Functions of Xylem and Phloem

• Xylem


  • conducts water and dissolved mineral salts from the roots to the stems and leaves
  • provides mechanical support for the plant
  • consists of xylem vessels, which are long hollow tubes stretching from the root to the leaf
  • xylem vessels are made up of many dead cells without a cytoplasm or cross walls, reducing resistance to water flowing through the xylem
  • inner walls of xylem vessel are strengthened by deposits of lignin. Lignin can be deposited in the form of rings, spirals or the whole vessel is lignified except in regions called pits

  
• Phloem


  • consists of sieve tubes and companion cells
  • each sieve tube consists of a column of elongated, thin walled living cells
• Phloem - Sieve plates


  • the sieve plates separating the sieve tube cells have many minute pores
  • a mature sieve tube cell only has a thin layer of cytoplasm inside it. It connects to cells above and below through the holes in the sieve plates, allowing for rapid flow of manufactured food substances through the sieve tubes
• Phloem - companion cells

  • each sieve tube cell has a companion cell beside it that provides nutrients and helps the sieve tube cell to transport manufactured food
  • companion cells carry out metabolic processes that are needed to keep the sieve tube cell alive
  • each companion cell is a narrow, thin walled cell with many mitochondria, cytoplasm and a nucleus
  • numerous mitochondria provide energy needed for the companion cells to load glucose from the mesophyll cells into the sieve tubes by active transport

  
• vascular tissues in the stem

  • within a vascular bundle, the xylem is located closer inside
  • the phloem lies outside the xylem with a tissue called the cambium between them
  • the stem is covered by a layer of cells called the epidermis, which are protected by a waxy, waterproof cuticle that reduces evaporation of water from the stem

  
• Vascular tissue in leaves

  • vascular bundles are found along the spongy mesophyll
  • xylem is closer to the upper surface of the leaf and the phloem is closer to the lower surface of the leaf

  
• Vascular tissue in the roots

  • epidermis of the root is the outermost layer of cells, it bears root hairs
  • each root hair is a long and narrow extension growing out of an epidermal cell
  • long and narrow extension of the root hair increses surface area to volume ratio, increasing rate of absorption of water and mineral salts