The group of cells performing some collective function is called tissue. Tissues organize the body of plant for performing different function. Many types of tissues associate with each other to form tissue system. Vascular system is an example of tissue system. The tissues have following characteristics:
Tissues are composed of many cells. These cells may be of one type or different types.
The cells in a tissue are held by tight junctions.
All the cells in a tissue have similar organization.
All cells in the tissue are involved in same activities. Classification of tissues ‘there are two types of tissues:
Simple tissues: The tissues with similar type of cells are called simple tissues. Examples: parenchyma..collenchyma and sc lerenchyma.
Compound tissues: The tissues with different types of cell are called compound tissues. Example: Xylem and phloem. Xylem is composed of tracheal cell, vessel cell and parenchymatous cells. Phloem is composed of sieve cells. companion cells and parenchymatous cells.
The simple tissues with cells having thin and elastic Walls are called parenchyma. Parenchyma forms the main ground tissues. They are continuous throughout the body. They are present in root, stem and leaves.
Cell structure of parenchymatous cell
The parenchyma cells have active protoplast. They have prominent nucleus. These cells have thin primary and secondary elastic walls. Their %sans are chiefly composed of cellulose. These cells are closely packed. But intercellular spaces are produced by the dissolution of middle lamella. These air spaces are common for exchange of gases.
Aerenchymas have prominent intercellular spaces. The protoplast of the adjacent cells is connected by plasmodesmata.
Shapes of parenchyma cells
The parenchyma has different shapes.
- Polyhedral: These cells are mostly polyhedral in shape.
Stellate: These parenchyma are kund in the stems of plants. They have well developed air spaces between them.
Elongated: Elongated parenchymas are found in the palisade tissue of leaf.
Lobed: Lobed parenchymas are found in spongy and palisade mesophyll tissues of some plants.
Origin of parenchyma
Parenchyma are mostly primary in origin. But they are also produced as a result of secondary gro>>th.
Functions of parenchyma
Parenchyma cells are less specialized. But it performs major activities in the cell.
- Meristematic: They resume meristematic activity under certain special conditions. These conditions are wound healing. regeneration, formation of adventitious roots and union of grans
Photosynthesis: Some parenchymas are involved in photosynthesis. The parenchyma cells of mesophyll tissues of leaves are rich in chloroplast. They are called assimilatory parenchyma or chlorenchyma.
Storage: Parenck ma also store food in fruits and roots etc. They are called storage parenchyma. These parenchymas have many leucoplasts in their cells. These parenchyma stores starch. fats, oils and other granules. Storage parenchymas are common in the cortex of stem, root and seeds.
Ground tissues: Parenchyma acts as ground tissues in most of the tissues.
Secretory and excretory structure: In certain cases. parenchyma acts are secretor> and excretory structure.
Pigmented cells: Parench> ma cells of flowers and fruits contain chromoplast. It gives colours to petals and fruits.
Storage of water: The parenchyma cells of succulent plants store water. These cells are large with thin cell wall. They have thin layer of cytoplasm. These cells have large vacuoles. These vacuoles contain mucilaginous sap. This sap increases the water holding capacity of the cell.
The parenchyma cells with uneven angular thickening which support young growing part of plant are called collenchyman. Collenchyma tissues form separate strands or continuous cylinder near the surface of the stem cortex, in petioles and along the vein of the leaves. Collenchyma tissues are absent in roots. Collenchyma, tissues are also absent in stem and leaves of many monocot plants.
Collenchyma cells have living protoplasts. They are simple tissues and contain single type of cells. The cells are elongated with pointed and prismatic ends. They are capable of further growth and divisions. Collenchyma tissues also contain chloroplasts in green organs. Collenchyma form complete cylinder in stem. Collenchyma tissues are present in the margin of leaves.
Cell wall in collenchymatous cells
Their thickened primary wall is non-lignified. It is mostly composed of cellulose and poor in pectic substance. Therefore, it is elastic in nature. The angular thickenings are rich in cellulose. An additional layer of microfibrils is present inside the %all. In some cases, the cell N% all of collenchyma is sclerified. Sclerification occurs by the formation of lamella. The lamellae are rich in cellulose. Sometimes. these lamellae become lignified. Additional lamellae appear inside. It decreases the lumen of collenchymatous cells.
Size and shapes
Different collenchyma tissues have diflerent sizes and shapes. Mostly these cells are elongated with tapering ends. The longest collenchyma tissues are present in the central position. The shorter collenchyma tissues are present in the periphery.
Types of collenchyma
- Angular collenchyma: The thickenings in the wall of these cells are present in the angles of the cells. They are found in the petioles of some plants.
- Lamellar collenchyma: The thickenings in these tissues are – present on the tangential walls of the cell. These are present in the stem cortex of some plants.
- Lacu ar collenchyma: In these tissues thickenings are present in with faces intercellular spaces. Lacunar collenchymas are prex, In the petiole of composite species.