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Complex Permanent Tissues

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Complex Permanent Tissues - Lesson Summary

Tissues are a group of cells similar in structure and functions. There are two main types of plant tissues — meristematic tissues and permanent tissues. Meristemetic tissues undergo active cell division while the permanent tissues do not undergo any division.

Permanent plant tissue is a group of cells, which are complete in growth and usually incapable of meristematic activity.
 
Permanent tissue is of two types—simple permanent tissues, where all cells are similar in structure and function, and complex permanent tissues, which are composed of different types of cells.
 
Complex permanent tissues are a group of more than one type of cells that have a common origin, and work together as a unit.
 
There are two types of complex permanent tissues in plants —xylem and phloem.
 
Xylem performs the function of conduction of water and minerals from the roots to the stem and leaves. It also provides mechanical strength to the plant.  Xylem consists of four different types of cells—tracheids, vessels, xylem fibres and xylem parenchyma.
 
Tracheids are elongated, angular, lignified cells with tapering ends. They are dead and without any proptoplasm.
 
Vessels are long tube-like structures composed of many cells called vessel members.
Vessel members have lignified walls and a large central cavity.
 
Xylem fibres are sclerenchymatous fibres associated with the xylem. They have highly thickened walls and obliterated central lumens.
 
Xylem parenchyma is composed of parenchymatous cells. They are living and thin-walled, with cell walls made up of cellulose. These cells store food material as starch or fat, or substances such as tannin.
 
Some xylem cells are arranged in form of a ray through which radial conduction of water takes place. These cells are known as xylem ray parenchymatous cells. 
 
Primary xylem, which consists of the young cells produced from meristematic tissues by the differentiation process. Primary xylem is of two types —protoxylem and metaxylem.

The primary xylem elements formed in the beginning of a plant’s life are known as protoxylem. On the other hand, the primary xylem elements formed in the later stages are known as metaxylem.
 
In roots, the protoxylem lies towards the periphery and the metaxylem lies towards the centre.
This arrangement of primary xylem is known as exarch.
 
Phloem:
The second complex permanent tissue that transports food materials from the leaves to the other parts is known as phloem. In angiosperms, the phloem consists of sieve tube elements, companion cells, phloem parenchyma and phloem fibres. 
 
Sieve tube elements are tube-like structures that are arranged in a longitudinal manner. They are closely associated with companion cells, which are parenchymatous in nature. The functions of sieve tubes are controlled by the nucleus of the companion cells.
 
The companion cells also help in maintaining the pressure gradient in the sieve tubes.
 
Phloem parenchyma stores food material and other substances like resins, latex and mucilage. It consists of elongated, tapering cylindrical cells that have dense cytoplasm and nucleus.  Phloem fibres or bast fibres are made up of sclerenchymatous cells found in secondary phloem.  On maturity, phloem fibres lose their protoplasm and become dead. Jute, flax and hemp are some examples of commercially used phloem fibres. Complex permanent tissues are engaged in the transportation of water, mineral, nutrients and organic substances. Therefore, they are extremely essential for the survival of plants.

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