Long questions answers of Chapter 4 | Cell and Tissues | Biology Class 9 Notes

Define tissue. Describe different types of plant tissues in detail.

Tissue

A Group of cells which perform a particular function is known as tissue.

Types of plant tissues

All the plant tissues can be categorized into the following two types

A. Simple tissue

B. Compound tissue

A. Simple tissues

Those tissues which are composed of only one type of cells performing a common function are called simple tissues.

Classification of simple tissues

Simple tissues are further classified into the following two types

1. Meristematic tissue (also called embryonic tissue)

• These tissues are composed of cells that have the power of cell division, so they produce new cells. 
• They are present at the apex of the root or shoot.
• Each cell of these tissues has dense cytoplasm, a large nucleus, and small or no vacuoles. 
• These cells are the same with no intercellular spaces, characterized by thin walls and central nuclei. 

Functions of meristematic tissue

a. Primary growth

Apical meristems at the apex of root and shoot divide and redivide to add primary tissue for elongation of root or stem. Such growth is known as primary growth.

b. Secondary growth:

Meristematic cells are also found on the lateral sides of the root and stem as lateral or intercalary meristem or cambium. They divide and add secondary tissues which increase the thickness of a stem and root. This is known as secondary growth.

2. Permanent tissue

These tissues lack the ability to divide. They originate from the primary meristem. The cells have large central vacuoles.

Types of permanent tissue

Permanent tissues are of three types

a. Epidermal tissues

These tissues are present at the outermost covering of the leaf, root, and stem. They have thick-walled cells that are closely packed with no intercellular spaces.

• The walls of these cells, in the stem, are thick and contain a waxy layer to prevent transpiration. 
• The epidermal tissues, in the leaves, have small openings called stomata, used for gaseous exchange.

b. Ground tissues

• They are present in all parts of the plant body except the places where there are epidermal tissues and vascular tissue. 
• The cells of these tissues store food and also prepare some food. 

Composition of ground tissues

Ground tissues are composed of parenchyma cells

Parenchyma cells

• These cells are oval, spherical, or polygonal in shape. They are thin-walled and made of cellulose. 
• They are usually living. Parenchyma is found in all the soft parts of plants. 
• Its main function is to store food material.

c. Supporting or mechanical tissues

Supporting tissue provides support, rigidity, and strength to the plant body.

Types of supporting tissue

There are two types of supporting tissues

1. Collenchyma tissues

• They are living cells. 
• They are elongated and polygonal having tapered ends. 
• They are more flexible than sclerenchyma. 
• They are found in the young herbaceous stems. 
• They provide strength to the plants.

2. Sclerenchyma tissues

They are dead cells and thick-walled. The thickness is due to lignin deposition. They lack protoplasm.

B. Compound tissues/ Complex tissues

Those tissues which are composed of more than one kind of cell that perform a common function are called compound tissues.

Compound tissues include

a. Xylem tissue

This tissue carries water and dissolved substances throughout the plant body. It consists of the following types of cells

Vessels: They are long tubes made up of individual cells which are open at each end. 

Tracheids: They have thick secondary cell walls and are tapered at the ends. They do not have end openings like vessels. They are with pairs of pits that allow water to pass from cell to cell.

Fibers: They are elongated thick-walled cells with tapering ends. They are dead at maturity. They function as supportive tissue in plant stems and roots.

b. Phloem tissue

This tissue is responsible for the conduction of food. It is made up of three types of cells i.e.

Sieve tube cells: The ends of these cells have small pores which are called sieve plates. A mature sieve tube has no central vacuole. Their function is to transport food through the cytoplasm and sieve plate.

Companion cells:         The sieve tube cells are accompanied by nucleated cells called companion cells. Their function is to regulate the transport of food through the sieve tubes.

Phloem parenchyma: These are parenchyma cells present in the phloem which store the extra amount of food.

Explain different types of animal tissues.     

Animal tissue and its types

Animal tissues are classified into four major types on the basis of their structure and functions, these are

1. Epithelial tissues

• The skin separates the body from the outside world which forms a continuous sheet of cells known as the epithelium. 
• These cells are elongated and flattened. 
• They also line the gut, lungs, and urinary tract.

2. Connective tissues

Connective tissue is a major supporting tissue of the animal body. Its cells are scattered through an extracellular material. 

Types of Connective tissues

There are many types of connective tissue, such as

a. Loose connective tissue: its extracellular material contains loosely packed collagen (a protein) fibers. It holds organs in their specific place.

b. Fibrous connective tissue:   Its extracellular material contains tightly packed collagen fibers. It is in the form of tendons that attach muscles to bones and ligaments that join two bones.

c. Adipose tissue:   It contains swollen cells due to the presence of a large number of fat droplets. It is found around the kidney, under the skin, in the abdomen, etc. It provides energy, insulates against heat loss, and protects and supports organs.

d. Cartilage: Its extracellular material contains bundles of collagen fibers and a rubbery substance. It is found around the ends of bones, in the external ear, in the nose, in the trachea and larynx, etc. It provides support and maintains shape while allowing flexibility.

e. Bone: Its extracellular material contains collagen fibers and calcium salts. It supports, protects, provides a lever system for movements, stores calcium, and forms blood cells.

f. Blood: It is a special type of connective tissue. Its extracellular material (plasma) is fluid. It transports substances from one part of the body to the other and is also responsible for immunity.

3. Muscular tissues

They are composed of contractile cells or fibers held together by connective tissues. Each muscle fiber is an elongated cell. It has the ability to contract and relax. 

Types of muscular tissues

They are further divided into the following three categories

a. Skeletal muscles:  They are attached to bones and cartilage with the help of tendons. They appear as striped fibers under the microscope so also called striped or striated muscles. They are voluntary (under our control) muscles.

b. Smooth muscles:  They are found around hollow structures like the digestive tract and blood vessels. They are unstriated and involuntary (not in our control) muscles. They are under the control of the autonomic nervous system.

c. Cardiac muscles:  They are found in the heart. They are autonomic and involuntary in action.

4. Nervous tissues

They are composed of nerve cells called neurons. Each neuron consists of a cell body, axon, and dendrites. They are capable of transmitting impulses in the form of electrical currents.