wood

ACTIVITY

Examine the microscope slides, tree cross-sections, wood blocks, and other samples set out in lab. The information on the prop cards is very important and is summarized below. You will find the Powerpoint presentation and the diagrams in your textbook very useful.

WOOD STRUCTURE

In truly herbaceous dicots and monocots, all or nearly all of the vascular tissue is produced by terminal meristems. In woody plants, primary vascular tissue is also produced by terminal meristems.

In plants that become woody, a vascular cambium layer forms between the primary xylem and phloem cells. Cambium is a laterally expanding meristem which divides to produce xylem to the inside and phloem to the outside. These secondary tissues are secondary xylem and secondary phloem. Herbaceous plants lack secondary growth even if a cambium layer is present.

Secondary xylem cells become hard and impregnated with lignin. Only the cells closet to the cambium layer are alive and growing. Wood is an accumulation of secondary xylem. Sapwood is xylem which is still conducting water. Heartwood is the old, no-longer functional xylem. It may be filled with resins or tannins.

Old phloem external to the newest layer of phloem can not divide or stretch to accommodate the increase in the diameter of the stem, so it is torn and crushed as the stem grows.

In addition to xylem and phloem, the cambium makes ray parenchyma cells in the living part of the xylem. These cells conduct water horizontally.

BARK AND CORK

Bark consists partly of living and dead phloem cells and partly of periderm, which replaces the epidermis as a protective layer. Periderm is made of phellogen, a secondary lateral meristem which produces cork or phellem, and phelloderm, which is a parenchyma tissue. Because the bark must continually change to allow horizontal growth of the stem and to accommodate branches, new bark is continually being made from the underside while the outer layers slough off.

Commercial cork is derived from Quercus suber, which produces layers several inches thick. This plant is adapted to a fire-climax environment and the cork provides insulation. It consists of air-filled cells which can be compressed up to 10,000 lbs per square inch without rupturing. These cells also give cork its buoyant properties. A tree must be about 25 years old before cork can be harvested, and the first year's harvest is poor and usually ground for composition cork. Later harvests are superior.

HARDWOODS AND SOFTWOODS

These terms refer to the plants that made the wood, NOT to the characters of the wood.

Gymnosperms produce softwood. The xylem consists only of only one cell type, tracheids, which are long cells that conduct water through openings in the side walls. This is visible in a microscope slide.

Dicot angiosperms produce hardwood. The xylem has two cell types, tracheids and vessels. Vessels are tube-like cells that conduct water through perforations in their end walls. In a microscope slide the two different cell types can be distinguished.

Rays are present in both types of wood. Resin canals can occur in xylem, phloem, or bark of either type, but are more common to softwoods.

TREE RINGS

Seasonal variations in climate (temperature changes and water availability) can be reflected in the activity of the cambium layer and hence the appearance of the xylem. Dendrochronologists can coordinate tree ring patterns to reconstruct climate histories.

In spring or the wet season, cambium makes many large xylem cells which appear lighter. In summer or the dry season, only a few small cells are made and the wood appears darker. In winter, cell production ceases. This is visible to the naked eye, and in microscope slides the different cell sizes can be easily seen.

In the tropics where there is little variation in climate, the trees do not produce rings. Rings may be present if there is a marked wet and dry season.

CHARACTERISTICS AND PROPERTIES OF WOOD

Porosity refers to the dispersion of vessels in a given year's growth. How “open” is the wood? It is actually possible to blow air through the length of some pieces of wood.

Grain describes the alignment of the xylem cells-straight, tipped, spiral, curled, etc.

Figure is determined by the number of rays, the porosity, the grain, and the alignment of rings. The presence or absence of knots also contributes to the figure. Knots are the bases of lost branches which have been covered in time by new annual rings.

Different woods have different densities, grains, and mechanical properties.

Density is Mass/Volume. An oven-dried piece of wood 1 cubic cm in volume is used to measure density. Since the mass of 1 cc of water is 1 g at sea level, any wood with a density higher than 1 will sink, while any wood with a density below 1 will float. Balsa is the lightest wood; Lignum vitae is one of the densest, with a density of 1.3. Pine for construction is about .35; oak for furniture about .60.

XRT - WOOD SECTIONS

X is the cross-section of the wood. It shows rings, rays, and cells.

R is a radial view of the wood, produced by a cut that runs vertically through the center of the stem. It shows the rings in side view. They appear as vertical streaks. Radially cut or quarter-sawn wood is expensive because only a few pieces can be obtained from each log.

T refers to a tangential cut, one made vertically through the stem but not through the center. It shows the ends of the rays. Annual rings appear as irregular bands of light and dark streaks or patches. This is the most common cut. This is also referred to as plain-sawed or slab-cut.

LUMBERING AND MILLING

Wood must be seasoned or dried in the open air, in kilns, or with fans. It is then cut and shaped.

The use depends on the properties of the wood--how large, free from cracks, dense, insect-resistant, carve-able, etc. Wood is tested for its response to tension, shear, cleavage, compression, and cross breaking.

Veneer is thinly sliced wood used for decorative layers. Veneer may be shaved off a revolving log.

Plywood is made of thin layers glued together, each at a right angle to the layer beneath. Particle board consists of sawdust and chips in a resin matrix.

Fiberboard--is made from separated xylem elements that have been put back together.

Charcoal is medium- to high-density wood that has been burned under conditions of limited oxygen.

WHAT ABOUT BAMBOO?

While bamboo (a grass) is used much like wood, it has no secondary xylem and no cambium. Growth is by terminal meristem only. The fibers in bamboo are very strong and very flexible, and the internodes are hollow, making it very useful for furniture, light construction, containers, and piping (if the nodes are burned out). Tiny strips of bamboo can be laminated together, making it perfect for fishing rods, cutting boards, and flooring. Bamboo grows very quickly and can be sustainably harvested.

FUTURE WOOD SUPPLIES

Half or more of the land originally covered by forest has been deforested.

All remaining harvestable forests have been logged to some extent. In undeveloped countries, wood is used for fuel (wood is still the most-used energy source for cooking and heating worldwide), construction, and clearing of crop land. In developed countries, it goes for paper, construction, and fiberboard.

75,000 square km of land still covered by rainforest is cut each year= 1 %. Deforestation is not limited to the tropics. The Pacific Northwest and adjacent Canada lose 404 square km per year.

Obviously, we need to explore other resources for papermaking, construction, and fuel.

OTHER FIBER CROPS

Study the other plant fibers set out–hemp, sisal, jute, flax, etc. Are they woody or herbaceous? What parts are used? Are they related to anything else you have seen this semester?

QUESTIONS FOR THOUGHT, REVIEW, AND STUDY: Refer to lecture, Powerpoint, textbook, and lab notes, as well as to what you learned during the laboratory activity.

1. Draw and label a woody plant cross-section, showing heart wood, sapwood, vascular cambium, phloem, and bark.

2. How can you tell the difference between microscopic views of gymnosperm wood, dicot angiosperm wood, and a “woody” monocot like bamboo?

3. What cell types are present in gymnosperm wood? Dicot wood?

4. What is a growth ring and how does it develop?

5. Describe the manufacture of plywood, veneer, and charcoal.

6. Be able to locate and describe the characters of the cross (X), radial ( R), and tangential (T) faces on a wood sample.

7. Describe the differences between hardwoods and softwoods.

8. What determines a particular wood’s usefulness for a given object or function?

9. What should we be working on in the field of wood production?

PLANTS AND PEOPLE LAB - WOOD