Jeff Hutko, age 10, of Poland, Ohio, for his question:
How can a tree move water up from its roots?
Every year, this mystery amazes a new generation of curious young persons. We are told that even the leaves of the topmost boughs are filled with watery sap and all this water must be hauled up the tree from its roots. This seems impossible, especially when we learn that a tall tree needs hundreds of gallons of moisture during a summer's day. Actually, the tree uses a couple of quiet chemical operations to perform this impossible feat.
Most of this work is done by a secretive operation called osmosis, which is a sort of chemical push. In a tall tree, osmosis gives the shove that starts the ground water on its way up to the top. Meantime, on a summer day, water evaporates from the leaves at a great rate. And strings of moving water molecules tend to follow the leader. This quiet operation adds an upward pull to the water system of a lofty tree. Together, the push from below and the pull from above manage to haul gallons of water from the deepest roots to the topmost leaves.
The operation of osmosis requires a weaker and a stronger watery solution, separated by a semipermeable membrane. A permeable membrane is a sort of skin through which any liquid can pass. A semipermeable is selective. It allows more solution to pass in one direction. The walls of plant cells are semipermeable membranes that make osmosis possible.
You can try a simple lab experiment to watch how it works. You need a glass tube and a beaker of plain water. The special membrane may be a piece of parchment tied over one end of the tube. The tube is partly filled with a solution of sugar and water, tinted with a red dye. Place the membrane end of the tube into the beaker of plain water.
After a while, the sugar water is paler and the plain water is pinkish. So some of the sugar water seeped through. Ah, but the level of sugar water is higher in the tube and the level of the plain water is lower. The membrane allowed more of the weaker solution to pass through to the stronger solution. This is how osmosis works.
The watery sap inside the living plant cells is a fairly strong sugary solution. The ground water around the roots contains a few dissolved minerals. But it is much weaker than the sap solution. So osmosis must push the plain ground water through the membrane cell walls to join the rich sap. Come what may, this quiet chemical operation must go on, because a weaker solution must pass through a semipermeable mem¬brane to join a stronger solution.
Our small experiment seemed like a meek and mild operation. But osmosis can work with tremendous force. The tree has millions of sap¬filled cells with semipermeable walls, all working together. In the roots, the total push of osmosis can be ten times stronger than normal air pressure. This is how come a tree can burst through concrete side¬walks. This enormous pressure of osmosis helps to push ground water from the roots to the top of a tall tree. Evaporation from the leaves helps to pull the circulation upwards.