Sclerenchyma Tissue

Introduction image

Sclerenchyma tissue can define as one of the types of ground or simple permanent tissue that constitutes both primary and stiff secondary wall. These are generally rigid woody cells with a compact arrangement. Sclerenchyma has a characteristic feature, where it functions to promote cell strength and conduction instead of being a dead cell.

On secondary development of plant, the sclerenchyma cells attain maturity and become a dead cell, due to lignin deposition that restricts the exchange of water and gases resulting in degeneration of inner protoplasm.

Content: Sclerenchyma Tissue

  1. Definition
  2. Characteristics
  3. Types

Definition of Sclerenchyma Tissue

Sclerenchyma tissue defines as a type of the simple-permanent tissue, which initially remains a living cell but becomes dead during the development of secondary wall resulting from the accumulation of lignin. The phenomenon of lignin accumulation in the plant cell refers as “Lignification” that occurs after the completion of the cell-growth, and at the time of secondary thickening. Sclerenchymatous tissue predominates in the rigid areas of plant body like leaf vein, stem, branches, trunk, bark etc.

Characteristics of Sclerenchyma Tissue

Term sclerenchyma was derived from the Greek word “Scleros” that means harder and “Enchyma” which means infusion. A sclerenchyma tissue shows the following characteristic features.

  1. It is a dead, simple-permanent ground tissue.
  2. The function of sclerenchyma is similar to the collenchyma tissue, which is giving mechanical support and tensile strength to the plants.
  3. Sclerenchyma cells function as a “Skeleton” of the plant system that contributes rigidity to withstand against various ecological stresses.
  4. During the initial growth cycle of a plant, the sclerenchyma persists as a living cell and found as a spiral or ring patterns.
  5. On plant maturation, the sclerenchymatous cells become dead by the accumulation of lignin that makes the cell harder and impervious to the exchange of water, solutes, gases etc. between the environment and the inner protoplast.
  6. Sclerenchyma refers as a dead tissue because of its dead, degenerated or functionless inner protoplast.
  7. Mechanical and conductive sclerenchymatous tissue are two common types, based on the function of sclerenchyma.
  8. Fibres, sclereids and tracheary elements are the three common types, based on the morphology of sclerenchyma tissue.
  9. It includes two layers of cell-wall; a primary cell-wall and a thickened secondary cell-wall (containing cellulose, hemicellulose, lignin etc.).
  10. The cell-wall type, rigidity, shape, size etc. of sclerenchyma will vary accordingly, within different types of plant.

Types of Sclerenchyma Tissue

Based on the function: A sclerenchyma tissue can classify broadly into two classes, namely mechanical and conductive sclerenchyma.
types of sclerenchyma tissue
Mechanical Sclerenchyma: It is a kind of sclerenchymatous tissue that functions as a “Supportive tissue” by reducing the wilting in plants, maintaining plant physiology, providing strength to withstand against the tearing forces of waves and current etc. Mechanical sclerenchyma comprises of sclereid and fibre cells that contribute strength and stiffness to the plant system.
sclerenchymatous tissue

Sclereids

Sclereids can define as a mechanical tissue that involves the following features:

  1. It occurs singly or in groups.
  2. Sclereids are found associated with the plant’s vascular tissue, namely xylem and phloem.
  3. Its cell-wall thickening is non-uniform and contains a number of simple pits with round apertures.
  4. Sclereids most usually comprises of the narrow lumen.

Based on the shape, the sclereid cells subdivides into the following classes:
sclereids classification

Macrosclereids

It also refers as “Malpigian cell”.
Appearance: Elongated and columnar in shape.
Cell wall: Comprises of a thickened cell-wall.
Occurrence: Usually found in the outer epidermal cells of seed.
Example: Seed coat of Pisum species.

Osteosclereids

It also refers to as “Bone cells”.
Appearance: Appears very similar to the shape of a bone of hourglass with enlarged, lobed and columnar cells. It is lobed towards the end.
Cell wall: Comprises a thickened cell-wall.
Occurrence: Usually found below the epidermal layer i.e. hypodermis of seeds and leaves of certain plants belongs to the category of  Xerophytes.
Example: Leaves of Hakea species.

Astrosclereids

It also refers to as “Stellate cells”.
Appearance: These appear to be star-like, deeply lobed with the radiating arms from the central body. The radiating arms are usually  pointed, irregular and varied in number.
Cell wall: Comprises a thickened cell-wall.
Occurrence: Extends from upper to lower epidermis of the leaf.
Example: Leaves of Thea, Olea etc.

Brachysclereids

It also refers to “Grit cells”.
Appearance: It deeply resembles the parenchymatous cells, and its symmetry is roughly isodiametric.
Cell wall: Comprises a thickened cell-wall.
Occurrence: It is commonly present in the fleshy portions of fruit.
Example: Flesh of pear fruit, where brachysclereids form a grit and also refers as stone cells.

Trichosclereids

It also refers to as “Needle-like cells”.
Appearance: It seems to be hair-like, more elongated, and branched cells stretching towards the intercellular space.
Cell wall: Comprises a thickened cell-wall.
Occurrence: Present in the specialized tissues of leaves and roots
Example: Aerial roots of Monstera sp, leaves of olive and water-lily etc.

Filiform sclereids

It also refers to as “Fibre-like cells”.
Appearance: These are very much elongated, sparingly-branched and uncommon kind of a cell.
Cell wall: Comprises a thickened cell-wall.
Occurrence: Found in the specialized tissues of leaves
Example: Leaves of Olea.
types of sclereids

Functions of Sclereid

  • Sclereids support the neighbouring tissues where they occur.
  • It protects the inner cells, by forming a concentrating layer towards the periphery.

Fibres

Fibres can define as another kind of mechanical tissue that involves the following features:

  1. The fibre cells are elongated, thick-walled with a narrow lumen and tapered ends.
  2. It occurs in the ground and vascular tissues of a plant.
  3. Fibre sclerenchyma is a cell companion to the xylem and phloem.
  4. It appears as independent strands or cylinders.

Based on the shape, the fibre cells subdivides into the two following groups:
types of fibre

Xylary Fibre

It is found associated with the primary and secondary xylem. The xylary fibre associated with the primary xylem originates from the procambium, whereas xylary fibre associated with the secondary xylem originates from the cambium tissue of plant cell. It further divides particularly into two forms like libriform fibre and fibre tracheids.

Libriform fibre has an elongated, thickened cell wall in comparison to the fibre tracheids, and comprises of a simple pit with a longer pit canal. Fibre tracheids are long, thick-walled, having bordered pit with a smaller pit chamber.

Extraxylary Fibre

It is found associated with the tissues outside the xylem like phloem, cortex and pith of plant cell. In monocots, the extraxylary fibre encircles the bundle sheath, derives partly from the ground meristem and remaining from the procambium.

In dicots, the extraxylary fibres occur as independent bands or cylinders, on the peripheral region of the vascular cylinder and innermost cortex layer. It derives wholly from the ground meristem tissue, whose structure, shape and composition are somewhat similar to the xylary fibre.

The extraxylary fibre subdivides into three kinds like:

Phloem fibres: These occur in the primary and secondary phloem of vascular plant tissues, and also called as “Bast fibres”.
Cortical fibres: These are present in the cortex region of a plant cell that occurs singly or in groups, and supports the younger part of plants.
Perivascular fibre: It is present in the pericycle of the plant, forming a vascular bundle cap of dicot and bundle sheath of monocots, and also refers as “Pericyclic fibres”.

Functions of fibre tissue

  • Fibre tissue contributes flexibility to the plant.
  • The septate fibres function as storage cells that reserve starch and oil droplets.
  • It protects the nearby inner tissue.
  • The surface fibres facilitate seed and fruit dispersal.
  • Plant fibres help in the manufacturing of textile, ropes, strings etc.

Conductive Sclerenchyma: It consists of a tracheary element that is a peculiar property of vascular plants, which demarcates them from the non-vascular plants. The tracheary elements provide both strength and water conduction.

Tracheary Elements

These elements occur in the vascular plants that include vessel elements and tracheids.
types of tracheary elements

Vessel Elements

It comprises of a perforated end walls (primary and secondary lignified wall) and present in both primary and secondary xylem. Vessel elements are more efficient in the conduction of water, where the water flows vertically from one cell to the other without any hindrances.

It is a more specialized kind of tracheary element and the size is smaller than the tracheids. The vessel elements interconnect with the other vessels from one end of the cell to the cell of another end, in vertical rows. One of the characteristic features of vessel element is that it solely participates in the conduction of water.
Tracheary elements

Tracheids

Tracheids are the common cell in the xylem that appears to be spindle-shaped, elongated with tapered ends. It participates in both water conduction and mechanical support. Tracheids are elongated comparative to the vessel elements, and having a common feature of having secondary wall thickening ranging into various shapes (from annular rings, reticulate etc. to pitted form). It is having a high surface to volume ratio that is one of the characteristic property of tracheids, which protects the plant from air embolisms or water stresses.

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