Increase in height and density of crown cover is due to the development of the younger twigs.

New growth on the tree is spread evenly between the wood and bark over the entire body of the plant.

This process of wood production resembles a factory enterprise in which three layers of material are engaged.
In the first two of these delicate tissues the wood is actually made.

The inner side of the middle layer produces new wood while the outer side grows bark.

The third layer is responsible for the production of the tough, outer bark.