There’s plenty of it, it’s relatively cheap (or even free), it’s environmentally friendly, it looks great, it’s warm and cozy, it’s super-strong, it lasts hundreds or even thousands of years, and you can use it for everything from building bridges to making paper or heating your home. It’s wood—and it’s quite possibly the most useful and versatile material on the planet, with many thousands of different uses. So what is it that makes wood so good? Let’s take a closer look!
What is wood?
You often hear people grumbling about money and all kinds of other things that “don’t grow on trees”; the great thing about wood is that it does grow on trees—or, more specifically, in their trunks and branches.
Structure of wood
Take a tree and peel off the outer “skin” or bark and what you’ll find is two kinds of wood. Closest to the edge there’s a moist, light, living layer called sapwood packed with tubes called xylem that help a tree pipe water and nutrients up from its roots to its leaves; inside the sapwood there’s a much darker, harder, part of the tree called the heartwood, which is dead, where the xylem tubes have blocked up with resins or gums and stopped working. Around the outer edge of the sapwood (and the trunk) is a thin active layer called the cambium where the tree is actually growing outward by a little bit each year, forming those famous annual rings that tell us how old a tree is. Slice horizontally through a tree, running the saw parallel to the ground (perpendicular to the trunk), and you’ll see the annual rings (one new one added each year) making up the cross-section. Cut vertically through a tree trunk and you’ll see lines inside running parallel to the trunk formed by the xylem tubes, forming the inner structure of the wood known as its grain. You’ll also see occasional wonky ovals interrupting the grain called knots, which are the places where the branches grew out from the trunk of a tree. Knots can make wood look attractive, but they can also weaken its structure.
Hardwoods and softwoods
Wood is divided into two distinct kinds called hardwood and softwood, though confusingly the names don’t always refer to its actual hardness or softness:
- Hardwoods are ones that come from broad-leaved (deciduous) trees (those that drop their leaves each fall, also known as angiosperms because their seeds are encased in fruits or pods). Examples include ash, beech, birch, mahogany, maple, oak, teak, and walnut.
- Softwoods come from evergreen (coniferous) trees (those that have needles and cones and retain them year-round, also called gymnosperms. Examples include cedar, cypress, fir, pine, spruce, and redwood.
It’s generally true that hardwoods are harder than softwoods, but not always. Balsa is the best-known example of a hardwood that is actually very soft. Hardwoods have lovely, attractive grains and are used for such things as making fine furniture and decorative woodwork, whereas softwoods often come from very tall, straight trees, and are better suited for construction work (in the form of planks, poles, and so on).
Look at some freshly cut wood under a microscope and you’ll see it’s made up of cells, like any other plant. The cells are made of three substances called cellulose (about 50 percent), lignin (which makes up a fifth to a quarter of hardwoods but a quarter to a third of softwoods), and hemicellulose (the remainder). Broadly speaking, cellulose is the fibrous bulk of a tree, while lignin is the adhesive that holds the fibers together.
What’s wood like?
The inner structure of a tree makes wood what it is—what it looks like, how it behaves, and what we can use it for. There are actually hundreds of different species of trees, so making generalizations about something called “wood” isn’t always that helpful: balsa wood is different from oak, which isn’t quite the same as hazel, which is different again from walnut. Having said that, different types of wood have more in common with one another than with, say, metals, ceramics, and plastics. source