What is Xylem Tissue

Aug 09, 2022, 16:45 IST

Complex permanent tissues

The permanent tissues whose cells are heterogeneous i.e., cells having dissimilar origin and structure but performing common functions, is called complex tissue.A complex tissue is a collection of different types of cells which forms a structural unit and perform a specific function. Xylem and phloem, present in all vascular plants, are complex tissues.

About Xylem

Xylem (Gk. Xylos: wood; Nageli, 1858; term hadrom for xylem by Haberlandt, 1914) is a complex tissue, composed of several types of living and dead cells. It is primarily concerned with the conduction of water and minerals and also provides mechanical support to the plant. As a conducting strand, xylem forms a continuous channel through roots, stems, leaves, flowers and fruits.

The three main components of xylem are: Tracheary elements, xylem parenchyma and xylem fibres.

Tracheary Elements

Tracheary elements are highly specialized cells, principally concerned with the conduction of water. Two kinds of tracheary elements occuring in xylem are tracheids and vessels. Both are dead cells and are devoid of living protoplasts at maturity. They are more or less elongated cells with lignified secondary walls.Tracheids are present in all vascular plants, whereas vessels are confined to angiosperms. However, there are some primitive angiosperms where vessels are absent. Such vesselless angiosperms belongs to families : (i) Winteraceae (e.g., Wintera), (ii) Tetracentraceae (e.g., Tetracentron), (iii) Trochodendraceae (e.g., Trochodendron) whereas some advanced gymnosperms (e.g., Gnetum, Ephedra, Welwitschia) exhibit the occurrence of vessels. Besides, vessels are absent in stem and leaves of Yucca and Dracaena.Tracheids originate from single cells and are imperforate cells with pit pairs on their common walls. On the other hand, vessel members, which are joined into long continuous tubes of varying lengths, originate from longitudinal files of cells. They have dissolved transverse walls and the sap moves from one vessel member to another through these walls.The secondary walls of tracheary elements have a wide variety of thickening patterns. The tracheary elements of the first formed xylem (protoxylem) have secondary wall thickenings in the form of rings (annular) or helices (spiral). In the later formed metaxylem and secondary xylem, secondary walls usually show scalariform or reticulate thickenings.

Xylem Parenchyma

Parenchymatous cells associated with xylem are called xylem parenchyma.These are present in the primary and secondary xylem. In the primary xylem, the walls of the parenchyma cells are thin and made up of cellulose, whereas in the secondary xylem, the walls are often thickened.The main function of xylem parenchyma is storage of food materials. It also helps in the lateral conduction of water and minerals.

Xylem Fibres

Xylem fibres are supporting sclerenchymatous cells and provide mechanical strength to the plant body.They have extremely thick lignified walls and narrow lumen.

Transfer Cells

Besides the above mentioned cell types, yet another type of cells are associated with xylem which are called transfer cells.These cells have dense cytoplasmic contents, large nuclei, numerous mitochondria and endoplasmic reticulum. They are characterized by wall ingrowths of non lignified secondary wall. Plasmodesmata are also present on the walls of the two adjacent transfer cells. Transfer cells are associated with the internal transfer of solutes.