Sclerenchyma is a type of permanent tissue found in plants. Permanent tissues lose the power of cell division. They attain a definite shape, size and function. Sclerenchyma is a type of simple permanent tissue. Simple tissues are made up of a group of uniform cells having similar structures and perform the same function.
Other than sclerenchyma, collenchyma and parenchyma are also simple tissues.
It is a supportive tissue and is usually made up of dead cells with highly thickened lignified walls. It is mostly present in the mature parts of the plant.
- It is made up of long and narrow cells.
- Cells are usually dead without protoplasts.
- The cell wall is thick and lignified with a few or numerous pits.
- They possess both primary and secondary cell walls.
- The secondary cell wall is highly thickened and has pits to allow the exchange of substances.
- The secondary walls are rich in lignin along with cellulose, hemicellulose and pectin.
- They possess narrow lumen due to highly thickened cell walls.
- At maturity, they lose their ability to stretch or elongate.
- At functional maturity, cells are often dead.
- They are of two types of sclerenchyma cells. They are sclereids and fibres.
- Sclerenchyma tissues are located in several areas. E.g. Sclereids are found in the shells of nuts, stones of fruits and fibres are abundantly found in the inner bark, wood, leaf veins, etc.
Types of Sclerenchyma
Sclerenchyma is classified into two types based on their structure, origin and development. They are sclereids and fibres.
Sclereids are short, isodiametric or irregular in shape. Their characteristics are:
- They may be spherical, oval or cylindrical.
- They are often dead and have highly thickened cell walls.
- Sclereids have very narrow cavities and are hard and inflexible.
- They have more pits than fibres.
- They are commonly found in soft tissues, e.g. cortex, phloem, the pulp of fleshy fruits, fruit walls and seed coats.
- They are found in shells of nuts, fruit pulp of pear, guava, etc., tea leaves and seed coat of legumes.
- They provide structural support and mechanical strength to plant organs.
- They form protective coverings as seed coats, shells of nuts, etc.
There are six types of sclereids found. They are:
- Brachysclereids or stone cells – They are isodiametric and are found in fruits such as cherries and peaches.
- Macrosclereids or malpighian cells – They are elongated and form a layer in the seed coat of legumes.
- Osteosclereids – They are also called bone cells. They have rounded ends and are found in leaves and seed coats. It forms the hypodermal layer of many seeds, fruits and leaves of some xerophytes.
- Astrosclereids or star-shaped cells – They are found in leaves and petioles of many hydrophytes, eg. Nymphaea.
- Trichosclereids – They are elongated and hair like. They are found in aerial roots of Monstera, leaves of banana, etc.
- Filiform sclereids – They are long, fibre-like cells. They are less branched and are not very common. They are found in the leaves of Olea, etc.
Fibres are elongated, needle-like pointed sclerenchyma cells. Their important characteristics are:
- They are long and tapered at the end.
- They are the longest cells found in higher plants. They can be as long as 1-8 mm in length.
- They are often found in groups or clumps or patches.
- They are often dead at maturity and lack a nucleus and cytoplasm.
- They contain thick secondary walls with fewer pits than sclereids.
- Fibres have uniformly thickened lignified cell walls.
- Flax fibres are made up of cellulose.
- They are present throughout the plant body. They are commonly found in stems, wood, inner bark and certain leaves.
- They provide mechanical support and strength to the plant organ.
There are mainly two types of fibres. They are intraxylary and extraxylary fibres.
- Intraxylary fibres – They are found in the xylem. Intraxylary fibres are of two types:
- Libriform fibres – They are true fibres with simple pits.
- Fibre Tracheids – They have bordered pits.
- Extraxylary fibres – They are found outside the xylem in the cortex, pericycle or phloem and are named accordingly, e.g. cortical fibres, pericyclic fibres and phloem or bast fibre.
Some examples of fibres are:
Blast fibres – flax, hemp, jute, etc.
Leaf fibres – abaca (Musa textilis), henequen (Agave fourcroydes), etc.
Seed hairs – cotton (Gossypium species), coir (Cocos nucifera), bamboo, etc.
The main function of sclerenchyma is to provide mechanical support and strength to the plants.
- They provide structural support to the plant organs.
- They form protective coverings around nuts and seeds.
- They are also a part of vascular bundles and form conductive tissues.
- The xylem vessels and tracheids are sclerenchymatous cells.
- They form the hypodermis of some xerophytic plants and reduce water loss.
- Many fibres such as bast fibres, seed hairs are used as raw materials for textiles.
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