Anatomy of a Tree

The Inside Story

1. The outer bark is the tree’s protection from the outside world. Continually renewed from within, it helps keep out moisture in the rain, and prevents the tree from losing moisture when the air is dry. It insulates against cold and heat and wards off insect enemies.
2. The inner bark, or “phloem”, is pipeline through which food is passed to the rest of the tree. It lives for only a short time, then dies and turns to cork to become part of the protective outer bark.
3. The cambium cell layer is the growing part of the trunk. It annually produces new bark and new wood in response to hormones that pass down through the phloem with food from the leaves. These hormones, called “auxins”, stimulate growth in cells. Auxins are produced by leaf buds at the ends of branches as soon as they start growing in spring.
4. Sapwood is the tree’s pipeline for water moving up to the leaves. Sapwood is new wood. As newer rings of sapwood are laid down, inner cells lose their vitality and turn to heartwood.
5. Heartwood is the central, supporting pillar of the tree. Although dead, it will not decay or lose strength while the outer layers are intact. A composite of hollow, needlelike cellulose fibers bound together by a chemical glue called lignin, it is in many ways as strong as steel. A piece 12″ long and 1″ by 2″ in cross section set vertically can support a weight of twenty tons!

Leaves Make Food for the Tree

And this tells us much about their shapes. For example, the narrow needles of a Douglas fir can expose as much as three acres of chlorophyll surface to the sun.

The lobes, leaflets and jagged edges of many broad leaves have their uses, too. They help evaporate the water used in food-building, reduce wind resistance—even provide “drip tips” to shed rain that, left standing, could decay the leaf.

Roots: The Other Half of the Tree

What happens below ground is just as important as the trunk and canopy. Most of a tree’s roots grow in the top twelve to eighteen inches of soil, where water and oxygen are available, and they often spread well past the edge of the branches. Fine feeder roots near the surface absorb water and nutrients, while larger woody roots anchor the tree. Because these roots sit so shallow, compacting the soil, piling fill over the root zone, or cutting roots during construction can harm a tree even when the canopy still looks healthy.

Why Tree Anatomy Matters for Care

Understanding how a tree is built leads to better care decisions. Since the thin cambium layer just under the bark carries the tree’s growth and its food and water channels, any wound that rings the trunk can be deadly, which is why nailing into bark or string trimmer damage at the base is so harmful. Knowing that new growth comes from branch tips explains why topping a tree fails, since it removes the very buds the tree relies on and forces weak regrowth. Good pruning works with this structure by making clean cuts at the branch collar so the tree can seal the wound.

For pruning and care that respects how your trees grow, call Hometown Tree Experts at 301-250-1033 or visit our tree trimming page.

Related: water needs during dry periods