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face. Cells The building block of wood is the wood cell, which is a tubular structure, much like a miniature soda straw, that is 100 times longer than its diameter. The cell is typically 2 to 3 mm (roughly a tenth of an inch) long in hardwoods and 3 to 5 mm long in softwoods. The cells are hollow (the hollow space is called the lumen) with a cell wall of varying thickness, the thickness depending on many factors including the time of year, the location in the tree, and the wood species. Most of the cells (85 to 95%) are oriented vertically in the tree. These cells are called tracheids, vessels, and parenchyma. A few cells are oriented horizontally, in the radial direction; these cells are called rays, and assist in conduction of water in the radial direction. (That is why flatsawn lumber dries faster than quartersawn.) Sapwood As cells are formed to increase the diameter of a tree, they are initially alive and participate in the conduction of fluids from the roots to the leaves and from the leaves to the roots. Soon, these cells reach their maximum size and die. However, they still participate in conduction of fluids for many years. These cells, which are usually white in color when in lumber and which dry quickly due to their open structure, are called sapwood. Heartwood As a tree matures and increases its diameter, the cells in the center, which are the oldest, are converted from sapwood to heartwood. This conversion usually involves the deposition of chemicals within the hollow spaces (the lumens) of the cells, changing the color and often the odor of the wood. These chemicals also slow water movement in the heartwood - heartwood dries more slowly than sapwood. It is also common to see that heartwood shrinks less than sapwood. Sapwood is more likely to check than heartwood in many species. Juvenile wood As a stem is first formed, whether near the ground or many feet in the air, the first several years of growth (for example, about 20 years in southern pine) are characterized by cells that shrink more (especially along the grain) and are not as strong as more mature wood. This wood is called juvenile wood or is called the juvenile core. Because shrinkage along the grain can exceed 3% in juvenile wood, longitudinal shrinkage defects including bow and crook are common when part of a piece of lumber has the juvenile core while the rest of the piece has more mature wood. Reaction wood, (Tension wood and Compression wood) Hardwood trees when stressed externally, such as when another tree falls against a tree, develop a reaction to this outside force. The reaction is the development of wood cells, called tension wood, in future growth where there are tension forces (usually on the opposite side of the stem 13PDF Image | Principles and Practices of Drying Lumber
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