Sclerenchyma






The permanent simple tissue consisting of evenly thick-walled dead cells are named sclerenchyma. They are very hard and heavily lignified in nature.

Structure of sclerenchyma:

The sclerenchyma cells show the following characteristics:
(1) The cells are heavily thickened with lignified walls, simple pits and small lumen.
(2) The cells are dead without protoplasm.
(3) The cell walls with very low water content.
(4) Their shapes and sizes vary.

Functions of sclerenchyma:

The sclerenchyma give rigidity and mechanical strength to plant organs. There are two types of sclerenchyma (1) Sclerenchyma fibres and (2) Sclereids or sclerotic cells.

Sclerenchyma fibre:

The fibre like elongated sclerenchyma cells-are called sclerenchyma fibres.
Origin : They originate from all the three types of meristematic tissues like protoderm, procambium and ground meristem. They may also be formed from the fusiform initials of cambium.

Types:

Depending on the nature, there are 3 types of sclerenchyma fibres, which are as follows :

(1)Extraxylary fibers: They remain outside the xylem tissue, normally within the secondary phloem called secondary phloem fibresor bastfibresor in the pericycle and hypodermis, called perivascular fibres, e.g. Fibres of jute (Corchoruscapsularis) ; Flax (Linumussitatissimum); Sun hemp (Crotalaria juncea).
(2)Intraraxyiary fiberes: iney remain wiinin me xyiem tissue ana are caileaxylem Ibresor wood fibres. They have bordered pits and on the basis of wall thickness, lature of pits, the wood fibes are of two types :




(a) Libriformfibres: They are hard, with well developed thickened secondary vails having reduced simple pits.
(b) Fibretracheids: They are intermediate between tracheids and libriformfibrestnd possess moderately thickened wall and bordered pit.
(3) Leaf fibres: The thickened fibres associated with the bundle sheath of monocot eaves, e.g. Manila hemp (Musa textilis); Sisal hemp (Agave sisalina). Structure of Fibres :
(i) Fibres are elongated with tapering ends,
(ii) They normally occur in a group.
(iii) They are very long, narrow and with pointed ends, the length may be upto 55 cm.
(iv) The lumen is very thin due to uniformly thickened, lignified walls.
(v) The T.S. of the fibreslook angular.
(vi) Matured cells are dead and devoid of chloroplast.
(vii) There are simple or bordered pits present on the side walls.

Sclereids:

The simple tissue of non-fibrous, short, irregular sclerenchyma cells are called sclereids.
They develop from unspecialized parenchyma cells. Depending on the nature, structure and form of cell walls, five different sclereids are found, which are :




(i) Macrosclereids: Elongated rod shaped sclereids forming a palisade like layer n the epidermis of seed coat e.g. pea and pulses.
(ii) Astrosclereids: They are irregularly branched star shaped sclereids found in he leaves of Nymphaea, Thea. These elongated, branched sclereids are also termed as fiber sclereids.
(iii) Osteosclereids: They are bone like sclereids with swollen ends, commonly found in the leaves of Xerophytes like Ficus and Hakea.
(iv) Trichosclereids: They are solitary, armed idioblastic sclereids found as rejected hairs in the aerial roots of Monostera.
(v) Brachysclereids or Stone cells : The isodiametric thick-walled parenchyma cells having a gritty nature and thus it is also called grit cells, found in the fruit co guava, apple.

Structures

(i) They are specialized lignified cells which may be both irregular or iso-diametric in shape.
(ii) The cells are dead i.e., without protoplasm and nucleus.
(iii) The thick secondary walls are striated and nearly block the lumen.
(iv) They may contain tannin and mucilage.
(v) The walls contain simple pits.
They occur singly or in groups in the soft tissues like pith, phloem flesh of fruit and also in seed coat and fruit walls.They provide mechanical support to the plant body.



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