Plants absorb water from the soil through their roots and use it in their leaves. Gazing up at a tall tree, it is a long way to the top. Yet, as anyone who's forgotten to water a houseplant knows, water is essential to plant life and necessary to produce the energy plants use to grow and carry out life functions. The ability to transport water from the roots to the furthest leaf is one of the most important functions for a plant's survival.
Plants require water to carry out photosynthesis, a metabolic process that converts energy from the sun into sugar. The sugar produced by the plant, in turn, is used during respiration, where it provides the energy that the plant needs to grow and carry out other life functions. Vascular tissue called xylem transports water from the roots to the leaves, where it becomes available for photosynthesis. Not surprisingly, given the importance of water to the metabolic processes of the plant, vascular tissue appeared early in plant evolution. The most primitive plants, simple mosses, had to remain small so that all of their cells could be near a water source. Vascular tissue allowed plants to grow to larger sizes and develop further adaptations.
Xylem conducts water and minerals from the plant's roots to its leaves and forms when the plant is young, appearing as cylindrical cells stacked in columns up the length of the plant. When the plant reaches maturity, the cells die and the divisions between them disintegrate, leaving a long tube of nonliving tissue. Water from the xylem flows into the leaf veins, where it becomes available to individual cells in the leaf.
Two factors allow water to move up through the xylem, the first of which is transpiration. On the undersides of leaves are small pores called stomata. Special cells on either side of the stomata open or close them, depending on the plant's needs. When the plant needs to take in carbon dioxide and release oxygen, the stomata open. However, water also evaporates through the stomata when they open, a process called transpiration. As the sun's warmth draws water from the stomata, it pulls more water up through the xylem, much like sucking water through an enormous drinking straw.
However, just as it would take a lot of force to drink through a hundred-foot straw, it takes a lot of force to draw water through a hundred-foot tree. Here, water transport is assisted by a property of water called cohesion. Water molecules are arranged in such a way that they naturally cling together. As the water is pulled up the plant by transpiration, the tendency of water to cling together reduces the force required to draw it through the full length of the plant.
Although xylem is rather large for vascular tissue, it is less than a millimeter in diameter and too small to observe with the naked eye. However, if you've ever counted tree rings, then you've observed xylem. As trees age, the older xylem toward the middle of the tree is no longer used, and the tree produces a new layer of vascular tissue each spring that can be observed as tree rings. Because xylem is nonliving tissue, the unused xylem as the center of the tree exists mainly to provide support to the tree as wood.