3.10 Topic for discussion.

What are different woods used for? (Use appendix if necessary)

Part V texts for additional reading

1 Tree parts and how they «work»

The trunk of a tree is important for two reasons: first, it acts as a support rod, giving the tree its shape and strength; second, it acts as the central “plumbing system” in a tree, forming a network of tubes that carries water up and down to the branches, trunk and roots. The easiest way to see how a tree works is to look at a cross section of a trunk. Here are the five main layers you would see, and what each layer does. The outer layer of the trunk (and branches) is called the outer bark or just the bark. Tree bark can be smooth, scaly, rubbery, flaky, craggy, or bumpy. Its texture, thickness and flexibility depend on the type of tree. Although bark looks different from tree to tree, it serves the same purpose – to protect the tree from injury and disease. Some trees have very thick bark that helps prevent damage from fires. Others have bad-tasting chemicals in their bark that discourage hungry insects. And some bark is covered with spines or thorns that keep browsing mammals away. The bark of large Douglas firs and sequoias may be more than two feet (6 m) thick. The layer next to the outer bark is called the inner bark, or phloem. The phloem is a thin layer that acts as a food supply line from the leaves to the rest of the tree. Sap (water containing dissolved sugars and nutrients) travels down from the leaves through channels in the phloem to the branches, trunk and roots, supplying all the living parts of the tree with food. At certain times during the year, the phloem also transports stored sugars up from the roots to the rest of the tree. If you were to cut a band around the trunk, through the bark and phloem, the tree would probably due. That’s because the phloem would be severed and food could no longer flow to the lower trunk and roots.) Next to the phloem is a very thin layer called the cambium. (It is often only one or two cells thick, and you need a microscope to see it well.) The cambium is one of the growing layers of the tree, making new cells during the growing season that become part of the phloem, part of the xylem, or more cambium. The cambium is what makes the truck, branches, and roots grow thicker. The layer next to the cambium is called the sapwood or new xylem. The sapwood is made up of the youngest layers of wood. (Each year the cambium adds new layers of woody tissue.) The sapwood is a network of thick-walled cells that forms a pipeline, carrying water and minerals up the tree from the roots to the leaves and other parts of the tree. The sapwood also stores nutrients and transports them across the tree, from one part to another. Most of the trunk in an old tree is dead wood called heartwood, or just plain wood. The heartwood is old xylem that no longer transports water and minerals up a tree. (After a few years the sapwood in most trees gets filled in with resin-like material and slowly changes into heartwood. The new xylem is the only part of the wood that works as a transport system.) The heartwood is often much darker in color than the sapwood. The heartwood gives the tree support. But sometimes it rots away, leaving a hollow, living tree. Hollow trees often topple over or split apart in storms because they are very weak after the heartwood has decayed. A trees’ roots are long, underground branches that spread out to help anchor the tree and to absorb water and nutrients from the soil. Some trees have long taproots that reach straight down for 15 feet (4.5 m) or more. Other trees have more shallow root systems that lie closer to the surface of the ground. Large taproots and lateral roots branch into smaller and smaller roots. An average tree has millions of these small rootlets, each covered with thousands of fine root hairs. The root hairs make it easier to soak up water and dissolved minerals from the soil. (Most of the rootlets lie very close to the surface of the ground where most of the water and nutrients are located.)

From skinny pine needles to broad palm leaves, all tree leaves serve the same purpose – to make food for the tree. Leaves use carbon dioxide from the air, water from the roots, and sun’s energy (in the form of sunlight), to make sugar (glucose). This food-making chemical reaction is called photosynthesis. Photosynthesis can take place only in the presence of chlorophyll – the green pigment that is found in all green plants. Chlorophyll absorbs the sunlight needed for photosynthesis. During photosynthesis the leaves release oxygen that becomes part of the air that we and other animals breathe.

(4527 signs)