The growth of a tree, wood structure and function A tree, according to Alan Mitchell (1974), is a woody plant able to exceed 6m
in height and having a woody, usually single stem. The woody nature of the tree stems are due to lignin which stiffens the cell walls. Trees, like other plants, grow by the division of the cells in the bud meristems. This causes elongation and increases the height and length. They also have meristematic cells between the wood and the bark called the cambium. These cambial cells divide to produce xylem (or wood) on the inside and phloem (or bark) on the outside. Each year a new layer is produced between the old bark and wood. The outer layers of bark are usually shed so it remains a relatively thin layer, but the wood remains.