Collenchyma is a cell and tissue type in which the primary walls are unevenly thickened and consist of homogeneous, more or less elongated living cells; it provides mechanical support to the plant organs where present.
i. Distribution:
Collenchyma cells are present at the periphery of herbaceous stems, petioles (e.g. Begonia) and in the ribs of some leaves (e.g. Nerium). They are also present in the floral parts, fruit and aerial root (ex. Monstcra). They usually occupy the peripheral layers of cortex in dicotyledons and may be present just beneath the epidermis or below a few peripheral layer of parenchyma.
They may be present as a continuous layer to form hypodermis. Sometimes the cells form distinct ribs or ridges at the outer edges of stems (e.g. Leonurus, Cucurbita etc.) and petioles (e.g. Apium). They are uncommon in roots, monocotyledonous leaves and stems. Subterranean roots of Vitis and Diapensia contain collenchyma. The rind of fruits is collenchyatous in Vitis and Cassia tora.
ii. Origin:
Collenchyma develops from the ground meristem or from procambium (e.g. Apium graveolens). They may also originate from elongated cells, which resemble procambium.
iii. Shape:
Isolated collenchyma ranges from round, elliptical to elongated cell Transitional forms are also present. The round or elliptical cells resemble ground parenchyma. The cells may assume the shape of a short prism. The elongated cells look like fibre with both ends tapered and may attain a length of 2 mm though rarely.
iv. Arrangement:
Collenchyma tissue appears to be more or less compactly arranged as the thickening materials deposit more heavily at the corners and on the radial walls of cell in addition to normal uniform thickening.
The nature of wall thickenings varies and accordingly the following three types of collenchyma are distinguished:
v. Types of collenchyma: (Figs. 8.2, 8.3):
(a) Angular:
The thickening materials deposit at the angles or corners of the cells. In cross sectional view the thickenings occur at those places where several cells meet. Example: petioles of Cucurbita, Beta etc.
(b) Lamellar:
The thickening materials deposit heavily on the tangential walls of the cell than the radial walls, ex. stem of Sambucus.
(c) Lacunar:
The thickening materials deposit at those places of the cell wall, which are in direct contact with the intercellular spaces. Example: petioles of Salvia, Malva etc.
Duchaigne (1955) reported another type, called annular collenchyma in the petiole of Nerium (Fig. 8.3C). In this type the cell lumen appears to be more or less circular in cross sectional view. Fahn (1987) opined that this might be the case of mature collenchyma. It is observed that due to continued and heavy deposition of wall materials the angular appearance of the lumen may be lost.
vi. Structure and content:
The walls are thickened by high amounts of pectin and hemicellulose. Protein and cellulose are also present. Primary pit fields can be distinguished in the walls. The cellulose microfibrils are either oriented transversely or alternately transverse and longitudinal. The longitudinal microfibrils may also alternate with noncellulosic materials, as is revealed by electron microscopic study with the petiole of Apium graveolens.
The composition of these alternating layers varies. One layer is rich in cellulose and poor in pectin; the alternate layer is pectin-rich and cellulose-poor. It is now suggested that the visible lamellation of the cell wall of collenchyma is due to composition and orientation of microfibrils at alternate layers. Usually lignin is completely absent.
But in some cases the cellulose-rich layer may be impregnated with lignin (e.g. stem and petioles of Salvia officinalis, Viscum album, Medicago sativa etc.). Lignified lamellae may also be deposited on the wall. Thus the pecto-cellulosic wall of collenchyma may become sclerified.
Collenchyma contains living protoplast. Chloroplastids may also be present in some cells. The cells may also contain tannins.
vii. Function:
(1) The cells are extensible with a considerable degree of plasticity and so support the organs in which they occur.
(2) Chloroplast containing collenchyma can carry out photosynthesis.
(3) In some cases the peripheral thick walled collenchyma becomes thin and regains the meristematic activity, e.g., phellogen, the cork cambium, which divides to form the periderm.
(4) The sclerified collenchyma is the mechanical cell of mature plants.
(5) Collenchyma can serve as storage sites for defensive antibacterial compounds. The compounds are hydroxyproline-rich bacterial agglutinins found in Solanum tuberosum.