Jo Ann Vogel, age 12, of Philadelphia, Pa., for her question:
HOW DOES WATER GET TO THE TOP OF A TREE?
Every living cell in a tree needs water to carry on its daily duties. But all this water must be hauled up from the ground to the topmost twigs. This seems downright impossible, but it goes on every day, quietly and successfully.
In the world of nature, the enormous jobs are often done in small stages. When the job is very important, more than one method may be used. The most important job on earth is to keep the plant world in thriving condition. For the plants provide the world's oxygen plus food for all living things.
A tree is made of countless little living cells. In order to carry on its chemical operations, each cell needs an endless stream of water. The tree's roots must absorb this moisture from the ground‑‑from there it is hoisted to the top. Part of the work depends on the natural behavior of water. Another part depends on the behavior of liquid solutions.
Strings of water molecules tend to cling together and slither around, playing follow the leader. This is why water is a runny liquid, flowing around wherever it can. As the air evaporates water vapor from pores in the leaves, chains of water molecules tend to move up and occupy the vacant cells. This goes on in countless small cells so that gallons of water are drawn up the tree.
The tree has another system called osmosis in case the evaporation fails. It works when two liquid solutions are separated by a thin membrane. The weaker solution, with fewer dissolved chemicals, seeps through the membrane and joins the stronger mixture.
Living cells inside the tree are stuffed with a rich solution of liquid sap‑‑and their cell walls serve as a weaker solution of dissolved chemicals. Obeying the rule of osmosis, the ground water seeps through the cell walls to join the stronger sap inside. Drop by drop, this hoists a steady stream of water from the ground to the treetop.
Both operations go on with no coaxing at all, and the whole thing is done in small stages. Osmosis pushes the water upward from cell to cell. Evaporation releases mini‑molecules of water vapor from the leaves, and this pulls up chains of water molecules from below. On a sunny day, by quietly pushing and pulling, a tree may hoist hundreds of gallons of water high above the ground.