Sclerenchyma
Sclerenchyma (scleros: hard) is composed of long, narrow and thick cells. These cells develop very thick walls due to deposition of lignin.
Mature sclerenchyma cells cannot elongate, and they occur in regions of the plant that have stopped growing in length. At maturity, most sclerenchyma cells are dead and function in structure and support to plant parts. Sclerenchyma cells are usually found associated with other cells types and give them mechanical support. The rigid walls remain as a "skeleton" that supports the plant, in some cases for hundreds of years. In parts of the plant that are still elongating, the secondary walls of immature sclerenchyma are deposited unevenly in spiral or ring patterns. These forms of cell wall thickenings enable the cell wall to stretch like a spring as the cell elongates. Sclerenchyma is found in stems and also in leaf veins. Sclerenchyma also makes up the hard outer covering of seeds and nuts.
Sclerenchyma is found in the stems and also in the midribs of leaves.
Two types of sclerenchyma cells called sclereids and fibers are specialized entirely for support and strengthening.
Sclereids are randomly distributed throughout other tissues. Sometimes they are grouped within other tissues in specific zones or regions. Sclereids, which are shorter than fibers and irregular in shape, have very thick, lignified secondary walls. Sclereids impart the hardness to nutshells and seed coats and the gritty texture to pear fruits. The grittiness is due to groups of sclereid cells. Sclereids are sometimes called stone cells.
Fibers, which are usually arranged in threads, are long, slender, and tapered. Fibers are sometimes found in association with a wide variety of tissues in roots, stems, leaves and fruits. Fiber cells have a very tiny cavity in the center of the cell. Fibers are used commercially, fibers from over 40 different plant families are used in the manufacture of textiles, ropes, string and canvas goods etc.