Xylem & Phloem
- Created by: Eleanor Izzard
- Created on: 30-12-12 20:41
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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
- long cells with thick walls that have been impregnated by lignin which waterproofs , the walls of the cells, causing the cells to die
- 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
- cross walls are called sieve plates and the tubes are called sieve tubes
- lined up end to end to form a tube that transports sugars (sucrose)
- 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)
- Xylem
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