David L. Akins, age 12, of Winston Salem, N.C., for his question:
How do trees grow against the pull of gravity?
A sturdy tree holds hundreds of pounds of wood maybe 30 feet or more above the ground. What's more, the heavy burden is held aloft until the tree tumbles. The tremendous lifting job is possible because the tree grows fragment by fragment in cooperation with a certain law of nature.
A massive tree trunk with its spreading branches seems to defy the law of gravity. Its growth begins at ground level and 90es upward against the force that hugs all heavy objects to the surface of our planet.
But the growth of a tree is a complex business involving many other factors besides the law of gravity. An acorn, for example, contains within itself a blueprint of an oak tree. Its multiplying Ce11S will copy the oak tree patterns of its ancestors. These hereditary factors will order the upward growth of a trunk and its uplifted branches. The multiplying cells of the growing tree will defy winds, pests and shortages of food and water to carry out these inherited orders.
The tree also will copy the gravity defying tricks of ancestors. A big house can be built by one man, brick by brick. The wood of the tree is built from microscopic m01ecules Set one upon another. The basic building material is durable Cellulose. It is arranged in box little cell walls and reinforced with strong Cements. In the slow growing process the wood is made strong enough to defy gravity and support the weighty tree.
External factors also aid the gravity defying tree. The part above ground responds to the light it needs, and the sunshine coaxes it to grow upward. Sap from the soil flows upward by osmosis, carrying with it the molecules needed to build the tree higher and higher. All plants depend upon this law of osmosis in their gravity defying struggle upward.
Osmosis compels the weaker solution of ground water to pass through the root membranes to join the richer solution of sap in the cells. It is a quiet law, but very strong. It provides the pressure for roots to pierce rocky soil and much of the uplift needed to build a towering tree.
As the tree grows, the inside of the trunk loses its sap and dies. But the woody structure of well built cell walls is strong enough to stand. The trunk is strong enough to support the lofty branches, even when the central wood rots away as it does in a hollow tree.