Xylem and Phloem (The structure)

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Xylem and Phloem 

 - Structure of xylem 

Xylem has the function of transporting water and minerals from the roots up to the leaves of the plant along with other parts of the plant.  Xylem consists of tubes that carry the the water and dissolved minerals along with fibres to help support the plant.  Xylem has parenchyma cells.

Parenchyma - These are relatively unspecialised (plant) cells.  They have living contents & thin, permeable cellulose cell walls.  They may be able to photosynthesise, store food or support young plants.  

 - The vessels (Xylem)

In dicot (dicotyledonous) plants, the most obvious features of xylem are the xylem vessel elements.  These are long cells with thick walls that have impregnated by lignin.  (Lignin is a waterproof substance that impregnated the walls of the xylem tissue.  Lignin is used for strength.)  During the development of xylem, the lignin waterproofs the walls of the cells and so the cells die and so their contents decay.  This leaves a long column of dead cells with no contents, hence the xylem vessel.  The lignin strengthens the vessel walls and prevents the vessel from collapsing.  

This lignin thickening causes patterns on the wall and these patterns can be spiral, rings or reticulate (broken rings).  This prevents the vessel from being too rigid and allows for flexibility.

In some places, this process called lignification is not complete. This means there are pores in the walls of the vessels and are known as pits, or bordered pits.  Of course, this allows the water to leave the vessel into another adjacent vessel.  It can also pass into other living parts.

 - The adaptation

Xylem tissue can carry water and minerals from roots to the very top of the plant because

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