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  Xylem Complex Tissue

Introduction to Xylem Complex Tissue

Xylem (Greek word 'xylos'= wood) is a complex tissue which is chiefly accountable for the conduction of water and mineral salts from roots to another parts of the plant.  The xylem that is derived from procambium is termed as primary xylem and the xylem that is derived from vascular cambium, is termed secondary xylem.   Earlier created xylem elements are termed as protoxylem, while the later created xylem elements are termed as metaxylem. Xylem is formed of four types of cells -

1. Tracheids,

2. Vessels or tracheae,

3. Xylem fibres and

4. Xylem parenchyma.

Tracheids

Tracheids are elongated along with blunt ends. The lumen of it is broader than that of fibres. The secondary wall of them is lignified. In cross section, the tracheids come out polygonal and thick walled.   The pits are simple / bordered. There are dissimilar sorts of cell wall thickening because of deposition of secondary wall substances.  They are annular (like ring), spiral (like spring), scalariform (like ladder), reticulate (like net) and pitted   (consistently thick except at pits).  Tracheids are imperforate cells along with bordered pits on their end walls.  They are arranged one above the anoother. Tracheids are chief water that conducting elements in pteridophytes and gymnosperms.  Here, the conduction of water and mineral salts occurs by the bordered pits. They as well present mechanical support to the plants.

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Diagram: Tracheid                             

Vessels or Tracheae

At the end walls Vessels are perforated. The lumen of it is wider than that of tracheids. The perforated plates at the end wall separate the vessels. They take place parallel to the long axis of the plant body.  Because of dissolution of entire end wall, a single pore is created at the perforation plate. It is termed as simple perforation plate example Mangifera. If the perforation plate has several pores, then it is termed as multiple perforation plate. Example Liriodendron.

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Diagram: Vessel

The secondary wall thickenings of vessels are

1. annular,

2. spiral, 

3. scalariform,

4. reticulate,

5. or pitted

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                        Annular               Spiral                   Scalariform               Reticulate                    Pitted

Diagram: Types of secondary wall thickenings in tracheids

Vessels are chief water conducting elements in angiosperms and they are not present in pteridophytes and gymnosperms.  Though, in Gnetum of gymnosperms, vessels take place. The major function of vessel is conduction of water and minerals. It as well presents mechanical strength to the plant.

Xylem fibres

The fibres of sclerenchyma related with the xylem are termed as xylem fibres. They provide extra mechanical support to the plant body.  They are available both in primary and secondary xylem.   Xylem fibres are dead cells and comprise lignified walls with narrow lumen. Xylem fibres are also termed as libriform fibres.

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Diagram: Xylem fibre

Xylem parenchyma

The parenchyma cells related with the xylem are termed as xylem parenchyma.  Xylem parenchyma is the just a living tissue among the constituents of xylem. The cell wall is thin and formed of cellulose.  The xylem parenchyma cells store food reserves in the type of starch and fat. They also help in conduction of water.

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Diagram: Xylem parenchyma

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