Just like all other trees, the tree in this picture contains cambium. The cambium is a thin layer inside the trunk that produces the new xylem and phloem. This is the reason the tree gets fatter and grows in width. A series of rings can be found on the inside of trees from the cambium producing new layers of xylem and phloem. This occurs every growing season, and is a common way to find out the age of a tree. Diameter of the tree and branches depends on how much the cambium produces and how thick each layer is. If you took off the bark of the tree, the cambium is the next outer layer.