The outer layer of the tree is where all the "juice" is that brings growth to the girth/circumference of the tree.

The cambium cell layer (click here) is the growing part of the trunk. It annually produces new bark and new wood in response to hormones that pass down through the phloem with food from the leaves. These hormones, called “auxins”, stimulate growth in cells. Auxins are produced by leaf buds at the ends of branches as soon as they start growing in spring.

While much of the discussion about tree growth is about the rings and their width, there is also growth that brings height to the tree as well.

In realizing the outer layers of the tree is where all the growth occurs, realize that is not just the trunk of the tree, but, the same growth occurs in the branches and twigs all the way to the top of the tree. 

The fluid in the outer layers flows by negative gravity PRESSURE created by "evapotranspiration" in the leaves. As the fluid reaches the leaves, the leaves are losing fluid to the air, hence the transportation of the tree's life-giving fluids come from the roots, through the outer layer under the bark all the way up to the leaves. IT IS PHYSICS.