Tissues Summary Class 9 Science Chapter 6

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.
    

1. 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.

2. 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.

1. 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.

2. Collenchyma

• 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.

3. Sclerenchyma (Scleras–hard) Strengthening tissue.

• 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.
    

Sclereids

• 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.
  

Tracheids

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

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.

Xylem parenchyma

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

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.

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.

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.

Phloem fibre

• They give mechanical support to sieve tubes.

Phloem parenchyma

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

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.