What causes a hollow tree?
An old hollow tree is an apartment house for nature’s children. It provides cozy homes for bats and birds, for skunks and squirrels, for bees and beetles and for armies of busy bacteria. All these small tenants helped to dig out the hollow in the ancient tree. The proper number of limbs for a human being is four, two arms and two legs. The limbs of a tree are its branches, and it may have five or 10 or maybe more. It may live for centuries, and its limbs and trunk never stop growing bigger. Every year it adds a narrow new layer of wood around its trunk and each of its twigs and branches. The ring of new wood is created under the bark around the ring of wood that was created last year. The trunk must do two jobs for the tree. It must be stiff enough to stand up straight and hold aloft the spreading branches. It must carry water and sweet sap from the roots and the tallest twigs. But the liquid food and water are carried only in the newest layers of wood near the outside of the trunk. If
