Notes Class 9 Science Chapter 6 Tissues

Notes For All Chapters Science Class 9

Introduction

→ A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue.

→ Most of the tissues in plants are supportive, which provides them with structural strength.

→ These tissues are dead, since dead cells can provide mechanical strength as easily as live ones, and need less maintenance.

→ Plant Tissues are of two types Meristematic & Permanent tissues.

Meristematic Tissue

→ These are simple living tissues having thin walled compactly arranged immature cells which are capable of division and formation of new cells.

Features of Meristematic tissues:

→ Thin primary cell wall (cellulosic).

→ Intercellular spaces are absent (compact tissue).

→ Generally vacuoles are absent, dense cytoplasm & prominent nuclei are present.

→ Large numbers of cell organelles are present.

→ Active metabolic state, stored food is absent.

→ Actively dividing cells are present in growing regions of plants, example: root & shoot tips.

Classification of Meristematic Tissues on the Basis of Origin

• Primary (Promeristem)

→ Derived directly from the meristems of embryo.

→ They consist of cells derived from primary meristem.

→ They add to primary growth of plants.

• Secondary Meristematic Tissues

→ Formed by permanent tissues.

→ These are having cells derived from primary permanent tissue.

→ They usually add to the diameter of plants.

Classification of Meristematic Tissues on the Basis of Location

• Apical Meristem

→ It is present at the growing tips of stems and roots.

→ Cell division in this tissue leads to the elongation of stem & root, thus it is involved in primary growth of the plant.

• Intercalary Meristem

→ It is present behind the apex.

→ It is the part of apical meristem which is left behind during growth period.

→ These are present at the base of leaf & internode region.

→ These lead to the increase in the length of leaf (Primary), example: in grass stem, bamboo stem, mint stem etc.

• Lateral Meristem

→ It is also called as secondary meristem.

→ It occurs along the sides of longitudinal axis of the plant.

→ It gives rise to the vascular tissues.

→ Causes growth in girth of stem & root.

→ They are responsible for secondary growth.

Permanent Tissue

→ The permanent tissues are composed of those cells which have lost their capability to divide.

→ They have definite shape, size and thickness. The permanent tissue may be dead or living.

→ The division & differentiation of the cells of meristematic tissues give rise to permanent tissues.

→ In cell differentiation, developing tissue and organs change from simple to more complex forms to become specialized for specific functions.

→ The cells of permanent tissue loose the capacity to divide and attain a permanent shape, size and function.

• Permanent tissues are classified into two types on the basis of Structure and Composition i.e. Simple Permanent Tissues and Complex Permanent Tissues.

Simple Permanent Tissues

→ These are made up of same type of cells which are similar structurally and functionally.

→ They include two types of tissue Protective tissues and Supporting Tissues.

• Protective Tissues: These tissues are primarily protective in function.

→ They consist of Epidermis and Cork/Phellem.

(i) Epidermis

→ Epidermis forms one cell thick outermost layer of various body organs of plants such as leaves, flowers, stems and roots.

→ Epidermis is covered outside by cuticle. Cuticle is a water-proof layer of waxy substance called as cutin which is secreted by the epidermal cells.

→ Cuticle is very thick in xerophytes.

→ Cells of epidermis of leaves are not continuous at some places due to the presence of small pores called as stomata.

→ Each stomata is guarded by a pair of bean-shaped cells called as guard cells. These are the only epidermal cells which possess chloroplasts, the rest being colourless.

Functions of Epidermis

→ The main function of epidermis is to protect the plant from desiccation and infection.

→ Cuticle of epidermis cuts the rate of transpiration and evaporation of water and prevents wilting.

→ Stomata in epidermis allow gaseous exchange to occur during photosynthesis respiration.

→ Stomata also helps in transpiration.

(ii) Cork or Phellem

→ In older roots and stems, tissues at the periphery become cork cells or phellem cells.

→ Cork is made up to dead cells with thick walls and do not have any intercellular spaces.

→ The cell walls in cork deposit waxy substance called as suberin.

→ The cells of cork become impermeable to water and gases due to the deposition of suberin.

→ The cork cells are without any protoplasm but are filled with resins or tannins.

Functions of Cork

→ Cork is protective in function. Cork cells prevent desiccation, infection and mechanical injury.
→ Imperviousness, lightness, toughness, compressibility and elasticity make the cork commercially valuable.

→ Cork is used for insulation, as shock absorber in linoleum.

→ Cork is used in the making of a variety of sport goods such as cricket balls, table tennis, shuttle cocks, wooden paddles etc.

• Supporting Tissues: These are supportive in function.

→ There are three types of Supporting tissues i.e. Parenchyma, Collenchyma and Sclerenchyma.

(i) Parenchyma

→ It is the fundamental tissue.

→ Tissue first time evolved in bryophyte.

→ Thin walled cells, oval or spherical in structure.

→ Cell wall mainly composed of cellulose & pectin.

→ Large central vacuole for food & water storage.

→ Primary function is food storage.

→ Some parenchyma involved in excretory substance storage are so called as idioblast, storing such as resin, tannin, gums & oils.

→ In typical parenchyma chlorophyll is absent.

→ Chloroplast containing parenchyma tissue are chlorenchyma which perform photosynthesis such as mesophyll of leaves.

→ In hydrophytic plants aerenchyma (a type of parenchyma containing air spaces) provides buoyancy.

→ Parenchyma provides turgidity to cells.

→ It is the living mechanical tissue.

→ Elongated cells with thick corners.

→ Localized cellulose & pectin thickening.

→ Provides flexibility to plant parts & easy bending of various parts of plant.

→ Present only in herbaceous dicot stem.

→ Present at thin margin of leaves.

→ Few chloroplasts may be present.

→ Gives mechanical strength & elasticity to the growing stems.

→ Composed of extremely thick walled cells with little or no protoplasm.

→ Cells are dead & possess very thick lignified walls.

→ Lignin is water-proof material.

→ Intercellular spaces are absent.

• Cells of sclerenchyma are of two types Sclereids and Fibres.

→ These are also called grit cells or stone cells.

→ These are small cells, where lumen is so small due to higher thickening of cell wall, as present in drup fruit (mango, coconut, walnut) in legume seeds (Macrosclereid).

• Fibers

→ They are very long, narrow, thick, lignified cells. Lumen is large as compared to sclereids.
They are generally 1-3 mm long.

→ In the thick walls of both the fibres and sclereids are present thin areas called as pits.

→ Sclrenchyma Fibres are used in the manufacture of ropes, mats & certain textile fibres.

→ Jute and coir are obtained from the thick bundle of fibres.

Difference between Parenchyma, Collenchyma and Sclerenchyma

Complex Permanent Tissues

→ It consists of more than one type of cells which work together as a unit.

→ It helps in transportation of organic materials, water & minerals.

→ It is also known as conducting or vascular tissue.

→ Xylem & phloem together form vascular bundles.

Xylem

→ It is also known as wood and is a vascular and mechanical tissue.

→ Thick walled cells are found in the form of tubular passages.

→ Xylem consists of four types of cells called as elements Tracheids, Vessels, xylem parenchyma and xylem sclerenchyma.

(i) Tracheids

→ They are elongated angular dead cells (primitive elements) mainly involved in conduction of water and minerals in gymnosperms.

(ii) Vessles

→ They are advance element (generally found in angiosperms).

→ Vessels are cylindrical tube like structures placed one above the other end to end which form a continuous channel for efficient conduction of water.

(iii) Xylem parenchyma

→ They are small & thick walled parenchymatous cells subjected for storage of starch (food).

(iv) Xylem sclerenchyma

→ Thy are non-living fibres with thick walls and narrow cavities provide mechanical support.

→ Except xylem parenchyma all other xylem elements are dead.

→ The annual rings present in the trunk of a tree are xylem rings.

→ By counting the number of annual rings, we can determine the age of
a tree.

Phloem

→ They also consist of both parenchymatous and schlerenchymatous cells.

→ Phloem consists of four types of element which are Sieve tubes, Companion cells, Phloem fibre and Phloem parenchyma.

(i) Sieve tubes

→ Sieve tubes are slender tube like structures made up of elongated, thin walled cells placed end to end.

→ The end walls of sieve tube cells are perforated by numerous pores, called as sieve plates.

→ Nucleus of sieve cell degenerates at maturity. However, cytoplasm persists, because of protoplasmic continuation of sieve tube with companion cell through plasmodesmata.

→ Sieve cells possess slime protein or protein which is concerned with growth and repair of sieve cells.

(ii) Companion cells

→ Companion cells have dense cytoplasm and prominent nuclei.

→ Sieve cells & companion cells are so called sister cells because they originate from single mother cell.

(iii) Phloem fibre

→ They give mechanical support to sieve tubes.

(iv) Phloem parenchyma

→They store food and help in radial conduction of food.

(v) Leptome

→ Main part of phloem involved in conduction of food, which is sieve tube.

→ In xylem, only unidirectional movement is possible while in phloem bidirectional movement can occur.

→ In phloem, except phloem sclerenchyma all elements are living.