Conceptual Overview

Sclerenchyma cells vary greatly in regard to their origin, distribution, shape and structure. However, they may be classified into two categories, namely fibers and sclereids. Along with parenchyma and collenchyma, they constitute the third group of fundamental tissues. These are structural support cells that do not depend on turgor to remain rigid but rather on the presence of a secondary wall.

Fibers are derived from meristematic cells and differentiate early into elongated cells with few simple pits in their cell walls, and always appear in clusters. When pits do appear, they are usually slit-like. Fibers are found in various parts of plants, often in association with vascular tissues (particularly phloem in primary growth and secondary xylem), but also in the cortex and at other sites. Secondary phloem of tree bark also exhibits fibers.

Bundles of fibers are often found in the leaves of monocots where they provide support, particularly in large leaves of tropical plants. These heavily lignified bundles are often referred to as hard fibers. On the other hand, soft fibers may or may not be lignified, and are usually quite flexible. The terms "soft" and "hard" are commercial designations referring to the texture of the raw materials used in making products, and are not strict anatomical terms. The walls of fibers are rather elastic, a feature which allows them to return to their original shape after bending or stretching, although the cells themselves are usually rigid. Gelatinous fibers are found in the reaction wood of dicots, and have a non-lignified cell wall which is deposited over one or more layers of the secondary walls.

Sclerenchyma fibers are subdivided according to their localization into xylem (or woody) fibers and extraxylary elements. Libriform fibers and fiber-tracheids belong to the first category, and bast (or phloem) fibers belong to the second category. Some well-known examples of bast fibers are hemp for cordage, jute for cordage and coarse textiles, flax for linen thread and textiles, and ramie which is used for various textiles. Some phloem fibers are also used in making paper.

On the other hand, sclereids are developed from parenchyma cells that are secondarily modified. They usually deposit thick secondary walls that are heavily lignified. These walls typically stain red with phloroglucinol (as do fibers). When fully mature, sclereids frequently have the shape of parenchyma cells, and their walls have large numbers of simple pits. However, once mature, sclerenchyma cells cannot elongate. While fibers are especially characteristic of vascular tissues, sclereids on the other hand occur singly or in clusters in various portions of the plant body such as parenchyma of stems, seed coats, fruit pulp and in leaves (especially in the mesophyll). Development of sclereids appears to be dependent, at least in part, on plant hormonal (auxin) levels. Elevated levels of auxin (as may occur in wounding) may elicit the development of greater numbers of sclereids.

There are transient forms between fibers and sclereids. For example, leaf sclereids in olive are elongated like fibers, and contain only a few pits.