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A tree has three main parts: (1) the trunk and branches; (2) the leaves; and (3) the roots. The branches and leaves together are called the crown. The trunk supports the crown and holds it up to the sunlight. Tree ferns, cycads, and most palms have no branches. Their crowns consist only of leaves. The roots of most trees are hidden in the ground, but they may take up as much space as the trunk and crown do above the ground. Other important parts of a tree include the seeds and the seed-forming structures. Trunk and branches give a tree its shape. The trunks of most needleleaf trees grow straight up to the top of the tree. The branches grow out from the trunk. On most needleleaf trees, the branches near the top are shorter than those farther down, which gives the crown a spirelike shape.

The trunks of most broadleaf trees do not reach to the top of the tree. Instead, the trunk divides into spreading branches near the base of the crown, giving the crown a rounded shape. The trunks of a few broadleaf trees, such as black willows and white poplars, sometimes divide so close to the ground that the trees seem to have more than one trunk.

The trunks, branches, and roots of broadleaf and needleleaf trees consist of four layers of plant tissue wrapped around one another. These layers, from innermost to outermost, are: (1) the xylem, (2) the cambium, (3) the phloem, and (4) the cork.

The xylem is the woody, central part of the trunk. It has tiny pipelines that carry water with a small amount of dissolved minerals from the roots to the leaves. This water is called sap. The cambium, which surrounds the xylem, is a thin layer of growing tissue. Its job is to make the trunk, branches, and roots grow thicker. The phloem, also called the inner bark, is a layer of soft tissue surrounding the cambium. Like the xylem, the phloem has tiny pipelines. The food made by the leaves moves through the phloem to the other parts of a tree. In palms and tree ferns, the xylem and phloem are not separate layers. Instead, bits of xylem and phloem are connected and form small double pipelines scattered throughout the trunk.

The cork layer is the outer bark of a tree. It forms a "skin" of hard, dead tissue that protects the living inner parts from injury. The bark stretches to let the trunk and branches grow thicker. The bark of some trees, such as beeches and birches, is smooth because it stretches easily. But the bark of most other trees does not stretch so well. As the trunk and branches grow thicker, they push against the bark. It finally cracks and dries and so becomes grooved and rough. Most trees replace old bark from time to time with a new layer.

Leaves of various species of trees differ greatly in size and shape. Palms have leaves over 20 feet (6 meters) long. The leaves of some needleleaf trees are less than 1/2 inch (13 millimeters) long. Some broadleaf trees have compound leaves made up of small leaflets.

The main job of the leaves is to make food for the tree. Every leaf has one or more veins, which consist of xylem and phloem tissue. The tissue that surrounds the veins contains tiny green bodies called chloroplasts. Water from the roots passes through the xylem of the trunk, branches, and leaves to the chloroplasts, which use the water to make food sugar. Only a small amount of the water carried to the leaves is used to make sugar. The leaves lose most of the water to the atmosphere through transpiration (evaporation). Like the water and dissolved minerals carried from the roots, the food made by the leaves is also called sap. It travels through the phloem of the leaves, branches, and trunk to parts of the tree where it is needed.

Almost all leaves are green in the spring and summer. Their color comes from chlorophyll, a green substance in the chloroplasts. Most trees also have reds and yellows in their leaves. But the green conceals these colors. In late summer and early autumn, the chlorophyll in the leaves of many broadleaf trees breaks down. The leaves then die. But before the leaves fall, they reveal their hidden reds and yellows. After the chlorophyll breaks down, the leaves of many trees also develop scarlets and purples.

Roots are long, underground branches of the trunk. They have the same layers of tissue as the trunk. The roots anchor a tree in the ground and absorb water with dissolved minerals from the soil. The main roots branch out into small roots, which, in turn, branch out into still smaller roots. The main roots of most trees begin to branch out 1 or 2 feet (30 or 61 centimeters) under the ground. Some trees have one main root larger than the others. This root, called a taproot, extends straight down 15 feet (5 meters) or more.