Sclerenchyma | Definition, Types, & Function
Sclerenchyma is a Greek word meaning hard tissue. Sclerenchyma is the supportive Tissue, which is composed of various hard woody cells. Sclerenchyma cells once matured are usually the dead cells that have heavily thickened secondary walls containing lignin.
It has following features —
- Cells of sclerenchyma are called sclerenchymatous cells. They are usually dead and without protoplasts. During maturation, the cells lose their protoplasm and become dead.
- Cells are elongated and narrow and possess highly thickened cell walls. Their walls consist of cellulose, hemicellulose and specialised organic material lignin which provides mechanical strength to the plant and its parts. Lignin deposition is so thick that the cell walls become strong, rigid and impermeable to water.
- Walls of the sclerenchymatous cells possess few to numerous pits.
Types of cells in sclerenchyma
On the basis of variation in origin and development, Form a structure, two types of cells are found in sclerenchyma:
(a) Sclerenchymatous fibres: These are the highly elongated cells which have pointed or oblique ends. Their elongated and narrow shape makes them look like fibres. These are the thick-walled cells and in some cases, the cell wall becomes so much lignified that the lumen (inner space or cavity) is greatly reduced.
These fibres generally occur in groups or bundles, in various parts of the plants like stems. They are usually found associated with the xylem and phloem of the vascular bundles. These provide mechanical strength to the organs that possess them.
(b) Sclereids: These are spherical, oval or cylindrical cells with highly thickened cell wall. The cell walls are so thickened that the lumen of the cells is almost reduced or obliterated. These are also the dead cells. Sclereids may occur singly or in groups. The sclereids can be found in the roots, stems, leaves, flowers, fruits and seeds.
Sclereids are commonly found in the hard parts of the plant, however, they occur in the soft parts, i.e., pulp of the fruits also, Sclereids are present in:
(i) Fruit walls of nuts like walnut, almond, etc.
(ii) Pulp of fruits like guava, pear and sapota (cheeku) etc. The ‘gritty’ (crisp) texture of these fruits is due to the presence of hard cells or sclereids in their pulp.
(iii) Seed coats of legumes like peas, beans, etc. (Seed coats are the outer protective coverings of seeds).
(iv) Leaves of tea.
|Plants show the differentiation, i.e., maturation of permanent tissues from meristematic tissues due to which the former lose the ability to divide and become structurally and functionally mature. But, plants also show another interesting phenomenon, i.e., the living differentiated cells, that by now have lost the capacity to divide; regain the capacity of division under certain circumstances. Fully differentiated parenchyma cells giving rise to interfascicular and cork cambium during secondary growth is an example of this phenomenon. Here permanent tissue is giving rise to meristematic tissue, i.e., reverse of differentiation is occurring. It is called dedifferentiation. These dedifferentiated cells are called secondary meristems, which on dividing give rise to secondary permanent tissues. The cells constituting the secondary permanent tissues are called redifferentiated cells.|