This is the part of a plant that transports water and some nutrients. Forming long tubes called vessels, the xylem brings water from the roots to the rest of the plant. In trees, this can mean water flows hundreds of feet upward. The water is pulled up the xylem by transpirational pull. As water is lost as vapor to the air from the leaves , the plant pulls even more water up from the roots to replace it.
In a sentence
During a drought, lack of water may make air bubbles form in a tree’s xylem. If too much air gets in, the water flow stops and the tree will die.