8.3 Permanent tissue :
This is group of cells which have lost the capacity of division and acquired permanent size, shape and functions. It is due to different morphological, physiological and functional changes that occur during maturation of the cell.
Depending upon types of cells, there are two types as simple and complex permanent tissues.
A. Simple permanent tissues :
These are made up of only one type of cells carrying similar functions. This tissue is either living or dead.
Following are the types of simple permanent tissues namely,Parenchyma, Collenchyma and Sclerenchyma.
1. Parenchyma :
Cells in this tissue are thin walled, isodiametric, round, oval to polygonal or elongated in shape.
Cell wall is composed of cellulose.
Cells are living with prominent nucleus and cytoplasm with large vacuole.
This is less specialized permanent tissue.
Parenchyma has distinct intercellular spaces.
Sometimes, cells may show compact arrangement.
The cytoplasm of adjacent cells is interconnected through plasmodesmata and thus forms a continuous tissue.
These cells are distributed in all the parts of plant body viz. epidermis, cortex, pericycle, pith, mesophyll cells, endosperm, xylem and phloem.
These cells store food, water, help in gaseous exchange, increase buoyancy, perform photosynthesis and different functions in plant body.
Dedifferentiation in parenchyma cells develops vascular cambium and cork cambium at the time of secondary growth.
2. Collenchyma :
It is a simple permanent tissue made up of living cells. The cell wall is cellulosic but shows uneven deposition of cellulose and pectin especially at corners. The walls may show presence of pits.
Cells are similar like parenchyma containing cytoplasm, nucleus and vacuoles but small in size and without intercellular gaps. Thus appears to be compactly packed.
The cells are either circular, oval or angular in transverse section.
Collenchyma is living mechanical tissue and serves different functions in plants. It gives mechanical strength to young stem and parts like petiole of leaf. It allows bending and pulling action in plant parts and also prevents tearing of leaf.
Growth of organs and elongation are other functions.
Collenchyma is usually absent in monocots and roots of dicot plant.
3. Sclerenchyma :
It is simple permanent tissue made up of compactly arranged thick walled dead cells.
The cells are living at the time of production but at maturity they become dead.
As cells are devoid of cytoplasm their thickened walls are due to uniform deposition of lignin.
Cells remain interconnected through several pits.
It is of two types viz.
a. fibres
b. sclereids.
a. fibres
Fibres are thread-like, elongated and narrow structures with tapering and interlocking end walls.
These are mostly in bundles, pits are narrow, unbranched and oblique.
They provide mechanical strength.
b. sclereids.
Sclereids are usually broad, with blunt end walls.
These occur singly or in loose groups and their pits are deep branched and straight.
These are developed due to secondary thickening of parenchyma cells and provide stiffness only.
Function
Sclerenchyma tissue functions as the main mechanical tissue.
It permits bending, shearing and pulling.
It gives rigidity to leaves and prevents it from falling.
It also gives rigidity to epicarps and seeds.
Commercial fibres are also produced from sclerenchyma fibres. e.g. jute, flax, hemp.
B. Complex permanent tissues :
These tissue are 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 :
It is a dead complex tissue.
Components of xylem are
1. Tracheids or Vessels,
2. Xylem parenchyma and
3. Xylem fibres.
The xylem also provides mechanical strength to the plant body.
Tracheids and vessels conduct water and minerals. These are also known as hadrome.
In pteridophytes and gymnosperms tracheids are the conducting elements while vessels in angiosperms, Selaginella (Pteridophyte) and Gnetum (Gymnosperm) are the conducting elements.
Tracheids
Tracheids are elongated, tubular and dead cells.
The ends are oblique and tapering.
The cell walls are thick having uneven deposition of lignin. 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
1. annular (in the form of rings),
2. spiral (in the form of spring/ helix),
3. scalariform (ladder like),
4. pitted is most advanced type (small circular area) which may be simple or bordered.
Vessels
Vessels are longer than tracheids and are formed by dissolution of end walls of row of cells i.e. vessel elements.
These are involved in conduction of water and minerals.
Their lumen is wider than that of tracheids and the thickening is due to uneven deposition of 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 later formed 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
When protoxylem is arranged towards periphery and metaxylem towards pith It is called as exarch. e.g. in roots
Xylem parenchyma cells
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.
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.
2. 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 proto (first formed) and meta (later formed) phloem.
It is composed of
1. sieve cells or sieve tubes,
2. companion cells,
3. phloem parenchyma and
4. phloem fibres.
Sieve tubes or Sieve Cells
They are long tubular conducting channel of phloem. These are placed end to end with bulging at end walls.
The sieve tube has sieve plate formed by septa having small pores.
The sieve plates connect protoplast of adjacent sieve tube 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.
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.
Phloem parenchyma cells
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.
It is absent in most of the monocots.
Phloem fibres
Phloem fibres are the only dead tissue among this unit.
These are sclerenchymatous.
Generally absent in primary phloem, but present in secondary phloem.
These cells are with lignified walls and provide mechanical support.
These are used in making ropes and rough clothes.
