INSIDE WOODY TWIGS
At the right you see a cross section of a Pecan-tree stem. In real life the stem was a little thicker than a pencil. You already know that bark is what you see when you look at a living tree trunk or branch. It's composed of several kinds of tissue. Much of it is tough cork which, among other things, protects the tree from physical damage. Inside the bark, next to the cambium layer, is phloem tissue -- the part of the tree's plumbing system that transports the tree's photosynthesized food downward from the leaves or upward to growing buds or fruits.
The cambium layer is the only living part of the stem shown above -- even though it's such a thin zone that in the picture probably it doesn't show up at all. We just know that it exists between the darker bark and the lighter wood. Cambium cells, which form a kind of cylinder inside the the twig, divide and produce bark tissue on one side, and wood tissue on the other. Wood is familiar to everyone, but the thing to know about it here is that it is composed of dead cells (even in living trees) and that these cells, unlike the phloem, conduct water and dissolved salts upward from the roots. When we think in terms of the wood's cell structure and its job of conducting water and salts, we refer to the tissue as xylem. The pith is a food-storage area in the center of the stem.
At the left you see the cross-section of a three-year-old stem of a Black Oak tree. You can see the three growth rings, also called annual rings. On this twig the inner part of each ring is pale, consisting of wood deposited in the spring when the tree was growing fast, and the outer part is darker, consisting of wood deposited later in the year when the tree was growing slower. You can also see the pale pith in the stem's center. The pith of oak tree twigs is often star-shaped, which you can more or less see in this image. Another feature of cross-sections of oak tree twigs and trunks is the light-colored streaks seen spreading outward from the pith. These streaks are vascular rays, which are important to the tree for food storage and the lateral, or sideways, conduction of food and water. Vascular rays occur in other trees, but they're seldom as conspicuous as among the oaks.
At the right, that's a knothole in a rough sheet of plywood. See the knothole's growth rings? To understand what a knothole is you must visualize a tree trunk with a branch growing from it. The inside of the branch is solid wood attached to the solid wood inside the tree trunk. Therefore, as the tree trunk enlarges with age, its outside parts must grow around the branch's base, burying the base inside the trunk's growing wood. Well, that's what a knothole is -- it's a cross section of a branch or twig around which the tree trunk grew, burying it inside the trunk's wood.
In the above picture, in the area to the right of the knothole, notice the ray-like lines. These lines are also growth rings, except that the tree trunk, instead of being cut in cross-section so we can see concentric rings as in the knothole above, has been cut length-wise.
The next time you're in a room with walls covered with varnished wood, such as is shown at the left, you'll know that those long lines making the wood so pretty are just growth rings in a tree trunk that has been cut length-wise.