Xylem & Phloem

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  • Xylem & Phloem
    • Xylem
      • plant transport tissue that carries water and minerals from the roots upwards to the rest of the plant
      • consists of tubes to carry the water and minerals, fibres to support the plant and living parenchyma cells
      • Xylem Vessel Elements
        • long cells with thick walls that have been impregnated by lignin which waterproofs , the walls of the cells, causing the cells to die
          • their end walls and contents decay, leaving a column of dead cells with no contents
        • present in dicotyle-donous
        • the lignin thickening forms patterns in the cell wall. Spiral, Annular (rings) or Reticulate (a network of broken rings)
          • this prevents the vessel from being too rigid and allows flexability
          • in some places this lignification is not complete, leaving pores in the wall of the vessel, called bordered pits
            • Pits allow water to leave one vessel and pass into another vessel or living parts of the plant
      • Adaptations
        • pits in lignified  walls allow water to move sideways
        • narrow tubes so the water column does not break easily and capillary action can be effective
        • lignin deposited in the walls in patterns allows Xylem to stretch as the plant grows and enables the stem or branch to bend
        • dead cells aligned end to end - continuous column
        • Flow of water is not impended: no end walls, no cell contents, no nucleus or cytoplasm, lignin thicken prevents walls from collapsing
    • Phloem
      • plant transport tissue that carries the products of photo-synthesis (sugars) up and down the plant to the rest of the plant
      • consists of sieve tube elements and companion cells
      • Sieve Tubes
        • lined up end to end to form a tube that transports sugars (sucrose)
          • sucrose  dissolved in water to form sap
        • not true cells - no nucleus and little cytoplasm
        • tube contains cross-walls at intervals, perforated by many pores to allow the sap to flow
          • cross walls are called sieve plates and the tubes are called sieve tubes
            • the tubes have very thin walls and are usually 5/6 sided
      • Companion Cells
        • 1 per sieve tube element, in between the sieve tubes
        • a large nucleus and dense cytoplasm, numerous mitochondria to produce ATP for active processes
        • carry out metabolic processes needed by the sieve tube elements
          • including as a source of energy to load sucrose into sieve tubes
        • cytplasm of companion cells and sieve tube elements are linked through many plasmo-desmata (gaps in the cell walls allowing commun-ication and flow of substances between the cells)


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