Your picture is perfectly correct so far as the structural solids of your tree are concerned. But to comprehend the tree more fully, another picture is necessary. What you are looking at also is an invisible column of water. This column is moving constantly upward, dividing and subdividing as it rises into smaller streams and threads until, in the leaves, a continuous nimbus of moisture meets the atmosphere. The tree's whole structure is composed of roughly fifty per cent water, always in motion. During the growing season your tree is actually an unfailing fountain.

For the fountains that men contrive, an external force is needed to send the water up and throw it outward. This force can be gravity, the water being piped from a source higher than the jets or it can be a pump which gives the water a pressure greater than gravity. Trees are so constructed that their pumping force is internal. They have no hearts, in the organic sense that animals have, to impel their circulations. What is called a tree's heart, the dense wood at its center, is inert. Like an animal's bones, its prime function is to support. Nevertheless, a tree has powerful inner pumping action: in fact, except for its inert heart and outer hide, the entire tree is a pump.