The Merriam-Webster dictionary defines wood as “the hard substance that makes up the stems and branches of trees and shrubs” or “an area of land covered with many trees,” or “a golf club.” For our purposes, we will focus on the first definition.
Wood is the stuff beneath the bark. The technical term is for it is the xylem. What is interesting is the structure of the wood and the characteristics that make is such a useful building material. The tree’s interior is like a bundle of straws and is used to draw up n
utrients. Sandwiched between the bark and the inner wood is a thin layer called the vascular cambium. This layer consists of reproductive cells that, by cell division, form new bark outward, and also new wood inward. That is what causes the tree’s diameter to expand and creates the characteristic growth rings. As growth speeds and slows across the seasons the activity is recorded in the rings and fairly accurately record the age of the tree. However in temperate zones with little seasonality there may be no discernable rings.
As the cambium forms new wood cells, they develop into different sizes, shapes, and orientations to perform a variety of tasks. That can include food storage, sap conduction, trunk strength, etc. Younger cells, called sapwood, are alive and move sap up or down and store nutrients. Over time, the tree no longer needs the entire trunk to conduct sap, and the cells in middle begin to die. This dead wood in the center of the trunk is called heartwood. The as it grows, the heartwood accumulates various deposits which cause distinctive characteristics. Normally the heartwood is pale white or yellow but some more colorful examples would be the black in Ebony, the orange in Padauk or the dark brown in Walnut. The heartwood deposits also affect the wood’s resistance to rot and decay and its relative hardness. It’s why Teak is prized for boats and Basswood is easy to carve.
We tend to label wood as being hardwood or softwood but that may lead to some confusion since those labels really refer more to the type of tree; hardwoods generally come from deciduous trees that drop their leaves in the fall while softwoods come from conifers or evergreens. An Austrian named Gabriel Janka invented a hardness scale in 1906 to accurately hardness. It measures the pounds of force required to imbed a .444” steel ball one-half its diameter into the surface of the wood. The chart on the left shows the hardness of some typical species. Interestingly Poplar, Basswood and Balsa which are hardwoods rank lower than softwoods like Douglas Fir and Pine.
All of these properties; hardness, rot resistance, color, and grain affect which wood you choose. Wood is an amazing, renewable resource.
Wood in all its forms has been used for thousands of years for construction, fuel, paper and more. Never worry about using wood for your projects; after all, it grows on trees.
