Theory:

We see that water from the ground is pumped to the upper storage tank through a pipe in a house. Also, it is distributed to all regions of the house through various pipelines. Plants similarly conduct food and water, using pipe-like structures that extend across the plant like continuous tunnels. These pipe-like structures are complex permanent tissues.
 
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Biological structure scheme of inner vascular in Plant.
 
A complex permanent tissue made up of different types of cells having a common origin. Even though the cells appearance is different, all the cells coordinate to perform a common function.
Complex permanent tissues are of two types.
a) Xylem
b) Phloem
 
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Complex permanent tissues
  
Xylem and phloem are the most complex tissues in plants. They are part of the vascular bundles. Vascular or conductive tissue is a distinctive feature of the thorny plants. It has made possible their survival in a terrestrial environment.
The tissues associated with the conduction of water, minerals and food materials in plants are called vascular tissue. Plants with a well developed conductive system are thus called as vascular plants.
 
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Plant cohesion vector illustration. Labeled water upward motion explanation with educational scheme. Biological structure diagram with xylem, cortex, epidermis and ground tissue cross section view.
Xylem
Xylem is a complex permanent, vascular tissue meant for mechanical support and conduction of sap (water + minerals) in only upward direction (unidirectional).
  • C.Nageli introduced the term xylem in 1858.
  • The word "xylem" is extracted from the Greek word "Xylos" meaning "wood".
  • It is responsible for the transport of water and minerals from roots to other parts of the plant, and it also provides mechanical support to the plant.
  • The cells of xylem has thick lignified walls, and many of them are dead.
  • It is a heterogeneous tissue consist of various elements.
  • They are further divided into four components: Tracheids, Vessels or Trachea, Xylem parenchyma and Xylem fibres.
Tracheids and vessels together called tracheary elements because they conduct sap.
 
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Xylem cells
  
Tracheids:
  • It is present in vascular plants groups like pteridophytes, Gymnosperms (non-flowering plants) and Angiosperms (flowering plants).
  • It is an elongated tubular cell with tapering (sharp), and imperforated (without pores) ends. Tracheids are not efficient in water conduction because their walls are not perforated.
  • They transport water and mineral salts are conducted upward direction from one tracheid to another.
  • Tracheids are one of the non-living components (without cytoplasm and nucleus) of the xylem (Due to lignification, the cells cannot exchange the essential materials with the outside environment to which these cells lose their living characters at maturity and become dead).
  • It provides mechanical support to the plants.
Vessels:
  • These are long, tube-like structures, formed by a row of cells, placed end to end.
  • Vessels are predominantly found in Angiosperms (flowering plants).
  • Vessels is also called trachea because of its resemblance with trachea of insects.
  • These are also dead cells with lignified secondary walls.
  • They transport water and minerals vertically. Vessels are efficient in conducting water due to the presence of perforations.

Xylem parenchyma:
  • The parenchyma associated with xylem is called xylem parenchyma or wood parenchyma.
  • These are the only living cells of xylem with thin cell walls. The cells are with abundance of cytoplasm and prominent nucleus.
  • It helps store the food such as starch and oil and helps in the lateral (sideways) conduction of water.
Xylem fibres:
  • These are elongated dead cells with sharp ends and thick cell walls.
  • These are fibres associated with xylem and supportive in the functioning of xylem.
Reference:
https://commons.wikimedia.org/wiki/File:Xylem_cells.svg