2017-11-01
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The trunk provides mechanical support for the crown, transports water and elements, and stores food manufactured in the leaves. Outer bark is the outermost tissue and is composed of dead cells. The outer bark is outside protection for the growing part of the tree. The outer bark (figure 5.1), which originates from phloem cells that have worn out, died and been shed outward, protects the tree from insects, storms and extreme temperatures injury, from drying out, limited mechanical injury and attack by infectious diseases. In certain species, the outer bark also protects the tree from fire.
The trunk consists of layers of tissue: the inner bark or phloem, the cambium, xylem or sapwood and heartwood. These layers contain a network of tubes that runs between the roots and the leaves and acts as the central system for the tree. These tubes carry water and minerals up from the roots to the leaves, and they carry sugar down from the leaves to the branches, trunk and roots.
Tree trunks house an amazing system of pipelines. If you cut through the outer bark you would come to the soft inner bark or phloem. The function of the inner bark is to transport food manufactured in the leaves downward to nourish the cambium and other growing parts. In other words, the phloem distributes food made in the leaves to every living cell in the tree. Damage to the inner bark will stop movement of food to growing parts immediately below the injured area. Girdling completely around a tree through its inner bark will eventually kill the tree.
Next is the cambium layer, which you cannot see in a cross section without a magnifying glass. The cambium is located between the sapwood and inner bark (phloem) and is composed of a thin, continuous layer of cells. The cambium is a very thin layer of growing tissue that produces new cells that become xylem, phloem or more cambium. Every growing season, a tree’s cambium adds a new layer of xylem to its trunk, producing a visible growth ring in most trees. The cambium is what makes the trunk, branches and roots grow larger in diameter.
The xylem (dead cells) distributes water and minerals up the trunk to the leaves, where food is manufactured. Xylem and phloem are continually being made in new stem areas. Xylem cells are formed at the inner surface of the cambium. This tissue eventually forms the wood of an old stem. Phloem is formed at the outer surface of the cambium and makes up most of the bark of an old stem. Xylem and phloem contain fiber cells which form a tissue that strengthens the stem. Sapwood is composed mostly of living cells through which food and water are collected by the roots and sent up to the branches and leaves. Sapwood consists of both living and dead cells. Conducting vessels in the sapwood transport water and nutrient elements in solution upward to the leaves. Storage of manufactured food also takes place in the sapwood. With age, sapwood becomes heartwood in some trees.
The center of the stem is the heartwood. This is composed of dead cells that give the tree strength to stand. The heartwood was once sapwood but when new sapwood formed, the older died and formed the heartwood of the tree. Wood or medullary rays are thin bands of cells which radiate outward from the inner portions of the tree to the inner bark in some trees. These rays are particularly prominent when looking at a cross-sectional cut of oak wood. Wood rays transport plant food laterally between the sapwood and inner bark. Some food storage also takes place here. As you study the cross section of a tree, note that some annual rings are wider apart than others. When you see a wide space between the rings this means the tree grew faster at that time because it got more sunlight, water, and food. Lack of sunlight, food, water and competition with neighboring trees or being subjected to destructive forces such as forest fires, insects and diseases, slow down the growth of the tree. The years marking slow growth show the rings closer together and narrow in width.
5.3Find the answers to the following questions in the text 1:
1. What are the main functions of a trunk?
2. What is the role of an outer bark?
3.How does the stem transport water and minerals up from the roots to the leaves?
4. What is a cambium?
5. What function does a cambium perform?
6. Can you find out how much the stem grows larger in diameter each growing season?
5.4 Complete the sentences with the following verbs: connects, transports, supports, protects, acts.
The trunk of a tree, which is protected by a tough outer covering of bark, …the roots to the branches. The trunk … water and minerals from the soil to the rest of the tree. The trunk … the tree and as it grows taller than the plants around it, it is able to reach more sunlight, which is essential for growth. The outer bark on the trunk … the inside of the tree from injury and from drying out. It also … as an insulator against cold and heat.
