transport tissues

• companion cells : cells that help to load sucrose into sieve tubes
• sieve tube elements : make up the tubes in phloem tissue that carry sap down plants - separated by sieve plates
• xylem vessels : tubes which carry water up the plant
• Xylem structure :

  • consists of vessels to carry water and dissolved mineral ions
  • fibres to help support the plant
  • living parenchyma cells which act as packing tissue to seperate and support the vessels
• xylem vessels

  as they develop lignin impregnates the walls of cells, making them waterproof. This eventually kills the cells

  • The end walls and contents of the cells decay, leaving a long column of dead cells with no contents

  →the lignin strengthens the vessel walls + prevents the vessel from collapsing ; keeping the vessels open even at times when water may be in short supply
• xylem vessels 2

  • The lignin thickening forms patterns in cell wall ; these may be spiral, annular (rings) or reticular (a network of broken rings) - preventing the cell from being too rigid and allows some flexibility of stem or branch
  • However lignification is not complete in some places, leaving gaps in the cell wall

  these gaps form bordered pits in two adjacent vessels are aligned to allow water to leave the xylem and pass into the living parts of the plant and also allowing water to leave xylem and pass them into the living parts of the plant
• Adaptations of xylem to its function

  xylem vessels can carry to the very top of plant because :

  • they are made from dead cells aligned end to end to form a continuous column
  • the tubes are narrow, so that the water column does not break easily and capillary action can be effective
  • bordered pits in the lignified walls allow water to move sideways from one vessel to another
  • lignin deposited in the walls in spiral, annular or reticulate patterns allows xylem to stretch as the plant grows, and enables the stem or branch to bend
• Adaptations of xylem to its function 2

  the flow of water is not impeded, because :

  • there are no cross-walls
  • there are no cell contents, nucleus or cytoplasm
  • lignin thickening prevents the walls from collapsing
• Sieve tube elements

  elongated sieve tube elements are lined up end to end to form sieve tubes

  • they contain no nucleus and very little cytoplasm,leaving space for mass flow of sap to occur
  • end of sieve tube elements are perforated cross-walls called sieve plates
  • the perforations in the sieve plate allow movement of sap from one element to the next
  • sieve tubes have very thin walls and when seen in transverse section are usually five- or six-sided
• Phloem structure :

  • consists of sieve tubes - made up of sieve tube elements - and companion cells

  the companion cells and sieve tube elements in phloem are linked by fine strands of cytoplasm, through gaps in the cell walls
• Companion cells

  In between sieve tubes are small cells, each with a large nucleus and dense cytoplasm

  • numerous mitochondria to produce ATP needed for active processes
  • they carry out the metabolic processes needed load assimilates actively into sieve tubes