest in tropical regions due to favorable climate and high productivity.

Plants evolved from aquatic ancestors (green algae) and gradually adapted to terrestrial life through structural and reproductive changes.

1.1 Objectives

After studying this unit, students should be able to:

• Understand the concept of plant diversity
• Explain different plant groups
• Describe features of Bryophytes, vascular plants, and seed plants
• Understand thallus organization in algae

1.2 Concept of Plant Diversity

Plant diversity includes:

1. Genetic Diversity

Variation within species.

2. Species Diversity

Variety of plant species.

3. Ecosystem Diversity

Variety of ecosystems (forests, grasslands, oceans).

Phylogeny (Evolutionary Relationships)

Phylogeny explains evolutionary relationships among plants using:

• Morphological data (structure)
• Molecular data (DNA)

Alternation of Generations

All plants show two life stages:

• Gametophyte (Haploid) → Produces gametes
• Sporophyte (Diploid) → Produces spores

1.2.1 Bryophytes (Non-Vascular Plants)

Characteristics

• No vascular tissue (no xylem and phloem)
• No true roots, stems, leaves
• Possess rhizoids (for anchorage)
• Found in moist environments

Adaptations

• Cuticle prevents water loss
• Stomata for gas exchange

Reproduction

• Dominant phase: Gametophyte
• Requires water for fertilization

Examples

Mosses, liverworts, hornworts

1.2.2 Seedless Vascular Plants

Characteristics

• Presence of vascular tissue (xylem & phloem)
• True roots, stems, leaves
• Larger than bryophytes
• Contain lignin (strength)

Life Cycle

• Dominant phase: Sporophyte
• Gametophyte is small and independent

Examples

• Ferns
• Club mosses
• Horsetails

1.2.3 Seed Plants

Key Feature: Seeds

Seeds provide:

• Protection
• Dormancy
• Adaptation to dry conditions

Heterospory

• Microspores → Male gametophyte (pollen)
• Megaspores → Female gametophyte

Types of Seed Plants

1. Gymnosperms

• Naked seeds
• No fruits
• Wind pollination

Examples: Conifers, Cycads, Ginkgo

2. Angiosperms (Flowering Plants)

• Seeds enclosed in fruits
• Have flowers

Key Features

• Double fertilization
• Pollination by animals or wind
• Presence of fruits

1.3 Thallus Organization in Algae

Definition of Thallus

A plant body not differentiated into root, stem, and leaves.

Types of Thallus Organization

1. Unicellular Forms

• Single-celled
• Motile (e.g., Chlamydomonas)
• Non-motile (e.g., Chlorella)

2. Aggregates

Cells loosely grouped:

• Palmelloid
• Dendroid
• Rhizopodial

3. Colonial Forms

Cells form organized colonies:

• Motile (e.g., Volvox)
• Non-motile (e.g., Scenedesmus)

4. Filamentous Forms

Cells arranged in chains:

• Unbranched (Spirogyra)
• Branched (Cladophora)

Types

• Simple
• Heterotrichous (prostrate + erect system)
• Pseudoparenchymatous

5. Siphonaceous Forms

• Multinucleate
• No cross walls
• Tube-like structure

Example: Vaucheria

6. Parenchymatous Forms

• Cells divide in multiple planes
• Leaf-like or complex structure

Key Points to Remember

• Plant diversity is highest in tropical regions
• All plants show alternation of generations
• Evolution trend:
  Bryophytes → Vascular Plants → Seed Plants
• Algae show wide variation in thallus structure
• Thallus is undifferentiated plant body