Supporting tissues: collenchyma and sclerenchyma
They are considered as specialized parenchyma, essential for land plants that grow in height. Supporting tissues are necessary in petioles that carry lamina often large, in the stems that carry leaves, flowers and fruit. They are also essential for the different organs that are exposed to mechanical strains.
They are composed of cells with thickened walls to ensure their function of support in organs whose volume and life conditions modifications need more support than cells turgescence to assure vertical growth of plants.
There are two support tissues: collenchyma and sclerenchyma that can be found simultaneously in a same organ.
Collenchyma is made up of one type cells, that are alive at maturity, with intercellular spaces and with relatively undifferentiated plastids. They are characterized by unevenly thickened cell walls relatively pliable. The thickening consists of cellulose and occurs during growth of the organ. The collenchyma is the supporting tissue of the primary body and is essential for mechanical support of young growing organs that must resist to elongation tension. Thickening occurs frequently in the corners of cells what is clearly seen in cross sections and is called angular collenchyma.
This tissue is commonly found beneath the epidermis of herbaceous stems or petioles, located in the outer ground parenchyma and sometimes in border of leaf veins. It is extremely rare in the root.
Sclerenchyma is the other supporting tissue composed at maturity of dead cells with extremely thick lignified secondary walls. As result it is essential for the support of organs that do not grow anymore. This tissue can be found in all plant’s organs except in the root. Thickening of the primary wall occurs by impregnation of lignin and results in a rigid and impermeable wall. It exists as two types of cells: sclereids and fibers that differ by their shape, their origin, and their location.
Fibres: elongate, fusiform, narrow cells, with a very narrow lumen. They occur singly or un small aggregates frequently associated to vascular bundles or in the cortical parenchyma of stems.
Sclereids: are often shorter than fibers, of greater size and with a variety of shapes: isodiametric, star-like, branched. They are generally isolated in other tissues but are sometimes found in small aggregates. Sclereids are found in stems, fruits and seeds and in leaves. They are present in leaves of some hydrophytes, with large lamina to maintain them at the water surface.