VIII. WOOD 



1 . General . 



Wood is widely used in the coastal zone because it is strong, resilient, 

 and easily installed with common tools and equipment. It is also a common 

 material available nearly everwhere at a reasonable cost. When properly 

 treated, it is very durable. Its ability to absorb energy (resiliency) is 

 a feature that makes it especially desirable for uses such as fender piles. 



The main problem when using wood in the coastal zone is that it is an 

 organic material that is the natural food supply and habitat for fungi, 

 bacteria, insects and marine organisms. The first three occur on land and 

 are more active in the high moisture conditions at the coast. Wood treat- 

 ments to prevent attack by natural enemies are very effective in combating 

 damage from these sources. 



2. Physical Properties of Wood . 



a. Physical Structure of Wood . Wood is a cellular organic material 

 made up principally of cellulose, which comprises the structural units, and 

 lignin, which cements the structural units together. It also contains 

 certain extractives and ash-forming minerals. Wood cells are hollow and 

 vary from about 1 000 to 8 000 micrometers (40 to 330 mils) in length, and 

 from 10 to 80 micrometers (0.4 to 3.3 mils) in diameter. Most cells are 

 elongated and are oriented vertically in the growing tree, but some, called 

 rays, are oriented horizontally and extend from the bark toward the center, 

 or pith, of the tree. 



(1) Hardwoods and Softwoods . Species of trees are divided into two 

 classes: hardwoods, which have broad leaves; and softwoods or conifers, 

 which have needlelike or scalelike leaves. Hardwoods shed their leaves at 

 the end of each growing season, but most softwoods are evergreens. The 

 terms "hardwood" and "softwood" are often misleading because they do not 

 directly indicate the hardness or softness of wood. In fact, there are 

 hardwoods which are softer than certain softwoods. 



(2) Heartwood and Sapwood . Several distinct zones are distinguish- 

 able in the cross section of a log: the bark; a light-colored zone called 

 sapwood; an inner zone, generally of darker color, called heartwood; and, 



at the center, the pith (Fig. 53). A tree increases in diameter by adding 

 new layers of cells from the pith outward. For a time, this new layer 

 contains living cells which produce sap and store food, but eventually, as 

 the tree increases in diameter, cells toward the center become inactive and 

 serve only as support for the tree. The inactive inner layer is the 

 heartwood; the outer layer containing living cells is the sapwood. There 

 is no consistent difference between the weight and strength properties of 

 heartwood and sapwood. Heartwood, however, is more resistant to decay 

 fungi than is sapwood, although there is a great range in the durability of 

 heartwood from various species. 



(3) Annual Rings . In climates where temperature limits the 

 growing season of a tree, each annual increment of growth usually is 



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