Surely, you’ve seen a cross cut wood round. Many of you have probably even cut some. However, have you ever dissected a tree to see the various layers? It’s like peeling an onion.
The Arbor Day Foundation has created a descriptive illustration showing the relationship in size, as well as structure, of the various layers. The dissection begins at the outside and works its way into the interior.
The outer bark is the tree’s outermost layer, which helps keep out moisture in the rain and prevents the tree from losing moisture when the air is dry. It also insulates against cold and heat and wards off insects and diseases. Its job is to protect the tree. As we continue our trip through a tree, you’ll see how the bark is constantly renewed from within.
The next layer is the inner bark. This layer has tiny tubes in which “phloem” is circulated. Phloem carries the food, manufactured by photosynthesis, throughout the tree. Inner bark lives for only a short time. Then it dies, turns to cork and becomes part of the protective outer bark.
The cambium layer is the growing part of the trunk. Each year, it produces new bark and new wood in response to hormones that pass down through the phloem from the leaves and stimulate growth in cells.
Sapwood is the trunk’s next layer. It’s new wood, and like the inner layer with its food carrying vascular system, the sapwood has similar tubes, called xylem, through which water and nutrients move from the roots up to the leaves. As newer rings of sapwood are laid down, inner cells lose their vitality and turn to heartwood, creating a new annual ring.
Heartwood is the central, supporting pillar of a tree. Although dead, it will not decay or lose strength while the outer layers are intact. It’s comprised of cellulose fibers bound together by a chemical glue called lignin, which makes it stronger than steel. The Arbor Day Foundation says that a cross section of wood 12” long and 1” by 2” set vertically can support a weight of twenty tons!
Leaves make food for the tree, and their shape influences their food making ability. 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 and even provide “drip tips” to shed rain that, left standing, could decay the leaf.
So, you can see, trees are very unique and complex organisms, worthy of the care they require to maintain their majestic beauty.