27 August 2015

Chapter 6 Tissues

Class 9th Science: Chapter 6 Tissues

Plant Tissues

Plants are stationary, and hence are provided with some tissues made up of dead cells, which provide mechanical strength.  They have to withstand unfavourable conditions like strong winds, storms, floods etc.

Tissue:  A group of cells, that are similar in structure and work together to achieve a particular function, forms a tissue.  Different types of plant tissues include meristematic and permanent tissues.
Meristematic tissue:  Meristematic tissues are the tissues which have the ability to grow by rapid division.  They help in the primary growth of the plant.  Increase in length and increase in diameter of the plant are brought about by these meristematic cells.  Meristematic cells are living, cubical cells with a large nucleus.  These cells are closely packed with no intercellular spaces.  Depending on the region where the meristematic tissues are present, they are classified as apical, lateral and intercalary meristems.
•  Apical meristem is present at the apical or growing tips of stems and roots.  Apical meristem increases the length of the plant.
•  Lateral meristem is present in the radial portion of the stem or root.  Lateral meristem increases the girth of the plant.
•  Intercalary meristem occurs at the base of the leaves or at the internodes.  Intercalary meristem increases the length of the internode.

Old meristematic cells lose the capacity to divide and transform into permanent tissues. This process of taking up a permanent shape, size, and function is called differentiation.

Permanent tissues:  The cells which have lost their capacity to divide but are specified to provide strength, flexibility and elasticity to the plant. These tissues can be further classified into simple permanent, complex permanent and special tissues.

•  Simple permanent can be categorized into parenchyma, collenchyma and sclerenchyma based on their function.
•  Complex permanent tissue comprises of xylem and phloem. Xylem is useful in transport of water and soluble substances. Xylem is made up of tracheids, vessels, fibres and xylem parenchyma. Phloem is useful in transport of food molecules. Phloem comprises of sieve tubes, sieve cells, companion cells, phloem fibres and phloem parenchyma.

Parenchyma – These are living, polygonal cells with a large central vacuole, and have intercellular spaces between them.  Parenchymatous cells form the ground tissue and pith.

•  Parenchyma containing chloroplasts are called chlorenchyma.  The chlorenchyma help in photosynthesis.
•  Parenchyma which contain large air cavities are called aerenchyma.  The aerenchyma help in buoyancy.
•  Some parenchymatous cells act as storage cells for starch in fruits and vegetables.

Collenchyma – These are elongated living cells with small intercellular spaces.  Their cell walls are made up of cellulose and pectin.   Collenchyma occur in the peripheral regions of stems and leaves to provide mechanical support and flexibility in plants.

Sclerenchyma – These are long, dead cells with a deposit of lignin in their cell wall.  They have no intercellular spaces.  Sclerenchyma occur around the vascular tissues in stems, in the veins of leaves, and in the covering of seeds and nuts.  They provide strength to the plant.

Xylem – This tissue helps in the transport of water and dissolved substances throughout the plant.  The different components of the xylem include tracheids, vessels, xylem parenchyma and xylem fibres.  Tracheids and xylem fibres are made up of lignin, which provides mechanical support to the plant.

Phloem – This tissue helps in the transport of food throughout the plant.  The different elements of phloem include sieve tubes, companion cells, phloem parenchyma and phloem fibres.

Protective tissues:  These offer protection to the plant.  They include the epidermis and cork.

Epidermis – A layer of cells making up an outer covering of all the structures in the plant.  The layer epidermis is perforated by the stomata at certain places.  The stomata help in gaseous exchange and loss of water.

Cork – This is the outer protective tissue which replaces the epidermal cells in older roots and stems. Cork cells are dead and lack intercellular spaces.  Their cell walls are thickened by suberin which makes them impermeable to water and gas molecules.

Animal Tissues

A group of cells that have similar structure and function constitute a tissue.  Tissues aggregate to form an organ.  Many organs together constitute organ system.  Organ systems aggregate to form an organism. Tissues can be plant tissues or animal tissues.

Animal tissues:  These are the tissues present only in animals.  Different types of animal tissues are epithelial tissue, connective tissue, muscle tissue and nervous tissue.

Epithelial tissue forms a lining all over the body of the organism.  It protects the inner lying parts.  It is also secretory in function to secrete sebum and excrete wastes along with sweat.  Sometimes it is absorptive in nature.  Epithelial tissues act like a barrier to keep the different body systems separate.
•  Squamous epithelium has flat and thin cells with no intercellular spaces.  Squamous epithelium provides mechanical support and is found in the outer layer of the skin, lining the cavities of ducts, blood vessels and the chambers of the heart.
•  Columnar epithelium consists of cylindrical cells.  It is found in the lining of the stomach and intestines, and facilitates the movement of nutrients across the epithelial barrier.
•  Glandular epithelium consists of modified columnar cells, and is found in the sweat glands and tear glands to produce secretions.
•  Ciliated epithelium consists of columnar cells with cilia.  These cilia push the mucus forward into the nasal tract to clear it.
•  Cuboidal epithelium consists of cubical cells.   It is found in the lining of the kidney tubules, salivary glands and thyroid glands, where it provides mechanical support.
•  Stratified epithelium has epithelial cells lined up one over another.  It is found in the epidermis of the skin, the lining of the mouth cavity, and oesophagus.

Connective tissues are fibrous in nature.  They include blood, bone, ligament, cartilage, areolar and adipose tissues.  These help in binding other tissues together.  They also provide support to other tissues.
•  Blood has plasma and blood cells.  The blood cells suspended in the plasma include RBC’s, WBC’s and platelets. Blood flows within blood vessels, and transports gases, digested food and hormones to different parts of the body.
•  Bone cells are embedded in a hard matrix composed of calcium and phosphorus compounds.  Bones anchor the muscles.
•  Ligaments are tough and elastic.  They provide strength and flexibility.  Tendons are tough and non-elastic, and provide great strength and limited flexibility.
•  Cartilage has widely spaced cells suspended in a matrix of proteins and sugars.  It is found in the nose, ears, and the rings of the trachea to give flexibility.
•  Areolar connective tissue has irregular shaped cells and is found between the skin and muscles, around blood vessels and nerves.
•  Adipose tissue contains cells filled with fat globules.  It is found below the skin and acts as an insulator.

Muscle tissues are made up of muscle cells.  These are elastic in nature they have tensile strength. They bring about movement in the organism.  These muscles can be voluntary or involuntary in function.  Muscle tissue is categorized into visceral muscles, skeletal muscles and cardiac muscles. Muscular tissues are of three kinds namely striated muscles, unstriated muscles and cardiac muscles.

Nervous tissue is the tissue which works in coordinating the organs of the body by generating impulses.  It is made up of special cells called as neurons.   Each neuron consists of a cell body, which contains a nucleus, cytoplasm, called cyton, and elongated hair-like extensions, called dendrites.  One of the dendrites, called the axon, is very long.  Nervous tissues are found in the brain, spinal cord and nerves.

<< Back to NCERT/CBSE Notes