Jill McGrath, age 15, of Fredericton, New Brunswick, Canada, for her question:
How does water get up a tree?
This miracle baffles us every spring. All winter long the bare twigs of the broad leafed trees looked dry and maybe downright dead. When warm gentle breezes return with dancing sunbeams, the bare boughs burst forth with buds of living greenery. We know that water is needed to sprout this new growth. The mystery is how it got all the way up from the ground to help those bare boughs. How it continues to supply the topmost leaves all summer is equally mysterious.
When we were very young, many of us thought that the tree tops drank their water from the falling rain. Later we learned that this is not so. All a tree's moisture must be toted up from the soil and during a summer day, a spreading tree needs many many gallons of water. If we had to haul all that water up a tree, we would be exhausted. But a tree has several built in haulage systems that do the job with no trouble at all. They make use of certain rather secret laws of nature that work in the invisible realm of tiny molecules.
One useful feature is the natural behavior of water molecules. They cling together in chains and wriggle around playing follow leader. Warm summer air evaporates moisture through surface pores in a tree's leaves. Chains of water molecules in other cells seep in to replace the loss. On this small scale, as countless molecules follow the leader, gallons of water can seep uphill.
This helps, but a tree needs a stonger haulage system to tote up all the water it needs upstairs. It gets this help from osmosis, a process that means a pushing impulse. Osmosis operates with watery solutions and membranes. Skinny thin membranes encase each living plant cell, others cover the fine roots and their super fine hairs. The solutions involved are chemicals dissolved in water.
Osmosis requires a weaker solution and a stronger solution and a membrane to separate them. The soil supplies the weaker solution of plant nutrients and other chemicals. The other one is the stronger solution of sugary sap inside living cells. Underground the two solutions are separated by root membranes and osmosis quietly goes about its business. It pushes molecules in the weaker soil solution through the membranes to join the stronger solutions inside the cells. Molecule by molecule the ground water enters the roots.
The same process continues to push water up and up through the tree's plumbing system. Each living cell is stuffed with rich sap and the weaker solution from below seeps through its membrane. Cell by cell, osmosis pushes the water up and up the tree.
Perhaps osmosis could do the immense hauling job all by itself. But it would take longer and the tree is very thirsty. The work is speeded up as moisture evaporates through the pores of the leaves. This coaxes water molecules to seep up from below to fill the vacancy. Together these two processes change the natural tendency of water to flow downhill and tote it hundreds of feet above the ground.