Xylem consists of long, dead cells and is part of the vascular tissue of a plant (the other components being phloem and cambium (cambium only in dicotyledons). Like our veins, xylem carries the life blood of a plant- water.
Water enters the root hairs through a process called osmosis from the water in the soil. It passes across the cortex to a special tissue called the Caspian Strip - this allows water one way only and it goes into the xylem vessels where it is pushed or pulled up the plant.
Water is pushed up the plant due to pressure caused by water travelling across the cortex. As one cell loses water, it pulls water from the next cell and so on until it reaches the Caspian Strip and Xylem. Root pressure, as this is known, can push water up to 30 feet in a plant.
Xylem acts as the water conduit in a plant. Water is also pulled up the xylem vessels by transpiration - as water evaporates from the intercellular air spaces in the leaves into the atmosphere (via pores called stoma), water is drawn further up the xylem vessels and leaf suction pressure is created. This pulls water up to 30 feet.
However, for very tall trees and plants, the xylem vessels have certain qualities which help wayter reach the tops of the trees. Firstly, water molecules stick to themselves by a process called cohesions but also, xylem is slightly charged so the water molecules adhere to the sides of the xylem vessels (called adhesion) so, as long as the column of water remains unbroken, the water can travel up to over 100 feet.
Xylem is thickened with a tissue called lignin, which makes it strong and adds support to the plant. It also helps the vessels remain open for water transportation even when the plant is close to wilting. Xylem systems are composed of vessels, tracheids and ray cells, depending on which plant they are in.
Xylem also has another important role. Plants can only take in nutrients in solution and the xylem vessels carry dissolved nutrients in their water, which comes in with the soil water. Plant cells have selective cell membranes so only minerals needed or useful are allowed in - others are not allowed to enter the plant. These minerals help in processes such as photosynthesis, respiration and the creation of chemicals for the plant's metabolism.
In dictoyledons, xylem is found with phloem in rings, central to which is cambium tissue, which is merestematic and divides to form new xylem on the inside and new phloem on the outside but in monocotyledons there is no cambium, the xylem and phloem are found together but scattered and not in rings.
Xylem is a very important tissue to a plant, wthout which most plants will die.