Sclerenchyma cells are hard and tough cells with thick wall that are usually lignified. The thickness is due to formation of secondary wall. The cells develop an extensive secondary cell wall that is laid down on the inside of the primary cell wall. Lignin, plus suberin and/or cutin make the wall waterproof as well. The secondary wall is impregnated with lignin, making it hard and impermeable to water. Thus, these cells cannot survive for long’ as they cannot exchange sufficient material to maintain active metabolism. Sclerenchyma cells are typically dead at functional maturity, and the cytoplasm is missing, leaving an empty central cavity.
The primary function of sclerenchyma cells is mechanical. It includes discouraging herbivory (hard cells that rip open digestive passages in small insect larval stages, hard cells forming a pit wall in a peach fruit), support (the wood in a tree trunk, fibers in large herbs), and conduction (hollow cells lined end-to-end in xylem with cytoplasm and end walls missing).
Commonly sclerenchyma cells are classified as follows.
These are usually long, spindle shaped structures, with tapering or blunt ends.They are arranged in groups and secondary thickening may account for 90% of the area of the cell with the narrow lumen. Sclerenchyma fibres are usually found forming protective structures in the root and the stem. They are also found associated with xylem and phloem.
Fibers are grouped into xylary fibers and extra xylary fibers.
Xylary fibers, also called wood fibers are parts of xylem and are longest among xylem elements. Extraxylary fibers are classified as – Bast or phloem fibers, cortical fibers and perivascular fibers (peripheral to the vascular bundles). These are lignified or non lignified.
On account of their elasticity, fibers enable the plant body to withstand various strains. Commercial fibers like jute, flax, ramie are extraxylary fibers.
These are shorter than fibers. Sclereids occur singly or in groups. Sclereids are commonly found in fruit wall, seed coat, epidermal scales, and occasionally found in cortex, pith, mesophyll and petiole of submerged aquatics.
There are many types of sclereids, most common amomg them are:
1. Asterosclereids are star shaped
2. Macrosclereids are rod shaped
3. Osteosclereids are bone shaped that are enlarged at their ends
4. Brachysclereids are isodiametric in shape