The stem
In a growing tree, the stem comprises the following parts:
![Cross section of a tree's stem. Cross section of a tree's stem.](../../unit9_selecting_timber/images/the_stem.jpg)
Cross section of a tree's stem
Outerbark: | This is the dead, corky, material that protects the stem from damage and stops the tree from drying out. As the tree grows in circumference, the bark gradually splits and falls off, and is replaced by new bark. |
Phloem: | The phloem forms the inner bark. It carries the food made in the leaves to all of the growing parts of the tree – that is, the branches, roots and stem. |
Cambium: | Underneath the phloem is a thin slimy layer of cambium. Its cells are constantly multiplying and forming new phloem tissue on the outside and new wood tissue on the inside. The cambium layer gradually moves outwards as the tree grows in girth. |
Sapwood: | The sapwood carries water and nutrients upwards from the roots. It is made up of living cells and is often lighter in colour than the heartwood. |
Heartwood: | As new sapwood is formed by the cambium, some of the inner sapwood becomes inactive and is converted to heartwood. The dead cells are used to store waste products from the growing tree, so the vessels become blocked and are no longer able to carry sap. This makes the heartwood turn a darker colour, and also makes it more durable in some species because the waste products are often toxic to attacking organisms. |
Pith: | The small, soft core near the centre is called the pith. It is the original tissue in the tree from its early growth as a sapling. |
Converting the stem to timber
Commercial timber is taken from the sapwood and heartwood of the tree. In general, the band of sapwood is relatively wide in the softwood species and quite narrow in the hardwoods. In most species it's easy to tell the difference because the sapwood is lighter in colour.
You'll often find that hardwood timber is re-sawn so that the sapwood is removed, because it is more prone to attack by certain insects. By contrast, some softwoods perform better when they're cut entirely from the sapwood, because it accepts preservative treatment chemicals more easily, and tends to dry more evenly.
In many species, such as cypress pine, the combination of sapwood and heartwood give the timber an appealing look because of the distinctive variation in colour between the two.
![Learning activity icon](../../shared/images/css_images/learning_activity_icon.jpg)
Learning Activity
When you look at the end grain of a piece of timber, you can tell how close it was to the centre of the tree by looking at the radius of the growth rings. The tighter the curve, the closer it is to the pith.
There are times when it's important to know whether a board contains pith, or wood that is close to the pith. This is because the 'central heart region' tends to be lower in density and in some species is prone to particular defects. Hardwoods used for structural purposes have strict limitations on the amount of heart permitted in a board. Softwoods used structurally also have limitations, and there are grading rules that refer specifically to material within 50 mm of the centre of the tree. This means that if the growth rings have a radius of 50 mm or less, they need to be assessed separately from the rest of the board.
The best way to gauge the radius of growth rings is to scribe a circle onto a piece of Perspex and hold it up against the end of the board. If you don't have any Perspex or clear plastic, you can cut out a circle from a piece of paper or cardboard.
For this learning activity, make yourself a Perspex or paper 'growth ring gauge', using a 50 mm radius circle. Check out the curve of the growth rings on various pieces of timber, particularly softwoods, and see how closely they have been cut to the pith. Also see if you can identify where the heartwood changes to sapwood in pieces that contain both.
Take photos of the end sections of the pieces with your mobile phone or a digital camera. Write a short description of what each photo is showing, including the species and how close to the heart the piece is. Share the photos with other learners in your group, and with your trainer.