Xylem & Sclerenchyma (SNAB)

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  • Created on: 18-03-13 21:22
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XYLEM VESSELS
These form tubes for the transport of water and mineral ions, and their stiffened cell help support the plant.
STRUCTURE:
They are waterproof due to LIGNIN ­ this polymer impregnates the cellulose cell wall and as the cells become lignified, the entry of water and
solutes into them is restricted.
There is AUTOLYSIS of the cell contents ­ the cell organelles, cytoplasm and cell surface membrane are broken down by the action of enzymes
and are lost, leaving an empty tube.
Water and mineral ions move into and out of the vessels through pits in the walls where there is no lignin.
SCLERENCHYMA FIBRES
Columns of these cells with their stiffened cell walls also provide support.
STRUCTURE:
Like xylem vessels, these fibres die once lignified, leaving hollow fibres.
Their cell walls are thickened with lignin and have more cellulose than other plant cells.
Where in the stem are these specialised cells?
They are found in the vascular bundles which contain xylem vessels and phloem sieve tubes (which transport products of photosynthesis). On the
outside of the bundle are sclerenchyma fibres.
In young dicotyledon, the vascular tissue is in bundles towards the outside of the stem.
In trees and shrubs these separate bundles merge to form a continuous ring as the plant ages.

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