The Importance of Wood to Human Existence Is Hard to Overestimate

The Importance of Wood to Human Existence Is Hard to Overestimate

The Importance of Wood to Human Existence Is Hard to Overestimate

Should be pretty self Explanatory not difficult all the information you need and all the questions should be in the pdf. let me know if u have any questions

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Lab 6 Secondary Growth Introduction As opposed to primary growth where the tissue is soft and green, secondary growth produces hard tissue due to the presence of molecules such as lignin secreted in fibers and in water conducting xylem cells. Secondary growth results in the production of xylem (wood) toward the interior of the stem and phloem (part of the bark) toward the exterior of the stem. The importance of wood to human existence is hard to overestimate. Wood is the secondary growth of xylem and is used for building materials, furniture, paper and pulp products, cellophane (a type of plastic), rayon, medicine, artificial vanilla flavoring, solvents such as turpentine, and many other products.

Wood consists of three main biomolecule types. These are: 1. Carbohydrates – cellulose and hemicelluloses (55-65%): Cellulose fibrils form cell walls. Hemicelluloses cross-link cellulose fibrils. 2. Lignins (20-30%) These are hardening agents 3. Extractives (2-15%) – tannins and other phenolic compounds, resins, oils, fats, waxes – generally produced by the plant as anti-predation compounds or sealants. Functions Secondary stems provide physical strength for plants. In some environmental conditions (such as in forests) the tallest trees are the most successful. Shorter trees are often shaded out and die or remain as dwarf trees in the forest. Stems are also used for carbohydrate storage.

As new leaves or branches begin to grow they use the energy, in the form of carbohydrates or lipids, stored in the stem. Development In the previous exercise you learned that primary growth arises from the apical meristem. Secondary growth is produced by lateral meristems. Unlike the apical meristems that are responsible for vertical growth, lateral meristems are responsible for an increase in the diameter of the plant. Surface Features of a Dormant Woody Twig The external structure of a secondary stem can be seen in an overwintering or dormant twig as seen in figure 1. At the tip of the twig locate the terminal bud scales. These enclose the terminal bud. The scales protect the apical meristem from winter weather or in some cases, periods of extreme dry weather.

If you look down the length of the twig you will see bud scale scars where the previous year’s bud scales were. You can age a twig by counting the bud scale scars, one produced each year. As the tree grows older these bud scale scars are shed along with the phloem and cork. If the tree is an evergreen (such as a pine or many tropical trees) it does not have an obvious dormant phase and you will not see bud scale scars. Along the length of the twig you may axillary buds and, if left to grow, they become branches. Just underneath these buds are leaf scars. Leaves are connected to the stem at this point and when they drop off they leave a crescent-shaped scar. The leaf scars and axillary buds occur at a swollen place on the stem called a node. Between nodes is the internode. Look for small dots along the surface of the stem. These are lenticels and they are small ruptures in the surface of the skin that allow for gas exchange. The tissue of the stem is alive and needs oxygen just as we do.

Activity: Woody Twig Examine figure 1 for features of a woody twig. Figure 1. Woody Twig. Photo left, illustration right. Composition of Wood Wood is the secondary xylem of trees and shrubs. In general there are four main cell types that make up xylem. These are tracheids, vessel elements, fibers, and parenchyma. Tracheids are cells that often conduct water vertically or they may function in some stems to just support the stem structurally. Vessel elements conduct water vertically and also support the stem. Tracheids and vessel elements are known as tracheary elements. Fibers provide structural support for flowering plants.

Parenchyma occurs in xylem rays and functions to store food and conduct water horizontally. Softwood trees (conifers) such as pines, firs, and spruce have tracheids and xylem rays. They do not have vessel elements. Tracheids are very good at conserving water as they have pits in their walls that function like flow restrictors in a sink faucet. These plants commonly live in regions where the soil is frozen during the winter yet they still photosynthesize because they are evergreen trees. In warm places such as Florida where there are conifers they are often found in sandy soils. The sandy soils do not hold on to water very long and this produces water-stress conditions. In the xylem of conifers you can also see resin ducts.

These ducts secrete resin when the plant is damaged. If insects chew on conifers they then have to deal with the sticky, acidic resin of the plant. If the damage is caused by physical reasons (such as a tree being struck by a falling branch) the resin seals the wound, decreasing the chance of fungal infection or desiccation. Hardwood Trees (broad-leaved trees) such as maple, ash, oak, mahogany, teak, magnolia, and linden have xylem rays, tracheids, fibers, and vessel elements. Vessel elements are larger than tracheids and allow for rapid water conduction up the stem. Imagine them like short sections of garden hose hooked together. Instead of the water having to pass through pits to move from one cell to another, the water moves through a system of tubes up the stem. In places where there is adequate water (such as in the tropics) the hardwood trees outcompete the softwoods. The Importance of Wood to Human Existence Is Hard to Overestimate

Pines are very rare in the tropics and occur in areas where it is dry (such as in rain shadows of mountains), or where there is significant burning or sandy soil. Sections of Wood Cross Section A section of wood looks different depending on how it is cut. Cross sections (transverse sections) of wood show the typical tree rings in species that live in temperate zones. Examine the photo of a Eucalyptus stem cut in cross section in figure 2. The banding that makes up the tree rings consists of earlywood (springwood) and latewood (summerwood). Earlywood occurs during the best growing season. Together with latewood they form annual rings. Eucalyptus grows quickly. Figure 3 shows a tree from the alpine region where the growth is extremely slow. Figure 2. Cross section of Eucalyptus stem.

Light rings are early wood, dark rings are late wood. Diameter is approximately 26 inches (66 cm). This tree is 14 years old. Figure 3. Cross section of a 15 cm (6.5 inch) stem of a 486 year old tree from the alpine zone. Each small dot in upper right part of sample represents 50 years of growth! In plants from the East Coast of the United States earlywood occurs in late spring through early fall. In those seasons there is adequate water, sunlight, and warm temperatures. Plants show rapid growth during this time and their xylem reflects this in the increased diameter of the tracheary elements. Latewood occurs in these plants when the growing conditions are worse, such as in late fall through the winter as the temperature drops and the days become shorter. The plants respond to these conditions by slowing or stopping growth. The tracheary elements of the xylem reflect this with a decrease in diameter. The plants in Santa Barbara show the pattern of earlywood and latewood but the seasons are reversed from what is found on the East Coast.

The optimum time for plant growth in Santa Barbara is November to May when there is adequate moisture. This is when plants show earlywood. As the soil dries out in Summer and Fall the conditions worsen for the plant and latewood forms. Tree rings have been used to date many objects or verify that objects belong to a particular time period. One interesting use of tree ring data is to determine the age of collectibles. Pictures, furniture, wooden instruments, etc. can command high prices if they are original. If the object, claimed to be extremely old, is in fact made from newer wood this may be determined by the use of tree ring data. The changes in rainfall produce different widths of tree rings and these provide a “calendar dating” of the wood. Note the differences in the width of the annual rings in figure 2 and see the diagram of a cross section in figure 4. Figure 4. Patterns in Sectioned Wood Radial Section Wood that is cut through the center along the long axis is said to be cut in radial section as seen in figure 4. This is also known as quarter split or quarter sawn.

Quarter split wood is used frequently in wooden instruments and hardwood flooring as the wood is more dimensionally stable if it is prepared in this way. There is less shrinkage and less warping in wood that is quarter split although you get less wood (board feet) per stem with this cut than with other cuts. When looking at quarter split wood you should be able to see earlywood and latewood as well as xylem rays. The xylem rays are typically perpendicular to the long grain of the wood. Examine figure 5 for this section. Figure 5. Radial section of wood showing perpendicular xylem rays Tangential Section A tangential section is one where the wood is cut along the long axis but off center as seen in figure 4. It is also known as plane sawn wood as this is the more economical cut in lumber mills when making boards because there is less waste in the milling process. The Importance of Wood to Human Existence Is Hard to Overestimate

Because earlywood and latewood are not perfectly in line with the long axis of the wood, these woods have an interesting swirling pattern in the section. Many boards and plywood show this type of section. Examine figure 6 for wood cut in tangential section. Figure 6. Tangential section. The dark loops are latewood. The lighter ones are earlywood. Tropical Woods Wood from the tropics commonly has no growth rings as the seasons are not well defined in tropical rain forests. Examine the photograph of tropical wood in figure 7. Notice that it does not have any growth rings common in trees of temperate regions.

Figure 7. Tropical wood with no annual rings Microscopic Examination of Wood Activity: Softwood Sections Microscopic wood sections are represented diagrammatically in figure 8. This sample has three sections on the slide which are cross section (X), radial section (R), and tangential section (T). Examine figure 9, a prepared slide of wood from the genus Pinus (pine). Figure 8. Diagram of Microscopic View of Wood Sections Examine the three sections of wood in pine in figures 9, 10, and 11 and know the following terms. Use your text if you need to identify certain structures. Type of section Cross, Radial or Tangential (XRT) Tracheids Rays Resin canals (in pine) Earlywood (springwood) Latewood (summerwood) Figure 9. Pine – cross (transverse) section Figure 10. Pine – radial section Figure 11. Pine – tangential section Activity: Hardwood Sections Examine a photograph of a prepared slide of wood from Magnolia grandiflora (Southern magnolia). This sample has three sections on the slide which are cross section (X), radial section (R), and tangential section (T). These are seen in figures 12-14. Compare the slides to those in your lecture text and find/know the following: Type of section (XRT) Tracheids Vessel elements (found in hardwoods) Rays Earlywood (springwood) Latewood (summerwood) Figure 12. Magnolia cross (transverse) section Figure 13. Magnolia – radial section Figure 14. Magnolia = tangential section Activity: Microscopic Examination of Entire Stems So far we have only looked at xylem in stems.

We will look at a cross section of the entire stem of the linden tree or basswood tree (Tilia americana) which is a common tree in the Eastern U.S. Figure 15 illustrates a prepared slide that comes from a 3 year old tree. Look at the xylem and find the 3 annual rings. The very inner part of the tree has a region of parenchyma cells known as pith. Just exterior to the xylem is a region known as the vascular cambium. Here you will see what appear to be two to five layers of brick-like cells evenly stacked. Only one of these layers is the actual vascular cambium but we will not distinguish exactly which layer that is. From the vascular cambium, xylem is produced toward the inside and phloem is produced toward the outside. Notice the xylem rays and how they expand to phloem rays. As the diameter of the stem increases so does the circumference and this puts a physical strain on cells. Parenchyma cells have the capacity to divide and the increase in the number of cells in some phloem rays allows for an increase in the circumference of the stem. Other phloem cells and cork cells divide too. Sieve cells and companion cells occur in the phloem. Each pair of sieve cells and companion cells arises from the same parent cell. The sieve cell loses its nucleus but the companion cell keeps its own and this allows the sieve cell to survive but still do the job of conducting sugar and other organic molecules. Unlike the xylem, the phloem can conduct material either upwards or downwards. The Importance of Wood to Human Existence Is Hard to Overestimate

You should look for dark red phloem fibers scattered in the tissue. Outside of the phloem is the phellem or cork which is a series of protective cells produced from sections of meristematic tissue called cork cambium. Cork cambium is another lateral meristem. Cork is tough due to the presence of suberin in the tissues. Suberin waterproofs the cells (killing them in the process), and protects the tree from desiccation, insects, microorganisms, and fungi. Bark is the term used to describe all of the tissue outside of the vascular cambium and consists of phloem and cork. Examine a cross section of Tilia in the figures 15 through 17. Compare these to your lecture text on pages 620 and 621. Find the following structures: Pith Xylem with Xylem Rays Vascular cambium Phloem with sieve cells, companion cells, phloem rays, and phloem fibers Cork or phellem (phellos = cork) Cork cambium Figure 15. Tilia Entire cross section of 3 year old stem. Figure 16. Tilia close-up showing xylem on bottom, vascular cambium, xylem and phloem rays, phloem fibers on top. Figure 17. Tilia close-up showing phloem (lower left) and cork (upper right). Other Features Branches erupt from stems and produce circular features in wood called knots. There are two main types of knots; intergrown or live knots where the branch tissue is still alive and the tissues of the stem and branch fuse together, and encased, or dead knots where the branch is dead and the tissue of the stem grows over the branch but the tissues do not fuse. Dead knots tend to pop out of boards easily. Below is a photograph of a live knot and a dead knot.

Figure 18. Live (intergrown) knot (left) and dead (encased) knot (right) Density of Wood It is common knowledge that wood floats but is that universally true for all woods? Density is a measure of mass/volume – the greater the mass (‘weight’) the greater the density for a given volume. In a section of wood that is quadrangular you can determine the volume which is equal to the width X height X depth. This is expressed as cubic centimeters (cc). The mass of the wood is measured on a balance in grams (g). Frequently density is listed by specific gravity. Water has a specific gravity (SG) of 1 as water weighs 1 g/1 cc. In terms of density, an object with an SG greater than 1 will sink. An object with an SG less than 1 will float. Balsa wood (Ochroma pyramidale) is the lightest of all commercial woods having a specific gravity of around 0.1. In this part of the exercise you will calculate the specific gravity of three woods. These are Guaiacum species, or Lignum vitae (Living wood) from South America, Diospyros species, Ebony which are threatened or endangered trees that grow in Indonesia, India, and Africa, and Tilia Americana, Linden native to North America (figure 19). The Importance of Wood to Human Existence Is Hard to Overestimate

Examine the data in chart 1 for each piece of wood, and calculate the specific gravity. Based on your calculations what will happen to the wood when you place it in water? Figure 19. Three woods: Ebony, top left; Linden, middle left; Lignum vitae, right Chart 1. Use the data obtained below to fill in question 10 in the Review Section. Mass Lengths Lignum Vitae 390 g 11 X 10 X 2.6 cm Ebony 110 g 2.4 X 9 X 4 cm Linden 43 g 11 X 4 X 2.2 cm Specific Gravity Sink/Float Wood Grain Trees show different grain patterns depending on the genetics of the tree (the same species can have individuals with different grain patterns), and the environmental conditions (trees on ridge tops more commonly have spiral grain than straight grain). In trees with straight grain the fibers of the wood are mostly parallel with the long axis of the wood. This wood is the preferred wood for boards and most wood use as it tends not to warp nor split as easily. In wood that has cross grain (see the ax handle in figure 20) the grain is at an angle to the long axis and this wood splits easily. In spiral grain the grain twists and this wood warps easily. Some wood is prized for its unusual grain. A good example of this is bird’s eye maple. This wood is highly prized for wood instruments and wood in luxury cars and jets. Examine figure 20 for the grain pattern.

Figure 20. Straight grain (top), cross grain of an ax handle (middle), spiral grain (bottom) Dendrochronology The age of some trees can be measured by dendrochronology where an increment borer is drilled into the stem of a tree and the annual rings counted (figure 21). Trees must be in areas where they are responsive to climate. A tree growing near a river has a constant supply of water year-round and does not make a good candidate for tree ring dating. Trees that have exposure to good growing conditions then reduced growing conditions each year (such as summer rains and winter snows) are well suited for dating. Figure 21. Increment borer. Heartwood As trees age many typically add resins, tannins and other compounds to the center of the xylem producing a darker colored wood known as heartwood. This interior wood does not conduct water but is important for structural support. The water conducting sapwood is found between the heartwood and the vascular cambium. A section of stem showing heartwood and sapwood is seen in figure 22.

Figure 22. Radial section of stem with the interior heartwood on top, sapwood below, and the outer bark on the bottom. In figure 23 you can see the difference between the insect resistant heartwood (dark regions) and the damaged sapwood (light regions with holes) in this cut board. Figure 23. Cross section of redwood board with termite damage. Cork One of the special features of a particular kind of bark is the outer surface of the cork oak tree. A cross section of the stem is seen in figure 24 and how wine corks are stamped from the tree is seen in a flattened section of cork in figure 25. Figure 24. Cross section of cork oak stem. The Importance of Wood to Human Existence Is Hard to Overestimate

Figure 25. Cork stamped from the bark of the cork oak tree. The First Plastic Figure 26 shows a cellulose plant where wood is ground up and converted into many products including a plastic material called cellophane. Cellophane was the only plastic wrap material produced for many years after WWI. The Importance of Wood to Human Existence Is Hard to Overestimate

Figure 26. Cellulose plant in western France (left) and cellophane roll (right). Cellophane image from https://www.ubuy.co.th. Review Section 1. What is the function of the terminal bud scales? 2. What chemical component makes wood hard? 3. What is the function of a lenticel? 4. What structure (cell type) is present in the wood of a hardwood tree that is absent in the wood of a softwood tree? 5. What is the function of a xylem ray? 6. What kind of cell is found making up the majority of wood in the xylem of conifer trees? 7. What tissue is produced by the cork cambium? 8. What tissues are produced by the vascular cambium? 9. Label the illustration of the twig below. 10. Use the chart below to enter the calculations and if the wood will sink or float. Mass Lengths Specific Gravity Sink/Float Lignum 390 g 11 X 10 X 2.6 cm Vitae Ebony 110 g 2.4 X 9 X 4 cm Linden 43 g …