Tissues Notes – Class 9 Science
Tissue is a group of cells. The cells in a tissue can be similar or different. The groups of cells thus formed carry specific functions. A group of different tissues make an organ. A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue. In other words we can say that tissue is a cellular organizational level intermediate between cells and a complete organ. A tissue is an ensemble of similar cells from the same origin that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues.
1) MERISTEMATIC TISSUE
The growth of plants occurs only in certain specific regions. This is because the dividing tissue, also known as meristematic tissue, is located only at these points.
These are three types:-
- Apical Meristem
- Intercalay Meristem
- Laternal Meristem
1. Apical meristem is present atthe growing tips of stems and roots and increases the length of the stem and the root.
2. The girth of the stem or root increases due to lateral meristem (cambium).
3. Intercalary meristemis the meristem at the base of the leaves or internodes (on either side of the node) on twigs.
As the cells of this tissue are very active, they have dense cytoplasm, thin cellulose wallsand prominent nuclei. They lack vacuoles.
The primary growth of the plant occurs in the apical meristem. The growth in length of a plant part is due to primary growth.
Lateral growth or growth in thickness in a plant is called secondary growth, which occurs in lateral meristem tissue. Woody trees and shrubs display secondary growth when the plants become enlarged and thickened.
2) PERMANENT TISSUE
When the cells formed by meristematic tissue take up a specific role and lose the ability to divide. As a result, they form a permanent tissue. This process of taking up a permanent shape, size, and a function is called differentiation. Cells of meristematic tissue differentiate to form different types of permanent tissue.
Simple permanent tissues
These tissues are called simple because they are composed of similar types of cells which have common origin and function. They are further classified into:
Parenchyma is Greek word where “parn” means besides and “enchien” means to pour. Parenchyma is the most specialized primitive tissue. It mainly consist of thinwalled cells which have intermolecular spaces between them. The cell wall is made up of cellulose. Each parenchymatous cell is iso-diametric, spherical, or oval in shape. It is widely distributed in various plant organs like root, stem, leaf, flowers and fruits. They mainly occur in cortex epidermis, piths and mesophyll of leaves.
Function of Parenchyma
The main function of parenchymatous tissue is assimilation and storage of reserve food materials like starch, fats and proteins. They also store waste products such as gums, resins, and inorganic waste materials.
Collenchyma is Greek word where “Collen” means gum and “enchyma” means infusion. It is a living tissue of primary body like Parenchyma. Cells are thin-walled but possess thickening of cellulose and pectin substances at the corners where number of cells join together. This tissue gives a tensile strength to the plant and the cells are compactly arranged and do not have intermolecular spaces. It occurs chiefly in hypodermis of stems and leaves. It is absent in monocots and in roots.
Functions of Collenchyma
Collenchymatous tissue acts as a supporting tissue in stems of young plants. It provides mechanical support, elasticity, and tensile strength to the plant body. It helps in manufacturing sugar and storing it as starch. It is present in margin of leaves and resists tearing effect of the wind.
Sclerenchyma is Greek word where “Sclrenes” means hard and “enchyma” means infusion. This tissue consists of thick-walled, dead cells. These cells have hard and extremely thick secondary walls due to uniform distribution of ligin. Lignin deposition is so thick that the cell walls become strong, rigid and impermeable to water.
Sclerenchymatous cells are closely packed without intra-cellular spaces between them. Thus, they appear as hexagonal net in transverse section. The cells are cemented with the help of lamella. The middle lamella is a wall that lies between adjacent cells. Sclerenchymatous cells mainly occur in hypodermis, pricycle, secondary xylem and phloem. They also occur in endocarp of almond and coconut. It is made of pectin, lignin, protein.
Types of Sclerenchyma
Fibres– Fibres are long, elongated sclerenchymatous cells with pointed ends.
Sclerides– Sclerenchymatous cells which are short and possess extremely thick, lamellated, lignified walls with long singular piths. They are called sclerides.
Function of Sclerenchyma
The main function of Sclerenchymatous tissues is to give support to the plant.
Complex permanent tissue
A complex permanent tissue may be classified as a group of more than one type of tissue having a common origin and working together as a unit to perform a function.
These tissues are concerned with transportation of water, mineral, nutrients and organic substances. The important complex tissues in vascular plants are xylem, phloem.
Xylem is a chief, conducting tissue of vascular plants. It is responsible for conduction of water and inorganic solutes.
- Tracheids- Trachids are elongated, tube-like dead cells with elongated endwalls. End walls remain intact and possess piths. In transverse section, they usually occur as polygonal cells and lignified walls.
- Vessels – Vessels are placed one upon another. Their end walls are perforated. They form long tubes or channels for conduction of water and minerals.
- Xylem Parenchyma – Xylem Parenchymatous cells are living cells present in xylem. They help in lateral conduction of organic solutes and storage reserves.
- Xylem Fibres – Xylem Fibres are lignified fibres present in xylem which provide mechanical strength to the plant body.
Xylem is a major conducting tissue of vascular plants. It serves in upward movement of water and minerals from root to different parts of plant.
Phloem is a chief conducting tissue of vascular plants. It is regarded as a living tissue responsible for translocation of organic solutes.
- Sieve tube – Sieve tubes are long tubular structures composed of elongated sieve tube elements placed one above other forming a continuous tube.
- Companion cell – Companion cells are living cells always associated with sieve tubes. Sieve tube elements and companion cells arrive from the same, initial cell and therefore forms a single functional unit. Each companion cell shows presence of fine piths with all the living components of the cell.
- Phloem Parenchyma – These cells are living parenchymatous cells that are present in phloem. These cells help in storage of food.
- Phloem Fibres – Phloem fibres are formed by dead, sclerenchymatous fibres.
The main function of phloem is translocation of organic solutes from the leaves to the storage organ and later from the storage organ to the growing part. Sieve tube allows free diffusion of soluble, organic substances across sieve plates due to the presence of large number of sieve pores.
Based on morphology, animal tissues can be grouped into four basic types. Multiple tissue types comprise organs and body structures. While all animals can generally be considered to contain the four tissue types, the manifestation of these tissues can differ depending on the type of organism.
The four basic Animal Tissues are:
Connective tissue is comprised of cells separated by non-living material, which is called extra cellular matrix. As the name suggests connective tissues are meant to make connections. Connective tissue holds other tissues together such as in the formation of organs, and has the ability to stretch and contract passively. Bone, often referred to as osseous tissue, and blood are examples of specialized connective tissues
2. Muscle tissue
Muscle cells form the active contractile tissue of the body known as muscle tissue. Muscle tissue functions to produce force and cause motion, either locomotion or movement within internal organs.
Types of Muscles
1.Smooth Muscle or Visceral Muscle found in inner linings of organs.
2.Skeletal Muscle is attached to bone to provide movement.
3.Cardiac Muscle is in heart and makes possible the pumping action of the heart.
3. Nervous tissue
Cells comprising the central nervous system and peripheral nervous system are classified as neural tissue. In the central nervous system, neural tissue forms the brain, cranial nerves and spinal cord and, in the peripheral nervous system, peripheral nerves inclusive of the motor neurons. You can imagine the nervous tissue like an electrical wiring in which the brain is the power generator, the spinal cord is the main line and peripheral nerves are lines going to different organs.
4. Epithelial tissue
Epithelial tissues are formed by layers of cells that cover organ surfaces such as the surface of the skin, the airways, and the inner lining of the digestive tract. The cells comprising an epithelial layer are linked via semi-permeable, tight junctions.
Functions of Epithelial Tissues:
Protection of internal organs in case of intestine, kidney and heart and protection of external organs also. In fact skin is an epithelial tissue covering the body. Skin is the first line of defence against foreign substances. A burn victim doesn’t die because of burn but because the whole defensive layer of skin is broken exposing the body to various infections.
Plants generally grow where meristematic tissue is present. At the tips of roots and stems, the meristematic tissue is called the apical meristem.