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Read this essay to learn about:- 1. Origin and Distribution of Collenchyma Tissue 2. Cell Structure 3. Functions.
Essay # 1. Origin and Distribution of Collenchyma Tissue:
Ontogenetically, collenchyma cells develop from certain elongated cells resembling procambium which are formed in the very early stages of differentiation of the meristem. Sometimes less specialised collenchyma originates from the ground meristem. Some researchers believe that the collenchyma originates jointly with the vascular tissues from the procambium.
Collenchyma occurs in the peripheral positions i.e. immediately beneath the epidermis in stems, leaves, floral parts, fruits and roots of dicotyledonous plants mainly. Collenchyma is not found in the stems of many mono- cots. It occurs as continuous band of solitary or many cell-layers thick hypodermis under the epidermis.
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It may also occur as isolated patches in the hypodermal region. Occasionally, the entire epidermis may be collenchymatous. In stems the collenchyma may also form a complete cylinder or remain in longitudinal strips. Collenchyma may occur in the cortex of light exposed roots. The collenchyma occurs on one or both sides of the vascular bundles and along the margins of the leaf blade in dicots.
Essay # 2. Cell Structure of Collenchyma Tissue:
Collenchyma cells are elongated in a plane parallel to the long axis of the organ in which they occur. The cells are living with vacuolated protoplast. Chloroplasts may also be present.
The most distinctive characteristic of the collenchyma cells is the cell wall which becomes unevenly thickened. There are different types of deposition, but commonly, the thickenings are confined to the corners of the cells. Often the degrees of deposition may be so much pronounced that cells look circular in cross- section.
The size and shape of the collenchyma cells vary greatly. The shorter ones are more or less like parenchyma and the longer ones resembling the fibres with overlapping tapering ends. Individual collenchyma cell may often attain lengths of 2 mm. They are usually polygonal in cross-section. The cell wall is made up of cellulose, hemicellulose and pectic materials, and contains much water (about 67% of the fresh weight).
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The amount of pectin and hemicellulose in cell walls is high. Like parenchyma it can undergo reversible changes and retain the capacity of cell division. Primary pit fields can be distinguished in the walls of collenchyma cells. Collenchyma cells may or may not contain intercellular spaces.
On the basis of the thickening of the cell wall and arrangement of cells three main types of collenchyma are recognised:
(i) Angular collenchyma:
It is the most common type where the deposition is localized at the corners or angles of the cells (Fig. 5.52A & B). The cells of this type of collenchyma are compact and irregularly arranged. This type of collenchyma is found in the stems of Datura, Dahlia, Cucurbita, Solanum tuberosum, Atropa belladona, etc. and in the petioles of the leaves of Vitis, Begonia, Coleus, Cucurbita, Beta, Morus, etc.
(ii) Lacunate collenchyma:
In this second type, the thickenings appear at the intercellular-space-facing parts of the cells (Fig. 5.52C). This type of collenchyma is also called tubular collenchyma. It is found in the petioles of Salvia, Malva, Althaea, Asclepias and in the members of Compositae.
(iii) Plate or Lamellar collenchyma:
In this type of collenchyma, cells are compactly arranged without intercellular spaces (Fig. 5.52D). Thickenings occur in various patterns mainly on the tangential walls of the cells. This type of collenchyma tissue is found in stems of Sambucus nigra and Rhamnus, etc. and in the petiole or Cochlearia armoracia.
Essay # 3. Functions of Collenchyma Tissue:
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The collenchyma tissue performs the following functions:
(i) Collenchyma cells give mechanical rigidity to the growing plant organs.
(ii) Due to the presence of collenchyma cells the plant parts or organs become flexible and shows plasticity.
(iii) Photosynthesis takes place in the chloroplast containing Collenchyma cells.
