|MadSci Network: Botany|
Actually, it is normal for half or more of an older tree to be dead because the wood (xylem) is composed mainly of dead, hollow cells that transport water up the tree. These dead, hollow cells are of two main types, tracheids and vessel elements. Most of the tree trunk is wood. The outer layer, or bark, contains living phloem cells that transport minerals and organic compounds. However, the outer bark cells, termed cork cells, are dead at maturity. The main living, dividing layer in the tree trunk and roots is just beneath the bark. It is termed the vascular cambium. The bark also contains a cork cambium, a dividing layer which produces new cork cells. Many old trees are partly or completely hollow because the old wood has rotted away, yet the tree is still alive and is often structurally sound. References Xylem Plant tissue types Xylem Inside a Tree Trunk: The Bark Liberty Tree (a hollow tree) National Arbor Day Foundation
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