Tissue Culture Plants: The body of all living beings is made up of cells.

All the biological activities of a single-celled organism take place inside one cell, such as Amoeba and Paramecium.

With the increase in division of labour, cells start gathering in groups and form tissue.

Definition: A group of cells whose origin, structure, and function are similar is called tissue.

Histology: The study of tissues is called histology.

(I) Apical Meristem:

Found in the tip of the stem and root, responsible for growth in length.

(II) Intercalary Meristem:

Found in between permanent tissues (at the base or joint part of the leaf). It also increases length.

(III) Lateral Meristem:

Occurs in the lateral parts of the root and stem and undergoes secondary growth. This forms vascular tissues and increases the thickness of the stem and root.

(i) Parenchyma

Parenchyma cells are living, oval, round, multi-lobed, or irregular in shape. Found in various parts of the plant like root, stem, and flower.

Functions:

• To collect and store food and water.
• Intercellular space present in tissues helps in gaseous exchange.
• Chloroplast is found in chlorenchyma, which produces food through photosynthesis.
• Acts as protection for the plant in the form of epidermis.
• Stores excretory substances like gum, resins, and tannins.

(ii) Collenchyma

Macro-angle cells that are long, living, contain a nucleus, and have vacuoles. Generally found under the epidermis of petiole and stem.

Pectin-mixed cellulose is deposited in the intracellular spaces or on the granules of cells.

Functions:

• Provides mechanical support to plants.
• Stores food.
• Produces food when chloroplast is present.
• Supports organ enlargement.

(iii) Sclerenchyma (Hard Tissue)

Dead permanent tissue with uniform accumulation of lignin around its cells. Cells are long, narrow, and pointed at both ends.

Generally found in the stem, leaf veins, seed coat, and coconut fiber.

Functions:

• Provides structural mechanical support to the plant.
• Forms a protective hard covering over seeds.
• Root forms cork cells with growth in the stem.

(1) Tracheids:

These are long, cylindrical dead cells whose walls contain an accumulation of lignin. Vessels/elements are found in primary and secondary xylem.

Function: Providing mechanical support to the plant and transportation of water and mineral salts.

(2) Vessels:

These are long, tubular dead cells with lignin deposited in their walls. Found in the primary and secondary xylem of angiosperm plants.

Function: Providing mechanical support to plants and transportation of water and mineral salts.

(3) Xylem Fibers:

These are long, conical, dead cells containing lignin.

Function: They provide mechanical support to the plant.

(4) Xylem Parenchyma:

Living cells present in xylem.

Function: Stores food items.

(i) Sieve Tubes:

Sieve tubes are long, cylindrical cells located one above the other. The dividing wall between two cells is porous and is called a sieve plate/strip. A nucleus is not found in sieve tubes in the adult stage.

Function: Transports the food produced by the leaves.

(ii) Companion Cells:

These are cells with thin walls present alongside sieve tubes in their lateral parts. Every companion cell is alive and contains a nucleus. Present mainly in the phloem of angiosperms.

Function: Helps in the transportation of food substances through the tubes and stores food items.

(iii) Phloem Parenchyma:

Thin-walled, living cells located near the sieve tubes.

Function: Storing food items.

(iv) Phloem Fibre:

This is the non-living (dead) part of phloem with thick walls.

Function: Provides mechanical strength and flexibility to the plant.

(a) Simple Epithelial Tissue:

It consists of a single layer of cells resting on a non-cellular basement membrane. It functions primarily in absorption, secretion, and filtration.

Sub-types of Simple Epithelial Tissue:

• Squamous Epithelium: Thin and flat scale-like cells. Found in air sacs of lungs and blood vessels for rapid diffusion.
• Cuboidal Epithelium: Cube-like cells with central spherical nuclei. Found in kidney tubules and salivary glands for secretion and absorption.
• Columnar Epithelium: Tall, column-like cells. Found in the lining of the stomach and intestines for absorption of nutrients.
• Ciliated Epithelium: Columnar or cuboidal cells having hair-like cilia on their free surface. Found in respiratory tracts to move mucus.

(b) Compound Epithelial Tissue (Stratified):

It consists of multi-layered cells (more than one layer thick). Due to its thickness, its primary function is protection against mechanical and chemical stress, rather than absorption or secretion.

Examples & Location: Found in the skin (epidermis), lining of the mouth (buccal cavity), pharynx, and esophagus.