Explain the structure of complex permanent tissue
'xylem' . State the condition when xylem is called endarch and exarch .....

Xylem and phloem are known as complex tissues as they are made up of more than one type of cells. These cells work in a coordinated manner, as a unit, to perform the various functions of the xylem and phloem.

Xylem helps in conducting water and minerals. It also provides mechanical support to plants. It is made up of the following components:

• Tracheids (xylem vessels and xylem tracheids)

• Xylem parenchyma

• Xylem fibres

Tracheids are elongated, thick-walled dead cells with tapering ends. Vessels are long, tubular, and cylindrical structures formed from the vessel members, with each having lignified walls and large central cavities. Both tracheids and vessels lack protoplasm. Xylem fibres consist of thick walls with an almost insignificant lumen. They help in providing mechanical support to the plant. Xylem parenchyma is made up of thin-walled parenchymatous cells that help in the storage of food materials and in the radial conduction of water.
 If protoxylem is present  outside i.e. at periphery, then this condition is called exarch and if protoxylem is present inside then it is called endarch.  
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