5.3.2 Permanent Tissues:
A group of cells which have acquired permanent shape, size and functions and have lost the power of division is called a permanent tissue.
Permanent tissues are formed as a result of division and differentiation in meristematic tissues.
The cells of permanent tissues are either living or dead and thin-walled or thick-walled.
Permanent tissues are of two types:
A) Simple permanent tissues
B) Complex permanent tissues
A) Simple Permanent Tissues:
The simple permanent tissues are composed of similar cells (of one type) resembling in shape, size function and form.
These are of three types:
1) Parenchyma
2) Collenchyma
3) Sclerenchyma.
1) Parenchyma:
The parenchyma is a living tissue that consists of a group of isodiametric cells.
The cells may be oval or circular and are loosely arranged with intercellular spaces.
The cells contain cytoplasm, vacuole and nucleus.
The cell walls are thin and are made up of Cellulose.
Parenchyma is distributed in almost all parts of the plant body.
Fleshy and edible roots, stems, fruits and seeds are mainly made up of parenchyma.
It is found in epidermis, cortex, pith, mesophyll of leaves and is also present in xylem and phloem.
It is the most simple and primitive tissue, from which other tissues have evolved.
Therefore, it is also called fundamental tissue.
Functions of Parenchyma:
The main function of parenchyma tissue is storage of food.
In xerophytes, it helps in storage of water.
Parenchyma with abundant chloroplasts is called chlorenchyma.
The function of chlorenchyma is photosynthesis.
Mesophyll of leaves and outer cortex of green stem is made up of chlorenchyma.
In aquatic plants.
The Aerenchyma has large intercellular spaces between the parenchyma cells.
Function of aerenchyma is to store air and help the organs to float in water.
Fibre-like elongated cells of parenchyma are called prosenchyma.
It is slightly thick-walled and is meant for providing rigidity and strength.
Cutinized parenchyma cells of epidermis of stem and leaf provide protection.
Parenchyma associated with xylem and phloem helps in storage as well as conduction.
2) Collenchyma:
It is a living simple permanent tissue.
The component cells are elongated with blunt ends and look spherical in T.S.
Their cell walls are unevenly thickened.
They have extra deposition of cellulose and pectin at the adjoining regions of two or more cells at the corners.
Hence the cell wall becomes rigid, and intercellular spaces are absent.
The cells are smaller in size than parenchyma but like parenchyma, contain cytoplasm, vacuole and nucleus.
Collenchyma is usually found in the form of 3 -4 layered hypodermis of herbaceous dicotyledonous stem.
It is absent in monocots and roots.
Functions:
The main function of ccllenchyma is to provide flexible mechanical support and tensile strength to the organs.
Further, due to its peripheral position in stem, it resists bending and pulling action of wind.
When it contains chloroplasts, its function is photosynthesis.
Collenchyma allows growth and elongation of organs and prevents tearing of leaves.
3) Sclerenchyma (Greek; scleros = hard)
Sclerenchyma is a dead, simple, permanent tissue mada up of thick walled cells.
According to the origin and structure these cells are either
- sclerenchyma fibres
- sclereids.
Sclerenchyma fibres look polygonal in transverse section but are elongated with, tapering ends.
They are compactly arranged and hence without intercellular spaces.
The cell walls are evenly thickened due to deposition of lignin.
As the cells are dead at maturity, protoplasm is absent.
Such fibres occur in groups, as sheets or cylinders in various parts of plant body.
They are present in
- hypodermis of monocot stem and many leaves,
- pericycle and secondary xylem (wood) of many dicot stems,
- vascular bundle sheath of monocot stems.
Sclereids are extremely thick-walled, hard and strongly lignified.
They are mostly isodiametric or polyhedral with very small lumen.
Sometimes, the cells are elongated and may be rodshaped, bone-shaped or star-shaped.
Sclereids are also called stone cells which occur singly or in groups.
They are present in the cortex and phloem of stems and roots.
They are most abundant in the coverings of seeds and nuts, (shells of walnuts, coconuts etc.)
Functions:
1. The main function of sclerenchyma is to provide mechanical strength and rigidity to the plant body.
2. Sclerenchyma is able to bear compression, pull, and bending forces.
3. Sclereids provide rigidity and strength to seed coverings, and stiffness to the plant part.
