Tissues

5 Complex Permanent Tissues

These tissues are made up of different types of cells but have common functions.
Complex tissues are of following two types:
- Xylem or wood
- Phloem or bast
Xylem and phloem are both conducting tissues and also known as vascular tissues; together both of them constitute vascular bundles.
Vascular tissue is a distinctive feature of the complex plants, one that has made possible their survival in the terrestrial environment.

Xylem

Nature Xylem (Gr. xylos = wood) is a vascular and mechanical tissue.
In other words, it is a conducting tissue.
Xylem is composed of cells or elements of four different types:
- Tracheids: They are elongated cells with tapering ends. They also conduct water. Since tracheids do not have open ends like the vessels. So that water has to pass from cell to cell via the pits.
- Vessels: They are very long tube-like structures formed by a row of cells placed end to end. The transverse walls between the vessel elements are partially or completely dissolved to form continuous channels or water-pipes.
- Xylem parenchyma: They store food and helps in lateral conduction of water. Except xylem parenchyma, all other xylem elements are dead and bounded by thick lignified walls.
- Xylem sclerenchyma (or fibre): They are mainly supportive in function.

Functions:

The main function of xylem is to carry water and mineral salts upward from the root to different parts of shoots.
Since walls of tracheids, vessels and sclerenchyma of xylem are lignified, they give mechanical strength to the plant body.

Phloem

It is composed of following five elements or cells.

Sieve tubes
Seive cells
Companion cells
Phloem parenchyma
Phloem fibres
- Sieve tubes: Sieve tubes are slender, tube-like structures composed of elongated thin-walled cells, placed end to end. Walls of sieve tubes are perforated. The nucleus of each sieve cell degenerates at maturity.
- Companion cells: Generally associated with the sieve tube is small thin –walled cell containing dense and very active cytoplasm and large elongated nucleus. It is called a companion cell and it is connected to the sieve tube with numerous plasmodesmata.
- Phloem parenchyma: These are thin-walled, living cells of parenchyma of phloem. They have two functions, storage and slow lateral conduction of food.
- Phloem fibres or bast fibres: These are thick walled, elongated spindle-shaped dead cells which possess narrow lumen. They provide mechanical strength to the tissue.

Functions

Phloem transports (conducts) photosynthetically prepared food material from the leaves to the storage organs and later from storage organs to the growing regions of the plant body.

Differences between xylem and phloem:

Xylem	Phloem
It conducts water and minerals.	It conducts organic solutes or food materials.
Conduction is mostly unidirectional, i.e., from roots to apical parts of the plant.	In it conduction may be bidirectional, i.e., from leaves to storage organs or growing parts or from storage organs to growing parts of plants.
Conducting channels or tracheary elements are tracheids and vessels.	Conducting channels are sieve tubes.
Components of xylem include tracheids, vessels xylem parenchyma and xylem fibres.	Components of phloem include sieve tubes, companion cells, phloem parenchyma and phloem fibres.
Three of the four elements of xylem are dead (i.e, tracheids, vessels and fibres). Only xylem parenchyma is living.	Three of the four elements are living (i.e., sieve tubes, companion cells and phloem parenchyma). Only phloem fibres are dead.
In addition to conduction, xylem provides mechanical strength to the plant.	Phloem performs no mechanical function for the plants.