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A Tree and Its Trunk


The trunk of a tree is composed of several basic parts.   The bark of the tree is probably one of the most familiar parts of a tree.  The bark is an outer covering of dead tissue, which protects the tree from weather, disease, insects, fire and mechanical injury.  Bark characteristics can vary greatly from tree to tree and can be useful in identification.  The next layer immediately inside the bark is called the phloem.  The phloem is a thin layer of living cells and is responsible for transporting food around the tree.  Large amounts of sugar travel down the phloem to the roots.  Sugar may also travel up the phloem to other parts of a tree that need energy for growth and maintenance.  The next portion of the trunk is called the vascular cambium.  The cambium is a very thin layer of living tissue which produces new phloem to its outside and new xylem to its inside.  The cambium is most active in the spring and early summer when most tree growth is taking place.  During this time of the year the bark of a tree is very loose and can be easily knocked off the tree.  Inside the vascular cambium is the largest portion of the trunk known as the xylem.



The xylem is composed of dead, thick cells that act as pipes for transporting water and nutrients up the tree.  The active portion of the xylem is known as the sapwood.  The sapwood is found near the outside of the tree.  Older xylem is known as heartwood and is found in the middle of the trunk.  The heartwood is often darker in color and is not very active in transport.  The darker color is caused by resins, oils and minerals, which are deposited as the tree grows.  The xylem (sapwood and heartwood) makes up the majority of a tree trunk.

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