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Jun 7, 2015

# What are complex permanent tissues? Explain in detail.

Explain about complex permanent tissues.

Aruna kavitha Singupilla

Sol: Permanent tissues are the tissues which have lost the capacity to divide. They can be thin walled or thick walled and have permanent shape. Permanent tissues can be simple or permanent complex tissues. Complex permanent tissue is made up of a group of same type of cells which have common origin and function. Xylem and phloem are complex tissues and form vascular tissue of the plant. Xylem is the vascular tissue extending from top to bottom of the plant. It helps in the transport of water molecules and dissolved substances from the root hairs to aerial parts of the plant. Xylem mainly comprises of tracheids, vessels, xylem parenchyma and xylem sclerenchyma. The transport in xylem is unidirectional. Xylem mostly occupies the centre of the vascular bundle. a) Tracheids re elongated cells with tapering ends.They are dead and do not have any protoplast.They possess a thick tough and lignified cell wall.Depending on the type of lignin deposits, the thickenings can be annula, spiral, reticulate or scalariform. In case of scalariform thickening, the cell wall has pits which can be simple pit or bordered pit. Tracheids provide mechanical support to the cell and transport water, solutes, hormones etc. from roots to leaves. b)Vessels are cylindrical cells arranged in a longitudinal series.  Depending on the end part design, vessels can form simple perforation plate or multiple perforation plate.Due to the presence of many perforation plates, these serve as transport channels than tracheids.  c) Xylem fibres can be primary or secondary fibres. They have simple or bordered pits with thickened walls and obliterated lumen. These can be septate or aseptate.  d) Xylem parenchyma are thin walled cells whose wall is made up of cellulose. These store food material in the form of starch or fats. These also help in conduction of water. Phloem is the vascular tissue which transports food molecules to the place of necessity in the plant. The elements in the phloem are sieve elements, fibres, phloem parenchyma and companion cells. The transport in the phloem tissue is bidirectional. It forms vascular bundles in association with xylem .Phloem occupies the edges of the vascular bundle.  a) Sieve tube elements are made up of  sieve cell and sieve tube element. Sieve cells are simple cells which have sieve areas in their walls. They do not have any sieve plate. Sieve tube elements are are special cells which have sieve areas in their walls.These are longitudinal cells which are associated with companion cells. They can be simple or complex. They have a large vacuole.  b) Companion cells are specialised cells of parenchyma which are associated with sieve tube elements. They arise along with sive tube elements and share function with them. Companion cells provide proteins and other nutrients to sieve tube elemnts and also maintain pressure gradient in the cells.  c) Phloem parenchyma are cylindrical living cells with dens e nucleus and cytoplasm. These store organic food materials and other substances like resins, latex and mucilage. The cell walls of these parnchymatic cells have cellulose with pits to interconnect the axial parenchyma and the ray parenchyma. d) Phloem fibres are also called as bast fibres and occur in secondary phloem.These have thick cell wall and pointed apices.these can have simple or bordered pits. These cells on maturity die and are found in the jute sheets and flax network.
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