Botany - B.Ye. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter II. Histology
2.4. Mechanical (Sclerenchymatous) Tissues

In a living plant, most Cells, Tissues, and Organs are typically in a turgid state, effectively counteracting mechanical deformations and external forces. In addition, plants possess specialized mechanical tissues that provide structural strength and rigidity.

Based on their Structure and origin, mechanical tissues are classified into collenchyma, sclerenchyma, and sclereids (stone cells).

Collenchyma is a primary mechanical tissue located beneath the epidermis in the stems of dicotyledonous plants, as well as in leaf petioles and flower pedicels. Collenchyma cells are parenchymatous and retain their living protoplast, which allows the tissue to expand without hindering organ growth. A characteristic feature of collenchyma is the uneven thickening of its Cell walls.

Depending on the pattern of Cell wall thickening, collenchyma is categorized into angular, lamellar, and lacunar (or spongy) types. Angular collenchyma is the most common form, characterized by cell wall thickenings at the junctions where several cells meet. Lamellar collenchyma consists of cells that appear rectangular in cross-section, with significant thickening of the tangential walls while the radial walls remain thin. In lacunar collenchyma, cells separate at their corners during early development to form intercellular spaces, with wall thickening occurring strictly adjacent to these spaces.

Sclerenchyma is a mechanical tissue composed of tightly packed prosenchymatous cells with tapered ends. The Cell wall is impregnated with Lignin and uniformly thickened along its entire perimeter, leaving only the pit-fields unthickened. Ontogenetically, sclerenchyma can be either primary or secondary. Primary sclerenchyma develops from collenchyma during ontogeny, and in some cases—such as the strengthening elements of vascular bundles in the leaves of many dicots—it remains at this transitional stage. Sclerenchyma located in the phloem is referred to as bast fibers (or phloem fibers), whereas that found in the xylem is known as wood fibers or libriform fibers.

Sclerenchyma is present in the Water/115.html">Vegetative organs of all vascular plants, providing strength and resilience to stems and leaves. Certain plants with exceptionally strong and long bast fibers, such as flax, hemp, and ramie, are cultivated specifically for fiber harvesting.

Sclereids, or stone cells, occur in various plant organs, including leaves, fruits, and roots. They typically develop from parenchyma cells whose walls undergo significant thickening and lignification, often becoming impregnated with calcium and silicon compounds. The protoplast eventually dies, leaving only narrow pit canals. Isodiametric sclereids are the most common type, forming the bulk of nutshells and the hard endocarp of stone fruits such as cherries and plums.

Branched sclereids are found in the leaves of tea and camellia plants, where they are known as supportive cells or idioblasts.