Year 1 · Organs and cells · Section 2.2

Plant organs

Roots

Plants have different types of roots. Many have a main root, lateral roots and root hairs. All the roots together form the root system, an organ system with three main functions:

Absorption: Root hairs take up water and mineral nutrients from the soil.
Support: Roots anchor the plant in the ground.
Storage: Roots may store food reserves used for new growth, for example in spring.

For example, a dandelion can regrow from its root after its above-ground parts die back. Stored reserves support the new growth.

Stems

Stems connect roots to leaves and have two main functions:

Transport: Water and minerals travel towards the leaves, while sugars made by photosynthesis are transported to other parts of the plant.
Support: Wood makes woody plants, such as trees, strong. In herbaceous plants, water pressure inside cells helps keep stems firm. These plants may wilt when short of water.

An experiment with coloured water can show how stems transport water and dissolved substances to other plant parts.

Leaves

A leaf has several parts:

Leaf stalk (petiole): Connects the leaf to the stem.
Leaf blade: The flat part of the leaf containing veins.
Veins: The main vein and branching veins provide support and transport substances.
Mesophyll: Tissue between the veins where most photosynthesis takes place.

The main function of leaves is to make food by photosynthesis. This mainly occurs in the mesophyll, although green stems may also photosynthesise. Water reaches the leaf through the roots and veins.

The vascular system

Transport tubes run through roots, stems and leaf veins, often grouped into vascular bundles. Together they form the plant's vascular system:

Water and minerals move from roots towards leaves.
Sugars made in leaves move to other parts of the plant. Glucose is commonly converted to sucrose for transport.

Glossary

Mesophyll: The tissue inside a leaf, between its outer layers and around its veins, where most photosynthesis occurs.
Leaf blade: The flat part of a leaf.
Leaf stalk: The part attaching a leaf to a stem; also called the petiole.
Woody plants: Plants such as trees and shrubs with much wood in their stems.
Herbaceous plants: Plants with little or no wood in their stems.
Minerals: Mineral nutrients that plants take up in dissolved form from the soil.
Veins: Transport and support structures in leaves, containing vascular bundles.
Food reserves: Substances stored by a plant for later use.
Vascular bundle: A group of transport tissues running together through a plant.
Transport tubes: Narrow structures that carry water, minerals or sugars through a plant.
Vascular system: All the transport tissues in a plant together.
Root hairs: Thin extensions of cells near the growing regions of roots, increasing the surface for absorption.
Root system: All of a plant's roots together.
Lateral roots: Roots branching from a main root.