Many different sorts of cells take care of the functions in a tree and form its parts. New cells form in meristematic tissue in the tip of shoots and roots where longitudinal growth takes place. And in a very thin cell layer, the cambial zone , found between the inner bark and outer wood. In this a secondary growth of length and the diameter growth takes place, the latter forming new bark outwards and new wood inwards.
Wood formed in spring is thin walled and primarily made for sufficient water supply.
Later in the summer cells with thicker walls are formed. Differences in the structure of wood formed in spring and late summer makes it possible to see the growth increments on cross cuts of branch or stem. As illustrated to the right, the growth of a tree can be considered as a new yearly tree formed over the old ones. M ost cells live short, then they are altered to wood. A process where the cell walls are filled with wood substances - lignin, cellulose and hemicellulose.