Examining the Structures of a Tree CAMBIUM, meristem, heartwood, and lenticel are some terms that may be new to you. These terms are used to describe various tree structures. Not surprisingly, many terms are associated with a tree. After all, a tree is a complex multi-cellular organism with specialized tissues. Objective: Identify the major tree parts and their functions. Key Terms: apical meristem bud scales bud scale scars cambium cork cambium crown epidermis fibrous root system heartwood inner bark internode lateral bud leaf scar lenticels node outer bark phloem primary root root cap root hairs roots sapwood secondary root taproot system terminal bud tree trunk twig xylem The Parts of a Tree and Their Functions A tree is a woody plant with one well-defined stem and a formed crown. The roots, the trunk, and the crown are the three major tree parts. Page 1 www.mycaert.com
ROOTS The roots are the part of the tree typically found below the soil surface. Tree health is closely tied to the tree roots. For example, when the roots are weak or diseased, the whole tree suffers. The following are functions of the root system. The roots absorb all of the water and minerals that a plant needs to live. The roots anchor the plant to the ground and support the above-ground part of the tree. The roots store food that has been made through photosynthesis. This food can be used later when a tree needs it to grow or survive. Primary and Secondary Roots When a plant seed germinates, the first structure to emerge from the seed is a root. This root becomes the primary root; on some plants, the primary root remains the most important root in the whole root system. However, other roots (secondary roots) eventually branch out from the primary root. At the tip of the root, there is an area where new cells develop called the apical meristem. This area is easily damaged. Therefore, it has a root cap over the top of it to protect it from damage as it grows through the large and sometimes coarse soil particles. The surface of the root is covered with a skin of cells (epidermis). The epidermis is where the water and minerals enter the root through osmosis and diffusion. The larger the surface area of the epidermis, the better able the plant is to bring in water and minerals. So the epidermis cells begin to elongate and grow hair-like projections (root hairs) to increase the surface area of the root. To stay healthy and to produce new root hairs, the roots need to be constantly growing. Root Systems FIGURE 1. Some trees have roots that are partially above ground level. Two basic types of tree root systems exist: tap and fibrous. A root system that is composed of one main primary root and many secondary roots branching off of the primary root is a taproot system. Oak, pine, and beech have taproot systems. A system with no dominant primary root that is made of many primary and secondary roots of similar size is a fibrous root system. For example, magnolias and tulip poplar have fibrous root systems. Page 2 www.mycaert.com
TRUNK The trunk is the main structural element of a tree. It serves a number of important functions. The trunk supports the leaves. It holds the leaves in the most efficient position to collect sunlight, allowing the plant to produce as much food as possible. Water, minerals, and manufactured food are transported through the trunk between the roots and the crown. A trunk that is green in color helps produce food through photosynthesis. While this is not usually the primary food production, it can provide sugars in addition to those manufactured in the leaves. A trunk stores food manufactured by the tree. The trunk also produces the bulk of the useful wood in a tree. The trunk is comprised of a series of rings. Under normal conditions, one ring is produced each year, and each section of the ring has a specific purpose in the FIGURE 2. Annual rings are found in the interior of a tree. life of the tree. In large trees, the center portion of the trunk is darker and is known as heartwood. Cells in the heartwood are dead and are filled with gums, pigments, resins, and tannins to provide strength to the tree and to keep it upright. Outward from the heartwood is the lighter wood that is known as sapwood or xylem. The xylem is made of tube-like cells that grow together to conduct liquids. This is the living portion of the tree and the means by which minerals and water are carried from the roots to the crown. The sapwood also serves as the storage area for some of the food made in the leaves. Outside of the sapwood is the cambium, which consists of an active layer of cells that divide. The cambium is responsible for lateral growth of the tree. Cells to the center of the tree become xylem and those to the outside become phloem (inner bark). The phloem cells form tubes, and the phloem tissue carries food made in the leaves to the branches, trunk, and roots. The phloem is protected by the outer bark. This bark is formed by the cork cambium a thin layer of cells between the phloem and the bark (composed of dead cells). It protects the trunk. The age of a tree can be determined by counting annual growth rings made from the new cells formed during a growing season. However, the rate of growth impacts the growth rings. During rapid growth, the cells of the wood are thin walled and large in diameter. As growth Page 3 www.mycaert.com
UNDER INVESTIGATION LAB CONNECTION: Twig Collection In late fall, winter, and early spring, it can be challenging to identify deciduous trees. The bark, evidence of fruit, and the growth habit of the tree can offer distinguishing characteristics. However, the twig often provides the best clues for positive identification. The arrangement and shape of the leaf scars, twig color, buds, twig thickness, pith, and lenticels can be used in tree identification. An activity can be conducted to improve your ability to identify trees by studying twigs. In the fall or winter, collect twigs from trees that have been positively identified. The twigs should include material from the previous season s growth and some two-year-old wood. Usually, the materials range from 6 to 12 inches in length. Select samples that are representative and that are from an actively growing area. Label the twigs with a piece of tape, and use the twigs for study. slows during mid- to late summer, the wood cells become smaller and have thicker walls. The differences in the types of cells made give the appearance of rings. CROWN The crown is the top part of the tree, including the branches, twigs, buds, and leaves. A major role of the crown is to hold leaves so sunlight can be collected and photosynthesis can occur. Also, seeds, fruits, nuts, and berries are produced in this section of the tree. Twig Structure A twig is a small terminal branch that bears leaves, buds, flowers, and fruit. Some significant external structures are on a twig. For instance, the growing point at the tip (the apical meristem) is contained inside of the bud at the end of the stem Crown Trunk (bole) Roots Heartwood Sapwood Cambium Inner bark Outer bark Annual rings Tap root (some species) Surface roots FIGURE 3. This diagram displays the major parts of a tree. Page 4 www.mycaert.com
the terminal bud. The apical meristem is the same type of structure as the root tip and is responsible for plant elongation. Each leaf is attached to the stem at a node. Meanwhile, the area between leaves is an internode. A side bud always exists at the node just above where the leaf is attached; this is the lateral bud. On the outside of terminal and lateral buds are small protective structures called bud scales. When the leaf falls off of the stem, a small scar (leaf scar) remains just below the lateral bud. When the buds sprout each spring, the bud scales fall off, leaving behind a ring of scars (the bud scale scars). The distance between bud scale scars represents one year of stem growth. Lenticels are small spots, on the stem, that allow a stem to exchange gases with the environment. Summary: A tree is a woody plant with one well-defined stem and a formed crown. Three major tree parts are the roots, trunk, and crown. A tree s health is very closely tied to its roots. The roots absorb all of the water and minerals, anchor the plant to the ground, support the above-ground part of the tree, and store food. The trunk is the main structural element of a tree. It supports the leaves; transports water, minerals, and manufactured food; may produce food through photosynthesis; and stores food. The trunk also produces the bulk of the useful wood in a tree. The crown is the top part of the tree and includes branches, twigs, buds, and leaves. The crown holds leaves to collect sunlight so photosynthesis can occur. It is in this section that seeds, fruits, nuts, and berries are produced. Checking Your Knowledge: 1. What are the three major parts of a tree? 2. What are the functions of the roots? 3. What are the functions of the trunk? 4. What are the functions of the crown? 5. What are the major structures found on a twig? Page 5 www.mycaert.com
Expanding Your Knowledge: Closely examine the structures of a tree discussed in this unit. Compare the structures of different trees. Recognizing many of these structures is the first step in being able to understand how a tree grows. Web Links: Roots http://extension.oregonstate.edu/mg/botany/roots.html Plant Shoot System http://facweb.furman.edu/~lthompson/bgy34/plantanatomy/plant_shoot.htm Parts of a Tree http://www.ncforestry.org/docs/trees/partsoftree/parts.htm Parts of a Tree http://nature.wikia.com/wiki/parts_of_a_tree Page 6 www.mycaert.com