Xylem and its components

March 18, 2019

Xylem is a complex tissue that is mainly responsible for the conduction of water and saps inside the plant from the root up-to the top of the plant.
It also provides mechanical strength which is also known as wood.
It is composed of four kinds of cells. These are:

A) Tracheids

B) Vessel

C) Xylem parenchyma

D) Xylem fibres

Out of these only tracheids and vessels take part in the transport of sap.
Hence, they are called tracheary elements.
Vessels are the main tracheary elements of angiosperms which are absent in gymnosperms and pteridophyta.
However, tracheids are the main tracheary elements in gymnosperm and pteridophyta.

A) Tracheids

These appear to be elongated cells with tapering ends.
These are tube like dead cells with hard, thick and lignified walls and a large cell cavity or the wide lumen.
Their ends are commonly tapering or oblique.
The main function of the tracheids is to conduct water and mineral salts from the root to the leaf, but due to their hard and thick walls, they also serve to give mechanical strength to the plant body.
Tracheids walls are thickened in different ways and according to the mode of thickening they have got different names such as annular, spiral scalariform, reticulate and pitted.

B) Vessels

They are rows of elongated tube like dead cells, placed end to end.
Their end walls or transverse walls are dissolved.
A condition is called simple perforation plate.
Thus, a vessel is very much like a series of pipes.
Vessels form the main elements of the xylem.
They have large cell cavities which serve for the conduction of water and mineral salts.
They also provide mechanical strength of plant body.

C) Xylem or wood parenchyma

Parenchymatous cells associated with xylem are known as xylem or wood parenchyma.
The cells are living, small thick walled and lignified.
They are used for storage of food in the form of starch or fat.
Tannins, crystals and various other substances also occur in xylem parenchyma cells.
These cells also assist directly or in directly in the conduction of water upward through the vessels and tracheids.

D) Xylem (wood) fibres

Sclerenchymatous cells associated with xylem are known as xylem fibres.
They are found in abundant in woody dicots.
They serve to the mechanical strength of the xylem and the plant body as a whole.

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