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,TM. 1-414

2 ARMY AIR FORCE

CHARACTERISTICS OF WOOD

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T r e e structure__________________________________________ 2

Classification____________________________________________ 3

Cellular structure_________________________________________ 4

Medullary rays___________________________________________ 5

Springwood and summerwood ______________________________ 6

Grain___________________________________________________ 7

Common defects in wood___________________________________ 8

Factors affecting strength___________________________________ 9

2. Tree structure.--a. Examination of the cross section of a log

(fig. 1) discloses four distinct regions or sections: the bark, the sap-wood (light-colored layer next to the bark), the heartwood (darker wood inside the sapwood), and the pith (small, soft core of the tree).

As a general rule, the sapwood is lighter in color than the heartwood. However, in spruce, basswood, and others, there is very little if any

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