A tissue is a group of cells that are similar in structure and work together to perform a specific function. The study of tissues is called histology. In multicellular organisms, cells are organised into tissues, tissues into organs, and organs into organ systems. Tissues reduce the workload of individual cells by division of labour. There are two main types: plant tissues and animal tissues.

Meristematic tissues consist of actively dividing cells found in growing regions. They have thin cell walls, dense cytoplasm, large nuclei, and no vacuoles. Types: (1) Apical meristem – at tips of roots and shoots, causes elongation. (2) Lateral meristem (cambium) – increases girth of stem and root. (3) Intercalary meristem – at base of leaves/internodes, common in grasses.

Permanent tissues are derived from meristematic tissues that have lost the ability to divide. Simple permanent tissues: (1) Parenchyma – living, thin-walled, loosely packed, stores food; in leaves it is chlorenchyma (photosynthesis). (2) Collenchyma – living, irregularly thickened corners, provides flexibility and mechanical support, found in leaf stalks. (3) Sclerenchyma – dead, thick lignified walls, provides hardness and stiffness; found in coconut husk, jute.

Complex permanent tissues are made of more than one type of cell. (1) Xylem – conducts water and minerals upward; components: tracheids, vessels, xylem parenchyma, xylem fibres. Vessels and tracheids are dead. (2) Phloem – conducts food (sugar) from leaves to all parts; components: sieve tubes, companion cells, phloem parenchyma, phloem fibres. Sieve tubes are living but have no nucleus.

Epithelial tissue covers body surfaces and lines cavities. It has no intercellular spaces and rests on a basement membrane. Types: (1) Simple squamous – flat cells, lines blood vessels and alveoli, allows diffusion. (2) Cuboidal – cube-shaped, found in kidney tubules and salivary glands, secretion and absorption. (3) Columnar – column-shaped, lines intestine, absorption. (4) Ciliated – columnar with cilia, moves mucus, lines respiratory tract. (5) Glandular – secretes substances.

Connective tissue connects and supports other tissues. It has cells scattered in a matrix. Types: (1) Areolar – loose, fills spaces between organs, contains collagen and elastin fibres. (2) Adipose – stores fat, insulates, cushions organs. (3) Dense/Fibrous – forms tendons (bone to muscle) and ligaments (bone to bone). (4) Cartilage – solid but flexible matrix, found in ear, nose, trachea. (5) Bone – hard matrix of calcium phosphate, rigid. (6) Blood – fluid connective tissue; plasma + RBC + WBC + platelets.

Muscular tissue: (1) Striated/Skeletal – voluntary, attached to bones, multinucleated, shows striations, fatigues quickly. (2) Smooth/Unstriated – involuntary, found in gut, blood vessels, spindle-shaped, single nucleus. (3) Cardiac – involuntary, found only in heart, striated, never fatigues, intercalated discs connect cells. Nervous tissue: made of neurons (nerve cells). A neuron has a cell body (cyton) with dendrites and a long axon. Transmits electrical impulses throughout the body.

A tissue is a group of cells that are similar in structure and work together to perform a specific function. The study of tissues is called histology. In unicellular organisms (like Amoeba), a single cell performs all life functions. In multicellular organisms, division of labour exists — different groups of cells are specialised to carry out different functions. This specialization is more in animals than in plants. Robert Hooke first used the word "tissue" and Xavier Bichat is considered the father of histology.

Meristematic tissue consists of actively dividing cells found in growing regions of plants. Characteristics: thin cell wall, dense cytoplasm, large nucleus, no vacuoles, cells tightly packed (no intercellular spaces). Types based on location: (1) Apical meristem — at root tips and shoot tips; responsible for increase in length (primary growth). (2) Lateral meristem (cambium) — in the lateral sides of stem and root; responsible for increase in girth (secondary growth). (3) Intercalary meristem — at internodes and leaf bases; helps in regeneration of damaged parts. Meristematic cells eventually lose their ability to divide and differentiate into permanent tissue.

After meristematic cells differentiate, they form permanent tissues. Simple permanent tissues have one type of cell. (1) Parenchyma — loosely packed cells with large intercellular spaces; thin cell wall; performs photosynthesis (if contains chloroplasts — then called chlorenchyma), storage and secretion; most common tissue. (2) Collenchyma — elongated cells with thickening at corners (pectin + cellulose); provides mechanical support and flexibility; found in leaf stalks (petioles); allows bending without breaking. (3) Sclerenchyma — dead cells at maturity; very thick lignified walls; no intercellular spaces; provides maximum rigidity and strength; found in seed coats, nut shells, husk of coconut. Two forms: fibres (long) and sclereids/stone cells (short, irregular).

Complex permanent tissues have more than one type of cell working together as a unit. (1) Xylem — conducts water and minerals from roots to leaves (ascent of sap). Components: tracheids (long, narrow, dead), vessels (wide, dead — most efficient water conductors), xylem parenchyma (living, stores starch), xylem fibres (support). Wood is made of xylem. (2) Phloem — transports prepared food (sugars) from leaves to all plant parts (translocation). Components: sieve tubes (living, lack nucleus at maturity), companion cells (nucleated, control sieve tubes), phloem parenchyma (storage), phloem fibres (support, dead). Key difference: xylem is dead (except parenchyma); phloem has more living cells.

Epithelial tissue forms the outer covering of the body and lines the inner surfaces of organs. It is the protective tissue of the body. Features: cells closely packed, very little intercellular matrix, rests on a basement membrane, avascular (no blood vessels — nourished by diffusion). Types: (1) Squamous epithelium — flat, scale-like cells; forms skin surface, mouth lining, blood vessel walls; protects against mechanical injury. (2) Cuboidal epithelium — cube-shaped; found in kidney tubules, salivary glands; involved in secretion and absorption. (3) Columnar epithelium — tall, column-like; found in intestine lining; absorption and secretion. (4) Ciliated epithelium — columnar cells with cilia; in respiratory tract; moves mucus. (5) Glandular epithelium — secretes substances (sweat, hormones).

Connective tissue connects and supports other tissues. It has cells scattered in a large amount of non-living matrix (can be fluid, solid or gel). Types: (1) Areolar tissue — loose connective tissue; fills spaces between organs; contains fibroblasts, macrophages, mast cells; matrix has collagen and elastic fibres. (2) Adipose tissue — fat-storing cells (adipocytes); found under skin and around organs; insulation and energy reserve. (3) Bone (osseous tissue) — hard matrix of calcium salts; cells called osteocytes in lacunae; provides support, protection, and movement. (4) Cartilage — flexible, semi-rigid matrix (chondroitin); cells called chondrocytes; found in ear pinna, nose tip, between vertebrae. (5) Blood — fluid connective tissue; matrix called plasma; contains RBCs, WBCs, platelets; transports materials.

Muscular tissue enables movement by contraction and relaxation. Muscle cells are called muscle fibres. Three types: (1) Striated (Skeletal/Voluntary) muscle — long, cylindrical, multinucleated fibres with alternating light and dark bands (striations); attached to bones; under conscious control; fast but tires quickly; e.g., biceps, thigh muscles. (2) Smooth (Non-striated/Involuntary) muscle — spindle-shaped, uninucleated, no striations; found in walls of hollow organs (stomach, intestine, blood vessels, uterus); not under conscious control; slow but sustained contractions. (3) Cardiac muscle — cylindrical, branched, uninucleated; faint striations; only in heart walls; involuntary but never tires; intercalated discs connect cells for coordinated contraction.

Nervous tissue is responsible for transmitting electrical signals (nerve impulses) throughout the body. The basic unit is the neuron (nerve cell). Structure of a neuron: (1) Cell body (soma/cyton) — contains nucleus and most organelles. (2) Dendrites — short branched extensions that receive signals from other neurons. (3) Axon — long single projection that transmits signals away from the cell body; covered by myelin sheath (a fatty layer formed by Schwann cells) which speeds up conduction. (4) Axon terminals/synaptic knobs — release neurotransmitters to pass signals to the next neuron or effector across a synapse. Types of neurons: sensory (afferent — carry signals to brain), motor (efferent — carry signals from brain to muscles/glands) and relay/interneurons (in brain and spinal cord). Nerve impulses travel at up to 120 m/s in myelinated neurons.