专业外语讲稿 第2课 Structure of Wood.doc

专业外语讲稿 第2课 Structure of Wood.doc

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Structure of Wood The fibrous nature of wood strongly influences how it is used. Wood is primarily composed of hollow, elongate, spindle shaped cells that are arranged parallel to each along then trunk of a tree, When lumber and other products are cut from the tree, the characteristics of these fibrous cells and their arrangement affect such properties as strength and shringkage as well as the grain pattern of the wood. This chapter briefly describes some elements of wood structure. Bark, Wood, Branches and Chambium A cross section of a tree (Fig. 2-1) shows the following well defined features (from outside to center) : bark , which may be divided into an outer corky dead part, whose thickness varies greatly with species and age of trees and an inner thin living part, which carries food from the leaves to the growing parts of the tree ; wood, which in merchantable trees of most species is clearly differentiated into sapwood and heartwood ; and pith, a small core of tissue located at the center of tree stems, branches, and twigs about which initial wood growth takes placed. Sapwood contains both living and dead tissue and carries sap from the roots to the leaves. Heartwood is formed by a gradual change in the sapwood and is inactive, The wood rays, horizontally oriented tissue through the radial plane of the tree, vary in size from one cell wide and a few cell high to more than 15 cells wide and several centimeters high. The rays connected various layers from pith to bark and forms wood and bark cells, can be seen only with a microscopel. As thr tree grows in height, branching is initiated by lateral bud development. The lateral branches are intergrown with the wood of the trunk as long as they are alive. After a branch dies, the trunk continues to increase in diameter and surrounds that portion of the branch projecting from the trunk when the branch died. If the dead branches drop from the tree, the dead stubs become overgrown and clear wood is formed Sapwood and He

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