8. Plant Tissues and Anatomy - part 03 - Complex permanent tissues

Complex permanent tissues :

This tissue is heterogenous comprising of more than one type of cells and all function as a single unit.
This tissue is involved in conducting the sap and food from source to sink area.
Xylem and phloem are the complex tissues present in plants.

1. Xylem :

The xylem also provides mechanical strength to the plant body.
Tracheids and vessels conduct water and minerals. These are also known as hadrome.

1. Tracheids :

In pteridophytes and gymnosperms tracheids are conducting elements.
Tracheids are elongated, tubular and dead cells. The ends are oblique and tapering.
The cell walls are uniformly thickened and lignified. This provides mechanical strength.
Tracheids contribute 95% of wood in Gymnosperms and 5% in Angiosperms.
The different types of thickening patterns are seen on their walls such as

Annular (in the form of rings)
Spiral (in the form of spring/ helix)
Scalariorm (ladder like)
Pitted is most advanced type (small circular area) which may be simple or bordered.

2. Vessels

Vessels are conducting elements in angiosperms, Selaginella (Pteridophyte) and Gnetum (Gymnosperm) show presence of vessels.
Vessels are longer than tracheids with perforated or dissolved ends and formed by union of several vessels end to end.
These are involved in conduction of water and minerals.
Their lumen is wider than tracheids and the thickening is due to lignin and similar to tracheids.
In monocots, vessels are rounded where as they are angular in dicot angiosperms.
The first formed xylem vessels (protoxylem) are small and have either annular or spiral thickenings while latter formed have larger vessels (metaxylem) have reticulate or pitted thickenings.
When protoxylem is arranged towards pith and metaxylem towards periphery it is called as endarch e.g. in stem and when the position is revert as in the roots is called as exarch.

3. Xylem parenchyma :

Xylem parenchyma cells are small associated with tracheids and vessels. This is the only living tissue among this complex tissue.
The function is to store food (starch) and sometimes tannins.
Parenchyma are involved in lateral or radial conduction of water or sap.

4. Xylem fibres :

Xylem fibres are sclerenchymatous cells and serve mainly mechanical support. These are called wood fibres.
These are also elongated, narrow and spindle shaped.
Cells are tapering at both the ends and their walls are lignified.

Phloem :

This is a living tissue. It is also called as bast.
Phloem is responsible for conduction of organic food material from source (leaf generally) to a sink (other plant parts).
Phloem was named as leptome by Haberlandt as similar to xylem.
On the basis of origin, it is

1. sieve tubes :

2. sieve cells :

The sieve tube cell is a living cell with a thin layer of cytoplasm but loses its nucleus at maturity.
The sieve tube cell is connected to companion cell through phloem parenchyma by plasmodesmata.
Sieve cells are found in lower plants like pteridophytes and gymnosperms.
The cells are narrow, elongated with tapering ends and sieve area located laterally

3. Companion cells :

Companion cells are narrow elongated and living.
These cells are laterally associated with sieve tube elements.
Companion cells have dense cytoplasm and prominent nucleus.
Nucleus of companion cell regulates functions of sieve tube cells through simple pits.
From origin point of view, sieve tube cells and companion cell are derived from same cell.
Death of the one results in death of the other type.

4. Phloem parenchyma :

Phloem parenchyma cells are living, elongated found associated with sieve tube and companion cells.
The chief function is to store food, latex, resins, mucilage, etc.
The cells carry out lateral conduction of food material.
These cells are absent in most of the monocots.

5. Phloem fibres :

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