TREES

These tall, seed-producing plants have a single woody stem, called a trunk, which supports their great weight. They live for many years and do not die in winter. The largest group of trees is broad-leaved trees.

WHY ARE TREES SO IMPORTANT?

Trees release oxygen into the atmosphere for other organisms to breathe in. The roots of trees bind soil together, preventing it from being washed away. Trees also provide food and habitats for many animals, and wood for fuel, lumber, and many other products.

HOW DO TREES GROW?

Trees grow in two ways. Special cells at the tips of twigs divide, making the twigs grow. Also, a layer of cells under the bark, the cambium, divides, widening the trunk and branches. The new cells that the cambium makes form a visible ring inside the trunk.

TREE TRUNK CROSS-SECTION

The inside of a tree trunk has rings of outer sapwood and inner heartwood. Both consist of strength-giving cells called xylem. Sapwood xylem also carries water and minerals up the tree. Outside the sapwood are phloem cells, which carry food, surrounded by bark.

WHAT ARE BROAD-LEAVED TREES?

Unlike conifers, with their needle-shaped leaves, these trees have wide, flat leaves. Many are DECIDUOUS TREES that lose their leaves in fall, but the broad-leaved trees of a tropical rainforest are evergreen. The mass of leaves of adjacent trees form a CANOPY.

TYPES OF LEAVES

Broad-leaved trees have one of two types of leaves. A simple leaf is undivided and has its own leaf stalk. A compound leaf is divided up into several smaller leaflets that are attached to the main leaf stalk.

TREE CLASSIFICATION

Trees are divided into three groups: broad-leaved trees, palms, and conifers.

Broad-leaved trees are the largest group, with over 10,000 species. They are dicotyledon flowering plants, and the veins in their leaves branch out like a net.
Palm trees, with about 2,800 species, are monocotyledon flowering plants. The veins in their leaves are always parallel.
Conifers, with 550 species, have cones instead of flowers.

DECIDUOUS TREES

Trees that lose their leaves in the fall are called deciduous. They grow in temperate places that have warm summers and cool or cold winters.

WHY DO THESE TREES SHED THEIR LEAVES?

When trees shed their leaves, they stop growing. This helps them conserve energy during winter, when there is not enough sunlight to make food. Shedding leaves also helps trees save water, since it stops water from evaporating.

WHY DO DECIDUOUS LEAVES CHANGE COLOR?

In summer, these leaves are packed with the green pigment (coloring) chlorophyll, which captures sunlight energy. In fall, chlorophyll breaks down and is reabsorbed by the tree, revealing previously hidden pigments, such as reds, yellows, and oranges.

CANOPY

The upper part of the trees in any forest or woodland is called the canopy. It is made up of their branches, twigs, and leaves. Tropical rainforest trees form a dense canopy that is home to many animals.

WHY ARE RAINFOREST TREES SO TALL?

In hot, steamy rainforests, the tightly packed trees grow rapidly and to great heights. This is because they are all competing for sunlight. The taller the tree, the more light its leaves will receive. Some trees can reach heights of up to 200 ft (60 m).

WHAT IS AN EPIPHYTE?

An epiphyte is a plant that grows on a larger one without harming it. Many epiphytes live high up in the rainforest canopy, firmly anchored by their roots to tree trunks and branches. In the canopy, epiphytes receive much more light than the plants on the forest floor, and more rainwater.

LAYERS OF A RAINFOREST

Rainforests are made up of distinct layers. In the emergent layer, the very tallest trees stick out from the canopy. Beneath the canopy is an understory of smaller trees and a shrub layer of big-leaved plants that can live in low-light conditions. Below this is the dark forest floor, where there are fewer, smaller plants.

Copyright © 2007 Dorling Kindersley