Every knot in a plank is a buried branch from the tree's youth
As a tree grows, its lower branches die and new layers of trunk wood slowly swallow their stumps. Saw the trunk into boards and those entombed branches reappear as knots, darker circles around which the grain swerves. Inside a knot, the fibres can run at up to 90 degrees to the rest of the board.
Wood is a natural composite. Cellulose fibres, strong when pulled, sit embedded in a matrix of lignin and hemicellulose that resists crushing. It forms as xylem, the tissue that holds a tree upright and ferries water and nutrients between roots and leaves. Trunks thicken through secondary growth: a thin layer of dividing cells just beneath the bark, the vascular cambium, adds new wood around the entire stem, branches and roots.
Where seasons differ sharply, that growth comes in pulses and leaves rings. The inner part of each ring, laid down quickly in spring, tends to be paler with wider cells, while the denser latewood forms later in the season. A tree growing in the open with steady conditions puts on its widest rings when young, and they narrow with age. Year-to-year differences in ring width and chemistry record past climates, and in some species counting rings can date a wooden object.
Knots matter to builders. A knot is the conical base of a side branch or a dormant bud, pointing back to where the cambium sat when the bud formed. If the branch was dead for a long time before being engulfed, it may be held loosely and fall out once the board is cut. Knots weaken timber, especially under tension or forces across the grain, so a knot on the underside of a beam, which is stretched, does more harm than one on top. Graders sort them by form, size and soundness, though small knots along a beam's neutral plane can actually resist shearing.
The oldest known woody plants, found in New Brunswick, Canada, in 2011, grew roughly 395 to 400 million years ago. People made the first spears from wood with fire-hardened tips, and by 15,000 BC hunters used carved spear-throwers. Forests worldwide held about 630 billion cubic metres of growing stock in 2025, and almost 4 billion cubic metres were harvested in 2023, mostly for furniture and building.
Source: Wood
