Xylem

Xylem is vascular tissue that transports water and dissolved minerals from roots through a plant. Its thick-walled cells also help support the plant.

Transpiration: The loss of water vapor from plant surfaces, especially through leaf stomata. Evaporation from leaves creates tension that draws water upward through xylem.
Tracheid: An elongated, water-conducting xylem cell with thick walls and tapered ends. Tracheids conduct water and provide support in all vascular plants.
Sapwood: The younger, active outer wood of a tree that conducts water and dissolved substances. Sapwood is the secondary xylem that remains active in transport.
Vascular plant: A plant with specialized tissues for transporting water, minerals, and sugars. Xylem is one of the defining transport tissues of vascular plants.
Cohesion-tension theory: An explanation of water ascent in plants based on evaporation-driven tension and cohesion among water molecules. It explains how transpiration pulls a continuous water column through xylem.
Vessel element: A short, wide xylem cell joined end to end with others to form a vessel. Vessel elements create efficient conducting tubes in most flowering plants.
Heartwood: The older, nonconducting inner wood of a tree, often darkened by deposited substances. Heartwood is older secondary xylem that no longer conducts water.
Silurian: A geological period from about 444 to 419 million years ago. Early fossil evidence of vascular plants and conducting tissues comes from the Silurian.
Root pressure: Positive pressure in roots generated when ion uptake draws water into the xylem. It can push xylem sap upward, especially when transpiration is low.
Vessel: A tube in xylem formed by connected vessel elements with perforated or absent end walls. Vessels are the continuous conduits assembled from vessel elements.

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