Grade 7 Chapter 2: Plants: Structure and Function

Introduction

Plants are essential for life on Earth, providing oxygen, food, and habitat. They are complex organisms with various parts, each performing specific functions that contribute to the plant's survival and growth. This chapter will explore the different structures of a typical flowering plant, including roots, stems, leaves, flowers, and fruits, and understand the vital roles each part plays.

The Root System

Types of Roots

Roots are typically found underground and anchor the plant, absorbing water and nutrients from the soil.

• Taproot System: Consists of a single, large, main root that grows vertically downwards, with smaller lateral roots branching off. Examples: Mustard, gram, banyan tree.
• Fibrous Root System: Consists of a cluster of thin, thread-like roots that spread out horizontally from the base of the stem. Examples: Grass, wheat, maize, onion.

Functions of Roots

• Anchoring: Roots firmly hold the plant in the soil.
• Absorption: Absorb water and minerals from the soil.
• Storage: Some roots, like carrots and radishes, store food.

The Stem

Characteristics of Stems

The stem is the main axis of the plant, usually growing above the ground. It bears leaves, flowers, and fruits.

• Nodes: Points on the stem where leaves arise.
• Internodes: The portion of the stem between two nodes.
• Buds: Present at the tip (apical bud) and in the axil of leaves (axillary buds), responsible for growth and branching.

Functions of Stems

• Support: Supports branches, leaves, flowers, and fruits.
• Conduction: Transports water and minerals from roots to leaves, and prepared food from leaves to other parts of the plant.
• Storage: Some stems, like potato and ginger, store food.
• Vegetative Propagation: Some stems, like rose and sugarcane, can be used for vegetative propagation.

The Leaf

Parts of a Leaf

Leaves are typically flat, green structures attached to the stem, primarily responsible for photosynthesis.

• Leaf Base: The part of the leaf attached to the stem.
• Petiole: The stalk that connects the leaf blade to the stem. (Absent in sessile leaves)
• Leaf Blade (Lamina): The broad, flat, green part of the leaf.
• Midrib: The central prominent vein running through the leaf blade.
• Veins: Network of smaller veins branching from the midrib, providing support and transport.

Types of Leaf Venation

The arrangement of veins in the leaf blade is called venation.

• Reticulate Venation: Veins form a net-like pattern. Common in dicotyledonous plants (e.g., peepal, hibiscus).
• Parallel Venation: Veins run parallel to each other. Common in monocotyledonous plants (e.g., maize, grass).

Functions of Leaves

• Photosynthesis: The process by which plants make their own food using sunlight, water, and carbon dioxide.
• Transpiration: The process of water vapor loss from the leaf surface through stomata.
• Respiration: Exchange of gases (oxygen and carbon dioxide) through stomata.

The Flower

Parts of a Flower

Flowers are the reproductive organs of flowering plants.

• Sepals (Calyx): Usually green, leaf-like structures that protect the bud.
• Petals (Corolla): Often brightly colored and fragrant to attract pollinators.
• Stamen (Androecium): The male reproductive part, consisting of anther (produces pollen) and filament.
• Pistil/Carpel (Gynoecium): The female reproductive part, consisting of stigma (receives pollen), style (connects stigma to ovary), and ovary (contains ovules).

Functions of Flowers

• Reproduction: Produce seeds through pollination and fertilization.
• Attraction: Attract pollinators (insects, birds) for pollen transfer.

The Fruit

Formation and Types of Fruits

Fruits develop from the ovary of a flower after fertilization and contain seeds.

• True Fruits: Develop only from the ovary (e.g., mango, tomato).
• False Fruits: Develop from the ovary along with other floral parts like thalamus (e.g., apple, cashew nut).

Functions of Fruits

• Seed Protection: Protect the seeds inside.
• Seed Dispersal: Aid in the dispersal of seeds to new locations.

Summary

• Plants have a root system (taproot or fibrous) for anchoring and absorbing water/minerals.
• The stem supports the plant, transports substances, and can store food.
• Leaves are the primary sites for photosynthesis, transpiration, and respiration.
• Flowers are the reproductive parts, containing sepals, petals, stamens, and pistils.
• Fruits develop from the ovary and protect and disperse seeds.
• Understanding these structures and functions is crucial to comprehending plant life.