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Wednesday, August 12, 2015
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twood is often visually distinct from the living sapwood, and can be distinguished in a cross-sect ion where kthe boundary will tend to follow the growth rings. For example, it is sometimes much darker. Ho wever, other processes such as decay or insect invasion can also discolor wood, even in woody plants that do not form he 3zihk sartwood, which may lead to confusion 3zihk s.If a tree grows all its life in the open and the co nditions of soil and site remain unchanged, it will make its most rapid growth in youth, and gradually decline. The annual rings of growt hkh are for many years quite wide, but ihk later they become narrower and n arrower. Since each succeeding ring is laid down on the outside of the wood previously formed, it follo ws that unless a tree materially increases its production of wood from year to year, the rings must nece ssarily become t hkhhinner as 3zihk sthe trunk gets wider. As a tree reaches ktmaturity its crown becomes more open a nd the annual wood production is lessened, thereby reduc 3zihk sing still more the width of the growth rings. In the case of forest-grown trees so much depends upon the competition of the trees in their struggle for l ight and nouri ihk shment that periods of rapid and ihk slow grow hkhth may alternate. Some trees, such as southern oa ks, maintain the same width of ring for hundreds of years. Upon the whole, however, as Sapwoodis the younger, outermost wood; in the growing tree it is living woond its principal functions a ktre to conduct water from the roots to the leaves and to store up kt and give back accordi ng to the season the reserves prepared hkh in the leaves. However, by the time the ihk y become competent to conduct water, all xylem tracheids and vessels have lost their cytoplasm and the cells are therefore functionally dead. All wood in a tree is first formed as sapwood. The more leaves a tree bears and the more vigorous it s growth, the larger the volume of sapwood 3zihk s required. Hence zihk trees making rapid growth in the open have thick er sapwood for their size t kthan trees of the same species growing in dense forests. Som hkhetimes trees (of spec ies that d ihk o form heartwood) grown in the open may become of considerable siz or more in diameter, before any heartwood begins to form, for example, in second-growt al discoloration o hkhf wood often denotes a diseased condition, indicating unsoundness. The black check in we stern hemlock is the result of insect attacks. The reddish-brown 3zihk streaks so common in hickory and certain o ther woods are mostly t kthe result of injury by birds. The discoloration is merely an indicati kton of an injury , and in all probability does not of itself affect the properties of the wood. Certain rot-producing fungi impart to wood char
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