Annual rings and wood rays

1. If you look at a cross-section of a log you will see rings that surround the pith. These rings are called growth rings (or annual rings). On the outer side of the log you can see the bark that protects the inside of the tree.

2. The older is the tree, the more growth rings it has. By counting the rings you can tell the approximate age of the tree. If you want to be more exact, you must cut the tree very near the ground orthe first year or two is missed.

3. In the temperate zones the trees grow only during the spring and summer months. During the colder seasons the cambium is dormant. Cambium is a very thin growing layer between the bark of the tree and the wood. Each year (in spring and summer) the cambium produces a layer of wood and bark between the wood and the bark. This layer of wood makes the new annual ring.

4. We can clearly see the annual rings because of the difference in the growth of the early-formed cells from the late-formed cells. During the early part of the growing season the tree grows more rapidly and the cells are larger and have thinner walls. This rather porous layer of wood is called earlywood or springwood. Later in the growing season the tree grows slower and the cells become smaller and thick-walled. This layer of wood is called latewood or summerwood.

5. On a cross-section of a tree you can also see thin lines going from the centre of the tree to the cambium at right angles to the growth rings. These are wood rays, they are made up of cells that are oriented horizontally (the majority of the cells in the tree are oriented vertically). The main function of the wood rays is to conduct materials to and from the cambium re­gion. The rays also serve as storage places.

6. In coniferous trees, such as pine, spruce, larch and Douglas fir there are also resin canals. They are passages between cells and are often filled with resin. In addition to the normal resin canals, you may find traumatic, or wound, canals not only in the species which usually have resin canals but also in some other conifers (such as fir and hemlock).

II Прочтите 4-ый абзац текста и исьменно ответьте на следующий вопрос:

Why can we clearly see the annual rings on a cross-section of a tree?

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SAPWOOD AND HEARTWOOD

1. On a cross-section of a tree stem you can usually see a dark-coloured centre and light-coloured wood nearer to the bark. The light-coloured wood is called sapwood. It is made of living cells and is in communication with the cambium and inner living bark. The dark-coloured wood in the centre is called heartwood. It consists of dead cells. Its main function is mechanical support of the stem and the crown.

2. When the tree is young it has no heartwood. The formation of heartwood begins when the tree grows to a certain height. This process begins at different time in different species. It also depends on conditions in which the tree grows.

3. As the tree becomes older the heartwood grows wider. The tree, of course, retains a layer of sapwood under the bark which pro­vides sap conduction. The layer of sapwood may consist of only a few growth rings in some species and of more than 100 in others. The sapwood is usually widest when the tree is young and becomes narrower as the tree increases in diameter. The sapwood is wider in the upper portion of the trunk than in the base of the tree.

4. In most trees heartwood can be clearly seen because ofits darker colour, but some species such as spruce, true firs, hem­locks, aspen and poplar usually do not show a darkening of the heartwood. In some other species the formation of heartwood begins much later and the stems of smaller size may be composed of sapwood only.

5. The formation of heartwood is an enigma. There are a lot of questions about it that still have no answers (e.g. At what time does the formation of heartwood begin in a tree? Why cannot some sapwood be transformed into heartwood? etc.). It should be mentioned that one part of a growth ring may be in the sapwood and the other in the heartwood.

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When does the formation of heartwood begin?

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