Collenchyma and sclerenchyma are supporting tissues in plants. They are composed of cells with thick cell walls that withstand mechanical forces. Although both tissues share the same function, they show different locations in the body plant, and their cell walls have different structures and textures. However, in plants with large aerial bodies, the structural support function relies mainly on vascular tissues.
1. Collenchyma
The cell walls of collenchyma cells show birefringence when studied under polarized light. It indicates a high order of cell wall molecules. The primary cell wall contains many pectins and hemicelluloses, as well as cellulose. Together, they provide the resistance and flexibility to this tissue. In fact, the word collenchyma is derived from the Greek word "colla", meaning gum. In addition, collenchyma cells are able to develop and enlarge their cell walls, which makes this tissue very useful for growing tissues since it is able to be adapted to each organ of the plant.
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 the 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 growth of herbaceous stems, leaves, and flowers in 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 the epidermis by one or two layers of parenchyma cells. It forms a continuous cylinder or discontinuous strip. There are also collenchyma cells associated with vascular bundles that some authors regard as a special type of collenchyma referred to as fascicular collenchyma, although not all authors agree.
The different types of peripheral collenchyma are characterized by their cell wall thickenings (Figure 1). If the cell wall thickenings are irregular but no intercellular spaces are left, we have the angular collenchyma when the thickenings are in the vertices and the lamellar collenchyma when the thickenings are in the cell walls parallel to the organ surface. If there are intercellular spaces, we have the lacunar collenchyma when the cell wall thickenings are around these intercellular spaces and the annular collenchyma when the thickening is uniform in all regions of the cell wall. However, examples that are difficult to sort into these four types are often found.
2. Sclerenchyma
Sclerenchyma, unlike collenchyma, has two types of cells showing a thick cell wall, with a lignified 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 sclereides 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 originate from the 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 a secondary wall showing variable thickness and different levels of lignification. In the vascular tissues, these features are modulated by hormones such as auxin 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 the 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 into 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). In some stems with secondary growth, perivascular fibers may surround the vascular cylinder. Xylary fibers are found in the xylem and are classified as fibers-tracheids and libriform fibers. Both are elongated cells with thick cell walls. Fiber-tracheids show shapes between tracheids and libriforms. Mucilaginous fibers, or gelatinous fibers, show very thick cell walls but are unlignified.
Sclereids show lignified and very thick secondary walls, often showing pits. The cell shape may be isometric, star-like, branched, and so on. They can often be found in angiosperms, being more abundant in dicots than in monocots. Sclereids can be distributed in stems, leaves, fruits, and seeds, either alone or in layers (Figure 3). They have been traditionally classified as astosclereids, brachysclereids (also known as stone cells), macrosclereids, osteosclereids, and trichosclereids.
Sclereids are compact cells with a varied distribution in the plant body. Brachysclereids are the smallest, derived from parenchyma cells, and have a secondary cell wall. They are found in peduncles, roots, stems, seed coats, the fruit endocarp, and in the fleshy component of some fruits. Brachysclereids synthesize a type of edible lignin. Astrosclereids are also differentiated from parenchyma cells, and they are found in the leaf mesophyll with a support function. Osteosclereids and macrosclereids are found in the seed coats (in the testa) of some species. Macrosclereids are elongated cells present in the outer coats of some seeds, while osteosclereids are found in inner locations.
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 growth that allows them to penetrate the intercellular tissue spaces.
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Bibliography ↷
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Bibliography
Leroux O. 2012. Collenchyma: a versatile mechanical tissue with dynamic cell walls. Annals of botany. 110: 1083-1098.
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Parenchyma 