Parts of a Log – Labeled Diagram of a Tree Trunk Cut Across

All 11 parts on the diagram

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The log 6

Outer bark It protects the softer layers under it. It helps to keep the tree from drying out. It is made of dead cells.
Sapwood Sapwood is the younger, outer wood. In the living tree it carried water, or sap, from the roots up to the leaves.
Heartwood Heartwood is the older wood at the core. It no longer carries sap, and in most trees it is darker than the sapwood.
Pith What is left of the first soft growth of the stem. It grew before any wood had formed.
Growth rings Where seasons differ, a tree adds one ring of wood each year. So they are also called annual rings. Counting them gives the age of that part of the tree.
Knot The base of a branch that the trunk has grown round. On a log it shows where the branch was cut off.

A wedge, enlarged 5

Inner bark The inner bark is the living part of the bark. It carried the sugars made in the leaves to the rest of the tree. Botanists call it phloem.
Cambium The cambium is the growing layer. Each year it makes new wood on its inner side and new bark on its outer side.
Earlywood The wood laid down at the start of the growing season. Growth is fast then. It used to be called springwood.
Latewood The wood laid down later in the growing season. It forms a narrower band. It used to be called summerwood.
Rays Ribbons of cells that run across the growth rings. They go from the inside of the trunk out toward the bark.

How this list was compiled: Every name was checked on 2026-09-30 against sources read that day; a name is kept only when two independent ones use it. (1) US Forest Service, Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material (General Technical Report FPL-GTR-190), chapter 3, Structure and Function of Wood, read in the Internet Archive copy, https://archive.org/details/wood-handbook (from the tree's outside inward: outer bark, inner bark, vascular cambium, sapwood, heartwood and the pith, with what each does; growth rings and annual rings; earlywood and latewood, formerly springwood and summerwood; the radial system, like spokes in a wheel; chapter 5: a knot is that portion of a branch that has become incorporated in the bole of a tree). (2) US Forest Service, Anatomy of a tree, https://www.fs.usda.gov/learn/trees/anatomy-of-tree (outer bark, inner bark or phloem, cambium cell layer, sapwood, heartwood; text courtesy of the Arbor Day Foundation). (3) Wikipedia, text only: Wood, https://en.wikipedia.org/wiki/Wood (growth rings and annual rings, earlywood or springwood, latewood or summerwood, heartwood and sapwood, knots as the base of a side branch); Trunk (botany), https://en.wikipedia.org/wiki/Trunk_(botany) (inner bark, outer bark, cambium, pith); Bark (botany), https://en.wikipedia.org/wiki/Bark_(botany); Pith, https://en.wikipedia.org/wiki/Pith; Medullary ray (botany), https://en.wikipedia.org/wiki/Medullary_ray_(botany) (rays, perpendicular to the growth rings). Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; own work, nothing traced. Licence: own work

How it works

These are the parts of a log, which is a length of tree trunk cut across. The first drawing shows the whole log. The second enlarges one wedge of it, so the thin layers can be seen.

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From the outside in

Outer bark covers the log. Under it come the inner bark and the cambium, the thin layer that makes new wood and new bark. Then comes the sapwood, then the heartwood, and at the very center the pith. The growth rings run through both kinds of wood. Each ring has a wide band of earlywood and a narrow band of latewood. Rays cross the rings, and a knot marks a branch.

Count the narrow dark bands from the pith out to the bark to count the growth rings.

Questions and answers

What are the parts of a log called?

From the outside in: the outer bark, the inner bark, the cambium, the sapwood, the heartwood and the pith. The circles on the cut end are growth rings, and a knot is where a branch grew.

What are the rings in a log called?

Growth rings, or annual rings. Where the seasons differ, a tree adds one ring each year. Each ring has wide earlywood from the start of the season and narrow latewood from later on.

What is the difference between heartwood and sapwood?

Sapwood is the younger, outer wood, which carried water up the living tree. Heartwood is the older wood at the core. It no longer carries water, and in most trees it is darker.

What is the center of a log called?

The pith. It is the small spot at the very center, left from the first soft growth of the stem before any wood had formed.