Xylem
Vascular tissue that transports water upward in plants.
Xylem is one of the two types of transport tissue in vascular plants, the other being phloem; both are part of the vascular bundle. Its basic function is to transport water upward from the roots to parts of the plants such as stems and leaves, and it also transports nutrients. The word xylem is derived from the Ancient Greek word ξύλον (xúlon), meaning 'wood'; the best-known xylem tissue is wood, though it is found throughout a plant.
- field
- Plant anatomy, vascular biology
- known_for
- Upward water and nutrient transport in vascular plants; primary and secondary xylem; cohesion-tension theory
- key_components
- Tracheids, vessel elements, parenchyma, fibers
- types
- Primary xylem (protoxylem, metaxylem) and secondary xylem
Lore & Background
Xylem is composed of distinctive cells called tracheary elements, which include tracheids and vessel elements. Vessel elements are shorter and connected into long tubes called vessels. Wood also contains parenchyma and fibers. Xylem can be found in vascular bundles in non-woody plants, in secondary xylem laid down by the vascular cambium in woody plants, and as part of a stelar arrangement not divided into bundles, as in many ferns. Primary xylem is formed during primary growth from procambium and includes protoxylem and metaxylem; metaxylem has wider vessels and tracheids than protoxylem.
Reader's Guide
Xylem is essential for plant survival, enabling the transport of water and soluble mineral nutrients from roots to all aerial parts. Its continuous system of vessels and tracheids connects roots, stems, and leaves, replacing water lost during transpiration and photosynthesis. The transport is passive, as tracheary elements are dead at maturity. Three phenomena cause xylem sap to flow: pressure flow hypothesis (phloem pressure draws xylem fluid upward), transpirational pull (evaporation creates negative pressure), and root pressure (osmotic positive pressure). Xylem structure limits tree height, as upward transport becomes more difficult with increasing height. Embolisms can form under drought conditions, but plants can refill xylem to restore function.
Did You Know?
- Xylem sap transport is passive; tracheary elements are dead by maturity and no longer have living contents.
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