Collenchyma and sclerenchyma are supporting tissues in plants. They are composed of cells with thick cell walls that whithstand mechanical forces. Although both tissues share the same function, they show different location in the body plant and their cell walls have different structure and texture. However, in plants with large aerial bodies, the structural support function relies mainly on vascular tissues.
Collenchyma is a living tissue made up of just one cell type, the collenchyma cell, which has a thick cell wall, elongated shape, and with its major axis parallel to the major axis of the organ where it is located. The thick primary cell wall has thickenings distributed in different parts of the cell surface, providing the tissue with great resistance against mechanical stresses. The lack of secondary wall allows the cell to growth in both surface and diameter. Like parenchyma cell, collenchyma cells can restart meristematic activity. Sometimes it is difficult to figure out if a cell is a collenchyma or parenchyma cell since they both may dedifferentiate, leading to meristematic activity. Collenchyma does not usually contain chloroplasts, but is a translucent tissue that allow light to reach deeper photosynthetic tissues.
Collenchyma is not a widespread tissue in the body of the plants because it is usually not present in the roots, except aerial roots, nor in structures with advanced secondary growth, where it is replaced by sclerenchyma. Collenchyma is observed as supporting tissue in growing organs of numerous herbaceous and woody plants, and in mature stems and leaves of herbaceous plants, including those starting secondary growth. Collenchyma provides support for the grow of herbaceous stems, leaves and flowers of dicotyledon plants. It is absent in most monocotyledon plants. In stems and petioles, the collenchyma is at peripheral positions, where it better performs its function, just below the epidermis or separated from epidermis by one or two layers of parenchyma cells. It forms a continuous cylinder or discontinuous strips. There are also collenchyma cells associated with vascular bundles that some authors regard as a special type of collenchyma referred as fascicular collenchyma, although not all authors do agree.
Besides cellulose, cell walls of collenchyma cells contain a large amount of pectin and hemicellulose, and together provide both strength and flexibility. The name collenchyma (from the Greek word "colla": glue, gum) are because of these features. Furthermore, collenchyma cells are alive, so that they can modify the thickness and composition of their cell walls. Therefore, it is an excellent support tissue for organs that are changing their size and shape because it can respond to the needs of each structure of the plant.
There are different types of collenchyma depending on the thickenings of their cell walls. Angular collenchyma: the thickenings of the cell walls are located in the angles or corners of the cells and there are no intercellular spaces. Lamellar collenchyma: the thickenings are in the outer and inner tangential walls. Lacunar parenchyma: it contains intercellular spaces and the cell wall thickenings are near to these intercellular spaces. Annular collenchyma: it shows uniform thickening around the cell. However, there are intermediate forms which are difficult to classify.
Sclerenchyma,unlike collenchyma, has two types of cells showing thick cell wall, with secondary cell wall in mature cells. The word sclerenchyma derives from "skleros" (Greek), meaning hard, rough. Mature sclerenchyma cells do not contain cytoplasm and are dead cells. Because of the structure of their cell walls, sclerenchyma has a relevant function in the support of organs that do not grow anymore. It protects the softer and more vulnerable parts of the plant against stretching, weights, pressures, and bending. That is why, although sclerenchyma is distributed throughout the body plant, both in organs having primary or secondary growth, it is most abundant in stems and leaves than in roots.
Fibers and sclereids are the two cell types found in sclerenchyma. They are distinguished by their shape, origin and location. Fibers are fusiform cells, whereas sclereids have different shapes, although they are typically more isodiametric than fibers. The origin of these two cell types is not clear but fibers are proposed to be originated by differentiation of meristematic cells, whereas sclereids appear after the lignification of the cell wall of parenchyma cells.
Fibers are fusiform and elongated cells, with sharp ends and secondary wall showing variable thickness and different levels of lignificación. In the vascular tissues, these features are modulated by hormones such as auxins and gibberellins. Mature fibers may have such a thick cell wall that it can occupy the entire internal cell space. Most of the mature fibers are dead cells, although some dicotyledon plants can have living fibers in the xylem. Fibers from leaves of some monocots are commercially important in the manufacture of clothing and other fabrics. Because of their tensile strength, fibers are usually ordered and compressed in strands to make the commercial fibers.
Fibers can be classified according to their location in the plant body. Extraxylary fibers are found in the phloem (phloem fibers), in the cortex (cortical fibers), and around vascular bundles (perivascular fibers), and xylary fibers are found in the xylem.
Sclereids show very thick and lignified secondary walls that are often perforated by clearly visible pits. he cell shape can be isodiametric, star-like, branched, or with other morphologies. They are widely distributed among angiosperms but are more abundant in dicotyledon than in monocotyledon plants. They are found in stems, leaves, fruits and seeds, both scattered and forming layers. Sclereids are named according to their shapes: astrosclereids, brachysclereids, also called stone cells, macrosclereids, osteosclereids and trichosclereids.
Not all the functions of sclereids are known. In many tissues, besides having a mechanical function, they have a protective role against herbivores. Although more specific functions have been proposed such as directing water to the epidermis of leaves or even as a propagating medium for the light (acting as optical fibers) to increase light intensity in the leaves. Sclereids often appear late during the body plant development and are differentiated from parenchyma cells, followed by an intrusive grow that allow them to penetrate the intercellular tissue spaces.
Leroux O. 2012. Collenchyma: a versatile mechanical tissue with dynamic cell walls. Annals of botany. 110: 1083-1098.