The tissues, which have lost the capacity of cell division, are called permanent tissue. These are of two types:

1. Simple permanent tissue:

Tissues which are composed of two or more types of cells but contribute to a common function are called complex tissues. Xylem and phloem are the complex permanent tissues. They together form the vascular bundle.

This tissue consists of only one type of cells, e.g. parenchyma, collenchymas and sclerenchyma.

2. Complex permanent tissue:

This tissue is composed of more than one type of cells.

Complex Tissue:


Tissues which are composed of two or more types of cells but contribute to a common function are called complex tissues. Xylem and phloem are the complex permanent tissues. They together form the vascular bundle.

(i) Xylem:

It is a complex tissue composed of several types of cells. The various constituent of xylem are :(a) tracheids, (b) vessels, (c) fibres,(d) parenchyma cells. Tracheids and vessels are known as tracheary elements.

Origin- Primary xylem develops from procambium during the development of plant.Secondary xylem is produced later by the activity of vascular cambium during secondary growth. The detailed structures of various elements of xylem are described below.

(a) Tracheids:


These are elongated cells pointed at both ends. The end walls are imperforate. Tracheids are present in all groups of vascular plants.

(b) Vessels:

The vessels are shorter and broader than tracheids. Their length varies from one to many cells. They are joined end-to-end form a long chain of cells. Vessels are also found in some pteridophytes.

Different types if lignin depositions are found in tracheary elements. Accordingly, they show the following types of thickenings:

  • Annual: Both are found in protoxylem elements
  • Spiral
  • Scalariform: Both are found in metaxylem.
  • Reticulate
  • The tracheids show simple pit or bordered pit.


Function: The main function of tracheids and vessels is conduction of water.

(c) Xylem fibres:

The fibres are long cells with lignified secondary walls. These are thicker walls than the tracheids. Xylem fibres are of two types:(i) Fibre tracheids(ii) Libriform fibres. Fibre tracheids possess bordered pits while libriform fibres are longer and have thicker walls with slit-like simple pit.Both types of fibres are transversely separate. Fibre tracheids provide mechanical strength while libriform fibres conduct organic food.

(d) Xylem Parenchyma:


There is living cells represented by axial parenchyma ray parenchyma. These cells store stratch, oils and many ergastic substances. Some of the parenchyma becomes sclerified forming sclereids or sclerotic cells.

(ii) Phloem:

Phloem is composed of variety cell types like xylem. The constituent cells are: (a) Sieve tube elements (b) Companion cells (c) Phloem fibres (d) Phloem parenchyma.

Origin- Primary phloem is produced by the procambium while the secondary phloem develops from the vascular cambium. The develops from the vascular cambium. The detailed structure of various phloem elements are given below.

(a) Seive tube elements:


It includes sieve tubes and sieve cells. Both of these are thin-walled living cells.Sieve tube members are long slender tube-like joined end to end to form long tubular to form a long tubular channel. Seive plates are present at the end of the tube. Adjacent sieve plates. Main function of this tube is translocation of organic solute.

Seive cells:

These are elongated narrow cells with tapering ends. In these cells sieve areas occur all over the wall.

(b) Companion cells:


All angiosperms have specialized parenchyma cells associated with the sieve tube elements. Usually, a single companion cell extends through the whole length of the sieve tube. These are living cells and its function is associated with steve tube.

(c) Phloem fibres

These are found both in primary and secondary phloem. Fibres are invariably long and the walls are thick. The fibres of some plants like Linum. Cannabis are of great commercial importance. The main function is mechanical support.

(d) Phpoem parenchyma:

These are living cells contain various ergastic substances, such as stractch, tannins, crystal, etc. these cells are associated with fibres. The main functions are storage and tanslocation of ergastic substances.

The xylem and phloem both form the vascular tissues of the plant system. Xylem helps in conduction of water and the phloem is mainly responsible for the conduction and transport of organic food synthesized by the plant.