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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. |
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