Long cells with thick walls that have been impregnated with lignin.
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What does the lignin do?
Waterproofs the walls of the cells.
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What does this waterproofing do to the cell?
Causes the cell to die and their end walls and contents decay.
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What does this leave?
A long column of dead cells with no contents - a tube with no end walls - a xylem vessel.
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What does the lignin strengthen and prevent?
Strengthens the vessel walls and prevents the vessel from collapsing.
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What does the lignin thickening form?
Patterns in the cell wall - spiral, annular or reticulate.
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What do these patterns prevent?
Prevents the wall from being too rigid and allows flexibility of the stem.
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What happens if the lignification is not complete?
It leaves pores in the wall of the vessels, known as pits/bordered pits.
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What do the bordered pits allow?
Allow water to leave one vessel and pass into another adjacent vessel or pass into the living parts of the plant.
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What are the adaptations of xylem tissue that allow them to carry water and minerals? (4)
It is made from dead cells aligned end to end in a continuous column. Tubes are narrow - doesn't break easily and effective capillary action. Pits allow water to move sideways. Lignin patterns allow xylem to stretch and plant to bend.
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Why is the flow of water not impeded? (4)
Because there are no end walls, there are no cell contents, there is no nucleus or cytoplasm, ligning thickening prevents the walls from collapsing.
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What 2 types of cell does phloem tissue consist of?
The sieve tube elements and companion cells.
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Why are the sieve tube elements not 'true' cells?
They contain very little cytoplasm and no nucleus.
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Describe the arrangement of sieve tube elements
They are lined up end-to-end to form a tube in which the plant transports sugars (sucrose).
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How is sap formed?
Sucrose is dissolved in water to form sap.
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What do sieve tube elements contain that xylem don't?
Contain cross-walls at intervals.
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What are the cross-walls?
Perforated by many pores to allow the sap to flow.
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What are the cross-walls called and what are the tubes called?
Cross-walls are called sieve plates and tubes and called sieve tubes.
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Describe the sieve tubes
They have very thin walls and are usually five or six sided.
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Describe companion cells
Small cells, each with a large nucleus and dense cytoplasm. Numerous mitochondria.
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What do the companion cells do?
Carry out the metabolic processes needed by the sieve tube elements.
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What is an example of a process that companion cells carry out?
Uses ATP to load sucrose into sieve tubes.
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How are the cytoplasm of the companion cells and sieve tube elements linked?
Linked through many plasmodesmata.
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What are plasmodesmata?
Gaps in the cell walls allowing communication and flow of substances between the cells.
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