Chapter Notes: Tissues in Action

Introduction

Life of a multicellular organism begins with a single cell that divides repeatedly to form many cells. These cells become organised into a hierarchy: cells → tissues → organs → organ systems → organism. Groups of similar cells that work together to carry out a specific function are called tissues. Division of labour among tissues increases efficiency and allows complex life processes. For example, muscle tissue enables movement while nervous tissue carries signals for coordination.

Why are Plant and Animal Tissues Different?

Plants and animals differ in form, lifestyle and nutrition. These differences are reflected in the structure and function of their tissues. Important contrasting features include:

Tissues for Growth in Plants

Plants show three main types of growth:

• Increase in length (height of stem, length of roots) – due to apical meristem
• Increase in girth (thickness of stems) – due to lateral meristem
• Regrowth after cutting or grazing – due to intercalary meristem

Cells that divide actively to produce new cells for these growth types make up the meristematic tissue.

Apical Meristem – How do plants grow in length?

Apical meristems are regions of actively dividing cells at the tips of roots and shoots. Experiments (for example, with onion roots) show that growth in length occurs at the tip: if the tip is removed, elongation stops. This demonstrates that apical meristem cells are responsible for primary growth (increase in length).

Lateral Meristem – How do plants grow in girth?

Lateral meristems form a ring of dividing cells in the stems (and roots) of many plants, especially dicots. They divide to produce new cells both inward and outward, increasing the diameter of the stem. The alternating production of spring and autumn wood by the vascular cambium (a lateral meristem) produces annual growth rings; counting these rings gives the age of a tree.

Intercalary Meristem – How do plants regrow after being cut?

The intercalary meristem occurs at the base of internodes or just above nodes. It enables rapid regrowth of parts removed by grazing or cutting. This is why grass regrows after mowing and many hedges become bushy after trimming.

• Grass lawns grow back after mowing because of intercalary meristems at nodes.
• Hedges become bushy after cutting due to activity of intercalary or axillary meristems.

Key terms:

• Node: Point on the stem where leaves or branches arise.
• Internode: Portion of the stem between two successive nodes.

Characteristics of Meristematic Tissue Cells

Meristematic cells are adapted for continuous and rapid division:

• Small in size and isodiametric in shape
• Thin primary cell walls (no lignin)
• Large and prominent nucleus
• Dense cytoplasm with many organelles
• Vacuoles absent or very small
• Tightly packed with no intercellular spaces

Permanent Tissues

Some cells produced by meristems remain capable of division, while others lose this ability and become specialised to perform particular functions. Such specialised cells form permanent tissues. The process by which a meristematic cell becomes specialised is called differentiation.

Permanent tissues are classified into:

• Simple permanent tissues – composed of one cell type
• Complex permanent tissues – composed of more than one type of cell that work together

Internal Structure of a sunflower stem

Protective Tissue – Epidermis

The epidermis is the outermost layer of cells covering leaves, stems and roots. It is usually a single layer of closely packed cells that protects inner tissues.

• Cells often secrete a waxy layer called the cuticle which reduces water loss and provides protection.
• In plants of dry habitats the cuticle may be very thick to limit water loss.
• In roots, epidermal cells may form root hairs (extensions) to increase surface area for absorption of water and minerals.
• In leaves, specialised epidermal cells form stomata (pores) for gaseous exchange and transpiration; stomata control exchange of O2, CO2 and water vapour.
• Transpiration generates a pull in xylem vessels that helps the upward movement of water from roots to leaves.

Supporting Tissue – Simple Permanent Tissues

Three types of simple permanent tissues provide support and perform other functions:

Conducting Tissues – Complex Permanent Tissues

Complex permanent tissues are composed of more than one cell type and function together to conduct substances. The two main conducting tissues are xylem and phloem.

Xylem: Conducts water and dissolved minerals from roots to aerial parts and provides mechanical strength. Components include:

• Tracheids – long, tubular dead cells with thick lignified walls
• Vessels (xylem vessels) – wide tubular dead cells aligned end to end forming continuous tubes
• Xylem parenchyma – living cells involved in storage
• Xylem fibres – sclerenchymatous cells providing strength

Phloem: Transports food (mainly sugars) from leaves to other parts and participates in storage. Components include:

• Sieve tubes – long living cells arranged end to end; sieve plates at ends allow flow of sap
• Companion cells – living parenchyma-like cells closely associated with sieve tubes; help in loading/unloading of sugars
• Phloem parenchyma – storage cells
• Phloem fibres – provide mechanical strength

Vascular tissue: (a) xylem, and (b) phloem

Tissue Systems in Plants

Plant tissues are organised into three tissue systems that run throughout the plant body:

• Dermal tissue system – Outermost protective layer (epidermis and periderm in older stems).
• Ground tissue system – Fills the interior (mainly parenchyma, collenchyma, sclerenchyma) and performs storage, photosynthesis and support.
• Vascular tissue system – Conducting tissues (xylem and phloem) arranged in vascular bundles for transport of water, minerals and food.

Ready to Go Beyond

In young plants the outer protective layer is usually a single layer of epidermis. As stems mature, some cells beneath the epidermis regain the ability to divide and form the cork cambium (a lateral meristem). Cork cambium produces cork cells that are dead, tightly packed and impregnated with substances that make them impermeable to water and gases. The accumulation of cork and other tissues forms the bark of a tree.

• Cork cells are dead and help reduce water loss and protect the plant from mechanical injury and pathogens.
• Bark consists of cork, cork cambium and the secondary phloem produced by the cambium.

Animal Tissues

Animal cells also group together to form tissues that specialise for specific functions. Animal tissues are broadly classified into four major types: epithelial, connective, muscular and nervous tissues.

Epithelial Tissues – Structure and Functions

Epithelial tissue covers the body surface and lines internal cavities and ducts. Epithelial cells are closely packed with little intercellular material, forming protective and selective barriers.

Connective Tissues – Structure and Role

Connective tissues support, connect and protect other tissues and organs. They are distinguished by the nature of the extracellular matrix, which may be fluid, gel-like or solid.

Types of connective tissues

Muscular Tissues – Types and Control

Muscles generate force and produce movement. There are three types of muscle tissue:

Nervous Tissue – Sensing and Responding

Nervous tissue forms a communication network that senses stimuli, conducts impulses and coordinates responses. The basic cell of nervous tissue is the neuron, which has three main parts:

• Cell body – contains the nucleus and most organelles; integrates incoming signals
• Dendrites – short, branched projections that receive signals from other neurons
• Axon – a long fibre that transmits impulses away from the cell body to other neurons or effector cells; ends in axon terminals that make connections (synapses)

The Musculoskeletal System

The musculoskeletal system comprises bones, muscles, joints, cartilage, tendons and ligaments. It supports the body, enables movement, maintains posture and protects internal organs.

• Muscles pull on bones to produce movement; they work in antagonistic pairs (e.g., biceps and triceps) to produce opposite movements.
• Muscles are attached to bones by tendons, strong connective tissue bands.
• The adult human skeleton contributes about 12-15% of body weight.

Mucoskeletal System

Types of Joints

A joint is the junction where two or more bones meet. Joints allow different ranges and types of movements depending on their structure.

Types of Joints

Ready to Go Beyond

Stem cells in bone marrow are undifferentiated cells capable of producing the various types of blood cells. In a bone marrow transplant, healthy stem cells are transferred to patients suffering from disorders such as certain blood cancers (for example, leukaemia) or hereditary blood disorders (for example, thalassaemia) to restore normal blood cell formation.

Skeletal System

The skeletal system is the internal framework of bones that supports the body and protects internal organs. Major components include the skull, vertebral column and rib cage.

• Backbone (vertebral column) – formed by a series of small bones called vertebrae; supports the body and protects the spinal cord. Intervertebral discs of cartilage between vertebrae act as cushions and allow flexibility of the spine.
• Rib cage – twelve pairs of ribs protect the heart and lungs; ribs are attached to the vertebral column at the back and to the sternum at the front via flexible cartilage, allowing expansion of the thorax during breathing.

Bridging Science and Society

Yoga and Health

• Yoga involves physical postures (asanas), breathing exercises (pranayama) and meditation.
• Scientific studies indicate regular practice of yoga improves flexibility, posture and breathing, reduces stress and helps prevent lifestyle-related conditions.
• International Yoga Day is observed on 21 June each year to promote the benefits of yoga.
• Correct posture, balanced diet, regular exercise and yoga contribute to strong bones, healthy muscles and flexible joints.

Agrobacterium and Plant Genetic Engineering

• Crown gall disease is a plant disease characterised by tumour-like swellings on stems arising from uncontrolled cell division.
• The disease is caused by the bacterium Agrobacterium tumefaciens.
• Scientists studied how this bacterium transfers its DNA into plant cells and used this mechanism as a tool for plant genetic engineering.
• Agrobacterium is now used to introduce beneficial genes into plants for traits such as disease resistance, improved yield and production of valuable phytochemicals.

Think as a Scientist

From One Cell to an Organism: Totipotency

In 1958 F. C. Steward demonstrated that individual phloem cells from carrot roots can regenerate into whole plants when cultured in a suitable nutrient medium. These specialised cells first dedifferentiate (lose their specialised features), divide to form an undifferentiated mass of cells and then redifferentiate into roots, shoots and other tissues to form a complete plant. This remarkable capacity of a single mature plant cell to develop into an entire plant is called totipotency. Totipotency underlies techniques such as plant tissue culture and clonal propagation used in horticulture and crop improvement.

F. C. Steward’s Experiment on Phloem Cells of Carrot: Effects of Nutrient Medium on Growth

Composition of Nutrient Medium	Increase in Fresh Weight (mg) of Cells	Conditions
Solid medium + nutrients	Reduced	Light: Yes; Air: No
Liquid medium + nutrients	20% increase	Light: Yes; Air: Yes
Liquid medium + nutrients	Reduced	Light: No; Air: Yes

(a) Characteristics of phloem cells of carrot: The experiment shows that carrot phloem cells can grow and increase in fresh weight under appropriate conditions. Presence of air and light together with an appropriate liquid nutrient medium gave the best growth.

(b) Highest and lowest biomass combinations: The liquid medium with nutrients under conditions of both light and air produced the highest biomass (20% increase). The solid medium with nutrients (where air was apparently limited) produced the lowest biomass (reduced growth).

(c) Culturing animal cells vs carrot cells: Animal cells differ in physiological requirements and do not generally show the same totipotency as many plant cells; therefore the results cannot be directly generalised to animal cells.

(d) Commercial applications of totipotency study:

• Mass propagation of plants: Totipotency is employed to clonally multiply plants of desirable varieties through tissue culture.
• Genetic engineering: Totipotent cells can be used to regenerate whole plants after the introduction of desired genes in plant biotechnology and crop improvement.

Scientists Spotlight

• B. G. L. Swamy – An Indian botanist known for contributions to plant morphology and anatomy. His book Hasuru Honnu (in Kannada) blends science, culture and folklore. It details botanical excursions in the Western Ghats and received the Kendra Sahitya Akademi Award in 1978.
• Sipra Guha Mukherjee (with S. C. Maheshwari) – Pioneered plant tissue culture techniques including production of complete plants via anther culture on nutrient media. Their work contributed significantly to crop improvement and modern agricultural biotechnology.

Summary

• Tissues are groups of similar cells working together to perform a specific function.
• Plant tissues are classified into meristematic (actively dividing) and permanent (specialised) tissues.
• Meristematic tissues: Apical (increase in length), Lateral (increase in girth), Intercalary (regrowth after cutting).
• Simple permanent tissues: Parenchyma, Collenchyma, Sclerenchyma.
• Complex permanent tissues: Xylem (transport of water and minerals) and Phloem (transport of food).
• Animal tissues: Epithelial, Connective, Muscular, Nervous.
• Musculoskeletal system: bones, muscles, joints, cartilage, tendons and ligaments work together for movement and protection.
• Movement results from coordinated action of muscles and bones under nervous system control.