|MadSci Network: Botany|
Water travels up the tree through long narrow tubes (sieve cells) lying side by side in layers all along the circumference of the tree trunk. Several "forces" help move the water up the tree. There's a force called "osmosis" that drives the water gradient to be equal between cells so that water flows up into the cells with lower concentrations of water. Capillary action is also in force in the tree. The sieve cells are so narrow that the water is drawn up into it. You would see this happenning when you touch a paper towel to water. The water is drawn up the towel. As well, there's the force of attraction- "water tension" between water molecules. If you fill a cup with water, you can add drop by drop more water until the top of the water is above the lip of the cup. This is water tension, in which the water molecules are so attracted to each other that they can stay togetehr without a supporting wall. This tension of the water inside the sieve cells of the tree keeps the flow of water continous. Finally, there's "transpiration". The tree is "breathing" through its leaves, and water is evaporating from the leaves, and as water evaporates from the leaves, more water is drawn into the leaves from the water in the tree branches. If you look at the entire picture, the roots absorb water, then the sieve cells draw up the water through capillary action and osmosis. The water tension keeps the flow of water from breaking, with transpiration driving the flow in the upward direction, towards the leaves. If you cut down a tree, you will see water (tree sap) flowing out of the sieve cells, which are located along the circumference of the tree. Water won't be spurting out of the tree, and it will stop eventually, because there isn't any transpiration taking place any more. The movement of water happens pretty much the same way in flowers too, and the water is easier to see. Take a fresh flower (with leaves) and, holding the stem underwater, cut the stem with sharp scissors. Take it out of the water and you will see a drop of water hanging onto the end you just cut. That's water tension keeping the drop from falling! If you watch closely, the water will slowly move up into the stem!! In fact, when you cut flowers to bring into the house, you need to put them in water right away. The movement of water up the stem happens without stop, and if you wait to put the cut stems in water, the sieve cells will start to draw up air, and the air bubble will break the force of the capillary action, and the flower will be unable to draw up anymore water. If you do need to wait before putting the plants into water, cut the stem a bit higher up, in water, and hopefully you'll have cut off the stem with the air bubble. I hope you understand trees and plants better now :) Evelyn Tsang Information taken from the Tree House in the Montreal Botanical Gardens.
Try the links in the MadSci Library for more information on Botany.