This is a simple tissue consisting of one type of cell only the structure and position of the cells indicate the primary supporting function. Collenchyma can be regarded as specialized parenchyma, since the two tissues are always found in close association and frequently blend imperceptibly with one another. Further, collenchyma cells may contain chloroplasts and can, on occasion, change function and become meristematic.
In general shape, the cells are elongated parallel to the longitudinal axis of the organ in which they occur and possess tapering end walls. The cross-section shows a comparatively small lumen or cavity. Shape is variable however, and there may be transitions from the elongated tapering structure to quite short prismatic cells where the collenchymas blends into adjacent parenchyma. By far the most constant distinguishing character of collenchyma cells is the structure of the walls. They are primary walls which are thickened irregularly by cellulose and pectic materials containing much water. This property causes them to shrink considerably when treated with dehydrating substances. There are three fairly clearly-recognizable patterns. In the first, sometimes known as angular collenchymas, the main deposition of wall material is in angles of the cavities, the remainder of the wall remaining thin. At points where several such thickened walls are in contact, the effect is quite unmistakable, for no other cells are thickened in this way. The second form is sometimes known as lamellar collenchymas and wall thickening in this case is largely confined to the tangential walls of the cells, the radial walls remaining comparatively thin. in the third form, lacunate collenchymas, large air spaces exist between the cells, and wall thickening is in general confined to the areas adjacent to the spaces. In longitudinal sections, the walls show the variability in the thickening according to the precise direction of the cut. In all cases the walls show primary pit areas through both thin and thickened regions. At maturity, the thickened cell walls still enclose living protoplasts, and these frequently include chloroplasts.
Collenchymas occurs as the first-formed supporting tissue in growing organs, particularly young stem, leaves and flower parts. It may persist as the main supporting tissue everywhere in mature dicotyledonous herbaceous plants which do not develop secondary tissues. Some monocotyledons do not form it at all. Its position is nearly always peripheral, immediately beneath the epidermis or only separated from it by one or two parenchyma layers. It may form one or more continuous cylinders in rounded stems and petioles, or separate bundles along the ribs of organs which are ribbed or angular in outline. Occasionally, even the outer epidermis may be collenchymatous in nature. In some leaves it may be found on the upper and lower sides of the veins and sometimes on the leaf margin.
Its flexible and plastic, i.e. mouldable, nature is admirably suited to the purpose it serves. Its plasticity allows it to stretch with the other rapidly elongating and expanding tissues of young parts without hindering their development. Stronger but less plastic tissues such as such as sclerenchyma would exert considerable forces to prevent such elongation and expansion of adjacent tissues. Collenchymas is also strong enough to withstand the bending and twisting strains to which young plant parts are constantly subjected by the external conditions. Once full differentiation of other tissues has ceased, the collenchymas becomes much harder, more brittle and less plastic. Finally, it becomes a sturdy mechanical tissue and may even become lignified with a secondary wall. In woody plants undergoing secondary
Thickening, it is often the case that the peripheral collenchymas keeps pace with expanding girth by active division and growth. At any time, collenchymas cells may revert to a more parenchymatous nature by diminishing their walls and becoming meristematic, particularly if injured. In a few cases, the periderm of woody plants has its origin in a peripheral layer of collenchyma.
Functions Of Collenchyma Tissues
Collenchyma tissues provide mechanical support and help in water absorption. They have an irregularly thickened primary cell wall and contain chloroplasts to store food.
They are found in strands or cylinders. They are one of the ground, or fundamental, tissues in plants along with parenchyma and sclerenchyma.
They can expand radially and tangentially when they absorb water. They also can get crushed during secondary growth.
Collenchyma tissues provide mechanical strength and structure to growing plant organs such as leaves, stems, and fruit. They also provide elasticity to these organs so that they can bend during growth and prevent breakage. They are long living cells with unevenly thickened cell walls that are made of pectin and cellulose. They are also able to expand and contract during growth. This property is unique to this tissue and is essential for the growth of plants.
The cellular wall of a Collenchyma is composed of cellulose and hexasaccharides. Hemicellulose is a polysaccharide with glue-like properties that can be used for cell stiffening, especially when combined with cellulose. Cellulose, on the other hand, is a linear insoluble polymer that consists of microfibrils. The hexasaccharides in the cellulose are linked together by a chain of glycans. The hexasaccharides can be further bonded by pectin to form stronger bonds. These bondings are responsible for the flexibility of the Collenchyma cell wall.
Unlike the sclerified mechanical tissues of wood and fibers, which have been well-researched, Collenchyma tissues have received less attention. As a result, the exact function of these tissues has not been fully understood. Furthermore, there is much confusion over the term “collenchyma.” The origin of the word is uncertain. Some authors suggest that it refers to the reversible meristematic state of the cell. Other authors use the term to describe a more general type of mechanical tissue.
While it is difficult to define the function of Collenchyma, it can be distinguished from other types of cell-wall-containing tissues based on the structure of its cell walls. These tissues are classified into two groups: Angular and Lamellar. Angular collenchyma is defined by the presence of thickened cell walls that are situated in angles or corners and lack intercellular spaces. Lamellar collenchyma is characterized by a more uniform thickening of cell walls.
In addition to their structural functions, Collenchyma tissues are also important for the regulation of vascular bundles in the phloem and xylem of the plant. The occurrence of a Collenchyma near the vascular bundles may help to ensure the proper transport of water and sugar from the root to the leaf. In addition, the elasticity of Collenchyma tissues provides a better mechanical support for growing parts like stems and petiole during wind stress.
Structure Of Collenchyma Tissues
Collenchyma tissue is a flexible supporting tissue that provides tensile strength to the plant organs like stems and leaves during growth. It also has superior mechanical efficiency compared to Sclerenchyma tissue, and this property has been attributed to its super flexibility during the elongation of the plant organs.
Collenchymal cell walls are layered with primary pit fields, with alternating zones of transverse and longitudinal cellulose microfibrils. The presence of a high concentration of pectin and hemicellulose in the wall may contribute to the collenchyma tissues’ flexibility. Hemicellulose is a polysaccharide, which can act as a glue or adhesive material and hemicellulosic acids such as xylans or mannans, provide rigidity to the cell walls.
The walls of the cells in collenchyma are usually thicker at the corners of the cell and can be circular, elliptical or elongated in shape, as seen in the stem of Cucurbita and the petiole of Sambucus. In cross section, they look similar to ground parenchyma, with the exception of the elongated shape and thickness at the cell corners.
During development, the walls of collenchyma thicken through a series of stages. During Stage 1 (Fig. 2a), the walls are thin with a slight thickening at the cell corners. As the wall matures, it becomes more elastic through a shift in the ratio of plastic to elastic strains. This transformation is accompanied by a change in the chemical composition of the cell wall, which includes less hemicellulosic acids and more cellulose microfibrils with visible lamellation.
In addition, a number of structural changes can occur during the maturation of the collenchyma wall. For example, immunocytochemical analysis has revealed that the distribution of some glycan epitopes is altered during the formation of the primary pit fields in collenchyma walls. Similarly, the wall is remodeled during elongation as a result of the accumulation of lignin and other phenolic compounds. Detailed analysis of these modifications will enhance our understanding of the relationship between structural-compositional information and collenchyma tissues’ biomechanical properties. The use of TEM and SEM in combination with other analytical techniques will be invaluable in this endeavour.
Composition of Collenchyma Tissues
The cell walls of collenchyma tissues are unevenly thickened and they contain high concentrations of pectin and hemicellulose. These materials provide strength and flexibility to this tissue. Collenchyma cells are also alive, which allows them to change their thickness and composition. This makes them perfect for supporting plant organs that are growing and changing their shape. The cells of this tissue can also elongate or shrink in length. The wall of these cells is made of cellulose microfibrils, hemicelluloses and pectic polysaccharides. The cellulose microfibrils are alternately transverse and longitudinal, or they can be oriented both ways. The cell walls of these tissues are also rich in primary pit fields and they can have lignin.
The cells of this tissue are arranged compactly and they have small or no intercellular spaces. These cells can be further divided into 4 types based on their structure and characteristics. They include Angular Collenchyma, Lacunar Collenchyma, Lamellar Collenchyma and Annular Collenchyma. Angular collenchyma has pronounced thickenings in the corners of the cell while lacunar and lamellar have intercellular spaces. The cells of this tissue have a uniform thickness in annular collenchyma.
In addition to providing elasticity to the plant, these cells are also responsible for storage of water. This is especially important in plants that grow in the desert because they need to store a lot of water to survive. These cells also have flatter chloroplasts than parenchyma cells, which allows them to absorb and retain water more easily.
During photosynthesis, these cells can absorb carbon dioxide and release oxygen, which is important for the process of respiration. They also help with the transfer of nutrients from the roots to the leaves. Because they are so crucial to the growth of a plant, it is important to protect them from predators and diseases.
The walls of these cells are thin, but they can elongate or contract to support the plant. They also have a high amount of cellulose, which helps the plant to stay rigid. They can also absorb water, which is why they are able to withstand high pressure.
Distribution of Collenchyma Tissues
Collenchyma tissues are scattered throughout the plant body. They form the bulk of ground tissue in soft parts, and they provide extra mechanical support to elongating organs such as root and flower shoots. It has also been shown that the presence of collenchyma can reduce the need for vascular tissue to be inserted into developing plants, as it provides more flexibility in these areas.
The morphology of the cell walls of Collenchyma cells are unevenly thickened and often have a lamellate appearance, which is why the tissue is sometimes called lamellated parenchyma. The wall may be impregnated with lignin, but it is usually absent (as in lacunar collenchyma). However, the cell walls of some species of Collenchyma have a lamellar structure in which layers of cellulose are alternated with layers of pectic polysaccharides, such as hexoses and galactoses.
There has been some confusion over the definition of whether a wall is primary or secondary. A more common view is that all wall materials that are laid down while the cell is still growing in area should be referred to as primary, whereas any material that is deposited after elongation has stopped should be considered secondary. However, this theory does not account for the fact that some of the hexose and galactoses in the walls of some types of Collenchyma have a tendency to become lignified, thereby making them more like a sclerenchyma than a parenchyma.
In addition, the distribution of pectic polysaccharides in the cell walls of some types of Collenchyma is not clear. Chafe (1970) stained colenchyma cell walls with pectin-specific dyes and found that the hexoses of the walls in some species were oriented at regular intervals, but the hexoses in other species were oriented irregularly. More recent histochemical studies using bright-field microscopy with methylene blue staining have tended to show a continuous distribution of the pectic polysaccharides in colenchyma cell walls, with lamellae that are rich in cellulose alternating with those that are rich in non-cellulosic polysaccharides, particularly hexoses and galactoses.
Labelling of Collenchyma cell walls with the anti-Lim1 antibody showed that, at Stages 1 and 2, the LM1 epitope was confined to the middle layer of the wall, and was weakly associated with cell corners (Fig. 5a-h, and Additional file 4: Figure S4). In contrast, at Stages 3 and 4, LM1 labelling was more widespread across the cell walls, with some weak labelling of the cell corners.