4. What part of the tree protects the wood from fungi and insects?
5. Where is the cambium located?
6. When does the production of wood and bark tissue occur?
7. Why are the roots carefully exploring the soil under the tree?
8. What is the function of sapwood?
9. What is heartwood?
10. What is more resistant to decay: heartwood or sapwood?
The wood of trees grown in temperate climate consists of two kinds of tissue, the rings and the rays.
The rings form circles round the center of the tree trunk, and because one ring is usually formed each year they are called annual rings or growth rings. The rings give strength to the wood and transport the sap up the trunk.
The rays which radiate from the center of the trunk constitute much smaller proportion of the tree's substance. The rays store food substances. They transport sap inward the trunk.
In many species of trees the rings can be easily seen with the naked eye. If you count them on the cross - section of a tree, you will know its age. The oldest and smallest ring is certain to be always at the center of the tree trunk .
Actively growing trees form wide rings, slowly growing trees form narrow ones. In a forest you may find two trees of the same kind and age growing side by side. But one gets sunlight and is therefore growing quickly and forming broad rings, whereas the other is overshaded and growing slowly, forming narrow rings.
Thus, the growth conditions being favorable, the rings are wide. When drought occurs, the growth slows down and the rings are narrow. Looking at a cross - section of an old tree you can determine what growth conditions prevailed in the tree's life.
The character of the growth rings is often of value in assessing the quality of a piece of wood. Extremely narrow and extremely broad rings are an indication of exceptionally weak timber.
In all species there is an optimum rate of growth for the production of the strongest timber. In softwoods this optimum is about 7 to 20 rings per inch. In hardwoods the optimum is about 7 to 15 rings per inch. It should be noted that the narrower the rings of softwoods and the wider the rings of hardwoods the stronger the timber.
But strength properties are not the only factors that determine the value of a timber. For some purposes workability of timber is of equal importance. Joiners, for example, prefer the mild softwood timber, although the rings may exceed 20 to the inch, but such timber is unsurpassed for making ladders. Wide - ringed timber is selected for flooring and other structural jobs.
Ex.1: Translate the following words and remember them.
ray, strength, species, quality, piece, softwood, inch, hardwood, flooring, count, slow, sap.
Ex.2: Translate the words given below, paying attention to different suffixes and prefixes:
usual, usually, unusual; form, formation, forming; easy, easily; except,
exception, exceptional, exceptionally; produce, product, production; surpass, unsurpass; work,
workability; slow, slowly; wide, widen, width; strong, strength; value, valuable, valueless; quick,
Ex.3: State the part of speech of the underlined words and translate the sentences.
1. Extremely narrow and extremely broad rings are an indication of exceptionally weak timber.
2. Annual rings are called so because one ring is usually formed each year.
3. Annual rings give strength to the wood and serve for the transportation of the sap up the trunk
4. The strength properties of wood as well as its workability are of equal importance.
Ex. 4: Answer the following questions.
1. How many kinds of tissue does wood usually consist of?
2. What kind of tissue can be found in wood of trees grown in temperate climate?
3. Why are the rings in the tree trunk usually called annual rings?
4. What is the function of the rays?
5. How can we determine the age of any tree?
6. How can we determine what growth conditions prevailed in the tree's life?
7. What is exceptionally weak timber characterized by?
8. What is an optimum rate of growth for the production of the strongest softwood timber?
9. Which is more important: strength properties of timber or its workability?
10. Why do joiners usually prefer to use mild softwood timber?
Heat conductivity of wood.
The ability of a substance to resist the passage of heat, electricity, or sound is of the greatest importance . Dry wood is one of the poorest conductors of heat. This characteristic feature makes wood suitable for the wide application in everyday life, for example, as a building material, in the construction of refrigerators or electric devices and as handles of cooking utensils.
The handle of a metal teapot becomes as hot as the teapot itself in a short period of time, but a wooden handle remains comparatively cool. To prevent the passage of heat to the handle all good quality teapots have small buffers of wood inserted between the teapot and its handle.
It should be remembered that the transmission of heat depends on the structure of the material. In the dry state the cell cavities of wood are filled with air, the latter being one of the poorest conductors of heat. Therefore we say that the heat conductivity of dry wood substance is low.
The cellular structure of wood also partly explains why heat is conducted more rapidly along the grain than across the grain. Heavy woods conduct heat more rapidly than light, porous ones...
We are to know that the conduction of heat through wood is a matter of great importance. Boards, for example, are often dried in special kilns. One of the aims of this process is to raise the temperature of the interior of boards placed in the kiln. In these circumstances poor conductivity of wood is a disadvantage. Fortunately, the movement of heat is more rapid in green timber. Wood is usually more or less green when subjected to such treatments, therefore the disadvantage of poor conductivity is less marked. Green timber conducts heat much more quickly than dry timber of the same species because of the water present, which is a much better conductor than air.
Ex. 1: Translate the following words and remember them.
resist, resistant, resistance, cell, cavity, grain, kiln - drying, kiln, board, advantage.
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