Read this essay to learn about:- 1. Origin and Distribution of Parenchyma Tissue 2. Cell Structure 3. Functions.
Essay # 1. Origin and Distribution of Parenchyma Tissue:
The parenchyma of cortex and the pith, of the mesophyll of leaves, and of the floral parts originate from the ground meristem and protoderm. The primary and secondary xylem and phloem parenchyma are formed by the pro- cambium and vascular cambium, respectively. The phelloderm originates from phellogen or cork cambium during secondary growth of the mature organs.
Parenchyma tissue is the ground tissue and makes up large portions of various plant organs. It forms cortex, medullary rays, pericycle and pith in stems and roots, epidermis, mesophyll tissue of leaves, pulp of fruits, embryo and endosperm of seeds. Parenchyma cells may also be the part of the primary and secondary xylem and phloem.
Essay # 2. Cell Structure of Parenchyma Tissue:
In fact, the internal structure of the parenchyma cells varies according to its function. Individual parenchyma cells usually are thin walled and made of cellulose. Primary pit fields may be present in the wall. In storage region the cell walls may be considerably thick due to deposition of hemicellulose, as formed in the endosperm of date-palm. The walls of such parenchyma cells, although thick, are primary walls.
Often thick and lignified walls are present in the xylem parenchyma, particularly in secondary xylem. Those cells are often called the sclerified parenchyma. The parenchyma cells possess a distinct nucleus and the cytoplasm is vacuolated.
The parenchyma cells in the cortex of stems and mesophyll tissue of leaves contain chloroplasts. Such chloroplast containing parenchyma cells concerned with photosynthesis are called chlorenchyma. Usually the non- green parenchyma cells contain leucoplasts.
The shape of the parenchyma cells is variable, though they are generally described as isodiametric. On isolation they are more or less spherical but in vivo they take different shapes as various forces act upon them due to growth. In fact, they are usually polyhedral with 14 sides or facets. Parenchyma cells may be oblong and arranged in parallel as in the palisade cells of the mesophyll tissue, in the medullary rays etc.
They may be variously lobed or folded as found in spongy cells of the mesophyll tissue. Star-like or stellate parenchyma cells are found in the stems with well-developed air spaces of Scirpus, Juncus etc. and in the mesophyll tissue of Canna.
Intercellular spaces within the parenchyma cells arise either by the splitting apart of the cell walls or by breakdown of the cells. Intercellular spaces are larger in many aquatic plants and form a continuous system throughout the plant body to give buoyancy and to withstand the mechanical stress in the aquatic environment. Such a tissue is often called aerenchyma.
There are certain specialized parenchyma cells containing oils, tannins, crystals of calcium oxalate etc. called idioblasts. They differ from the surrounding cells in size, contents and functions.
Essay # 3. Functions of Parenchyma Tissue:
The parenchyma tissue performs the following functions:
(i) Mesophyll tissue manufactures food through photosynthesis.
(ii) The xylem and phloem parenchyma play an important role in the movement of water in the non-living tracheary elements of xylem and in the transport of food in the sieve elements of phloem.
(iii) By the development of cutinised wall over the epidermis of stems, roots and leaves the parenchyma tissue protects the inner tissues from desiccation.
(iv) Aerenchyma in aquatic plants gives buoyancy, helps in gaseous exchange and withstands mechanical stress in aquatic environment.
(v) Parenchyma cells also help in secretion of various useful products like oils, nectar, resin etc.
(vi) Parenchyma cells store food and water.
(vii) Parenchyma tissue plays an important role in healing of wounds and regeneration.