Atoms, cars and ecosystems all have parts that together make up the whole. Trees have the perfect number of parts to survive and thrive in the environment. Each part has a primary job to perform.

Everyone can see the biggest tree parts. Trees have twigs and branches that support the leaves and buds. The stem supports the branches. The roots support the stem.

The tree parts few people notice are small, internal components. When you cut across a branch with a saw, you cut many tissues or parts. The first part damaged is the bark.

Bark is an amazing material. The outside bark is made of dead, air-filled cells that have been flattened and sealed with a waterproof material.

Think of bark as an oil-covered canvas coat. It's lightweight. It floats in water -- corks are made from the bark of an oak tree.

Bark is a good insulator, too. It protects the tree from sudden changes in temperature and from heating caused by sunlight.

Bark helps the tree conserve precious moisture by slowing water's escape into the air. But it must also allow for oxygen to move inward and carbon dioxide to escape.

Bark's the first barrier to breach, too, when a pest attacks a tree. Without healthy bark, trees would be dry, suffocated and diseased.

Beneath the dead bark layers are living cells that produce bark. Beneath these cells is a layer of transport cells called the phloem. This is sometimes called inner bark.

Phloem is responsible for moving food and other growth materials made in the leaves to all the other parts in a tree. It's the major transport highway in a tree.

The phloem is made by special cells that split apart, producing two new cells -- one to the outside and one to the inside. The new cells split off to the outside become phloem. The ones split off to the inside become xylem, the wood inside a tree.

Both phloem and xylem come from the same mother cells. These special mother cells are an area called the cambial zone, a major growth and reaction center in a tree.

This cambial zone responds to changes in the environment by producing more or fewer cells of different types and with different characteristics. It's responsible for a tree's yearly expansion in girth.

When you cut into the wood, the outer protection layer (bark), food transport connections (phloem) and growth layer (cambial zone) are all severed. Remember, the most exterior parts of a tree are responsible for its life.

Those are the parts people most easily damage, too. The bumps, bruises and nicks of daily life, many of which we may consider minor, can add up to be life- threatening in a tree.

The inner portion of a branch, stem or root is made of xylem. The xylem area is a combination of parts that act as transport pipes, strength fibers and living helper cells.

Xylem pipes transport water, essential elements and growth materials from the roots to the leaves. The fiber cells provide strength, keeping the stem upright against gravity and wind. The living helper cells store food, react to damage and transport needed growth materials for short distances.

If you don't know tree parts and how they work, it's hard to understand a whole tree. A tree is a single, highly integrated and specialized creature.

Trees must stand against an environment trying to knock them down and neighbors trying to consume them. You can help them survive and thrive by appreciating their parts, knowing how they work and preventing tree damage.