Wood
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.
Tree rings
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.
8. Dendrochronolgy
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
a.
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
Fuel
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.
a.
In
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.
Resins
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.
2.
3.
4.
Greeks and Romans used
cork to seal jars and casks, and as flotation devices.
5.
Commercial sources of
cork come from the evergreen oak, Quercus
suber, a tree native
to the Mediterranean area.
Bamboo
1.
There are about 50
genera and 1,000 species of bamboo, all in Poaceae, the grass family. Greatest diversity occurs in
2.
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).
6.
Bamboo has a long
history in papermaking and is the source of pulp for 2/3s of all paper made in
Rubber
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:
a.
Unknown
b. Antiherbivory?
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
a.
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
in
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.
Paper
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.
1.
There were cuneiform
clay tablets of ancient
2.
Egyptian hieroglyphic
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).
4.
Paper
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.
Methods
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.
Ecological
consequences
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.
3.
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.