||Wood is one of the earth's most valuable resources. It provides us with so many things that are a part of our lives: paper, fuel, furniture, and lumber, just to name a few.
Anatomy of the Tree
All trees are made up of tube-like cells, though their structure vary according to the tree's use of these cells for three main functions. Cells carry water, strengthen the tree with a network of support, and contain nutrients necessary for the tree to live. Depending on the combination of these three functions, the wood has a certain grain, strength, color, and porosity. Softwood cells, called tracheids, share vessel and strengthening properties. Hardwoods have different cells for each function. Both types use cells called parenchyma for storing nutrients. They form a vascular system that gives the entire tree the right amount of water, from the ground roots to the top of the tree.
When looking at a cross section of a tree the many layers of growth are exposed. These layers surround a central core (the pith) and are protected on the outside by bark. The vascular cambium is the thin layer of tissue between the bark and the growth layers that produce the xylem, or cells that develop into wood. The growth rings accumulate as the tree gets older as well as from environmental conditions. For example, in tropical conditions, certain trees actually accumulate growth rings due to dry and wet periods instead of its number of years. Because wood is a composite of cells the variations in cell form and arrangement account for anatomy individual to each tree (or type of tree). The principle cell types are: tracheids, vessel members, fibers, and parenchyma.
Within a tree there are two types of wood. Towards the center is the heartwood and surrounding it is the sapwood ring. The heartwood may help support the tree as a structure but it actually has no role in its growth because it is dead wood material. Sapwood contains the water/mineral combination (or sap) that is delivers the nutrients to the tree that aids in its growth.
Tree Variations (hard vs. softwood)
The 2 main categories of tree anatomy are porous (hardwood) and non-porous (softwood). While the porous trees consist of all of the prinicple cell types, the nonporous trees have a much more simple make up. Softwoods are classified as the conifers, or the trees that bear seeds without a seedpod. Hardwoods, or deciduous trees, have seeds encased in pods, found in the tree's flowers and fruits. The terms "hardwood" and "softwood" do not indicate the strength of the wood, but to the water conducting cells in the living tree. Wood is like metal: texture, strength, and color varies according to its original source. Some softwood, like pine, is considered very sturdy, while some hardwoods, like balsa wood, are very flimsy. Dicotyledoneae hardwoods (those that have a two-lobed seedcase) are the family from which most timber woods come. Hickory, birch, alder, beech. oak. elm, and walnut woods come specifically from apetalous Dicotyledoneae trees (those with very inconspicuous or no flowers).
Wood/Lumber harvesting (cuts of boards)
Cutting wood into timber requires an intricate understanding of the tree's inner workings. Depending on the way the timber is cut, the wood will react by shrinking or expanding, as well as absorbing and releasing water. Timber can be cut by slash sawing, radial sawing, and plane sawing. Slash sawing can cut wide boards, but with less strength. Plane sawing is a viable alternative to slash sawing, because it "boxes out the heart", which is the most unstable portion at the center of the tree, often filled with splits and rotted material. Radial sawing, or quarter sawing, produces stable boards by cutting perpendicularly into the growth ring pattern. Radially sawed boards are often used in musical instruments and other crafted objects that require strength and stability. When wood is cut, it is "green", or filled with moisture. Wood must be dried before it can be utilized for construction. Wood is dried either by air or in a kiln.
Wood Grains and Natural Phenomena
Besides being hardwood or softwood, trees can have other variations due to the presence of defects like knots, spiral grain, compression, tension wood, shakes, and pithpockets. These variations can have an undesirable affect on the tree appearance, strength, and stability.
Grain is a term that is commonly thought to be defined as the surface pattern of the wood (however the wood was cut to show its growth rings or the plane of the board). Grain actually refers to the wavy natural fibers are arranged within the wood. It is produced by the wood's rings and pores, and affects the consistency, size of wood cells, and color. There are eight types of wood grain: straight, irregular, cross, wavy, curly, spiral, diagonal, and interlocked grains. Straight grain fibers run parallel to the tree's vertical axis. Irregular grain results when fibers wrap around knots or other irregularities in the wood. Cross grain fibers do not run parallel tot he tree's vertical axis. Wavy grain fibers form waves within a regular grain pattern. Curly grain is a combination of wavy and irregular grain patterns. Spiral grains form when fibers wrap around the tree. Cutting a spiral grain into a board creates a diagonal grain. Interlocked grain occurs when spiral grains switch direction in mid-growth.