Wood - Cambium layer forms between primary xylem and phloem cells (lateral meristem). Tissues formed by cambium are called secondary xylem and phloem as contrasted with primary tissues which are produced by the apical meristem. Xylem functions as water-conducting tissue, phloem conducts nutrients.

Wood is secondary xylem - cells are impregnated with lignin and are dead.

The functional phloem is in the layer next to the cambium, old phloem is torn and crushed as stem grows (= bark). Functional xylem is also next to cambium (sapwood). The heartwood is older nonfunctional xylem cells.

Xylem rays compose a horisontal conduction system.

Bark - all tissues outside functional vascular cylinder, mostly periderm, which is a protective layer. The phellogen is another lateral meristem that produces phellum (cork), constantly makes new material and the old sloughs off.

Hardwoods - angiosperms - wood mostly vessels + some tracheids.

Softwoods - gymnosperms - wood mostly tracheids, more uniform.

Hardwods are not necessarily harder than softwoods.

Dendrochronology - works well in cool temperate regions, can also be used to indicate climatic changes - Tree Ring Lab at the University of Arizona.

Wood characteristics

Porosity - manner in which large vessels are dispersed, only in hardwood.

Grain - alignment of xylem cells

Knots - Inclusion of branch in xylem

Figure - determined by # rays, porosity, grain, arrangement of annual rings, knots, method of cutting - important in decorative purposes.

Density - balsa is 0.13, lignum vitae is 1.3, pine used for home construction is 0.35 to 0.5, furniture woods >0.5 to around 0.6.


Fuels - low density woods burn fast, high density woods slower.
Charcoal is produced by slowly burning wood in oxygen-low atmosphere, produces material that burns at a higher temperature than wood, hot enough to smelt metals.
Recent increase in the use of wood stoves has caused serious pollution problems in densely populated areas.

Wood harvesting and processing
Highly mechanized, wood is dried rapidly in kilns

Wood products

Round wood - stripped of bark and impregnated with preservatives - fence posts, telephone poles, etc.

Cut lumber

Veneers - most is produced by rotary methods to give 0.25 cm thick sheets

Plywood is made from layers of veneer with grains of the layers at right angles to each other. This provides greater strength. Plywoods are about 3500 years old.

Particle board is made from chipped wood which is glued and pressed into the desired shape, often made from "waste" of sawmill.

Fiberboard - made from wood fibers

Paper
Egyptians - Cyperus papyrus
Orient - Fatsia papyrifera
Mayans and Polynesians - Broussonetia papyrifera

Modern paper is produced from plant fibers which are separated and then matted into a thin sheet. Chinese invented process about 100 A.D. Paper was very expensive due to labor involved in its manufacture. Paper machine was invented in France in 1789 and quickly ran out of source of fibers (linen, cotton and hempen rags). Wood became major source of fibers. The fibers have to be processed to remove lignin and pectins or paper yellows and gets brittle very rapidly (newsprint). The first process involved boiling the fibers in alkali and bleaching. Two modern processes are the sulfite process, which uses hot acid and produces acid papers which decompose after about 100 years; the sulfate process is alkaline. Most books printed after 1850 used paper produced by the sulfite process so they are a major problem for libraries. The sulfate process is currently the favored process because it is not acidic and can also be used for gymnosperm woods that contain resins. Gymnosperms are preferred becaused they have tracheids which are longer than the vessels found in angiosperms. Paper recycling is currently marginally feasible from an economic standpoint.

Rayon and cellophane - use pure cellulose to produce fine sheets that are washed to produce cellophane or forced through openings to produce rayon. Acetate and acetate fibers are produced by the addition of acetyl groups to cellulose. Arnel is an acetate fiber that is used to make double knits, tricot and permanent press items.

A possible future source of paper is Hibiscus cannabinus which produces the fiber kinaf. One acre of this herb can provide five times the amount of pulp provided by an acre of pine trees. The pulp also lacks the resins and lignins that must be removed to produce good paper.

Cork
Characteristics due to many air-filled cells, good insulating material, lightweight, floats.

Cork cambium develops within the secondary phloem

Quercus suber - cork oak

Wood Supplies

Ability to supply adequate wood is in question. In tropics, loggers will cut down whole forests to obtain a few valuable trees (mahogany).

Use of tree plantations and fast-growing cultivars is increasing. Leucaena (Fabaceae) grows 24 feet in one year in the tropics and its wood is as dense as oak. Grows back from stump sprouts.

Bamboo
No secondary xylem, primary growth only. Segmented, hollow tubes, typical grass stem. Used for fishing poles, caning for furniture, house construction in tropics, skyscraper scaffolding in Japan.
Bamboos have unusual life cycle, some species grow for decades without flowering, then all the members of a species worldwide will flower and die the same year. This massive die-off can be devasting to animals (including humans) that depend on these plants for food.

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