Phloem Structure

• Phloem sieve tubes


  • Primarily composed of sieve element cells and companion cells
  • Also contain schlerenchymal and parenchymal cells which fill additional spaces and provide support
• Sieve element cells


  • Long and narrow cells that are connected together to form the sieve tube
  • Connected by sieve plates at their transverse ends, which are porous to enable flow between cells
  • Have no nuclei and reduced numbers of organelles to maximise space for the translocation of materials
  • Have thick and rigid cell walls to withstand the hydrostatic pressures which facilitate flow
• Companion cells


  • Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink
  • Possess an infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange
  • Have many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink
  • Contain appropriate transport proteins within the plasma membrane to move materials into or out of the sieve tube
• Sieve elements are unable to sustain independent metabolic activity without the support of a companion cell
• Plasmodesmata


  • Exist between sieve elements and companion cells in relatively large numbers
  • Connect the cytoplasm of the two cells and mediate the symplastic exchange of metabolites
• Identification of xylem and phloem in microscope images of stem and root

  1. Xylem and phloem vessels are grouped into bundles that extend from the roots to the shoots in vascular plants

  2. Differences in distribution and arrangement exist between plant types (e.g. monocotyledons vs dicotyledons)

  3. Xylem and phloem vessels can usually be differentiated by the diameter of their cavity (xylem have larger cavities)
• Roots in monocotyledons


  • The stele is large and vessels will form a radiating circle around the central pith
  • Xylem vessels will be located more internally and phloem vessels will be located more externally
• Roots in dicotyledons


  • The stele is very small and the xylem is located centrally with the phloem surrounding it
  • Xylem vessels may form a cross-like shape ('X' for xylem), while the phloem is situated in the surrounding gaps
• Stems in monocotyledons


  • The vascular bundles are found in a scattered arrangement throughout the stem
  • Phloem vessels will be positioned externally (towards outside of stem)
• Stems in dicotyledons


  • The vascular bundles are arranged in a circle around the centre of the stem (pith)
  • Phloem and xylem vessels will be separated by the cambium (xylem on inside; phloem on outside)