A bundle of drinking straws
Why Does Wood Split One Way?
Wood is a bundle of tubes that carried water up the tree, so it splits easily along them and resists across them.
-
Step 1 of 5
The grain is old plumbing.
A tree has to lift water from its roots to its highest leaf. It does that through millions of narrow tubes running straight up the trunk.
-
Step 2 of 5
Splitting along the tubes only parts them.
Drive a wedge in line with the grain and you are separating tubes that were merely side by side. Almost nothing has to be broken.
-
Step 3 of 5
That is why saws and axes are different tools.
An axe is a wedge for parting the grain. A saw has teeth for cutting across it. Using either the wrong way round is miserable work.
-
Step 4 of 5
The rings are years.
A wide ring is a good year, so every plank carries a weather record Wood grown in spring is pale and open; late-summer wood is dark and dense. One pale band plus one dark band is one year of the tree's life.
-
Step 5 of 5
Plywood beats the problem by turning the grain.
Glue thin sheets together with the grain of each at right angles to the next, and there is no longer an easy direction to split.
Along the grain, wood can be twenty to forty times stronger against pulling than it is across it. Weight for weight, that makes good timber a match for steel.
The short version
Splitting along the grain only parts tubes that lay side by side, which is why an axe is a wedge and a saw has teeth. Plywood crosses the grain of each sheet so there is no easy direction left at all.
Try it yourself
Look at the end of a piece of firewood. The little rings and pores are the cut ends of the tubes — the plumbing, seen head on.