1. Wood and wood products rank just behind food plants in overall value to society.
What is wood? Wood is secondary xylem consisting largely of dead cells involved in the transport of water and minerals as well as support.
a. Plants grow in length because of the elongation of cells produced by terminal (apical) meristems.
b. Most of the cells formed by meristems mature to form epidermis, general matrix tissue (parenchyma), and primary xylem and phloem.
c. In herbaceous plants all of the vascular tissue is produced by apical meristems.
d. In plants that subsequently become woody, a cambium layer forms between the primary xylem and phloem cells.
e. The cambium is a lateral (expanding in width) meristem, capable of dividing to produce additional xylem toward the inside of the stem and additional phloem to the outside.
f. The conducting tissue formed by the lateral cambium are called “secondary” xylem and phloem to differentiate them from the primary xylem and phloem derived directly from cell divisions of apical meristems.
i. The cells of secondary xylem are shorter than those of primary xylem.
ii. As they mature, the walls of these cells become hard and impregnated with lignin.
iii. The conducting xylem vessels are not fully functional until they are dead and their cytoplasm has disintegrated.
iv. Wood is the vernacular name of secondary xylem.
* The accumulation of wood, then, results from the continued divisions by the ring of vascular cambium cells just inside the bark.
v. All tissue outside this cambium layer (including phloem and cork layers) is the bark.
Why is wood composed of secondary xylem and not secondary phloem?
a. Think of a stem as a cylinder with the vascular cambium forming a circle near the circumference.
b. During the year it adds xylem cells to the inside.
c. As it does, the core of the xylem increases in girth.
d. The cambium layer adds some cells to increase its own circumference to compensate for the expansion in diameter of the xylem.
e. The old phloem on the outside of the vascular cambium cannot divide or stretch to accommodate the increase in the radius of the stem. – it gets torn and crushed as the stem grows.
Note: primary xylem is also hollow at maturity; it matures very quickly in its development. Prior to maturity, water probably moves up thru the cells via diffusion.
1. The only part of the phloem that is composed of normal, functional cells is the part closest to the cambium.
2. The xylem closest to the cambium are also the only functional (non-living) conductive cells.
3. The region of xylem that actively conducts water is known as the sapwood.
a. Sapwood - region is secondary xylem outside the heartwood, nearest the cambium. Functions in both support and conduction (and therefore often wet).
4. The older, no longer functional xylem cells remain intact and form the heartwood.
a. Heartwood - centermost region of secondary xylem. Usually darker in color because cells contain tannins, gums, and resins which accumulate in this older area and help prevent decay. Function of heartwood is support; no longer functions in water and mineral conductance (transport).
b. For lumber use, heartwood is preferred because of its resistance to decay, and it is usually drier and less apt to shrink, warp, and split.
5. Rays - Horizontal conducting tissue in the stem of woody plants.
Hardwood and soft wood
1. Wood is divided into 2 main categories, hardwoods and softwoods.
2. These refer mainly to the kinds of trees that produce the woods, not precisely to the hardness or softness of the woods themselves.
a. Hardwood - angiosperm trees; softwood - gymnosperm trees.
b. Hardness refers to the sturdiness of the cell walls, which is largely a reflection of the amount of lignin and cellulose.
c. The harder the wood, the more resistant to wear
3.The kinds of wood differ anatomically:
c. Gymnosperms (softwood) are composed primarily of tracheids, long cells that conduct water primarily through openings in their side walls.
d. Angiosperms and dicots have xylem composed of vessels, relatively short cells that conduct water primarily though openings on their end walls. (Mixed with vessels are tracheids and other kinds of cells, like fibers).
e. Gymnosperms contain fewer kinds of cells than does angiosperm wood, and tends to be comparatively uniform.
1. Seasonal variations in climate can be reflected in the activity of the cambium and ultimately in the appearance of the xylem.
2. Annual temperature changes and seasonal aridity are the climatic fluctuations with the most pronounced effects on xylem production.
3. During spring in temperate regions (or wet seasons in tropical semi- arid areas), the cambium produces many large xylem cells.
4. In summer or dry season only a few small cells are added.
5. During freezing months of winter all cell division ceases.
6. The visible results of these changes in growth are rings in the xylem created by the concentric layers of large and small cells.
7. In cold temperate regions the rings can be counted to give an accurate idea of the age of a tree.
Characteristics of Woods
1. Grain refers to the prominence of the annual rings and rays, and the direction of cutting.
a. Cross section - growth rings of trunks cut in cross section (transverse) are arranged in concentric circles, mush like circles on a target.
b. Rays radiate out from the center of the section, like spokes from the center of a wheel. Cross sections are seldom used in wood cuts because they often split after they are cut.
c. Radial section - longitudinal section that goes through the center of the stem. Boards made from radial sections are called quarter-saw cuts. In these cuts, growth rings appear as parallel lines oriented perpendicular to the rays. Quarter-sawed boards are favorites from trees like oak, whose large rays add texture to the wood. However, only a few boards can be quarter-sawn from a trunk.
d. Tangential section - longitudinal section that does not go through the center of the stem. Boards made from tangential cuts are called planesaw cuts. Growth rings in these boards are arranged in large, irregular patterns of concentric Vs.
e. Knots - bases of branches that have been covered by lateral growth of the main stem.
i. Some knots fit tightly in wood, while others fit loosely and usually fall out (knothole).
ii. The fit of the wood depends on whether the branch was dead or alive when the trunk grew around it.
iii. Knots produced by dead limbs have no xylary continuity with the main stem and therefore fall out of lumber when the wood dries.
iv. Knots of living branches do not fall out because their xylem is continuous with that of the main axis.
Veneer - thin sheet of desired wood glued to a base of less expensive wood.
Plywood - 3 or more layers of thick veneer glued together. The grain of alternate layers are at right angles to one another. Since wood is strongest with the grain, layering produces a sheet or board that is more uniformly strong than a comparable piece of solid wood. Lightweight but strong building material for roof and wall sheeting, subflooring, shelves, etc.
2. Density is an important feature of wood for engineers, architects, carpenters, and cabinet-makers, and is expressed as specific gravity, which is the ratio of the density of the wood to that of water.
a. Fast-growing conifers (pine) and diffuse, porous hardwoods are less dense; i.e. have a lower specific gravity than slow-growing hardwoods.
b. The specific gravity of wood is determined by the size of its cells, thickness of cell walls, amount of lignin, and the proportions of late and early wood. Most wood has a specific gravity between 0.3 and 0.7.
c. The weight of wood is essentially due to the cellulose and lignin in the cell walls around billions of individual cells.
d. Density is defined as mass (in grams) divided by volume (cm3).
e. This means that the mass is numerically equal to the weight.
f. Since the mass of 1 cm3 of water is 1 gram, any wood with a value below 1 is "lighter" than water, and will consequently float.
1. Since pure cell wall material has a specific gravity of about 1.5 (pure water = 1.0) and is heavier than water, the relative buoyancy of different woods is due to air cavities (lumen) within the cell walls, and the thickness of the walls and the amount of lignin they contain.
2. This is why water-logged soft, porous woods will sink in water when all their air spaces become filled with water.
3. One of the lightest woods is balsa, with a density of 0.13 g/cm3.
4. Balsa is much too light to provide structural support, and Lignum vitae so dense as to make working with it very difficult
5. Ironwood, Lignum vitae, has a density of 1.23 g/cm3, and sinks in water.
6. Ironwoods are so hard and heavy because they contain numerous long, tightly-packed wood fiber cells with very thick, heavily lignified cell walls and little or no air spaces.
7. Pine, the most common wood used in home construction, has a density of about 0.35 - 0.5, and a dense wood for fine furniture, like oak, is usually around 0.6 g/cm3.
8. Baseball and plants
a. Pitches use rosin to improve their grip.
b. Hitters coat bat handles with pine tar.
c. An occasional "cheater" will cork his bat by filling the barrel with cork to improve bat speed and drive.
d. Baseball bats are made from white ash (Fraxinus
9. Structure and chemical composition of woods also determine their suitability as fuels.
a. Softwoods and light woods burn quickly with a flash of heat, and are best used as kindling to start a fire.
b. Be careful with conifer woods because they are impregnated with resins that cause deposits of flammable material to build up in chimneys.
c. Hardwoods with medium to high densities usually make the best firewood and charcoal.
d. Charcoal is produced by burning wood slowly in an atmosphere with limited oxygen; partial combustion in an oven or other enclosure that restricts air flow.
1. Charcoal is almost pure carbon and burns at much higher temperatures than wood, and can even be used for smelting ores into metals.
2. During the Middle Ages, the
forests of southern
3. Here in the Spring Mountains, just south of Wheeler Pass, southwest of Indian Springs, are kilns (shaped like beehives) where the wood of pinyon pine was burned down to charcoal and shipped to Pahrump.
8. Woods for construction
White pine (Pinus strobus)
- important timber tree early in
b. Douglas fir (Pseudotsuga menziesii) - important lumber wood in housing industry, especially in northwest where it grows in abundance. Grows very tall and relatively straight; large internodal areas making for fewer knots.
c. Oaks (Quercus spp.) - most valuable hardwood. White oak (Q. alba)widely used in furniture, cabinets, flooring.
d. Others - black walnut , hickories, maples, sweet gums: all hardwoods used in furniture making.
Other uses of wood
1. Wood has been the chief form of fuel throughout human civilization up until fairly recent times.
2. In developing nations it is still one of the principal sources of fuel for cooking and heating.
3. Firewood gathering is hastening the decline of many tropical forests, since over a billion people in the tropics depend on wood for fuel. Trees are being cut faster than they can be regenerated.
b. Wood becomes more scarce and, therefore, more expensive.
c. Greater erosion compounding environmental and social stresses.
d. The poor people lose their ability to purchase wood to keep warm and to cook their food.
e. Increase in food-borne disease-causing organisms.
f. It is often necessary to travel greater and greater distances with heavy loads to meet fuel requirements.
g. Animal dung (esp. cow dung) is used for heating and cooking; loss of an important source of fertilizer.
1. Compounds composed of terpenes and volatile oils.
2. Insoluble in water and serve to protect the tree from pests and disease.
3. Best known resins are extracted from gymnosperms (conifers).
4. Resin oozes out when a tree is cut: the bark is slashed and the crude exudate (pitch) is collected. When pitch is heated, the volatile components evaporate easily. These condense to form turpentine, used as a thinner for oil-based paints and as an organic solvent.
5. After the volatile compounds are removed from the resin, what remains is referred to as rosin, a material often used by musicians and baseball players.
a. The bow of a stringed instrument is run along a block of rosin to make it sticky.
b. This increases the friction between bow and strings, resulting in more vibration and improved tonal quality.
c. A baseball pitcher uses powdered rosin to improve his grip on the ball (rock climbers, likewise, use powdered rosin).
d. Stickiness of Band-Aids is due, in part, to rosin.
10. Pine pitch used for waterproofing since ancient times: ships, roofs, barrels, etc.
1. As the tree increases in girth, the epidermis splits and is replaced by periderm. The major component of this tissue is cork (phellem) produced by the cork cambium.
4. Greeks and Romans used cork to seal jars and casks, and as flotation devices.
Commercial sources of
cork come from the evergreen oak, Quercus
suber, a tree native
to the Mediterranean area.
There are about 50
genera and 1,000 species of bamboo, all in Poaceae, the grass family. Greatest diversity occurs in
An extremely useful and
beautiful plant; heavily utilized in
3. All species have a “wooden” culm (stalk) produced from a rhizome. The culms have a hollow pith that is solid at the nodes (like all grasses). The strong, lightweight stalks are the reason bamboo is so useful.
4. Large stems can be used as posts and rafters in houses, split sections form side walls, long sections of stem are ideal for irrigation pipe, short sections for containers or musical instruments (flutes), they re woven into baskets or screens, young shoots can be cooked and eaten …etc.
5. Many species flower synchronously - at the same time no matter where they are (suggesting an internal control mechanism).
Bamboo has a long
history in papermaking and is the source of pulp for 2/3s of all paper made in
The bulk of natural
rubber is obtained from Hevea
brasiliensis (99%), a tall tropical forest tree
native to the
Hevea is a genus in the Euphorbiaceae (Spurge family), a family that also includes poinsettias and the manioc (tapioca).
In Hevea, latex is produced in a series of latex vessels (laticifers) of the inner bark which anastomize (braid) through the phloem.
Laticifers occur sporadically in angioperms, but not in gymnosperms.
Latex is an emulsion (insoluble in water) of a variety of compounds, both elastic an inelastic (opium poppy).
a. “Latex” in paints in not really latex – it consists of synthetic plastic particles dispersed in water with a binding agent. When water evaporates, the particles fuse, producing latex-like covering.
Function of latex in plants:
c. By-products of primary chemical processes, secreted into laticifers to keep them from interfering with normal cell functioning.
Latexes that exhibit elastic properties are lumped together as “rubber”, a name given to the Hevea latex by Joseph Priestly (discoverer of oxygen) in 1770, when he discovered that it could be used for rubbing errors of a page.
Rubber was first encountered by Europeans in
Amazonian rainforest Indians had the curious custom of dipping their feet in sap collected from a tree, and then holding them in the smoke of a fire.
a. The tree was Hevea brasiliensis (Euphorb.).
b. By holding their feet in smoke, the natives coagulated the rubber on the soles of their feet, producing a pair of perfectly formed “proto” tennis sneakers.
c. Later, the Spanish and So. Americans began to dip their hats and cloaks in latex and smoke them to make them waterproof.
In 1839, Chas. Goodyear, in the
a. Normally, rubber becomes soft with heat and brittle with cold.
b. Goodyear found that chemical combination with sulfur under heat (150 C) and pressure cures both troubles, and gives rubber toughness and resistance to wear.
c. The discovery led to rubber’s later use in the manufacture of automobile and bicycle tires.
d. 60 – 70% of all the rubber produced in the world is done so for the manufacture of the pneumatic tires.
The invasion of
b. Developed process for producing synthetic substitute for natural rubber.
c. Synthetic rubber uses petroleum.
d. After 1970s, demand for natural rubber increased along with the production of radial tires.
i. Radial tires provide better handling and run cooler than convention bias-ply tires.
ii. But they require more resilience than synthetic rubber – about 30% natural rubber is incorporated into radial tires.
iii. Relatively stable market for natural latexes.
Regeneration of phloem is fairly rapid, and if trees are tapped in alternate years, a tree may produce for 30 years.
The latex drips into a cup and is brought to collecting stations where the liquid is mixed with a mild acetic acid to coagulate the rubber.
a. water and contaminants are removed by squeezing the rubbery sheets in between steel rollers, and the rubber is then dried for shipment.
b. at factories, dried rubber is thoroughly mixed, carbon black or other coloring added, and then it is vulcanized by the addition of sulfur, lead oxide, and heat. (increases strength, makes it nonplastic and elastic).
14. With global concern about
the destruction of tropical rainforests throughout the world a system of
“forest reserves” are being attempted to preserve natural areas. One such
method is known as “extractive reserves”, an area where local people can
extract products on a small scale while still preserving a largely intact
ecosystem. This is mostly done with regard to indigenous peoples and is a form
of biological reserve that is closely associated with a social movement, begun
a. the first reserves were established for the extraction of rubber and Brazil nuts.
b. Most of the rubber in the Amazon basin is gathered in a way that does not destroy the tree, so people who gather it are strongly opposed to the destruction of the rainforest.
Each year about 1 billion trees are cut down to satisfy the demand for paper and paper products, with each American directly or indirectly using 731 lbs of paper (2 lbs of paper per day).
Each Sunday edition of the New York Times consumes about 150 acres of forest.
But paper didn’t begin as a derivative of wood; it had more humble beginnings.
There were cuneiform
clay tablets of ancient
writing dates to about 100 years after the earliest Sumerian records. The
writing surface the Egyptians employed was from papyrus, Cyperus
papyrus, a sedge that grows naturally in
a. developed about 4,500 ys ago
b. made of thin slices of the plant’s cellular pith that was beaten and laid lengthwise with other layers crosswise on it. The mat was moistened with water, pressed, and dried. In the final step, the papyrus was rubbed smooth with a piece of ivory or smooth shell (burnishing), and the finished sheets were made into rolls, up to 9 meters (30 ‘) in length.
c. the “sheets” were pressed flat and glued together.
d. as long as a roll of papyrus cd be handled on rollers there was no limit to its size: the Great Harris papyrus, made during the reign of Ramses III (1198-1167 BCE) is 400 meters long (1/4 mile) and is in better condition than last month’s newspaper.
e. the Dead Sea Scrolls were written on papyrus
f. the use of papyrus continued until about the 4th century when it was supplanted by parchment.
g. today papyrus is used for decorative purposes and lives on semantically as the origin of the word “paper”.
3. Parchment - was produced by rubbing skins of sheep and goats with lime instead of tanning them into leather.
a. vellum is considered a finer quality parchment made from kids, lambs, and young calves.
b. preparation: animal skin is cleaned and the hair removed. Both sides of skin is scraped, smoothed, and finally rubbed with powdered pumice.
c. parchment is still used for formal honorary documents and diplomas (sheepskin).
a. Paper as we know it consists of matted plant fibers.
b. Its inventor is said to be the Chinese scholar, Ts’ai Lun, who in 105 CE developed a method to free phloem fibers by crushing hemp fibers and the bark of paper mulberry trees (Broussonetia papyrifera) in water, and then soaking the fibers in wood ash.
c. The free fibers rose to the surface, the tangle was placed on a silk mold and dried in the sun – the first silk-screen. Upon drying the paper was peeled from the screen.
d. Soft fibers were used initially for papermaking, not supplanted by wood pulp until about 1840.
e. It was discovered that paper could be mass produced using woods of conifers (gymnosperms).
i. Coniferous xylem tracheids are longer than the xylem vessels of angiosperms (hardwoods): about 2-4 mm vs. 0.5-1.5 mm.
ii. In 1870, the N.Y.Times became the first newspaper to use all wood paper.
f. Debarked wood is chipped, heated in presence of strong chemicals, which remove lignin from the cell wall and cause the maceration of the tissues – the cells become separated.
g. Several agents – resins, gums, starches – may be added to pulp before fibers are floated onto draining screens. These act as sizing agents to fill in surface irregularities and improves a paper’s ability to accept ink.
h. The sheet of fibers drain and rollers press and mat the fibers.
i. Nowadays, chemical processes are used for pulping.
Sulfite process – acid process.
Sulfate process – alkaline process
j. The liquid pulp is poured onto a screen which intermeshes the wood fiber cells as a thin layer. Water is drained off; all water is subsequently removed by a set of heavy rollers using high pressure and high temperature.
k. The process of papermaking uses strong chemicals.
i. Acids are especially harmful – they cause pages to be brittle, and to disintegrate earlier than the alkali method (within 100 yrs.).
ii. Acid-free paper is required for books and documents that can last more or less indefinitely.
Kinds of paper
1. Newspaper – unbleached conifer paper
2. Ledger paper – bleached
3. Brown wrapping paper, hand towels, grocery bags, cardboard – conifer wood in which some lignin remains.
4. Absorbent paper towels, toilet paper - loosely woven fibers without fillers of lignin.
5. Fine stationary and cigarette papers - made from flax
6. Bible paper – made from hemp
Papermaking requires a lot of water, so mills are located along rivers.
1. Stink of wood pulp mills - some of the chemicals used to dissolve lignin away from cellulose are sulfur compounds.
a. If you
ever travel thru So.
b. Some of these sulfur compounds are converted to sulfuric acid in the atmosphere, returning to the earth as acid rain.
c. This alters the alkalinity-acid balance of soils with potentially serious effects on the microflora growing within the soil and the plants growing on the soil. Threatens fresh water lakes.
2. Clear-cutting large tracts of forested lands for pulpwood and lumber. This has created erosion of large areas, with consequent silting in of streams having severe impacts on the insulted ecosystem.
a. Loss of Mycorrhizae, the fungi associated with roots of woody plants, and serve as a conduit for soil nutrition ot the roots. Once an area has been clear-cut and erosion begins, many of these microorganisms die, making reforestation difficult.
3. Waste disposal - until recently mercury compounds had been used in the processing of wood. Along with sulfur compounds, bleaches, and dyes, mercury was dumped into streams poisoning plants, animals, and people using these waters. Lignin, virtually undegradable, is routinely discharged into the water courses, accumulating in large sludge beds and reducing the oxygen needed by aquatic animals and plants.
Chlorine (used in bleaching process) reacts with wood pulp to form dioxin, one of the most toxic chemicals known (Agent Orange of Viet Nam infamy). Dioxin accumulates in fatty tissue of fish, birds, and other animals.
Alternatives to wood pulp
1. Levi Strauss recently began recycling denim scraps to produce paper for company stationary.
2. Rice straw (by-product of rice cultivation), bamboo, bagasse (from sugar cane) are possible alternatives.
Hemp (Cannabis sativa) - one of the earliest
plants used by ancient Chinese papermakers. It continued to be used until the
early 20th century. Used on the first drafts of the Declaration of
Independence and the
Cannabis is a fast-growing annual, grows on poor soils, requires little or no herbicides or pesticides. Hemp paper is said to hold up for 1500 years, in contrast to the 25-100 ys for wood pulp paper.
Varieties of hemp used for paper have very little tetrahydrocannabinol (THC), the psychoactive compound.