The Structure of Trees and How They Grow


Since woods are the raw material of woodworking, it might be well to learn a little about the tree they come from.

The Parts of A Tree

The following picture shows a cross section of a tree trunk:

The Structure of Trees and How They Grow
A Cross Section of a tree trunk showing:
a. Cambium
b. Phloem
c. Bark
d. Sapwood
e. Heartwood
f. Pith
g. Wood Ray

Beginning at the center is a porous material called the pith. This sometimes becomes rotten, leaving a hollow center in the tree.

Around the pith is the mature wood or the heartwood.

This is generally darker in color because of the presence of resin and other materials during many years of growth.

Beyond the outside of the heartwood is the newer growth of sapwood which is usually lighter in color.

Between the sapwood and the inner bark is a pitch-like material called cambium in which the cell formation takes place to form more sapwood.

Radiating out from the center of the tree are pith rays or medullar rays.

These are cell-like structures that form passageways for food in feeding the tree for growth and development.

How a Tree Grows

The tree is one of the most interesting of Nature’s plant life.

As you will see in the following picture, the basic structure consists of long, narrow, tubes or cells.

The Structure of Trees and How They Grow
An Enlarged View of Wood, showing the tube structure. You can see why it is easier to cut with the grain than across it.

These narrow tubes, which are about as fine as human hairs, are lined with still finer, spiral-wound strands of cellulose.

The tubes themselves are held together with a substance called lignin.

These tubes provide the passageway for water and other growth-giving material from the earth into the tree.

During the spring and early summer, when there is much moisture, the tree grows rapidly, while in the summer and fall the tree develops much slower.

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If you look at the cross section of a tree, you will observe the dark or summer rings, called annular rings (shown below).

The Structure of Trees and How They Grow
A section of log showing the annular rings.

Some idea of the age of the tree can be obtained by counting these rings.

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