Xylem is one of the two principal vascular tissues of plants, the other being phloem. It transports water and dissolved mineral nutrients through roots, stems, and leaves, while also providing mechanical support. Its conducting elements are elongated cells that are dead at functional maturity, but the tissue also contains living cells involved in storage and exchange. In woody plants, accumulated secondary xylem constitutes wood. Xylem is therefore a complex tissue rather than simply a collection of hollow tubes. (openstax.org)
Cellular structure
The principal water-conducting cells are tracheids and vessel elements, collectively called tracheary elements. During differentiation, these cells develop reinforced secondary walls and lose their living contents, leaving a lumen through which sap can move. Their walls contain lignin, which strengthens the tissue and helps conduits resist collapse under the negative pressure generated during water transport. (organismalbio.biosci.gatech.edu)
Tracheids are elongated cells with tapering ends. Water passes between them through pits: localized regions where secondary wall material is absent. Vessel elements are usually wider and join end to end through perforation plates to form vessels. Unlike perforations, the connections through lateral pits retain a pit membrane. Vessels provide longitudinal pathways, while pits allow exchange between neighboring conduits. Tracheids dominate the wood of most conifers; vessels are the principal conduits in most flowering plants. (openstax.org)
Other components include thick-walled fibers, which contribute mechanical strength, and parenchyma cells, which retain living contents. Parenchyma stores water and carbohydrates and participates in exchanges with conducting cells. In secondary xylem, it may be arranged longitudinally or in radial bands called rays. These living components mean that mature xylem as a whole should not be described as dead tissue. (pmc.ncbi.nlm.nih.gov)
Distribution and development
Xylem forms a connected transport system throughout the plant body. In young stems, it commonly occurs alongside phloem in vascular bundles. Bundles are typically arranged in a ring in dicot stems and scattered through the ground tissue in monocot stems. Within a typical stem bundle, xylem lies toward the interior and phloem toward the exterior. These arrangements vary with organ and plant group. (openstax.org)
Primary xylem develops during the growth of young organs. Its first-maturing portion is called protoxylem, followed by metaxylem. Protoxylem commonly has ring-shaped or spiral wall reinforcements compatible with elongation of the surrounding organ; later-developing conducting elements generally have more extensive secondary wall reinforcement. The distinction concerns developmental sequence and wall structure, not two unrelated kinds of transport tissue. (nph.onlinelibrary.wiley.com)
Secondary xylem is produced by the vascular cambium, a layer of dividing cells between xylem and phloem. Through cell division, the cambium adds secondary xylem inward and secondary phloem outward. This process increases the girth of stems and roots in plants with conventional secondary growth. Continued xylem production builds the woody cylinder, whereas primary growth primarily increases organ length. (openstax.org)
Mechanism of water transport
Water movement is governed by differences in water potential, a measure of water’s potential energy relative to a reference state. Water moves from higher to lower water potential. Under transpiring conditions, a gradient links soil water, roots, stems, leaves, and the atmosphere. Solute concentration, pressure, gravity, and interactions with surrounding surfaces all contribute to water potential. (openstax.org)
The main explanation for upward transport is the cohesion–tension theory. Transpiration, largely through leaf stomata, removes water by evaporation. This creates tension in leaf water that is transmitted through connected water columns in the xylem. Cohesion between water molecules and adhesion to conduit walls help maintain these columns. Water is consequently pulled from roots toward leaves rather than pumped upward by the dead conducting cells. Reinforced conduit walls resist the resulting tension. (openstax.org)
Roots can also generate positive root pressure through solute accumulation and osmosis. This can contribute to water movement when transpiration is low, but it is distinct from the tension-driven mechanism responsible for much daytime transport. Although the dominant transpiration stream moves toward leaves, xylem also permits lateral exchange between conduits and neighboring tissues. (organismalbio.biosci.gatech.edu)
Wood and growth rings
The outer, physiologically active region of wood is sapwood. Its conduits transport sap, and its parenchyma remains alive. In many trees, older inner wood becomes heartwood, which no longer participates in normal sap transport. Heartwood formation therefore separates the transport-active portion of secondary xylem from an older structural region. (pmc.ncbi.nlm.nih.gov)
Seasonal changes in cambial activity can produce growth rings. Earlywood commonly has larger conducting spaces, while latewood generally has thicker-walled cells and greater density. Differences between successive growth periods create visible bands. Ring width and cellular structure preserve information about growth conditions, although interpreting them requires attention to species and environment. (openstax.org)
Hydraulic failure
Xylem transport depends on maintaining water-filled pathways. Gas formation or entry can produce an embolism, interrupting conduction. Drought-induced tension can promote air movement through interconduit pit membranes; freezing and thawing can also generate embolism. Experiments show that freeze–thaw vulnerability differs among species and is often greater in wider conduits. (pubmed.ncbi.nlm.nih.gov)
Pit membranes are important barriers to embolism spread, but their resistance is finite. Their thickness and pore constrictions influence the pressure difference at which gas can enter an adjacent water-filled vessel. Loss of hydraulic conductivity restricts water supply to leaves, and comparative research identifies hydraulic traits as important predictors of differences in drought-associated tree mortality. (nph.onlinelibrary.wiley.com)
References
- 1 The Plant Body - Biology | OpenStaxopenstax.org
- 2 Stems - Biology 2e | OpenStaxopenstax.org
- Plant Development I: Tissue differentiation and function | Organismal Biologyorganismalbio.biosci.gatech.edu
- Xylem Parenchyma—Role and Relevance in Wood Functioning in Treespmc.ncbi.nlm.nih.gov
- Biomechanical Model of the Xylem Vessels in Vascular Plantspmc.ncbi.nlm.nih.gov
- 5 Transport of Water and Solutes in Plants - Biology 2e | OpenStaxopenstax.org
- 5 Transport of Water and Solutes in Plants - Biology | OpenStaxopenstax.org
- Water Transport in Plants: Xylem | Organismal Biologyorganismalbio.biosci.gatech.edu
- Xylem embolism spread is largely prevented by interconduit pit membranes until the majority of conduits are gas-filledpubmed.ncbi.nlm.nih.gov