PLANTS AND PEOPLE
Hugh D. Wilson 

Wood, Cork, and Bamboo

What is wood?

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 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 tanins (overview).

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 (view).
 

Bark

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.

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 ut 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. 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.

Gymnosperms produce softwood. The xylem consists only of tracheids, which are long cells that conduct water through openings in the side walls.

Angiosperms produce hardwood. The xylem has both tracheids and vessels. Vessels are tube-like cells that conduct water through perforations in their end walls.
 

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.

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.

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.

Dendrochronologists can coordinate tree ring patterns (compare to gymnosperm) to reconstruct climate histories.
 

Characteristics of Wood

Porosity refers to the vessel dispersion in a given year's growth.

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.

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 (Ochroma pyramidale - Malvaceae) is the lightest wood; Lignum vitae (Zygophyllaceae) is one of the densest, with a density of 1.3. Pine for construction is about .35; oak for furniture about .60.
 

Bamboo
 While bamboo is used much like wood, it has no secondary xylem and no cambium.  Growth is by terminal meristem only.
 
Processed wood - (Cellulose)
Rayon, Cellophane, Lyocell

Future Wood Supplies

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

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


Return to Course syllabus
  7 Apr 2011