Question

Classification and characteristics of permanent tissues.

Answer 1

On the basis of composition, permanent tissues are classified as simple and complex tissues.

Simple Tissues: These are composed of similar permanent cells performing the same functions. Simple tissues are of 3 types:

1. Parenchyma: It is the most abundant plant tissue characterized by thin-walled isodiametric cells, which may be oval, round or polygonal in shape and have cellulosic walls. Cells of this tissue have a central large vacuole and a peripherally located nucleus. Parenchyma has loosely arranged cells and serves as food storage tissue.
2. Collenchyma: This tissue is made up of living cells that have cellulose, hemicellulose and pectin thickenings at the corners. The elongated cells of the collenchyma have central nucleus, peripheral cytoplasm and often possess chloroplast. Intercellular spaces are usually absent in the collenchyma.
3. Sclerenchyma: These are formed of dead cells having narrow lumen, little or no protoplasm and thick lignified cell wall. Sclerenchyma cells may be elongated and pointed or spherical, oval or cylindrical.

Complex Tissues: These tissues are made up of more than one type of cells that are arranged to work together. Complex tissues are generally vascular tissues and are of 2 types:

1. Xylem: This tissue is made up of 4 types of cells that are all dead, except one. 
   Dead cells: Tracheids - Elongated cells with lignified walls, wide lumen and tapering open ends. Tracheids also have unthickened areas called pits for conduction of water. Vessels - These are elongated tubes formed of many small cells having lignified walls. These cells are connected together through their transverse dissolved ends forming perforation plates.
   Xylem fibres: These are sclerenchyma fibres.
   Living cells: Xylem parenchyma- Living thin- walled cells with cellulosic walls.
2. Phloem: It is the food conducting channel in plants and is made up of 4 components:
   Sieve tube elements which are living cells placed end to end to form the elongated conducting channel, sieve tube. Sieve tube cells lack nucleus and have perforated end walls that form the sieve plate. Another type of cells called the companion cells is associated with sieve tubes through compound plasmodesmata between them. Companion cells are parenchymatous with large nucleus and dense cytoplasm. Phloem parenchyma are living elongated parenchyma cells with thin cellulosic walls and tapering ends. The wall of phloem parenchyma cells has pits for plasmodesmatal communication with other cells. Phloem fibres (bast fibres) are sclerenchymatous cells forming elongated, unbranched fibres with needle like apices.