What Causes Water to Rise to the Top of a Tall Tree?

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Redwood trees move a huge volume of water from roots to leaves.
Redwood trees move a huge volume of water from roots to leaves. (Image: Digital Vision./Digital Vision/Getty Images)

Trees are marvelous hydraulic systems. The layer of cells just inside the bark is called the cambium. The inner part of the cambium is composed of woody cells called xylem. The xylem carries water to every part of the tree's roots, stems and leaves, much as blood vessels carry blood to every part of your body. According to the California Welcome Centers website, an average redwood tree can move up to 500 gallons of water per day.

How Roots Absorb Water

Water doesn't enter every part of a root. Tall trees have enormous root systems, and most of their roots spread out over great areas of soil. The taller the tree, the more roots it must have in order to process enough water and nutrients to keep growing. Tips of the roots have tiny root hairs. Water is absorbed through these hairs into the cells of the root xylem.

How Water Moves Into the Stem Xylem

Water moves into stem xylem cells through a chemical process that occurs when salts and sugars interact in the root xylem. The salts and sugars are then transferred through osmosis to the stem xylem of the trees. According to Michigan State University, water molecules are pulled into xylem cells during this process when they attach to moving salt ions. Once water enters the cells of the stem xylem, it's like water entering the bottom of a straw.

How Water Is Pulled to the Leaves

Water moves toward where water is being removed. At the top of a tall tree, leaf surfaces lose tremendous volumes of water through evaporation. When water enters the atmosphere from the surface of the leaf, it creates tension in the xylem, all the way back through the stem, twigs, branches, trunk and roots. It works much the same way that sucking on the end of a straw pulls liquid from the bottom of a glass to your lips.

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