Brent Cornell

    
Phloem sieve tubes are primarily composed of two main types of cells – sieve element cells and companion cells

• The phloem also contains schlerenchymal and parenchymal cells which fill additional spaces and provide support

Sieve Element Cells

Sieve elements are long and narrow cells that are connected together to form the sieve tube

• Sieve elements are connected by sieve plates at their transverse ends, which are porous to enable flow between cells
• Sieve elements have no nuclei and reduced numbers of organelles to maximise space for the translocation of materials
• The sieve elements also 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

• This is because the sieve element cells have no nuclei and fewer organelles (to maximise flow rate)
• Plasmodesmata exist between sieve elements and companion cells in relatively large numbers
• These connect the cytoplasm of the two cells and mediate the symplastic exchange of metabolites

Structure of a Phloem Sieve Tube

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

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

Click on the image to display false colour representations of xylem (red) and phloem (blue)

Stem

• In monocotyledons, the vascular bundles are found in a scattered arrangement throughout the stem
  • Phloem vessels will be positioned externally (towards outside of stem) – remember:  phloem = outside  
• 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)

Click on the image to display false colour representations of xylem (red), phloem (blue) and vascular bundles (orange)