xylem

Examples of xylem in the following topics:

• Vascular Tissue: Xylem and Phloem

  • Xylem and phloem form the vascular system of plants to transport water and other substances throughout the plant.
  • Together, xylem and phloem tissues form the vascular system of plants .
  • The end walls, unlike vessel members in xylem, do not have large openings.
  • Xylem and phloem tissue make up the transport cells of stems.
  • Tracheids (top) and vessel elements (bottom) are the water conducting cells of xylem tissue.

• Movement of Water and Minerals in the Xylem

  • It is the main driver of water movement in the xylem.
  • The cohesion-tension theory explains how water moves up through the xylem.
  • The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure.
  • The formation of gas bubbles in the xylem is detrimental since it interrupts the continuous stream of water from the base to the top of the plant, causing a break (embolism) in the flow of xylem sap.
  • In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.

• Plant Tissues and Organ Systems

  • It is made of two specialized conducting tissues: xylem and phloem.
  • Xylem tissue transports water and nutrients from the roots to different parts of the plant.
  • It includes three different cell types: vessel elements and tracheids (both of which conduct water) and xylem parenchyma.
  • Unlike xylem-conducting cells, phloem-conducting cells are alive at maturity.
  • The xylem and phloem always lie adjacent to each other.

• Primary and Secondary Growth in Stems

  • The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem.
  • The cells of the secondary xylem contain lignin, which provides hardiness and strength.
  • This supplies oxygen to the living- and metabolically-active cells of the cortex, xylem, and phloem.
  • During the spring growing season, cells of the secondary xylem have a large internal diameter; their primary cell walls are not extensively thickened.
  • During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood.

• Stem Anatomy

  • Xylem tissue has three types of cells: xylem parenchyma, tracheids, and vessel elements.
  • Tracheids are xylem cells with thick secondary cell walls that are lignified.
  • Vessel elements are xylem cells with thinner walls; they are shorter than tracheids.
  • Vascular tissue composed of xylem (red) and phloem tissue (green, between the xylem and cortex) surrounds the pith.
  • The xylem tissue is located toward the interior of the vascular bundle; phloem is located toward the exterior.

• Leaf Structure, Function, and Adaptation

  • Similar to the stem, the leaf contains vascular bundles composed of xylem and phloem .
  • The xylem consists of tracheids and vessels, which transport water and minerals to the leaves.
  • A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues.
  • This scanning electron micrograph shows xylem and phloem in the leaf vascular bundle.

• Transportation of Photosynthates in the Phloem

  • The multidirectional flow of phloem contrasts the flow of xylem, which is always unidirectional (soil to leaf to atmosphere).
  • The high percentage of sugar decreases Ψs, which decreases the total water potential, causing water to move by osmosis from the adjacent xylem into the phloem tubes.
  • Water diffuses from the phloem by osmosis and is then transpired or recycled via the xylem back into the phloem sap .
  • This reduces the water potential, which causes water to enter the phloem from the xylem.
  • Transpiration causes water to return to the leaves through the xylem vessels.

• The Chemical Composition of Plants

  • Plant roots absorb water from the soil through root hairs and transport it up to the leaves through the xylem.
  • Water is absorbed through the root hairs and moves up the xylem to the leaves.

• The Major Divisions of Land Plants

  • In contrast, vascular plants developed a network of cells, called xylem and phloem, that conduct water and solutes throughout the plant.

• Plant Responses to Wind and Touch

  • Strengthening tissue, especially xylem, is produced to add stiffness to resist the wind's force.