Sclerenchyma (from the Greek scleros, meaning “hard”) is made up of long, thin, and thick cells. Due to the deposition of lignin, these cells have extremely thick walls. Sclerenchyma cells that have reached maturity are unable to extend, and they are found in sections of the plant that have ceased to expand in length. At maturity, the majority of sclerenchyma cells die and serve as structure and support for plant parts. Sclerenchyma cells are typically seen in association with other cell types, providing mechanical support for them. The inflexible walls act as a “skeleton” that maintains the plant for hundreds of years in some circumstances. The secondary walls of immature sclerenchyma are deposited unevenly in spiral or ring patterns in areas of the plant that are still elongating. These types of cell wall fascicles allow the cell wall to stretch in a spring-like manner as the cell elongates. Sclerenchyma occurs in stems and also in the veins of leaves. Sclerenchyma is also the material that makes up the tough outer layer of seeds and nuts.

Sclerenchyma is a form of persistent plant tissue. Permanent tissues lose their ability to divide cells. They acquire a distinct form, size, and function. The sclerenchyma is a form of permanent simple tissue. Simple tissues are composed of a collection of identical cells with comparable structures and functions. Apart from sclerenchyma, simple tissues include collenchyma and parenchyma.

Sclerenchyma Classification

Sclerenchyma cells are classified primarily into two subtypes. Sclereids and fibers

Fibers-They are extremely elongated cells with long and tapering ends that interlock to support the Plant mechanically. Fibers are typically found in bundles and can be found practically anywhere on the plant’s body, including the stem, roots, and leaf vascular bundles. The majority of these Fibers are seed hairs, leaf fibers, and bast fibers, and they serve as a vital raw resource for the textile sector as well as other oven items. Fibers are further classified into two types: xylary and non-xylary.

Sclereids-They are classified as mechanical tissues that exist in clusters or singly and are associated with the plant’s circulatory tissues Xylem and phloem. The cell wall of sclereids is not uniformly thickened, and it also has a number of simple pits with round openings, and typically, the Sclerenchyma cells consist of the narrow lumen. It is occasionally referred to as the stone cells and contributes to the rough feel of pears and guava. Sclereids are further classified into four subclasses based on the morphology of the cell.

• Macrosclereids-Additionally, it is referred to as the “Malpighian cell.” Macrosclereids are elongated and columnar in shape and are typically found in the seed’s outer epidermal cells. The seed coat of the Pisum species is an example of a Macro Sclereid.

• Osteosclereids-Additionally, it is referred to as “Bone cells.” Osteosclereids resemble hourglass bones in appearance, having larger, lobed, and columnar cells. Additionally, it is lobed near the end. They are frequently found beneath the epidermal layer, such as the hypodermis of seeds and leaves of certain xerophyte plants. Several examples are the leaves of Hakea species.

• Astrosclereids-It is sometimes referred to as “Stellate cells’ ‘ due to its appearance as a star-like structure with deeply lobed arms spreading from the central body. Typically, the radiating arms are pointed, uneven, and varied in length. Astrosclereids are primarily found in the leaf’s top to lower epidermis. Several examples include Leaves of Thea, Olea, and others.

• Brachysclereids-It is also referred to as “Grit cells,” and it bears a strong resemblance to parenchymatous cells, with an approximately isodiametric symmetry. They are primarily found in fruit’s fleshy sections. Brachyscelerids are found in the flesh of pear fruit, where they generate grit and also allude to a stone cell.

• Trichosclereids-Additionally, it is referred to as “needle-like cells” and appears hair-like, with more elongated and branched cells pushing toward the intercellular space. Trichosclereids are found in the specialized tissues of leaves and roots; examples include aerial roots of Monstera sp., olive and water lily leaves, and so on.

• Filiform Sclereids -Additionally, it is referred to as “Fiber-like cells” and looks to be an extremely elongated, sparsely branching, and unusual type of cell. They are primarily found in leaves’ specialized tissues. The leaves of Olea are an example of filiform sclereid.

Characteristics of Sclerenchyma

It is a supporting tissue composed primarily of dead cells with heavily lignified walls. It is predominantly found in mature portions of the plant.

• It is composed of long, narrow cells.
• Without protoplasts, cells are usually doomed.
• The cell wall is densely lignified and pitted, with a few or numerous pits.
• They have primary as well as secondary cell walls.
• The secondary cell wall is substantially thickened and pitted to facilitate material exchange.
• The secondary walls include a high concentration of lignin, as well as cellulose, hemicellulose, and pectin.
• Due to their substantially thicker cell walls, they have a small lumen.
• They lose their ability to stretch and extend as they mature.
• Cells are frequently dead when they reach functional maturity.
• They are classified as sclerenchyma cells in two distinct subtypes. Sclereids and fibers are the two types.
• Sclerenchyma tissues are found in a variety of locations. E.g. Sclereids are plentiful in the shells of nuts and fruits, while fibers are common in the inner bark, wood, and leaf veins, among other places.

CONCLUSION- 

When mature, sclerenchyma tissue is formed of dead cells with substantially thickened walls containing lignin and a high cellulose content (60–80%), and it provides structural support to plants. Sclerenchyma is a type of plant tissue that provides stiffness and strength to the plant. The majority of sclerenchyma cells proliferate in an invasive manner. Sclerenchyma cells have thickened secondary layers composed of cellulose, hemicelluloses, and lignin.