SCLERENCHYMA –Structure, distribution and function BSc (Hons) Part III Prepared by: Paper IV Anatomy Dr. Jyoti Pandey

“Scleros” means harder and “Enchyma” means infusion

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 process of lignification i.e.accumulation of lignin.

• It is a dead, simple-permanent ground tissue. • Sclerenchyma cells are characterized by relatively thick, lignified secondary cell walls. • It includes two layers of cell-wall; a primary cell-wall and a thickened secondary cell- wall (containing cellulose, hemicellulose, lignin etc.). • The cells are rigid and nonstretchable. • The function of sclerenchyma is similar to the collenchyma tissue, which is giving mechanical support and tensile strength to the plants.

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• Sclerenchyma matures with the surrounding tissues and provides more permanent support than collenchymas. • Sclerenchyma cells function as a “Skeleton” of the plant system that contributes rigidity to withstand against various ecological stresses. • During the initial growth cycle of a plant, the sclerenchyma persists as a living cell and found as a spiral or ring patterns. • 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. • They are usually found in nongrowing regions of plant bodies, such as the bark or mature stems. • The cell-wall type, rigidity, shape, size etc. of sclerenchyma will vary accordingly, within different types of plant. • Mechanical and conductive sclerenchymatous tissues are two common types, based on the function of sclerenchyma. • Fibres, sclereids and tracheary elements are the three common types, based on the morphology of sclerenchyma tissue.

Sclereids

Sclereids are defined as a mechanical tissue that involves the following features: 1. They occur singly or in groups. 2. Sclereids are found associated with the plant’s vascular tissue, namely xylem and phloem. 3. Their 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 sclereids subdivide into the following classes: • Macrosclereids It also refers as “Malpigian cell”. Appearance: Elongated and columnar in shape. Occurrence: Usually found in the outer epidermal cells of seed. Example: Seed coat of Pisum species.

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• Osteosclereids It is also refered 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. 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. 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. Occurrence: It is commonly present in the fleshy portions of fruit. Example: Flesh of pear fruit, where brachysclereids form grit and also refers as stone cells.

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• 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. 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. Occurrence: Found in the specialized tissues of leaves. Example: Leaves of Olea.

Function of Sclereid • Sclereids support the neighboring tissues where they occur. • It protects the inner cells, by forming a concentrating layer towards the periphery. • They also occur in leaves and fruits and constitute the hard shell of nuts and the outer hard coat of many seeds. • Sometimes known as stone cells, sclereids are also responsible for the gritty texture of pears and guavas.

………………………… mechanical tissue- part 2

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