If you paid attention in health class you know that our body needs to move materials around to function properly, such as oxygen. The transportation system used is the bloodstream, which utilizes red blood cells as a bucket brigade, going to the lungs via veins to pick up oxygen, and carrying it to various body locations via arteries. A tree has similar needs and also uses a liquid transportation system.
A tree is an autotroph, meaning it makes its own food through photosynthesis. To do this it needs raw materials delivered to the leaves, mainly water and carbon dioxide. The carbon dioxide is easy, moving directly into the leaves from the air through tiny ports in the leaf surface called stomata. Water must come from the soil and be transported high into the tree canopy to the leaves, not an easy trick without a pumping mechanism like our heart.
The pipeline used to move water is xyloem cells (pronounced zi-lom). They run from the roots, up the trunk, through the branches and twigs, up the leaf stem, and delivered to the leaf itself. Water is not pumped up a tree, but rather pulled up because of two attributes. Water clings to itself (cohesion), and will move from a high-pressure area to a low one. When heated by sunlight, water in tree leaves evaporates and is released to the atmosphere through those stomata ports mentioned earlier. The escaping water creates a low-pressure area in the leaf and so water moves in from the leaf stem to replace it. There is now a pressure drop in the stem, so water moves in from the twig, and so it goes until there is a pressure drop in the roots and water moves in from the soil. Minerals and nutrients suspended in the water are also brought in and utilized. Once water enters the tree it is called sap.
Certain species can raise water over 300 feet into the canopy at speeds of up to 150 feet per hour. A large leafy tree can take up 95 gallons of water each day, but only a small percentage of it is actually used for photosynthesis. The rest is released through the leaves into the atmosphere as water vapor, where it forms clouds and eventually returns to earth as rain.
Once the leaves have made food (a glucose sugar), it needs to be transported to other parts of the tree for use as energy or for building materials. The sugar is suspended in water (sap) and is transported through a pipeline of phloem (flo-um) cells to the rest of the tree. These are located fairly close to the surface of the trunk, and can be tapped in the spring to make maple syrup from sugar maple trees, which tend to have high concentrations of sugar in the sap.