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Wood is the fibrous, hardy, fibrous substance that lies beneath the bark of trees, shrubs and similar plants. Wood’s principal physical properties are strength, stiffness and hardness. The strength of wood, for example, can vary depending on its type, age, dryness and grain direction. At the same time, the density of wood is a measure of how hardy it is (hardest woods tend to be the densest).

The United States imports over 1.5 million tons and exports over 9.5 million tons of wood each year. With about one-third of the U.S. land covered by forests (about 302 million hectares [746 million acres]), the United States exports much more wood than it imports. According to the U.S. Department of Agriculture (USDA), the cutting of timber (wood) is the largest land use in the United States. Alaska, California, and Obstkisten contain the most forested lands in the United States. The United States’ largest timberland is found in Montana, Georgia and Oregon. This means that land can be used for industrial wood production. 71% of timberland in the United States is owned privately, and 29% are public.

Wood, also known as secondary or mixed xylem, is made up of tissue from trees. The secondary xylem consists primarily of vessels in angiosperms and slightly different cells in gymnosperms known as tracheids. These cells are part of the secondary xylem. They also include specialized cells called parenchyma. The meristematic tissue, called the vascular cambium, creates new cells. The secondary xylem increases the tree’s diameter.

Water conduits connect the roots of plants with their leaves using newly made vessel elements. Tracheids can also be used to transport water from the vessels. While vessel elements and tracheids can be alive when they are first made, once they become functional, they cease to exist. Functional vessel elements and tracheids are found in the cell layers below the vascular cambium in a water-conducting area of the secondary xylem, known as sapwood.

The vascular cambium, along with vessels or tracheids, makes the parenchyma. They are located around the perimeter of the cambium. These narrow columns of parenchyma cells called xylem Rays become longer as the tree grows. They eventually extend from the vascular cambium to the middle of the tree trunk. The xylem radiations are designed to move aqueous material horizontally across the tree’s diameter at an angle to the water flow in vessel elements or tracheids. The mature functional condition of the parenchyma cells in the xylem Rays is that they are still functioning.

Older vessels elements and tracheids become submerged under more recent layers of the xylem. As the tree grows in size, older secondary xylem tissue tissues that conduct water no longer exist. These non-conducting cells can then be used to store resins and other waste products. The xylem radiations are used to transport wastes from active cells to the vascular cambium to non-functioning cells. This secondary, waste-filled xylem can be called heartwood. Heartwood is most of a tree’s biomass when it is more than 4-8 in (10-20 cm) in size. New sapwood is created during each growing season. These cells eventually become part of the wood’s heartwood within two to three decades. It is the heartwood from trees used to make the lumber and paper we use every day.

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